Why do Sloths Hang Upside-Down?

The Sloth Sanctuary in Costa Rica not only provides a safe haven and home for dozens of sloths, but also carries out biological research on these little-known creatures. Here they provide an explanation as to why and how sloths can live their entire lives upside down.
“Three-fingered sloths have a unique system of multiple internal adhesions that anchor their abdominal organs against the lower ribs. These adhesions support the weight of a sloth’s stomach, kidneys and digestive tract when inverted, facilitating ease in breathing by preventing the lungs from being compressed.

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.

In addition to the adhesions, sloths have other anatomical features that allow them to remain comfortable when inverted. These include circulatory adaptations that prevent the blood rushing to the head and a highly specialized esophagus that allows them to swallow while upside down.”


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