The platypus stores fat in it's tail to help it survive when food is limited.
Picture from Amamoore Lodge
An adaptation also found in animals such as the Tasmanian devil and fat-tailed sheep, the platypus tail is mainly made up of a fatty tissue that is used to store energy supplies, which the animal can use when there is a shortage of food, such as in the winter months. It also acts as a form of insulation.
The platypus stores fat in its tail to help it survive when food is limited - See more at: http://www.livescience.com/27572-platypus.html#sthash.g5XSb8ez.dpuf