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Content Updated: 7th September 2008


What is hibernation?
Why help your parents raise their next generation of offspring?
Why extend your territory during the winter?
How is it that marine mammals are able to see underwater while we are not?
Why is the largest mammal bigger than the largest fish?
Is it okay for me to feed wildlife? Am I causing any harm by putting out table scraps or seed for local animals?

Q: What Is Hibernation?

Sleeping catA: The term “hibernation” probably stems from the Latin hibernare, meaning to “spend the winter”. I suspect that if you were to ask your mates in the pub tonight what hibernation actually was, those not robbed of the ability to form words or rational thought by the ravages of alcohol would say something along the lines of “sleeping the winter away”. As with most things in life, however, it is more complicated than that! Indeed, many scientists still disagree about precisely what hibernation entails. Generally, however, ‘true hibernators’ are characterized by a substantial drop in their body temperature – often to just above freezing! The hedgehog, for example, drops its body temperature to around 10-deg C (50-deg F), while some species of bat have been documented with body temperatures as low as 2-deg C during hibernation and dormice may have body temperatures below 4-deg C.

When an animal settles down to true hibernation, changes in heart rate -- leading to severe bradycardia (much reduced heart rate) -- and body temperature appear to be controlled by an area of the brain called the hypothalamus. The instigation of hibernation seems to be controlled by a blood-borne substance (Hibernation Inducement Trigger – HIT) that kicks in with declining day length and temperatures. (Photo: There's more to hibernation than a long nap.)

During hibernation, an animal relies on its reserve of white adipose tissue (normal fatty tissue) for energy. When a hibernating organism needs to ‘wake up’ it needs to generate a substantial amount of heat, in order to raise its body temperature up to a workable level. This heat is obtained from a type of fatty tissue found around the heart, back, shoulders and liver of hibernating animals and human babies, called brown adipose tissue (BAT).

BAT was first described in marmosets -- a type of fruit-eating monkey from South America of the genera Callithrix or Cebuella -- by Konrad Gessner during 1551, although its true function was unknown until much more recently. The precise mechanisms by which BAT operates are rather complicated, and I will bring it down to its most basic level here.  Under normal metabolic conditions, sugars (i.e. glucose) are broken down and converted to a compound called adenosine triphosphate (ATP), which is the energy currency of the body's cells. This process occurs in little organelles called mitochondria, which can be thought of as the ‘power houses’ of a cell. BAT contains a special protein that uncouples the proton pump that normally forms part of the ATP-making process, allowing the energy to be dissipated as heat, rather than going on to make ATP. For the biologists amongst you, the proteins allow mitochondria to uncouple oxidative phosphorylation (part of the ATP creation process involving an electron transport chain) and utilize substrates to produce heat rather than ATP. All you need to know, however, is that BAT is 'burnt' to produce heat, rather than energy that the body can use for growth. Protein uncoupling has recently been found in tissues other than BAT, suggesting that BAT may not be the only tissue that can produce heat in this way.

An intriguing five year study by a team at the North Carolina State University identified two genes that seem to control hibernation in mammals. One gene stops carbohydrate metabolism (so glucose is preserved for use by the brain and nervous system) and the other controls the production of an enzyme that breaks up stored fatty acids and converts them into a useable fuel source.

Many creatures reduce their activity during the winter months; often sleeping for prolonged periods. There are, however, none of the physiological changes associated with hibernation (e.g. considerable drop in body temperature, changes to heart rate, apnea, changes to metabolic rate etc.) – this is known as dormancy. In Britain, the only mammal species that truly hibernate are hedgehogs (Erinaceus europaeus), dormice (Gliridae) and bats (Chiroptera). Several others, including badgers (Meles meles) and squirrels (Sciurus vulgaris and Sciurus carolinensis), may go into a state of winter dormancy over parts of their range. Both dormancy and hibernation are usually triggered by decreasing light levels, lowering temperature and a growing absence of food.  (Back to Menu)

Fox with pram cartoonQ: Why help your parents raise their offspring, surely it is better for you (genetically speaking) to move away and have your own?

A: Intuitively this does seem like the best option – leave the family home, produce your own offspring, thus ensuring that your genes make it to the next generation. If you think about it, however, by helping your parents raise their subsequent broods (i.e. your brothers and sisters) you are getting some of your genes into the next generation. You have half your mother’s and half of your father’s genes, and so too will each of your younger brothers and sisters. Therefore, you are as closely related to your full siblings as you are to your own offspring. Consequently, by sticking around and helping with the maternal and paternal chores, you are increasing the survival rate of your younger brothers and sisters and thereby nursing your own genes into subsequent generations. Okay, so you’re not strictly getting your own genetic material into the next generation, but the genetic line -- of which you are a part -- is being maintained with your assistance. Obviously, there are factors that muddy the waters a little; relatedness is going to vary according to monogamy or polygamy, but you get the general idea! At this point, I feel I should point out that it is easy to disregard the above as a failure to acknowledge the emotions of animals: after all, do we think that foxes really count chromosomes? Of course not; foxes have no conception of chromosomes. Nonetheless, this process marches on regardless. Why? Simply because it happens without the animal's input: it is entirely subconscious. The emotions that we identify as love, attraction and cherish are all ways our genes have of 'helping themselves' live on down the generations.

There are, of course, other good reasons for staying with your parents. Three heads are better than one when it comes to finding a meal, and strength in numbers can be a big bonus for ‘seeing off’ any potential assailants. Moreover, helpers may gain vital parenting skills and even inherit the territory from their parents. Still, not all dispersal is voluntary! Certain species won’t tolerate their weaned offspring loitering about; female deer will, for example, readily chase away any of their progeny still around by the following rutting season. Indeed, it has been suggested that, in some species (foxes, for example), helping provision food and babysitting may serve as a form of 'rent', allowing subordinate individuals to remain on their natal territory. (Back to Menu)

Q: Why extend your territory during the winter?

A: Perhaps the main reason has to do with food availability. Generally, winter represents a tough time for wild animals – food is usually scarce (or buried under snow) and some creatures must survive almost entirely on the fat reserves they built up over the summer. Hence, increasing the size of your territory provides a greater distance over which to search for a potential meal. Although food is almost certainly a key factor in triggering an enlargement of territory, there may be an alternative explanation in species that mate during the winter months. Studies on Red grouse (Lagopus lagopus scoticus) by researchers at the Institute of Terrestrial Ecology in Scotland during the mid-1990s found that increased testosterone caused increased aggression and enlargement of territory in grouse cocks (males). In addition, cocks with added testosterone were observed to raise more chicks than their neighbours -- although their overall survival rate was decreased -- because females opted for the more aggressive ‘manly’ males. Recent research has, however, cast something of a shadow over the widely-held belief that females prefer the bigger, stronger or best-endowed males. Work by Jason Watters at the University of California at Davis revealed that female Coho salmon (Oncorhynchus kisutch) opt for the smaller, mild mannered jacks, rather than the aggressive boisterous hooknoses (same species, but with a different growth pattern). Dr Watters suggested that females might prefer Jacks because their earlier maturation -- males have to develop for a further year to become Hooknoses -- may signify increased quality and success. It seems that females with jacks worked harder on their nests and spawned for longer than those with hooknoses. Most intriguingly, Watters found that virgin females preferred Hooknoses, while experienced females opted for jacks – this suggests that females might learn through sexual experience.

Overall, it seems likely that there are several factors (including a scarcity of food leading to need for increased foraging area, and and increased testosterone levels) that interact to cause an animal to increase its territory during the winter period. Not all species expand their range during this season, however, so it doesn't appear to be an essential behaviour. (Back to Menu)

Q: How is it that marine mammals are able to see underwater while we are not?

Dolphin with binoculars cartoonA: Humans can see underwater, we just have difficulty focusing – hence objects appear blurred when viewed underwater without the aid of a scuba mask or goggles. Water is between 770 and 800 times denser than air and, as a result, light travels slower in water than in air. Indeed, light travels almost 23% slower in water than it does in air, which represents a reduction in speed of approximately 66,000 kilometres per second (or about 41,000 miles per second).

In the human eye, light passes through the cornea and is refracted (changes direction) -- because the cornea is of higher density than the air -- through the pupil and focused on the retina by the lens. Special muscles (called ciliary muscles) in the eye tighten and relax, distorting the shape of the lens and focusing the image. When the light hits the rods and cones -- special light-sensitive cells on the retina, rods sense shades and cones detect colour -- they ‘fire’, sending messages to the brain via the optic nerve. Under ‘normal’ circumstances, light passes from a gas (air) into a liquid (the vitreous humor in the eyeball), but when underwater, the situation is reversed. The great difference in density between the water and the cornea causes the human eye to lose most of its refractive power; the lens is unable to compensate for this and the image appears out of focus.

On account of the lens being unable to accommodate this difference in refraction, we become acutely hyperopic (long-sighted) underwater and objects appear blurred. Indeed, at distances more than five metres (about 16ft), humans are almost incapable of deciphering an object from the background underwater. Overcoming this problem requires a layer of air to be present in front of the eyeball (i.e. trapped by a scuba mask or goggles). This is not without its problems, however. If you imagine that our ray of light is happily travelling towards you at about 225,000 km per second (140,000 miles per second), suddenly it enters your scuba mask and accelerates to about 290,000 km per second (181,000 miles per second). This change of speed as the light passes from the water into the airspace of your mask causes its angle to shift slightly – this shift results in objects appearing about 25% bigger than they really are.

