Turning their lives upside down saves further energy. They have about half the skeletal muscle of a terrestrial mammal. They don't really have so many of the extensor muscles that are the weight-bearing muscles; instead, they rely on retractor muscles to pull themselves along. They have long, hooked claws and a high fatigue resistance, so they can literally hook on and hang like a happy, hairy hammock for hours on end. And sloths can do almost anything in this inverted position. They sleep, eat and even give birth. Their throat and blood vessels are uniquely adapted to pump blood and to swallow food against the force of gravity. They have sticky bits on their ribs that prevent their enormous stomach from crushing their lungs. And their fur grows the opposite direction, so they can drip dry after a tropical drenching. The only problem is, if you turn a sloth the other way up, gravity removes its dignity.