Survival tactics

25th February 2005 at 00:00
Ben Aldiss explains some of Nature's tricks that help animals cope in cold weather

What's the weather doing today? This winter has been unpredictable, with some mild weather interspersed with storms and a few sustained periods of bitter cold. Spring may have arrived early, it could be lashing rain, it may even be snowing. If you're reading this in a cosy staffroom and it's snowing outside you're probably not too worried as you enjoy the comfort of your usual chair with a warming cup of coffee. You can just sit back and thank technology for keeping you insulated from the fury of the elements.

The bleak outdoors can be conveniently ignored until you dash to the car.

But what if the car gets stuck and you're stranded on the way home, or the central heating packs up? Suddenly you realise how vulnerable you are.

Luckily, humans are resourceful and intelligent - by fashioning the world around ourselves like a protective cocoon, we've circumvented the design faults in Homo sapiens. From clothing to houses, and thermostats to transport, we've systematically solved the problems of survival by our inventiveness, allowing us to emerge from our ancestral home into hostile environments all over the planet.

Without these props we're pathetic creatures: it doesn't matter how fit, strong and intelligent we are - left naked to fend for ourselves in a snowy landscape, we would die of hypothermia in minutes. If humans can't manage, how do the plants and animals around us cope? Most are adapted to survive in such an environment. Whereas a naked human starts to thermoregulate when the temperature drops below 27C, many of our mammals and birds don't begin to feel the cold until it is much lower. This explains why mallard ducks in the park look relatively happy even when their pond is iced over. We simply ascribe human sensations to them and think they ought to feel cold. Other animals can cope with even greater extremes. That rugged working dog of the Arctic - the husky - only begins to feel uncomfortable when the temperature dips to minus 40C.

Every animal and plant in the countryside has its own special way of facing the winter. Low temperature is just one of the factors they're up against.

Others include dim light, strong winds, too much water (during floods), or too little (during extended freezes), but perhaps the most crucial is the lack of food. Insects depend on the ambient temperature to keep them warm and active: they are ectotherms. Most rely on green plants or living prey for food, so it's no coincidence that they tend to reach the end of their short lives by the first frosts as food becomes scarce. Feeding on the abundant food of summer and early autumn, they mature into adults, mate and lay eggs before the falling temperatures cause them to slow down and die.

Like chicken eggs, those of insects have a tough, protective outer covering and the development of the embryos inside can be arrested almost indefinitely until temperatures rise again. The eggs are effectively tiny time-capsules, holding their secret contents in suspended animation until the signs of spring welcome them into the world.

A few insects cope in other ways. Honeybees are a bit like humans - interlopers in an unsuitable environment. They originated in the hot steamy jungles of south-east Asia, but spread into more temperate climates by virtue of their sophisticated social set-up. Although they're ectotherms, like other insects, their activity can generate a lot of heat. Collectively they can use this heat to warm the hive and stay alive throughout the coldest of winters. Using honey made from nectar collected in the summer as their source of energy, they huddle together on the wax combs to create a living ball of bees, with the all-important queen at the centre. This ball pulses with life, responding to every change in the external conditions.

When the temperature plummets, the ball gets tighter to seal in the heat.

As the bees on the outside get cold, they eat some honey and wriggle towards the centre, while their warmer sisters take up duty on the outer edge. Conversely, in warmer weather the living ball gets looser to dissipate heat. It's a very sensitive system, with the bees able to use their internal thermostats to keep the centre of the hive at a remarkably constant temperature, slightly above that of the human body: 37C.

Wasps, hornets and bumblebees have a different strategy. Although they, too, live in colonies, their nests can't survive the winter. This might seem odd, as all these insects are native to Britain, unlike the honeybee.

The difference lies in the honey, or rather the lack of it. Wasps and hornets have enough individuals in each nest to huddle for warmth, but, because they don't make honey, they die of starvation before the end of winter. Bumblebees are more like their cousins, the honeybees, but they only store enough honey to tide them over periods of cool rainy weather in the summer. Their colonies are very small, so no amount of huddling would provide the heat necessary to sustain them until the following spring.

This means that the huge numbers of wasps, hornets and bumblebees we see throughout the spring and summer die as the days shorten and the leaves fall from the trees. The only survivors are the newly-mated queens. These can sometimes be seen busily visiting autumn flowers to find nectar and pollen. Over the course of a few weeks they gorge themselves with food to increase their fat-reserves, ready to hibernate in a cool, dry place for up to seven months. During this time they remain motionless, their metabolism barely ticking over and their body temperature never quite dropping to freezing. On their awakening in April, they will have burnt off all the fat that they stored during the previous autumn.

