Bat
Most bats rest, sleep and hibernate in an upside-down position. They hang on to branches or rocks with their feet. To do this they have a locking mechanism on the tendons in their feet which stops them from slipping. The energy they spend hanging on is greatly reduced. The most obvious gain is that they can sleep without falling off. Once the tendons are locked, the muscles in their legs and feet can relax. Even dead bats stay hanging.