Fish obviously don't wear goggles, so how do they manage? Well, they have evolved to see things in the underwater world with the help of a larger, more spherical lens than land vertebrates. Additionally, there is very little difference in density between the fish cornea and that of the seawater, so there is very little refraction and focusing is left up to the lens (refraction of light in the human cornea is part of the focusing process). It is commonly considered that marine mammals (i.e. whales, dolphins, otters etc.) can see equally well above and below the waterline. The mechanisms for this ‘dual-fluid’ vision are, however, largely speculative. A study by Ronald Schusterman and Barry Barrett in 1973 revealed some interesting insights into the visual system of the Asian Short-clawed otter (Aonyx cinerea - below, left). Schusterman and Barrett designed a series of experiments to test the visual acuity (i.e. the capacity of the eye to resolve fine detail) of otters in air and underwater at different light levels. They found that the otters had almost equal visual acuity in water and air under conditions of bright light, but their underwater vision was poorer than their vision in air under conditions of dim light. The authors consider that otters probably have a similar focusing mechanism to turtles, in that the front of the lens is squeezed by muscles around the iris. Hence, they suggest that poor underwater vision in the dark might be a result of pupil dilation leading to an insufficient squeezing of the lens. In optical morphology, the otter uses a similar method of maintaining visual acuity as fish (i.e. the lens is more spherical so that it can compensate for the different refractive index of water).

Asian Short-clawed OttersIn whales, the difference in refractive index between the cornea and seawater is slight (less than it is in humans), so there is little focusing taking place at the water-cornea interface. Some evidence also indicates that many whales lack interocular (between eyes) muscles, suggesting that focusing is either less important or accomplished differently. Dolphins apparently have eyes more similar to fish than humans – the lens is greatly increased, almost spherical in shape and situated further forward in the eye. According to chairman of the Information Committee of the European Association for Aquatic Mammals, Jaap van der Toorn, the dolphin retina is “organized differently than most mammal eyes”, having two yellow spots (areas of high sensitivity, technically referred to as the fovea) rather than the single spot found in human eyes; one spot is probably associated with forward vision, the other with lateral vision. Interestingly, a similar structure was observed on the retina of the Great White shark (Carcharodon carcharias) by Samuel Gruber and Joel Cohen in their 1985 paper; it's thought that it may increase the sharks’ visual acuity in poor light conditions (especially at dawn and dusk). van der Toorn also notes that the dolphin pupil has a covering that can slide over it in conditions of bright light, possibly giving the dolphin a greater depth-of-field above water.

A paper to the journal Cetology during 1972, found that their study dolphins were myopic (short sighted) in air. In the paper, William Dawson and his colleagues suggest that the aerial acuity in Bottlenosed dolphins (Tursiops truncatus) may be equal to their underwater acuity because they have a pupil that becomes tightly constricted in bright light (i.e. in air) compared to water. Several studies on dolphin vision have suggested that, in water and possibly air, dolphins were emmertrophic (i.e. focused at infinity). In an intriguing paper published in the journal Marine Mammal Science back in 2001, Thomas Cronin and Tricia Litwiler attempted to establish a mechanism by which dolphin eyes were able to adjust to focus on objects at various distances. Their measurements of the refractive state of the dolphin’s eyes in water revealed that two individuals were indeed emmertrophic, while a third was slightly myopic. Interestingly, Cronin and Litwiler found no evidence for accommodation in any of the subjects examined. The authors propose that underwater vision may be used merely to supplement echolocation and, as such, the ability to focus sharply may not be necessary.

A study looking at pinnipeds (seals and sea lions) found a similar response to that described by Schusterman and Barrett in otters. In his 1970 paper to the journal Science, Ronald Schusterman found that, in the Californian sea lion (Zalophus californianus), only when luminance levels were reasonably high (in the range of 100 to 200 mL) were the sea lions able to resolve detail equally well in air and underwater.

In conclusion, it seems that there has been a degree of convergent evolution of the eye in marine species, leading to a scenario where the lens is larger with a more spherical shape to compensate for the loss of corneal focusing input. It is often difficult for us, a species for whom vision is such a crucial sense, to comprehend how animals are able to perform in a blurry world. When you watch aquatic predators hunting (e.g. otters feel for their prey, dolphins echolocate, whale sharks feed on microscopic plankton, etc.), however, it becomes apparent that the ability to focus a pin-sharp image is less important to them than it is to us. (Back to Menu)

Q: Why is the largest mammal bigger than the largest fish?

Blue whaleA: An interesting paper published in Reviews in Fish Biology and Fisheries back in 2002 looked at the question of “why are there no really big bony fishes?” In the paper, Jonathan Freedman and his colleague David Noakes -- both at the University of Guelph in Canada -- looked at physiological, morphological and life history data for a variety of elasmobranchs (sharks and rays) and bony fishes. None of the hypotheses they investigated were able to, on their own at least, explain the fact that the largest cartilaginous fish species is significantly larger than the largest bony fish (the Sunfish, Mola mola). The physiologists concluded that the discrepancy in maximum sizes between the elasmobranchs and bony fishes may lie in smaller initial body size (a factor of oviparity) and slower growth rate (due to indirect development), both of which may mediate the ultimate limit to maximum size of bony fishes. Now, although this doesn’t relate directly to our question, in the same paper Freedman and Noakes mention that gill size may represent a proximal limit to fish size – in other words, there is a maximum body size that can be reached with a given gill size. This is interesting as it steers us on to one of the possible reasons behind how mammals can grow larger than fish.

The largest living mammal species in the world is the Blue whale (Balaenoptera musculus - right), reaching some 33.5 m (110 ft) and weighing a staggering 150 tonnes. Conversely, the largest extant fish species is the aptly named Whale shark (Rhincodon typus - below, left), which attains a maximum length of about 21m (68 ft) and can weigh an impressive 36 tonnes or more. There have been several theories put forward to explain these differences, one of which revolves around one of the most fundamental gasses in the atmosphere: oxygen. In a fascinating communication to New Scientist in September 1997, M. Pannevis suggested that the discrepancy in size between these two species might be a result of the way in which each takes up oxygen. As Pannevis notes, seawater at 10-deg C (50-deg F), with oxygen saturation of 50% contains about 4ml of oxygen per litre; the oxygen content of water then decreases significantly with increasing temperature. The air directly above the water is at atmospheric pressure (i.e. 760 mm Hg), however, and contains about 21% oxygen, which translates to about 260ml of oxygen. The point to all of this is that whales (and mammals in general) have access to between 20 and 50 times more oxygen when breathing air than fish can extract from the water.  Thus, oxygen probably represents a significant limiting factor to the growth of fish over mammals.

Whale sharkAnother theory proposes that heart design may be a limiting factor to overall body size. It has been suggested that the differences in circulatory ‘wiring’ and heart morphology between fish and mammals may restrict body size. The circulation found in fish is often referred to as “single”, because the blood travels a single circuit: from the heart, to the gills, to the body, and back to the heart. Mammals, on the other hand, have “double circulation”, whereby the blood travels through two consecutive circuits. First blood is pumped from the right-hand side of the heart to the lungs and then returns to the left-hand side of the heart (the pulmonary circuit), from where it travels around the body and back to the right-hand side of the heart (the systemic circuit). The benefit of this double circulation is that it allows blood to be pumped through the lungs at a low pressure (protecting their delicate capillaries) and then the re-oxygenated blood returns to the heart where it is pumped at high pressure through the veins and arteries of the body.

Double circulation is possible because the mammalian heart is divided into two sealed sections along its vertical axis (each “chamber” is then divided horizontally into the ventricle and atrium); this prevents the oxygen-rich blood coming from the lungs mixing with the de-oxygenated blood returning from the body. In fish, however, blood pressure in the entire body must be low (fish hearts aren’t divided into separately sealed ‘sides’ like the mammalian heart) – if the blood left the heart at high pressure it would rupture the capillaries of the gills and the fish would die. Consequently, the blood leaves the heart at low pressure on its travels to the gills and then has to continue on to the rest of the body at the same pressure. Thus, the theory is that, the larger the body size, the greater the pressure required to push the blood around the body – so, fish (with their low blood pressure) are prevented from getting too big by their hearts.

So, which is the correct theory? Well, I think that it would be rather naive to assume that only one theory accounts for the difference in size between whales and fish. I’m of the opinion that it is probably the combination of a barrage of different factors -- oxygen availability, heart design, size at birth, growth rate and perhaps more subtle physical and thermodynamic factors -- that interrelate to limit the size of the various creatures that roam the planet. (Back to Menu)

Q: Is it okay for me to feed wildlife? Am I causing any harm by putting out table scraps or seed for local animals?

Blue-tit on bird feederShort Answer: Perhaps! Feeding wildlife is common across the globe and many people see it as a chance to help their local wildlife out during tough periods when food is hard to find. Others argue that food is an invaluable tool in providing ‘close encounters’ of the wild kind; without the use of food many people would never see some species and many of the photos adorning glossy wildlife magazines would never have been taken. There are, however, some compelling arguments against feeding wildlife -- including building dependencies, habituating animals to humans, increased aggression of fed animals, high mortality on the roads, substantial (unsustainable) increases in population numbers, digestive problems and the attraction of predators -- and the practice is now illegal in some countries. If you are going to feed your local hedgehogs, foxes, badgers, birds etc., consideration should be given to what food is offered and how the feeding is carried out.

