WikiFindings

The energy costs of storing fat and being athletic mean that we can get all kinds of relationships between body size and air temperature among species of mammals and birds

Background

Animals that live further from the equator tend to be larger. The usual explanation for this is that large bodies lose heat at a slower rate per kilogram. However, in some species we see the opposite pattern, which is unexplained. Also, larger bodies still lose more energy in total than small bodies, so if finding food isn’t easy it might not be sensible to be larger. None of the existing hypotheses have considered the challenge of finding enough energy, nor do they consider that animals could have smaller bodies (lower costs) but store lots of fat (energy reserves). However, being fat would be bad if it makes it easy for predators to catch the animal.

Findings

Our model finds the optimal amount of muscle and fat across lots of conditions where the animal has to keep warm by burning calories. We predict that whether total body mass increases or decrease as the air temperature gets colder depends on many environmental factors: temperature range; the risk of predation; how often it gets hard to find food and how long this lasts; and whether the animal is nocturnal or diurnal. Combinations of these factors could interact, and some combinations enable us to predict that body size may first go up and then go down as animals get further from the equator.

Implications

Our work is the first to predict the whole range of patterns of body size we observe across the world among animals that maintain their body temperature (birds and mammals). This means that understanding how body size and composition has evolved depends on taking a holistic view of the lives of species, including aspects of their food supply, predators, and life style.