WikiFindings

Understanding how large and how fat different species are follows from predicting optimal bodies that avoid both starving and predation

Background

Animals differ in the amount of fat they carry around depending on their species, status and sex. However, the causes of much of this variation have been a mystery. An important reason for the storage of fat is as a back-up energy source if an animal fails to find food. However, carrying fat has a cost of reducing athleticism and so making animals more vulnerable to their predators. Animals can be stronger to compensate, but the energetic costs of extra muscle mean that the animal would starve quicker during a food shortage. We used a mathematical model to predict how the composition of animals’ bodies should depend on ecological factors.

Findings

We confirm the expected result that animals subjectted to high risk of predation should be less fat whereas those subjected to more food shortages should be fatter. More surprisingly, the model reveals that an important consideration is how much carrying fat increases the energetic costs of movement. If this cost is low then larger animals should be fatter, but if this cost is large when smaller animals should be fatter. If competition for food is intense then larger animals should be less fat, which means subordinate animals should be fatter than dominant ones.

Implications

We confirm the expected result that animals subjectted to high risk of predation should be less fat whereas those subjected to more food shortages should be fatter. More surprisingly, the model reveals that an important consideration is how much carrying fat increases the energetic costs of movement. If this cost is low then larger animals should be fatter, but if this cost is large when smaller animals should be fatter. If competition for food is intense then larger animals should be less fat, which means subordinate animals should be fatter than dominant ones.