Why does a platypus store fat inside of its tail?

Platypuses do not always have a big tail. The size of a platypus's tail is a indication of how healthy it is, and how much food has been available. A platypus stores most of its body fat in its tail to help it survive when food is limited, and to prepare for when the female must incubate her eggs and care for the young when they hatch.

The tail is also useful for other purposes, and needs to be a good size in order to perform these functions. During the breeding season, the female digs herself a chamber within her deep burrow, and this chamber is lined with leaves and other nesting material. The female with carry this nesting material rolled underneath her tail as she climbs up and down steep riverbanks. In addition, the tail of a platypus helps the animal to swim and acts as a rudder when it dives.