The Details: Before we look at motivations and impacts behind the feeding of wildlife, we should quickly define what we mean by the term “supplemental feeding”. Supplemental feeding is quite simply the deliberate feeding of wildlife: whether you put up a bird feeder in your garden, leave a saucer of pet food out for your local hedgehog or fox, or give your sandwich to a squirrel in the park, all are considered supplemental feeding. This definition does, however, exclude things like Yogi Bear or a cousin breaking into your car and nicking your lunch, or animals breaking into a feed shed.

There is no doubt that the practice of feeding wildlife is widespread and big business – in 2006, the British Trust for Ornithology estimated that Britons alone spend around £200 million each year feeding garden birds. I have met people who feed their local foxes a bag full of meat every night; when you factor in these cases along with the thousands of other people who feed various other fauna in their backyards, I would not be surprised to see the total annual bill approach £1 billion! Elsewhere in the world, equally substantial amounts are spent on wildlife. The results from the 2006 National Survey of Fishing, Hunting and Wildlife-Associated Recreation (carried out by the Unites States Fish & Wildlife Service) show that 87 million people took part in wildlife-related recreation (71.1 million were “wildlife watchers”) and spent $122 billion (about £63 billion or €79 billion) doing so. Similarly, according to a report to the Texas Parks and Wildlife Department, during 2001, one million “Texas residents and nonresidents spent $1.28 billion [nearly £657 million or €830 million] in Texas on equipment and services related to their wildlife watching activities.”

So why are people willing to spend so much money on providing food for wildlife? Well, the reasons can be broadly spilt into two groups: helping animals out; and prolonging interaction/viewing time or likelihood. Let’s consider each in turn.

Helping animals out: Most people are aware that humans are having a significant impact on the environment and in far too many cases the impact isn’t a positive (or even neutral) one. The term ‘habitat destruction’ refers to any process that results in one habitat type being removed and replaced with another (all too often a ‘concrete jungle’ or ‘wasteland’) – in more common parlance, the term is extended to include overexploitation or pollution of a habitat. BirdLife International currently rank the destruction of habitat as the most serious pressure on the world’s bird species – some 86% of the globally threatened bird species are in decline because of habitat destruction. Habitat destruction combined with other factors -- including climate change, which whether one agrees that the current warming is largely a result of man’s activities or not, is causing a shift in the distribution and reproductive cycles of hundreds of species -- have been implicated in the decline of species across the globe.

Closer to home, here in the UK, we have seen a significant decline in the numbers and distribution of once common farmland and woodland bird species through changes to farming and woodland management practices as well as an increased need for housing. None of the aforementioned has gone unnoticed by the newsmedia and conservation issues are making the headlines on almost a daily basis, with the urge for people to ‘go green’ (i.e. to try and reduce their impact on the environment wherever possible). Consequently, it is generally considered that many people feed wildlife (and particularly birds) because they believe that the animals need this food – as we shall see, this is something of a double-edged sword. Indeed, the bird feeding industry is geared towards this exact presumption, selling different types of food at different times of the year (e.g. high fat during autumn and winter and high protein during summer) and to attract specific bird species.

Prolonging interaction: Wildlife watching is often painfully difficult. Most wild animals are either nocturnal (as in the case of many British mammals) or have a healthy fear of humans – in the majority of cases, both! So, if you want to get that fox, or those badgers to stay that little bit longer in your garden (perhaps for enough time to allow you to get the camera or camcorder out), what do you do? You put out food and, if you’ve done your homework, you probably scatter it around the lawn to prevent the animal in question from just picking it up and taking it away to eat it in a ‘safer’ location. Similarly, some of the most spectacular images of wildlife have been taken when the animal has been lured to the photographer on the promise of something to eat. Educational tours use the same philosophy to ‘guarantee’ close encounters with their chosen subjects; as most badger watchers have found, brock spends very little time milling around the sett of an evening, often emerging and taking a few sniffs, before heading straight out to feeding grounds – a couple of handfuls of nuts sprinkled around the sett helps keep the badgers about to permit observation.

Holidaymakers and film crews are drawn to some locations in the tropics and sub-tropics on the promise of being in the water with sharks – again, a guide with a block or box of fish help to ensure that the animals turn up.

Hand-feeding a badger
Food is a crucial factor in the lives of animals and plants and is often a limiting factor in the growth of a population. Putting out supplemental food may lead to an increase in the population that cannot be sustained should the food source be removed.

The belly rules the mind
The late opera singer Luciano Pavarotti once said: “One of the very nicest things about life is the way we must regularly stop whatever it is we are doing and devote our attention to eating.” This sums up nicely the role of food in the lives of animals (from the smallest of invertebrates to humans) – food is crucial to survival, without it we cannot survive and for this reason it is called a ‘limiting factor’. Limiting factor is an ecological term for anything that an animal (and consequently, the species) needs in order to survive – food, water, shelter, oxygen, etc. Different species may have different limiting factors and some are invariably more crucial than others -- typically, we can survive for up to about three weeks without food, but only about three days without water and three minutes without oxygen -- but, overall these factors can be usually be used to explain changes in a population.

Hand-in-hand with limiting factors goes the maximum number of individuals of a given species that an area of land can support (known as the ‘carrying capacity’). A population will increase in size until it reaches the habitat’s carrying capacity, at which point one or more limiting factors will step in to prevent it getting any higher. Let’s take an example.

Food is a crucial factor in the lives of animals and plants and is often a limiting factor in the growth of a population. Putting out supplemental food may lead to an increase in the population that cannot be sustained should the food source be removed.

Fox feastYou got to the supermarket every week for your grocery shopping; it’s always been well stocked and pretty quiet, but suddenly a new housing estate built nearby leads to an influx of other people, all trying to do their shopping. The supermarket only had a limited amount of food and when the last watermelon, say, is gone then that’s it – they may order some more but it’s going to take a couple of days for them to arrive (just like wild food takes time to replenish). So, you need to get one of those melons, but everyone else in the shop also has a desire for fruit salad, what do you do? Well, you try and get to the fruit first: maybe you get to the shop earlier, or barge your way through knocking pensioners and small children out of your way – this is known as competition (you and your neighbours are striving for the same thing and, invariably, some will succeed at the expense of others).

What happens if you don’t get to the melons in time, or can’t wrestle one away from another shopper? You either need to go somewhere else (another shop) and look or you have to change your plan and chose another species of fruit (assuming there are still some left!). This may be an inconvenience to you, but to a wild animal not being able to find sufficient food can spell disaster because, without it, death is sure to follow and the population will decrease in size until there are fewer animals than food items (more melons than shoppers, if you will) – at this point, more animals can survive in the area (more babies get the food they need and grow to adulthood) and the population rises until the shortage happens all over again. Limiting factors and carrying capacities join together to form a part of what ecologists call ‘density dependant’ population control.

Where am I going with this? Well, two of the main arguments against feeding wildlife are that it leads to artificially high population sizes -- which cannot be sustained should the supplementary food source be removed -- and that supplemental feeding may lead to dependence – taking the above into account, it is not difficult to see how this can happen.  The idea that provision of food by humans leads to greater numbers of animals and could lead to them becoming dependent on our hand outs go hand-in-hand. To return to our watermelon example; if some kind-hearted person drives to the supermarket in the next town and buys ten additional melons (which they bring back to your shop) that represents ten more people who will get one and ten (or more) people who will survive another day who may not have otherwise. This is all well and good, but what about tomorrow? If there aren’t an additional ten melons tomorrow, or the day after, how will you feed yourself and your family? Hence, wildlife biologists say that if you’re going to put out food, you need to do so consistently.

Indeed, in his fascinating overview of the issues and impacts of feeding wildlife as a tourist attraction, published in the journal Tourist Management during 2002, Massey University at Albany (New Zealand) marine ecologist Mark Orams writes:

The ability of animals to find food is often determined by learned behaviour – such as where to go, how to approach potential prey and how to effect capture of that prey. When an animal does less of this, they quite simply become less efficient at it.

This statement would seem logical and it is easy to imagine how an animal fed entirely on ‘human food’ may lose the ability to hunt (although cat owners might disagree!). The point here is that even where liberal quantities of food are provided, it rarely constitutes the complete diet of a given animal. Indeed, you may be relating the above to your own experience and thinking that you don’t recall your local fox becoming dependent on your handout, nor dying when you failed to put food out every night for it. You’d be quite right. Most animals are omnivorous (i.e. they eat a variety of different foods) and this helps them to survive through variations in one or other food item.

Foxes, badgers, hedgehogs, squirrels and many other common garden animals will gladly accept our handouts, but there is little more than circumstantial evidence that they come to expect it. I have noticed that foxes will turn up to a feeding station spot on time (if food is left out at a given time each night) and have observed them to show up for several days even after food provision has stopped, as if expecting the food to return. After a few nights of not getting any titbits, the fox fails to turn up (or, more likely turns up later if my garden happens to be on his beat). One interesting finding of tracking studies on foxes in Bristol, however, is that they seem to visit the gardens where they know that they’re going to get fed first; in contrast, hedgehogs seem to forage as normal, whether food is put out or not.

