Internal energy storage

As continuous access to energy is necessary for survival, the level of energy stores has a strong effect on fitness and survival. Behaviors by which energy stores are acquired and built up are known as foraging, and this is a very common topic in animal behavior studies. It is especially important for flying organisms to finely tune the level of energy reserves they carry. While most animals will use carbohydrates for high intensity work and fat mainly for long-term storing, birds have the ability to build up and metabolize body fat deposits quickly. As fat is lighter than carbohydrates this is an adaptation to their flight ability. Optimal energy regulation has been especially well studied in two avian systems; small birds in cold environments and birds on migration. Wintering small birds will experience two sources of mortality, starvation and predation, and it is the sum of these that they must minimize. Many of these birds are large-scale food hoarders that depend on stored food for their winter sustenance. As these have reliable access to stored energy supplies they can afford to carry less fat than non-hoarding species. A decrease in body fat level will decrease predation risk. In cold environments body size is very important as small bodies will cool quicker than large ones. During cold nights many small birds will lower their night-time body temperature to a hypothermic state to decrease metabolism and save energy. In this way they can survive cold nights on smaller fuel reserves than needed at their normal body temperature. An extreme example is provided by some hummingbirds that may have a core temperature of 7°C during cold nights. The more fuel a migrating bird carries, the heavier it becomes. This means that the additional distance that a bird can fly non-stop will be described by a curve of diminishing returns when the fuel load increases. Yet some birds are able to make astonishingly long flights. Godwits, for example, can fly directly from Alaska to New Zealand. Some small songbirds seem to know beforehand when they need to load extra-large fuel reserves before passing a geographical obstacle, such as a sea. For this they use cues from the earth magnetic field. Also some mammals, for example large whales, will migrate far between foraging areas in cold seas and warmer reproduction areas. During reproduction, finally, it is important for females of many animal species to build up large fat deposits in order to increase the probability of successful breeding.