Sloth

Why so slow

A sloth’s metabolic rate is among the slowest of any mammal — about 40–45% of what would be expected for a mammal its size. The slow metabolism is an adaptation to a low-nutrient diet of tough rainforest leaves, which contain few calories and require slow, prolonged digestion to extract.

A meal can take up to two weeks to digest. Sloths have multi-chambered stomachs (analogous to a cow’s) where bacteria slowly break down the cellulose. The digestion is so slow that a leaf eaten on a Monday may not finish processing until late the next week.

This metabolic constraint shapes everything: sloths can’t move fast because they can’t generate the energy. Their muscle mass is unusually low. They can’t shiver to warm themselves, so they bask in the sun like reptiles.

Two-toed vs. three-toed

There are six sloth species, divided into two distinct groups:

• Three-toed sloths (genus Bradypus) — four species. Smaller, with three claws on each forelimb. Strict herbivores. The familiar sloth-of-meme.
• Two-toed sloths (genus Choloepus) — two species. Slightly larger, with two claws on each forelimb. More active at night. Will eat occasional small animals and eggs.

Despite the similar appearance and tree-hanging lifestyle, the two groups have been evolutionarily separate for over 30 million years — they evolved their similar body plans independently. They’re a remarkable case of convergent evolution.

A weekly trip to the ground

Sloths defecate roughly once a week, descending to the ground to do so. The trip is dangerous — sloths are nearly defenseless on the ground, where their slow speed and weak limbs make them easy prey. Why they don’t simply defecate from the canopy isn’t entirely clear; one hypothesis is that the burial of dung at the base of the tree fertilizes the food trees that sustain individual sloths over their lives.

The descent also creates an opportunity for a unique multi-species relationship.

Sloths as ecosystems

A sloth’s fur hosts an entire small ecosystem. Algae grows in grooves on the hairs, giving the sloth its slightly green tinge and providing rainforest camouflage. Moths (specifically Bradipodicola hahneli) live in the fur, lay their eggs in the sloth’s dung when it descends, and migrate back to the canopy on the next descending sloth. Beetles, mites, and other invertebrates round out a small population of inhabitants per individual sloth.

Studies have suggested the algae and microbes living in sloth fur may also produce compounds with antibacterial and even anti-cancer activity — though biomedical applications remain speculative.

Surprisingly good swimmers

Despite their tree-hanging lifestyle, sloths are surprisingly competent swimmers. They drop from canopy branches into rivers below, where they can move three times faster than they do on land. Their slow metabolism lets them hold their breath for up to 40 minutes underwater. Long-distance swimming between forest islands is part of how their populations persist in fragmented habitats.

Why claws

Sloth claws are massive — up to 8 cm long — and curved into hooks. The claws aren’t for digging or fighting; they’re entirely for hanging. A sloth’s resting muscle posture is “grip closed,” not the normal mammalian default of “muscle relaxed.” This means a sloth uses no energy to hang from a branch — it would actually require effort to let go.

Sloths sometimes die hanging from branches and remain in place, found by researchers months or years later, still attached.