Red squirrel in nut feederThere is also little empirical support for the ‘dependency theory’ in the literature. Studies on foxes in urban areas of Bristol and Zurich have demonstrated that despite the food put out by home owners, birds and small mammals still typically account for at least 20% of the diet.  Nonetheless, scavenge (intentional provision or otherwise) may represent an important component of the diet and, in Zurich, more than 60% of stomach contents were scavenged meat and cultivated crops (namely fruit). In the UK, the diet of foxes foraging in urban areas can consist of between 30% and 65% scavenged food – studies in London suggest that the proportion of scavenge can be related to the where in the city the animal forages, with inner city foxes eating fewer earthworms and more scavenge than those foraging at the edge of the city.

A study by Craig Shuttleworth on the Red squirrels (right) at a nature reserve in Formby (UK) found that, even given the option of peanuts, the bulk of their diet consisted of natural foods (i.e. conifer seeds, shoots, buds and flowers) – when the natural food became scarce, the squirrels started eating more peanuts. Similarly, a study by Rebecca O’Leary and Darryl Jones at Griffith University in Queensland on Australian magpies (Gymnorhina tibicen) found that the diet of birds provided with supplemental food still included 76% of natural food.  Perhaps more interesting was that the wildlife biologists observed that the birds fed their chicks with predominantly natural foods, rather than taking the provisions back to their nests – in their conclusion, the authors wrote:

Magpies were not reliant or dependent on supplementary food provided by wildlife feeders at any time during the breeding season. Although magpies did utilize suburban feeding stations extensively, they continued to forage for and provision their chicks with natural food.”

It may be true that many species show no sign of dependency on human provisions, but there are some tragic examples of others that do. When we think of an animal being dependent on us for food, it is easy to think that our provisions mean that they somehow lose their ability to search for their own food (as per Mark ram’s comments). While this may be a problem for very young animals raised solely by humans, it seems unlikely to be the case for wild adults. Importantly, the ability to forage is only of benefit if there is food to find in the first place.  If the food just isn’t there (when our provision has been removed) or the animal’s digestive physiology doesn’t permit a rapid switching of diet, a true dependency can arise. A couple of good examples come from the USA.

During the winter of 1996 in Montana, the practice of feeding local deer made the news. At one feeding station a local woman explained that they could no longer afford to feed the deer, telling a wildlife biologist “Every day more and more [deer] come to the feeders, but we’re already spending $100 [about £50 or €65] a month. Isn’t there something you can do?” The answer was, no – the feeder was drawing deer in from miles around and they weren’t used to the diet of corn and hay the feeders were supplying. Deer are ruminants, which rely on microorganisms (bacteria and protozoa) in their stomachs to breakdown the plant material they consume (mammals lack cellulase enzymes and so cannot digest cellulose). The downside to this is that different microorganisms are needed to tackle different plant material; a diet of new shoots and other spring growth will lead the deer to have a different stomach flora to a diet of bark and woody shrubs. Moreover, time is needed for the microorganisms to develop in the gut. It is for this reason that deer cannot switch foods at the drop of the hat and why carcasses can be found having starved to death with full stomachs.

Another example comes from Monterey in California during 1988, when vendors were selling fish to tourists so they could feed the pelicans and sea lions. Unfortunately, this superabundance of food meant that the pelicans didn’t migrate and during the winter when the tourists had gone there wasn’t enough food to support them all. Many birds starved or died from Erysipelotrix infection (Erysipelotrix is a genus of bacteria that the pelicans contracted when they raided garbage bags for rotting fish remains); some took to stealing food from local people. However you look at it, when our provisions allow an increase in a population, the removal of the provision must invariably lead either to a decline in local numbers (population changes) or a change in the animals’ behaviour/diet to compensate.

Population Changes
Above, we have looked at the main population change that can come about when food is provided for wildlife by humans: that, all other things equal, numbers increase to a level that cannot be sustained if the food is removed and this leads to increased competition and mortality among animals. So, are there any other problems that act at the population level? Well, yes!

Reproduction: One, almost universal, finding from studies on the supplemental feeding of wild animals is that it profoundly affects both breeding season and fecundity (i.e. number of offspring produced). In the majority of cases, it is an increase in the aforementioned features that is found. This is precisely what one might expect, given what we have already discussed about food as a limiting factor regulating the carrying capacity of an environment.

Wild Boar pigletA recent paper to the Royal Society’s journal Biology Letters by a team of UK-based biologists reports that supplemental feeding can alter the population dynamics of songbirds by increasing future breeding success. The research team found that birds living on sites with food supplementation during the winter laid eggs earlier and had increased fledging success (i.e. more chicks survived to leave the nest), even when the food was removed six weeks before the breeding season started.  Similarly, a study of wild pigs (Sus scrofa - left) and moose (Alces alces) between 1986 and 1998 by Evg Nedzel’skii at Russia’s Irkutsk State Agriculture Academy found more barren (no offspring produced) females of both species in non-supplemented groups than fed ones. Nedzel’skii also observed more embryos and corpora lutea (the clump of cells that forms from an ovarian follicle after the release of a mature egg) as well as higher embryo weights in the provisioned group – the conclusion of the study was that supplemental winter feeding improves the fecundity of wild ungulates.

The above are just two examples of a whole plethora of similar studies showing that supplemental feeding increases the length of the breeding season and fecundity in a range of species, from Australian magpies (Gymnorhina tibicen) to Red squirrels (Tamiasciurus hudsonicus), Red foxes and Mule deer (Odocoileus hemionus). As we have seen, however, food is not always the factor limiting a population – if food is plentiful but, for example, shelter from the elements is lacking, the population will fail to increase. A good example of this can be seen in hares. In a paper to the Journal of Animal Ecology during 1992, University of British Columbia zoologists Mark O’Donoghue and Charles Krebs report the results of their study on how supplemental feeding influenced the reproduction and leveret (young hare) growth of snowshoe hares (Lepus americanus) in the south-west Yukon. In common with other studies, the zoologists found that food addition increased the number of hares on the survey grids, brought the breeding season forward by one week and lead to 5% more females falling pregnant than on the control (un-fed) grids. The study failed, however, to find any significant increase in litter sizes, length of the male breeding season, leveret growth rates or the total number of leverets born. Moreover, it appears that the higher densities on supplemented grids may have caused the hares stress, perhaps accounting for the higher rate of stillborn leverets.

Perhaps the most potent evidence that food is not always the primary limiting factor is the observation that, while the addition of food can certainly impact the density and demographics of a population, it doesn’t seem to alter patterns of population change. As O’Donoghue and Krebs point out, in mammal populations that undergo cyclic changes (such as those of snowshoe hares and voles) the decline in numbers is not prevented by the addition of food – other factors (e.g. disease, predation, climate, etc.) are at work here.

Nutrition: In his 2002 Tourism Management paper, Dr Orams notes that:

Surprisingly, there are few scientifically substantiated reports of negative consequences for the health and viability of provisioned animals.

Indeed, despite some empirical evidence that the food we feed animals isn’t good for them (conditions such as “lumpy jaw” in kangaroos and possums and increased fat depositions in the livers of Great Barrier Reef fish fed by tourists) there are few studies on the health of provisioned animals outside of zoos. Nonetheless, there is a considerable amount of human food that isn’t even particularly good (and here I mean healthy) for us, let alone wildlife! The food that we readily label ‘junk food’ is often high in sugars, salts and saturated fats, which can lead to many potential health problems. It doesn’t take a great deal of imagination to believe that highly processed foods probably aren’t good for wildlife either. In my experience, however, there is often the same kind of ‘haven’t seen it, so I don’t believe it’ attitude that is seen with smoking.

I have often heard smokers say something along the lines of: ‘My grandmother smoked 20-a-day all her life and died at the age of 95 without ever contracting cancer’. This is perhaps an expected -- if flawed -- perspective because people tend to gauge risks based on their own perceptions and experiences (which often leave out any lifestyle or genetic factors that may make you more or less susceptible to a condition). I know several people who opt to continue feeding animals highly processed food -- especially chocolate -- because they have never seen or heard of an animal dying from ‘a bit of choccie’. Chocolate (or more specifically, theobromine) poisoning does, however, happen (and your local vet could probably confirm it happens more than most people think); there is even a case that made it into the scientific literature of a fox and badger dying after gorging themselves on chocolate.

Obviously, the problem we have with feeding wild animals food that isn’t good for them is that we don’t constantly monitor them to see if they get sick (unlike our pets). If you feed your local fox a chocolate bar, chances are it will take an hour-or-so for the theobromine to kick in, by which time the animal will probably be a couple of streets away. Feeding the wrong types of food can have other implications. Magpies have been known to suffer serious bacterial infection of their beak when fed tinned meat (the meat sticks to the edge of the beak and mouth, leading to bacterial proliferation) – in rare cases this has apparently even lead to the birds losing their beak! Similarly, foods that give humans high cholesterol seem also to give birds (and probably mammals) high cholesterol too. Bread provides wildlife with little nutritional value and if the stomach is full of such nutritionally-poor items, there’s no room for high protein foods like insects.

