A young man once asked Winston Churchill to what did he attribute his success in life. Churchill responded:
Conservation of energy. Never stand up when you can sit down. And never sit down when you can lie down.
Sloths take a similar approach. Unlike most mammals, they aren't endothermic--that is, their bodies produce little internal heat. This is just as well, as the tree leaves that they consume offer little caloric value. Camila Mazzoni of the Leibniz Institute for Zoo and Wildlife Research in Germany explained to the BBC:
“The thermo-regulation that most mammals have to do requires a lot of energy,” says Mazzoni. “But because sloths don’t have it, it means they require a lot less energy.
“But this means they can only live in the tropics, and not high up in the mountains where the temperature gets quite low.