Polar Exploration. Dixie Dansercoer

      Infrared illustrations of our body's heat radiation

      Polar bears can withstand lengthy periods without food. Male bears, for instance, are routinely forced to go without a major meal for three or four months each summer, when melting ice prevents them from hunting seals. Pregnant females apparently go without food for eight months – a record among mammals. Mothers even keep fasting for some weeks after their 1lb cubs, usually twins, are born between late November and March. By the time the cubs have left her care, one to three years later, however, the mother has rebuilt her energy stores and is ready for another litter.

      Staying cool and keeping warm

      Warm-blooded animals sweat or pant to lose heat through water evaporation. They can also cool off by moving into a shaded area or by getting wet. Only mammals can sweat. Primates, such as humans, apes and monkeys, have sweat glands all over their bodies; dogs and cats have sweat glands only on their feet. Whales have no sweat glands, but since they live in water don't really need them. Large mammals can have difficulty cooling down if they get overheated. This is why elephants, for example, have large, thin ears through which heat is lost quickly.

      Mammals have hair, fur or blubber, and birds have feathers to help keep them warm. Mammals with thick coats of fur which keep them warm in winter shed much of this in summer to help them cool off and maintain body temperature. Warm-blooded animals can also shiver to generate more heat when they get too cold. Some warm-blooded animals, especially birds, migrate from colder to warmer regions in the winter.

      Thus warm-blooded creatures try to keep their body temperature constant by generating their own heat when in a cooler environment, and by cooling themselves when in a hotter environment. To generate heat, warm-blooded animals convert food into energy; compared to cold-blooded species they have to eat a lot of food to maintain a constant body temperature. Only a small amount of food is converted into body mass; the rest is used to fuel a constant body temperature.

      Cold-blooded creatures take on the temperature of their surroundings: they are hot when their environment is hot, and cold when it is cold. In hot environments, the blood of cold-blooded animals can be much warmer than warm-blooded animals. Cold-blooded animals are much more active in warm environments, and very sluggish in cold environments: their muscle activity depends on chemical reactions which work quickly when it is hot and slowly when it is cold. A cold-blooded animal can convert much more of its food into body mass.

      Human heat regulation

      Over thousands of years we have evolved to a state where the most comfortable outside temperature (for a clothed human being) is 21°C. We need to be surrounded by air that is cooler than our body temperature by just the right amount so that heat flows away from our bodies at the same rate we generate it. We can work and play without being either cold or sweating profusely to maintain our core body temperature at 37°C. This temperature can only be maintained when we provide our bodies with enough energy (food and water) so that the heart can pump our energy-laden blood to our extremities and back.

      In a cold environment, we need to counter-attack the cold by ingesting more food and water, but that is clearly not enough. We also need suitable clothing to retain the heat produced by a moving body. A resting body produces a mere 100 watts of heat, whereas an intense workout can produce ten times as much.

      Of course, there comes a time when any living creature needs to rest or sleep. And when the cold gets a grip on a cooling human body, that body starts shivering as a reaction (shivering increases the amount of heat produced four or five times). At the same time, less blood will be delivered to both the extremities and to vital organs such as the heart and the brain.