PeanutsSome foods that are widely associated with use as wildlife feed and may seem rather innocuous to us can have serious impacts. Peanuts, for example, are provided in thousands of bird feeders (and around badger setts!) across the country, but it is important that they are “wild bird grade” nuts. Fungi of the genus Aspergillus (most commonly A. flavus and A. parasiticus) naturally occur in a wide range of foods, including peanuts. The fungi release mycotoxins called aflatoxins, which are metabolized in the liver and can cause serious liver damage – aflatoxins are also some of the most potent carcinogens known from the natural world. While humans have a considerable tolerance (no species is immune) to these mycotoxins, many other species are highly susceptible to aflatoxicosis. Wild bird grade peanuts cost more than regular ones because they are manually sorted, blanched and de-skinned before being tested for aflatoxin and rejected if any is found. Peanuts should only be offered in cage feeders or crushed; whole ones can choke birds and can also get stuck on the teeth of small mammals (e.g. hedgehogs), preventing them from feeding.

Animals may actively seek out specific natural foods to counteract some of the nutritional problems provisioned food can present. In a paper to the journal Wildlife Biology during 2000, biologist Craig Shuttleworth presents data from his study on the red squirrels (Sciurus vulgaris) at a 40 hectare (almost 100 acres) area of the National Trust’s reserve at Formby, West Lancashire (UK) between May 1994 and July 1996. Dr Shuttleworth found that, when the natural supply of conifer seeds was low, more than half a squirrel’s diet could consist of peanuts provided by visitors to the site or by residents in the gardens of nearby houses. Peanuts, however, have a high fat content (almost half of their dry weight), contain enzymes that prevent amino acids being absorbed and also have a high phosphorous content. Phosphorous plays an important part in the formation of bones and teeth and in a host of biochemical processes within animal cells; it should, however, be in an approximate one-to-one ratio with calcium and if the phosphorous level is too high it can cause problems for calcium metabolism (the process by which the body maintains its calcium level) and lead to hypertension (high blood pressure). Shuttleworth observed that when peanuts were eaten, so too was an increasing amount of “low energy” foods such as conifer buds – these buds are rich in calcium and the author suggests that the squirrels may eat them to compensate for the high phosphorous in the peanuts and help restore their calcium-phosphorous ratio.

Disease: Pathogens spread more rapidly through a population where individuals are packed closely together than in populations where the animals are more sparsely distributed. This makes sense when we think about it. The more crowded an area, the less distance germs have to move in order to reach the next potential host – I’m sure most people know that if one person gets a cold in the office, it’s not long before lots more come down with it. This is especially pronounced when individuals are stressed. For some time now we have known that stress has an influence on our immune system: while short-term stressors can stimulate the immune system, chronic long-term stressors (such as a shortage or food) can have a detrimental impact (it was recently demonstrated that chronic stress reduces the immune system’s capacity to respond to glucocorticoid hormones, which stop the inflammatory process after an injury).

An increased rate of disease spread through a population is one thing, but many wild animal species are also known hosts for pathogens and parasites that can be transferred to humans – so called ‘zoonotic’ diseases. For example, anthrax is an important zoonotic pathogen of humans, which is commonly contracted by ruminants (both wild and domestic) who consume the bacterial spores while grazing. Similarly, diseases can also be spread interspecifically (i.e. between species).  Perhaps the most topical example of this currently in the UK is bovine tuberculosis, which can be transferred from cow-to-cow and between cattle and (among other species) badgers – recent research in the USA has also linked supplemental feeding of deer -- White-tailed deer (Odocoileus virginianus) -- with the spread of bovine TB.

It is widely considered that feeding stations are an excellent reservoir for pathogens. In the struggle to get a handle on how squirrelpox virus is transferred both intra- and interspecifically (i.e. between members of the same and different species, respectively), the use of feeding stations has been implicated as a potential infection route (although empirical data are lacking). Feeding stations should be kept clean (i.e. disinfected regularly) and any un-eaten food should be removed.

Brown ratUnwanted Attention: It is almost impossible to put out food so that only one species is able to access it. When you hang up bird feeders in the garden you get the birds you want and those that you perhaps don’t (such as feral pigeons and magpies) – food falling to the ground (especially from seed feeders) also attracts rodents, which may not be a group that you wish to encourage. Leaving food out for badgers or hedgehogs will probably also attract foxes, which many people dislike.  If, like me, you don’t have anything against pigeons, magpies, rodents or foxes (quite the contrary, actually!) then, for you, feeding wildlife probably doesn’t come with any disadvantages (unless any of the aforementioned cause damage to your garden). It is always worth bearing in mind, however, that not everyone finds wildlife as endearing as we do – foxes may be fine in your garden, but your neighbour may not appreciate it when food put out in your garden is buried in their prize rose beds! (Photo: Feeders can attract species you like as well as those you may not. Brown rats are often attracted to food spilt from bird feeders.)

Non-target species come in many forms and another aspect to consider when you encourage wildlife in your garden is that some will invariably be predators – when prey is concentrated, predators are attracted. I have come across many comments from avid bird feeders that the smaller songbirds congregating in their garden attracts the attention of owls and other avian raptors. Additionally, in a recent (2008) paper to the Journal of Wildlife Management a team of biologists at the University of Wisconsin report that, on their study sites in Georgia -- where supplemental food was being provided for quail (Colinus virginianus) -- Red-tailed hawks (Buteo jamaicensis) were almost three times closer to supplemental feeding sites than control sites. The suggestion was that the quail were attracted to the provisioned food and the hawks were attracted to the quail.

Direct predation on the animals attracted to a feeder can be compounded by the more general potential increase in predation as a result of having the predator in the neighbourhood. In a paper to the Wildlife Society Bulletin during 2000, Susan Cooper and Tim Ginnett at the Texas A&M University (USA) report the findings of their study on the impact of providing deer feeders on rates of bird nest predation at a ranch in Uvalde County (Texas) between 1997 and 1999. The biologists found that when the ground cover was low (in very dry years), lots of their artificial nests were raided by predators and the lack of cover outweighed any influence that the deer feeders had. When the ground cover was sufficient to obscure nests from view, however, the presence of the deer feeders significantly reduced the number of eggs surviving. The researchers considered that the deer feeders attracted predators, which now in the vicinity, were able to find the nests. These data contradict previous studies, which have suggested that providing predators with supplemental food may actually reduce the number of nests raided.  As several authors have noted, however, one should bear in mind that supplemental feeding of predators is likely to lead to increased nutritional status (i.e. the predators are in better condition) and therefore potentially have greater survival and fecundity – this, leads to more predators in the area.

It perhaps goes without saying that feeding some predators in your garden (e.g. foxes, ferrets, mink, etc.) may put local inadequately-secured livestock (namely chickens) or pets (rabbits, guinea pigs etc.) at risk. In some areas it may even put you and your neighbours at risk! In the UK, we are fortunate (?) in that there are no longer any large carnivores (escapee big cats notwithstanding) roaming our landscape. In the USA, however, attraction of large predators can be a considerable problem. According to the Humane Society, during 1998 black bears broke into 1,100 vehicles in Yosemite National Park (California) causing $630,000 (£322,000 or €405,000) worth of damage. Similarly, on their website, the Colorado Division of Wildlife note how residents putting food out for foxes and deer attracted a mountain lion that had to be trapped and killed -- apparently at the residents’ request -- in order to make the neighbourhood safe again.

High population densities may also lead to increased fighting. In some species (e.g. deer), there is evidence that gathering at food sources in abnormally high numbers leads to increased aggression and competition between individuals. Such intensive competition often leads to the younger and weaker individuals (who invariably need the sustenance most) being excluded from the food. Provisioned animals may also be more aggressive towards people as a result of the feeding.  Several studies on primates (e.g. Macaque monkeys in Gibraltar and Thailand; baboons in Tanzania) have found that groups provided with food by humans tended to be more aggressive towards people. In the Thailand study, the macaques weren’t only more aggressive to people – they were also less healthy and less active than the control group (which weren’t given food by humans).

Nuisance: We have mentioned that not everyone is equally as appreciative of wildlife. I personally enjoy watching squirrels digging in the flowerbeds outside our window and tackling the bird feeders in my parents’ garden. I also know of several people who shoot squirrels (red or grey) that come into their garden; especially if they tackle their bird feeders. In an urban setting, the home range or territory of an animal is likely to be less (sometimes substantially so) than it would be in a more rural setting – this links back to the availability of their limiting factors. As such, if a squirrel, fox or other animal visits your garden it is not unreasonable to think that it will visit (however fleetingly) that of your neighbours and it probably lives fairly close by. The guy four doors down from you may not take kindly to foxes raising a litter or cubs under his shed and -- whether justified or not -- may blame your feeding station. Animals such as foxes, squirrels, rats and mice may also cause damage to the property, especially if they take up residence in the house.

There is some suggestion that where nuisance leads to persecution, animals may actually stay away, regardless of food put out for them; moreover, this might be an inherited trait. In a 1994 paper to the Wildlife Society Bulletin, a team from the Michigan Technological University studied the response of Grey wolves (Canis lupus) to the roads and human presence at the borders of the Kenai National Wildlife Refuge in Alaska. The researchers found that wolves avoided areas of heavy human traffic and suggested that wolves may teach their young to avoid areas that they associate with persecution. Brown bears have also been observed to shift their territories away from heavily-used highways and badgers in heavily persecuted areas show different emergence times to those subject to less persecution.