Apart from a few unusual species such as the December moth that flies on winter nights, and the hibernating tortoiseshell butterfly, most moths and many butterflies pass the winter as pupae - almost motionless, hard-shelled structures in which the miraculous transformation from caterpillar to adult takes place. Smooth, rounded and shiny, with a row of small openings called spiracles down each side, looking like portholes on a ship, many such pupae lie buried in the earth. They await an unknown signal to complete their metamorphosis while winter rages above them. To thwart the possible invasion of bacteria and fungi, each pupa remains hermetically sealed for all but a few minutes each month. During this time the slow metabolism that effects the change from caterpillar to moth creates a build-up of carbon dioxide that could prove fatal. When the concentration reaches a certain level, something remarkable happens - the pupa opens a valve in each of the 14 spiracles and the gas is pumped quickly out. Before a bacterium or fungal spore has time to get in, the valves shut again, creating an impenetrable barrier once more.

Mammals and birds also have to find ways of surviving the rigours of winter. What strategy they adopt depends on four major factors: size, mobility, insulation, and food availability. If food becomes scarce, very mobile creatures can migrate. Birds such as warblers and swifts are good examples. Being insectivorous, they might starve if they attempted to remain here in a cold winter, so they fly to southern Africa. The wren is an exception. You've almost certainly got a few of these tiny birds in your school grounds. Despite being insect-eaters and so small that they lose heat rapidly, these hardy little creature manage to survive our winters somehow. True, many die in particularly harsh years, but by huddling together in large numbers behind a protective screen of ivy, or in a nest-box, they can conserve heat and wait out the worst of the weather (see "Home-building", above, on how to make one).

Medium-sized and large mammals such as the fox or deer have enough fur to insulate themselves and can find sufficient food for their winter needs.

Smaller mammals have a problem, though. Their ratio of surface area to volume is relatively large, resulting in too much heat loss. On a cold day, the energy generated by their diet fails to keep pace with that required to stay warm. Such animals employ various degrees of torpor to cope with this difficulty. This is the ability to reduce their body temperature by a significant amount in response to certain signals - a drop in temperature or light intensity, for instance.

Squirrels do this during particularly cold weather, so saving energy as their metabolism falls. Hedgehogs, bats and badgers also go into periods of torpor. The hedgehog, being quite small and carnivorous, has difficulty finding enough food to boost its energy in very cold weather, so it makes a nest of dry grass and leaves, curls into a ball to reduce its surface area, then lowers its metabolism for weeks at a time. Bats hang motionless in dry caves throughout the winter and even in the height of summer they habitually enter torpor to conserve energy while sleeping.

Hibernation - the extreme form of torpor - is seen par excellence in the animal made famous by the mad hatter's tea party in Lewis Carroll's Alice in Wonderland. The dormouse is too small to remain active in prolonged cold weather, especially as its food (mainly nuts, berries and fruits) is hard to find in the depths of winter. From October to April, it remains in deep hibernation in a nest of leaf litter, its body temperature dropping from nearly 40C to around 5C. It hibernates curled up into a ball, on its back, with its hind legs exposed. It seems that its feet act like thermometers, signalling the dormouse's metabolism to vary according to the outside temperature. This ensures that the animal will never freeze, even in the harshest of conditions.

Bears are renowned hibernators, but recent research has shown that they really go into a lesser form of torpor, known as dormancy or seasonal lethargy. Rather than their body temperature dropping dramatically, it falls by only two to six degrees. Although they remain almost motionless throughout their six-month sleep, they can be roused frighteningly fast if their dormancy is interrupted - as one or two brave researchers have discovered. Whereas human muscle wastes away when not in use (as anyone who's been in plaster for a while will know), a bear's muscles remain in tip-top condition throughout its slumber, ready for immediate use in the face of a possible threat or a nosey researcher.

The last brown bear in Britain was killed in Roman times, but populations still exist in parts of Europe. It might be thought that such a big, furry animal could survive the winter without the need for hibernation, but the food supply required to keep that 300kg body warm would be so enormous, that bears would have trouble feeding themselves in the snowy mountains.

At the other end of the scale is a creature so strange as to be almost unbelievable. Called the "bear animalcule" because of its resemblance to the lumbering giant just described, the tardigrade is a microscopic creature found in cushions of moss, such as can be found on top of any old school wall. Its other names are "moss piglet" and "water bear". When asked to describe one, a fan of AA Milne once explained that it looked like "Pooh Bear with his head in a honey-pot".

We know very little about these creatures, but one thing is certain - their ability to survive drought and freezing conditions is legendary. By a process known as anhydrobiosis, tardigrades can survive for many years in a completely dry shrivelled state, known as a "tun". Within an hour of being immersed in water, they are moving again (see lesson ideas). They can also survive temperatures well below zero, so this makes them well-suited for life in the average school moss-cushion, throughout winter as well as summer.

From the huge to the tiny, every creature has to cope with winter if its numbers are to remain healthy the following year. As we look forward to spring and all these animals are released from their winter shutdown, spend a minute to consider the challenges faced by our familiar wildlife as we snuggle under duvets and and keep the central heating on high.


The Mammal Society: The Mammals Trust: ARKive (an excellent site, patrons include HRH Prince Philip and Sir David Attenborough): BBC science and nature (an excellent site, with information on hibernation): tts_index.shtml

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