Our Sloth Backpack Project reveals that sloths spend about 90% of their day inverted; the internal adhesions make that possible by reducing sloths’ energy expenditure. Because of their extremely slow metabolic rate and exceptionally low body temperature, it can take up to 30 days to digest a single leaf. Having a constantly full stomach means they can’t ingest much on a daily basis, so their caloric intake is very low. Sloths have virtually no flexibility in their energy budget: they generate just enough energy from their diet to move when and where required; beyond that, not much more is stored.
Why do sloths hang upside down for so long? Because they need to eat tender new leaves that grow on the outer tips of branches. These leaves often have a lower toxin content and are easier to digest. In order to reach these choice leaves, a sloth has to hang by its toes.