Road-kill badgerLocomotory Mortality: Some wildlife biologists have raised concerns that animals moving to and from feeding stations might be in greater danger of being killed than those foraging for ‘natural’ food. Between 1993 and 1996 zoologist Craig Shuttleworth studied the population dynamics of red squirrels (Sciurus vulgaris) in and around the National Trust reserve at Formby in Lancashire (UK). In a paper to the journal Urban Ecosystems, Dr Shuttleworth presents his results for the relationship between traffic mortality (i.e. the number of squirrels being run over) and the provision of supplemental food. Shuttleworth found that, in the presence of supplemental food (namely peanuts) the squirrels spent about half their time foraging on the ground – previous studies have found that the time spent on the ground by squirrels wholly reliant on natural foods was only 10% to 30%. The author suggested that the greater period of time spent on the ground, coupled with the observation that squirrels receiving hand-outs may cache (bury for later use) more than five times the number of items than those relying on nature’s bounty alone might lead to increased risk of being run over as they move from the reserve to, from and between gardens. From the data presented by Shuttleworth, there was no statistically significant correlation between the number of squirrels killed and the time they spent on the ground, but the highest road kill reported was during 1994, when animals spent the most their time on the ground (cf. the following autumn when only 37% of activity was on the ground).

We have already mentioned that, in some areas, bears have a habit or breaking into cars and we will consider the concept of habituation shortly. The potential for food provision to result in (potentially, at least) heavy mortality is, however, rather aptly demonstrated by Bighorn sheep (Ovis canadensis) in Colorado. On their website, the Colorado Division of Wildlife writes:

Drive up the Mount Evans Road [in the Rocky Mountains] just about any summer weekend, and you’ll see bighorn sheep – lambs and all – ready to romp onto the road as cars approach. The bighorns head straight for the car windows, often crossing right in front of the grills of four-wheel drive vehicles.

On their website, the Humane Society of the US point out that, in their experience, beggar animals in the Rocky Mountains tend to stay close to roads, where the risk of traffic collision is greater.

Behavioural Changes
Before we look briefly at the behavioural changes that may result from the provision of supplemental food to wildlife, we need to define a couple of terms – we need to establish the difference between “attraction” and “habituation”. In their 1998 paper to the Wildlife Society Bulletin Colorado State University biologists Doug Whittaker and Richard Knight define “attraction”, in terms of wildlife management, as:

“... the strengthening of an animal’s behaviour because of positive reinforcement, and implies movement towards the stimuli.

With this definition we have introduced a psychological concept known as “reinforcement”. Reinforcement is a pivotal concept in what psychologists and animal behaviourists refer to as “associative learning”; in other words, using ideas and/or experiences to enhance learning. The circumstance in which associative learning is most commonly found is “conditioning” (i.e. the process of modifying an animal’s behaviour) – there are two main types of conditioning: that which uses reinforcement of voluntary behaviours (“Operant Conditioning”) and those that cause an association between two stimuli and an involuntary response (“Classical Conditioning”). Reinforcement is achieved with the aid of a stimulus called a reinforcer – a reinforcer is classically defined (as per Nottingham University behaviourist Chris Barnard in his Animal Behaviour: Mechanism, Development, Function and Evolution) as:

“… any event that increases the probability that the behaviour it follows will recur in the future.”

I don’t want to get bogged down in the technical aspects of reinforcement -- it’s a fascinating subject, but it is easy to get caught up in the semantics surrounding the definitions of the different types reinforcement and their associated reinforcers -- but reinforcement (be it positive or negative) is coupled with punishment. Reinforcers make it more likely that an ensuing behaviour will happen again, while punishers weaken the behaviour, making it less likely that it will occur in future.

Food is a very potent reinforcer and is widely used in the conditioning of animals (including humans). Have you ever given your kid(s) (or, as a child been given) sweets for being good or brave? How about dog training – have you noticed that food is almost always used to reward the dog when it does as its master/mistress asks? These are all examples of operant conditioning (using positive reinforcement) – the animal is being trained to associate doing something (sitting on command or not making a scene in the supermarket!) with getting something to eat. Why is appropriate food so effective? It’s effective because it is a limiting factor: animals need it to survive. Having set the scene, we will return to this topic shortly, when considering the possible implications of teaching animals to associate humans with food.

It is important to distinguish the concepts of attraction and conditioning from that of habituation. According to Doug Whittaker and Richard Knight, the term “habituation” is commonly misapplied and confused with attraction. In their paper, they define habituation as:

“… a waning of response to a repeated neural stimuli …”

In other words, the animal in question starts to ignore whatever had previously caused it to respond. Our back garden borders a railway line and when we first moved in the train activity used to keep me awake at night; after a couple of weeks I was sleeping soundly and I now hardly notice the trains coming and going – I have become habituated to their presence. Animals too can become habituated to human presence – if you feed a fox for a few consecutive nights you will find that it will tolerate you moving a little closer. In most cases the animal will maintain a healthy distance from you (this is called the animal’s “flight zone”, beyond which it feels confident it could out-run you should the need arise), but I have seen some examples where foxes have become so habituated that they feed from the person’s hand (I cannot see how this does the fox any good).

SurferAttraction, Conditioning and Habituation of animals: There has long been concern that provisioning animals with food teaches them to associate humans with something to eat; this occurs as a result of operant conditioning – the animal learns that if it performs a particular behaviour (i.e. comes to a certain place at a certain time) it gets food. The concern is that the process may go a little further and teach the animal to associate people in general with a free meal.  Conservationists have taken this one step further and raised the issue that animals may even associate human-made inanimate objects with food outside of the scenario where they actually learnt the original association.

In 1998, New Scientist magazine carried an article telling of conservationists: “… worried by [shark-diving tour] practices which may lead great white sharks to associate food with items such as surfboards or children’s toys.” White sharks (Carcharodon carcharias) are notoriously curious creatures and it has long been established that they will investigate objects floating at the surface. A major shark attack theory of the 1980s (and still widely-followed in the media today) was that sharks might ‘mistakenly’ attack surfers and bodyboarders because their silhouette (against the bright surface, when viewed from below) resembles that of a pinniped (seal or sea lion). So, when it was discovered that objects such as surfboards elicited an investigatory response in great whites, it wasn’t long before they were being used by some researchers and shark-diving tours to draw in the sharks, which were then kept in the vicinity with the aid of a bait and chum (blood and fish bits suspended in water and ladled over the side of the boat).  In the article, George Burgess of the Florida Museum of Natural History is quoted as saying:

The sharks are getting the opportunity to find out that every time they see a surfboard there might be food around …”

Surfboards are one thing, but conservationists were also alarmed by reports that some operators were putting children’s toys into the water to encourage the sharks to investigate further and some “… gun their engines to ‘call in their babies’ upon arrival at dive sites.” All-in-all, the White Shark Research Institute’s (based in Cape Town, South Africa) Craig Ferreira told New Scientist: “Guaranteed there will be a death or bad injury”. So, the idea is that a great white used to visiting tour dive site may be swimming along one afternoon and, spotting a surfer sitting on his/her board waiting for a wave, associate the shape with being fed in the past and go to investigate further. If ever proven, this would be operant conditioning: the shark performs a voluntary behaviour (going to investigate), which may (or may not) result in getting something to eat (reinforcement). (Photo: Could using surfboards to attract Great white sharks to tour boats be teaching them to associate the boards with food?)

In the case of sharks, the suggestion that they may learn to associate humans with being fed doesn’t stop with Carcharodon: in the tropics, there has been much debate recently about whether shark dives (where tourists pay to dive and watch reef sharks being fed) should be allowed to continue. In August 2001 a holidaying Wall Street banker lost a leg in a shark attack when he was attacked by a shark off a beach in Grand Bahama – he successfully sued the beach resort for failing to notify their guests (of which he was one) that local dive operators run shark feeding dives just along the coast. Later that year (November), the Florida Fish and Wildlife Conservation Commission voted 6 to 1 in favour of banning shark feeding dives in their waters – the ban was challenged by some dive organizations (including PADI and DEMA), but they failed to have the ban lifted.

In order to circumnavigate the legislation, some dive operators simply took their tours further offshore. Consequently, there have been calls to ban so-called “interactive” shark diving on a much wider scale. The shark-diving debate was re-fired and received widespread media coverage during February of this year (2008), when an Austrian tourist was killed by a shark while on a diving tour in the Bahamas.

So, is there any evidence that feeding sharks (or any other animal for that matter) teaches them to associate humans in general with food providers? The short answer is “yes”, for some species at least. In his book Understanding Sharks, biologist Erich Ritter extols the virtues of using food (bait) to draw in sharks for study, writing that attitudes to sharks can’t be changed unless sharks and people can be brought together, which is almost impossible to do outside of an aquarium without the involvement of some form of bait. Dr Ritter goes on to say that -- outside of captivity -- it’s impossible to feed a shark sufficient a quantity of food to lead to dependency. Noting that the “dependency theory” assumes that every shark attending the dive eats (which is apparently not the case), Ritter writes that the arguments of conservationists imply that sharks are both somehow dim-witted and intelligent at the same time. Ritter’s argument is that on the one hand, it is said that when food isn’t provided, a conditioned shark would look for it from another human and bite a swimmer or surfer, because they’re “too stupid” to tell them apart from the food provider, but:

On the other hand, this kind of reasoning assumes that the shark has the ability to generalize. Sharks would be able to understand that the thing inside the diving suit providing food is exactly the same as that which sits at a large distance somewhere on a flat water in a swim ring and splashes.”

