Thermoregulation

There are two types of animals.

Poikilothermic (cold blooded) animals cannot control their body temperature. Their body temperature is influenced by their surrounding temperature. So when it is hot so is their body temperature and vice versa. Many of the animals are inactive during cold periods.
Homeothermic (warm blooded) animals can heat their bodies and keep their temperature constant no matter what the surrounding temperature. This means that they can be active even when it is cold.

In warm blooded animals the body needs to work within a strict range of temperatures. If the body cools to below 35°C then hypothermia follows. The body can no longer warm itself and death follows. If the body warms to above 42°C then the body over heats and this can lead to death. This is hyperthermia.

There is a need to control the temperature of the body so that it is within these acceptable limits. Temperature control is called Thermoregulation and is part of homeostasis.

There are many factors which can affect the amount of heat loss from animals.

The amount of insulation is an obvious one. Animals which live in cold climates generally have a thick layer of insulation (either fur, feathers or fatty blubber) to reduce heat loss.
The external temperature will also affect heat loss from animals. The colder it is the greater the temperature difference between the body and the surroundings (the temperature gradient) and the faster the animal will lose heat.
Small animals have a greater surface area in relation to their volume (surface area to volume ratio). This means that they have a greater amount of surface through which to lose heat in relation to their size. So large animals stay hotter for longer. As a result small animals need to eat a great deal to get enough energy to keep their bodies warm. This also explains the difference between African and Indian Elephants. African elephants live on the hot plains and need to cool quickly in the hot sun. Indian elephants live in the tropical rain forests where it is cooler. African elephants have very big ears and very wrinkly skin to increase their surface area so they can cool down. Indian elephants have smaller ears and less wrinkly skin as they do not need to cool down as much.