Ritter raises several good points in his argument. Dive operators and spectators note that not only is there no evidence to suggest that any of the shark attacks on swimmers or surfers are committed by animals that had previously attended feeding dives (although quite how you’d measure this, even if you wanted to, is something of a mystery to me!) and also that the sharks taking part in the dive seem perfectly capable of distinguishing those dishing out the fish from those sitting and watching. Unfortunately, however, the International Shark Attack File hold 24 accounts of people who have been injured while on shark feeding dives (although I would reserve judgement without being able to see all of the reports) and one might be forgiven for taking the view that even if the philosophy of feeding sharks is wrong, preventing such feeds is a ‘better safe than sorry’ approach.

As the debate about whether shark feeding should be allowed to continue rumbles on, what about other species? From a dependency perspective, I feel that it is very difficult to know whether an animal will become dependent upon your charity – even animals that we have raised from juveniles and we feel are dependent upon us are probably less so than we think if given suitable freedom. I think that the big exceptions here are the ruminants (as per the deer already discussed) – where we have altered their diet, they can be considered dependent upon us for food for a period at least. We have already spoken about the impacts of human feeding at the population level, but on an individual basis I am not aware of any studies to support the idea that (ruminants notwithstanding) animals fed regularly on human-provisioned food lose any of their hunting skills and are thus any less capable of providing for themselves if sufficient natural food is available. The idea of habituation, however, is a different matter.

Canada Goose with goslingsMillennia of persecution through hunting have driven a healthy fear of humans in most wildlife (and even some domestic animals). In some populations, however, repeated interaction with humans and reinforcement with food has altered this. You can take your lunch and sit in a park in almost any city on the UK and within minutes you’ll draw the attention of birds and squirrels – some will even sit on the bench next to you waiting for a hand out. Similarly, in parks where Canada geese (Branta canadensis - right) are regularly fed by humans, whole flocks waddle directly towards people in search of hand-outs. There is concern that being less afraid of humans makes them an easier target for those less endeared to wildlife: namely hunters and those with BB guns and too much free time on their hands.

Animals becoming habituated to human presence is just part of the story – there is also a problem with humans losing their respect for wild animals. In an article to the Friends of Monterey County Wildlife’s quarterly newsletter, Anne Muraski sums up the problem succinctly, writing:

For some reason, many people who would never consider petting a stray dog will readily approach a wild animal.

Indeed, habituation to wildlife can also lead us to take more risks than we might ordinarily accept with species to which we were less familiar. With few exceptions, humans are notoriously bad at judging the ‘mood’ and flight zone of animals, and getting too close can often result in problems, especially if the animal is already habituated to human presence and doesn’t run away or when food is involved. The results of such close encounters can be serious and there have been many cases of people being bitten, scratched or pecked by wildlife that they have gotten too close to. Perhaps one of the largest risks people take is hand-feeding wild animals. In his article to the Coastal Conservancy (California) website, California Fish and Game wildlife biologist Ron Jurek writes of an incident where a woman was hospitalized with a broken coccyx after being tossed in the air by a deer buck she was feeding apple slices. Worse, as Dr Jurek notes, is that each time a human is injured by a wild animal, there is often an outcry for the animal to be killed because it’s considered dangerous.

Ritter’s response to the conservationists’ argument that luring sharks to feeding stations teaches them to associate humans with food was that it assumes sharks can generalise – that they can link a human in one situation with a human in other. While I am not aware of any evidence to suggest that sharks can do this (and the situation is somewhat different for them), I recently had the opportunity to witness the ability of deer to generalise and how much of a nuisance it can make them.

My girlfriend and I visited a wildlife sanctuary in the New Forest (Hampshire, UK) and in one enclosure people were allowed in with a couple of Fallow deer (Dama dama). The sign on the gate asked that we didn’t allow the deer to eat our guide books and as soon as we were spotted, a deer came straight over and pushed up against us looking for food – I assume it was food she were looking for because, when she failed to find any, she began chewing on my girlfriend’s cardigan and my t-shirt. After repeatedly being pushed away, the doe eventually got the message and moved on to another couple in the enclosure. The deer could obviously recognise us as the same kind of animals that bring them food everyday (despite the different size, shapes and colours of the people visiting the enclosure and the fact that none of us were wearing the park’s uniform or pushing a wheelbarrow, which is how the food is brought into the enclosure).

Another aspect of habituation of wildlife through feeding is that the animals may fail to respond appropriately to predators – this was one argument against the rehabilitation and release of hedgehogs by Pat Morris. During their studies, Dr Morris and his team found that some animals became so used to being caught and weighed (which happened every night thanks to their radio transmitters) that they barely even bothered to curl up. Some of Morris’ critics suggested that this might have made them less likely to curl up when confronted by a badger. I’m not, however, aware of any data to suggest that a habituation to humans makes animals any less able to respond appropriately to any other predatory species. After all, the songbirds in our garden will readily go about their business while we sit out there or photograph them, but as soon as they sense a sparrowhawk (Accipiter nisus) in the vicinity they go quiet and retreat to their perches.

While habituation to humans may not change an animal’s response to a predator, it may cause it to put itself in greater danger than if it retained its ‘natural fear’ of man. Where I used to live in West Sussex, our neighbour used to feed a starling (Sturnus vulgaris) every morning in her garden. Over the years the bird became very tame; so tame that it would allow her to pick it up. On more than one occasion the starling came into our house and sat on the furniture looking at us. While this was rather charming to begin with, at the time my parents had three cats, any one of which would not have passed up the opportunity of a chance at catching the bird. I dare say that, had the cat arrived, the bird’s response would have been to fly away (as would be expected of any ‘non-habituated’ individual); however, by entering the house (which no other bird had ever done of its own free will), it had automatically put itself in greater danger than a wild one. Of course, perhaps I overestimated the danger; perhaps the bird was actually safer inside the house (despite being in an enclosed space). While outside the starling would be vulnerable to cats and other predators (e.g. birds of prey); in the house it was only vulnerable to the cats, which were perhaps more likely to be in ‘sleep mode’ than ‘hunting mode’. Nonetheless, I can’t help but feel that such ‘taming’ of wild animals does them few favours in the long run.

Do Not Feed the Seagull Signs
A selection of signs along the seafront at Penwith (Cornwall, UK) asking visitors not to feed the seagulls.

Home Range and Migration: We have spoken of how food represents an important limiting factor in the control of populations. Food is also intrinsically liked to another limiting factor: available home range. The area of land (or water) that a territorial animal uses is directly related to the amount of food available in the habitat. Red foxes (Vulpes vulpes) in our towns and cities (where food is abundant), for example, maintain a territory as small as 40 hectares (0.39 sq-km or 0.15 sq-mi.), while those in the highlands of Scotland (where food is considerably scarcer) may range over as much as 4,000 hectares (40 sq-km or 15.5 sq-mi.).

When a team of biologists at Bristol University studied the impact of supplemental feeding on the city’s fox population they found that when food was put out it was usually done so in quantities that far exceeded the amount required to support the local population. Moreover, the biologists observed that as a result of increased food availability, the foxes reduced the size of their territories – in one case so much food was available that the dominant pair confined their activity to half their original territory and the slack was taken up by their daughters. The combination of excess food and territory splitting lead to the population reaching a high of 30 adults per square kilometre (75 per sq-mi.) – the highest density ever recorded! The scientists also noted that where food was super-abundant, the foxes didn’t move very far and localized problems with foxes digging and defecating in gardens was reported by householders. Given that food availability seems to the most important factor determining the activity budget of an animal (i.e. how it spends its time), it follows that where supplemental food is provided, animals need to spend less time foraging and hunting or on other activities that may take them further afield.

The impact of supplemental feeding on home range use has also been studied in the Red squirrels, Tamiasciurus hudsonicus, of Canada by biologists at the University of British Columbia. Between June 1983 and June 1986, Thomas Sullivan studied the influence that supplemental feeding stations had on red squirrel populations and found that the average abundance of animals in the fed populations was three to four times higher than in the (non-fed) control. Dr Sullivan reports, in his 1990 paper to the Journal of Mammalogy, that more adults were recruited into the fed population, which had longer breeding seasons than the control. In a response to Sullivan’s findings, however, Rolf Koford of the US Fish and Wildlife Service suggested that the increase in squirrels recorded could actually be a result of individuals increasing their movements; effectively shifting their home ranges to get piece of the action (well, food!) – this has been demonstrated in grey squirrels (Sciurus carolinensis). Nonetheless, in their response Sullivan and Walt Klenner maintain that even when the transient squirrels (i.e. those that were only caught once) were removed from the analysis, there was still a three or four-fold increase in density and their other studies had demonstrated a close relationship between the capture frequency and the territorial status of the squirrels.

So, some species may decrease the territory they defend in the presence of supplemental food, while others (namely Sullivan’s red squirrels) seemed to actively defend feeding sites from intruders. This tells us that different species respond to supplemental feeding in different ways and it’s not always easy (even appropriate) to assume a particular response – the fact that supplemental feeders attract squirrels from miles and lead to a population increase, as we have seen, doesn’t mean the same is true of a sympatric tree squirrel species. Adjustment of home ranges and territories is one aspect of supplemental feeding, but it can also have deeper influences – influences on migration.

We have already mentioned the case of pelicans failing to migrate after tourists brought fish to feed them on. There are many more examples, especially from birds, of supplemental feeding altering migration timing or preventing it altogether. For example, according to the American wildlife charity Progressive Animal Welfare Society (PAWS), there have been significant problems with Canada geese in Washington because human food sources are so plentiful that many no longer migrate; instead they remain onsite and the population increases to the point where the birds are culled by the local authorities. Studies on deer have also demonstrated an alteration to movement behaviour in the presence of supplemental food. In a paper to the Journal of Wildlife Management during 2006, Chris Peterson and Terry Messmer at Utah State University report that Mule deer (Odocoileus hemionus) remained on their winter ranges when provided with additional food. In their conclusion, the biologists wrote:

Increased concentrations of mule deer for longer periods of time could impact winter browse and exacerbate human-wildlife conflicts

Hand-feeding foxWhy should animals fail to migrate when supplemental food is provided for them by humans? Well, first let’s sidestep the fact that defining what actually constitutes a migration is complicated (it’s not a simple definition to come up with, because Nature ‘refuses’ to be seen in absolute terms) and, for our purposes, call it the movement of predators outside of their home ranges to follow prey. This may sound a little odd to being with, but if we consider that, in its broadest sense, predation is the process whereby one organism feeds on another – consequently, herbivores can be considered predators (although not ‘true’ predators) of plants. So, for example, when a herd of wildebeest (Connochaetes spp.) move elsewhere looking for new prey (grass) they can be considered to be migrating; in the event that lions (Panthera leo) chose to follow them, they would also be migrating. So, the crux of our definition is that an organism is following its food – with this in mind, it is not difficult to see how supplemental feeding has the potential to significantly alter migration. If the food doesn’t move (or simply becomes substituted with something else), there’s no longer any need to embark on the often perilous journey looking for it! PAWS are also concerned that supplemental feeding may affect animals’ ability to decipher seasons (migration is often thought to be triggered by a decline in the availability of food, although it must be said that the jury is still out on this).

One final concern raised by conservation groups is that if animals fail to migrate, their old summer or winter stomping grounds may be developed by humans. For example, one of the many proposed sites for development in the UK is the Bristol Channel, where an energy company wants to erect 380 wind turbines to form the ‘Atlantic Array’. The plan is being challenged because the channel is an important feeding and breeding site for several species of migratory birds. One species in particular, the critically endangered (IUCN Red List, 2008) Balearic shearwater (Puffinus mauretanicus), migrates from breeding sites in Spain to feed in the estuary during the summer. If these birds failed to migrate, it would represent one less ‘arrow’ in the conservationists ‘arsenal’.

To feed, or not to feed…
Having read this far, you may be left with the feeling that there is a rather overwhelming case for you to remove that bird feeder or to stop putting out food for your local foxes, badgers, hedgehogs etc. An animal’s behaviour continually changes, however, as the animal adapts to the environment in which the individual finds itself. In settings where an animal is already significantly influenced by human activity (e.g. in towns and cities), accepting food that you’ve put out in your garden may simply be swapping one human-derived food source for another (e.g. scavenging from bins). So, does the food you’ve put out help improve the survival of the animals you aim to help and, if so, how can you ensure you provide them with the best possible fare?

Increasing survival
At the start of this article we established that, in the main, there were a couple of reasons people had for feeding wildlife: one of these was that they believed it helped the animals out by improving their chances of survival. So, is there any evidence for this? In some species there certainly is, while for others the data are less supportive. For example, during his study of red squirrels in British Columbia, Thomas Sullivan failed to find any consistent differences in the survival between those receiving supplemental food and those that weren’t. Conversely, in their 2006 paper to the Journal of Wildlife Management, Utah State University biologists Chris Peterson and Terry Messmer report that mule deer fed rations of whole corn, alfalfa hay and protein pellets suffered lower mortality (33%) than their non-fed conspecifics (55%). Similarly, supplemental feeding probably does little to improve ‘urban red fox’ survival (most are killed by cars rather than starvation), but it is generally considered to increase survivorship of songbirds.

For those wishing to put food out for their local wildlife, the important message here seems to be that, while appropriate supplemental feeding doesn’t always seem to improve survival probability, nor does it seem to decrease it. Nonetheless, if food is put out and then removed, the evidence suggests that the population will decline to the pre-provisioned level (the carrying capacity). How much of the decline is a result of death and how much of emmigration isn’t known for certain, but the possibility that the decline is largely a result of starvation is sufficient for many opponents of feeding wildlife to argue people shouldn’t put out food. It seems to me, however, that should provisions need to be removed, provided the food is declined gradually, the impact (in terms of mortality) can probably be minimized.

Middle ground…
Rowan berries
So, you really want to feed wildlife in your garden, is there a better way to go about it than putting the remains of your Sunday lunch out on the lawn? Well, yes, and the answer is probably in how you manage your garden.

Arguably, whether you garden in a manner that increases natural prey or provide a plate of mealworms, you’re still providing supplemental food for the animal(s) of your affection. I, however, see a crucial difference here. Not only are you providing the crucially appropriate food -- the stuff that the birds and mammals have evolved to hunt and forage for -- but you’re also providing some of the habitat that is disappearing rapidly on a global scale. Bramble bushes, trees, shrubs, flowers, lawns, log piles etc. attract invertebrates, which in turn attract their predators (birds, small mammals, reptiles, amphibians and the larger invertebrates) – small mammals, reptiles, amphibians and birds attract bigger (predatory) mammals, reptiles and birds. The greater the variety of ‘habitat types’ you can provide the better and, if you can avoid using pesticides (which are likely to be counterproductive to your original goal), all the better. Additionally, a pond serves as a valuable water source for all animals. (Photo: Ornamental Rowan trees are attractive additions to any garden and provide a bumper crop of berries that birds love.)

As we have seen, the general consensus seems to be that, if you plan to put food out for your local wildlife, it is best to leave out food that would form part of their natural diet. For example, if you plan to feed your local foxes, put out meats (chicken, steak, etc., avoiding highly processed foods like sausages, spam etc.), eggs, insects (perhaps mealworms or insect mix sold by pet shops for reptile owners), fruits, nuts and so on. Also avoid foods like bread and milk (hedgehogs will eat it, but it’s far from good for them) and, if possible leave out a bowl of clean water.

In his book Urban Wildlife, Wildlife Trust Director of Community Affairs Peter Shirley MBE suggests mixing up your bird feeders. Along with advocating the growing of native flowers, shrubs and trees to encourage insects and other creepy-crawleys, Shirley recommends using several places to feed the birds rather than just one – this way it spreads them out, making it more difficult for predators (including the local cats) to nab them and gives the less aggressive birds a chance. Shirley also suggests offering food in different ways; using hanging feeders, tables, ground-based feeders and pushing nuts and other treats into the bark of trees to stimulate natural foraging behaviour.

Wildlife and the law
The final point to consider when feeding wildlife is the local laws. To the best of my knowledge -- outside of nature reserves, zoos and wildlife parks -- there are no laws forbidding the feeding of wildlife here in the UK (although feeding some ‘pest’ species is discouraged in many large cities); however, this is not the case elsewhere in the world. It is illegal to feed wildlife in Northern Australia, for example, as it is in many American states – Monterey County, Ventura County, and Northwest Minnesota to name a few. Some areas have specific no-feeding laws. For example in Alaskan wildlife parks it is illegal to feed any animal except a songbird (which have registered a serious decline of late), while in Arizona it is currently illegal to feed any animal that isn’t a songbird or tree squirrel. Similarly, it is illegal to feed deer in Virginia (but some animals are allowed to be fed) and it is against the law to feed alligators in Georgia. So, the advice here has to be to check before you feed!

Hedgehog eatingIn conclusion…
So, where does all of the above leave us in respect to whether we should, or shouldn’t, feed wildlife? The general opinion of wildlife conservation groups and many government bodies is that supplemental food shouldn’t be provided and there is a substantial amount of evidence (all be some of it rather circumstantial) to support the decision. Many species are, however, experiencing drastic declines in numbers as a direct result of the impact humans are having on the Earth. Consequently, it is difficult to argue that we should not help them out if we can. The key here is that, if supplemental food is to be provided, it should be appropriate food, not highly processed (fatty, sugary and salty) ‘human food’. Feeding stations should be kept clean and any uneaten food should be removed – water should also be available if at all possible.

People who partake in wildlife feeding should observe the animals from a distance and should avoid direct interaction, however tempting it may be. These people should also be sympathetic to the concerns of their neighbours and be prepared to come to a compromise should any wildlife attracted by their food start causing damage to property. If the food needs to be removed at any time, it should be phased out gradually if possible, rather than being removed in one hit.

In the end, the best practice is to garden for wildlife. Maintain your garden so as to provide plenty of different plants, shrubs and trees for animals on which to live, hunt and shelter. By creating a wildlife garden you will not only have a beautiful area to sit and relax on a warm summer evening, but you’ll also have a host of different animals and plants to enjoy. Moreover, you don’t have to own a mansion with rolling grounds to make a wildlife garden – as Chris Baines writes in his landscaping opus How To Make A Wildlife Garden (I highly recommend this book if you’re looking to create a haven for wildlife in your back yard):

Don’t imagine you need a five-acre country estate before you can begin to plan for wildlife. Even a window box can provide a welcome resting place for passing butterflies if you plant the right flowers…

Ultimately, all of this comes down to doing what’s best for the wildlife in your area. In some cases, feeding wildlife can cause more problems than it solves. It is up to the individual to take a responsible approach to the matter, weigh up the pros and cons and make an informed decision on how to proceed. (Back to Menu)

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