Why Are Sloths Slow? Their Eyes Are Part of the Answer

Wild brown-throated three-toed sloths sleep 9.63 hours a day. The figure comes from miniature EEG recorders fitted to sloths living free in the rainforest canopy on Barro Colorado Island, Panama. A sloth is awake for roughly fourteen hours of every day, and still moves at the speed of a sloth.

The slowness therefore has to be explained by something other than sleep. Three measured constraints do the work: an energy budget nothing else in the class can match, a stomach that ferments for over a week, and a pair of eyes that stop working when the sun is up.

Why are sloths slow?

Sloths are slow because leaves pay badly. A three-toed sloth runs the lowest field metabolic rate ever recorded in a mammal, digests a single meal for a week or more, and sees the world as a colourblind rod monochromat that is effectively blind in daylight. Slow movement is what that budget and that eyesight allow.

The lowest energy budget ever measured in a mammal

Jonathan Pauli and colleagues measured field metabolic rate, the energy an animal actually spends living its life rather than resting in a lab, for two-toed and three-toed sloths sharing the same habitat in Costa Rica. Writing in The American Naturalist in 2016, they put three-toed sloths at 162 kJ per day per kg^0.734 and called it the lowest field metabolic rate recorded for any mammal. The odd exponent is standard allometric scaling, the correction that makes a 4 kg animal comparable with a 400 kg one.

They then compared field and basal metabolic rates across 19 arboreal folivore species spanning a spectrum of specialisation. The whole group ran lower basal and field metabolic rates than other mammals, and the more specialised the species, the lower its field rate went, while basal rate held steady. The saving came from thermoregulation and behaviour rather than from a smaller resting engine.

Temperature does part of that. Three-toed sloths in the same study were more heterothermic than two-toed sloths, meaning core temperature drifts with conditions instead of being defended at a mammal’s usual fixed setting. Constraint explains more than temperament does, in biology and elsewhere: the moon keeps a single hemisphere pointed at Earth for reasons that are entirely mechanical.

A stomach that ferments for days

Leaves are easy to find and expensive to process, and sloths eat very little of them. Rebecca Cliffe and colleagues followed three captive adult female Bradypus variegatus for 158 straight days and reported a mean intake of 17 g of dry-weight food per kg of sloth per day in PeerJ in 2015.

Whatever does go in stays. A 2022 trial in the Journal of Animal Physiology and Animal Nutrition fed four brown-throated sloths two leaf-based diets and timed how long a liquid marker and a solid-particle marker took to pass through.

Diet	Liquid marker	Particle marker
Mixed leaves, higher intake	133 h, 0.75% per hour	181 h, 0.55% per hour
Cecropia only, lower intake	204 h, 0.49% per hour	261 h, 0.38% per hour

261 hours is nearly eleven days to clear one mouthful. Cliffe’s group also found intake rose with ambient temperature and proposed the mechanism: a sloth whose core temperature tracks the air is running its gut microbes hotter, fermentation speeds up, and room appears for the next meal. Their paper points out that this cuts against other mammals, which tend to eat more in colder conditions.

The eyes are a speed limit

Christopher Emerling and Mark Springer reported in Proceedings of the Royal Society B in 2015 that the cone-photoreceptor genes of sloths are broken. In Hoffmann’s two-toed sloth, SWS1, PDE6C, PDE6H, GNGT2, GRK7 and GUCY2F are pseudogenes, inactivated and no longer coding for anything, while the rod machinery came through the same screen with no confirmed inactivation.

A retina with rods and no cones delivers what their paper calls colourblindness, low-acuity vision in dim light, and blindness in bright-light conditions. An animal that cannot resolve the branch it is reaching for in daylight has a hard ceiling on how fast it can cross a canopy without falling out of one. Eyes make confident mistakes in every species, which is also why the vessels under your skin look blue when nothing inside them is.

The damage is ancient. Emerling and Springer place the inactivation of PDE6C in the common ancestor of sloths and anteaters shortly after that lineage split from the armadillos, near the Cretaceous-Palaeogene boundary, and argue the loss fits an ancestor that lived underground. That boundary lines up with the impact linked to the end-Cretaceous mass extinctions, whose buried crater is traced by a ring of sinkholes in the Yucatan. Modern sloths run daylight canopy vision on a retina last optimised for a burrow.

The muscle is built for hanging

Skeletal muscle mass in sloths is reduced, and a 2018 Journal of Applied Physiology study by Kyle Spainhower and colleagues examined what remains. Across forelimb muscles in five Bradypus variegatus and four Choloepus hoffmanni, the dominant fibre type was the slow MHC-1 isoform, with moderate fast MHC-2A and almost no hybrids.

The enzyme results only half matched the authors’ own prediction. Creatine kinase and lactate dehydrogenase, both anaerobic markers, showed elevated activity, while the aerobic markers including citrate synthase came out low, the reverse of the oxidative profile the team expected from a muscle built to hang. Bodies keep the machinery their history handed them, much like a human jaw that still grows a third set of molars with nowhere to put them.

None of that makes a sloth feeble. A 2026 Journal of Anatomy analysis of Hoffmann’s two-toed sloth forelimbs estimated the digital flexors alone at roughly 2.2 times body weight in force, which the authors described as exceptional grip strength. A sloth arm is a clamp with a very long duty cycle.

The 15-hour sleep figure comes from zoos

The Smithsonian’s National Zoo page on the southern two-toed sloth states that sloths are “primarily nocturnal, sleeping for 15 hours during the day and waking up at night to eat.” The figure has a real measurement behind it: sloths in captivity had been clocked at 15.85 hours of sleep a day.

Niels Rattenborg and colleagues tested it. In Biology Letters in 2008 they published the first EEG recordings of sleep in unrestricted animals in the wild, using a miniaturised recorder on three adult female brown-throated three-toed sloths in the canopy at Barro Colorado Island. The wild sloths slept 9.63 hours a day, over six hours less than previously reported in captivity.

The sleepiest-mammal statistic was recorded on captive sloths. When Rattenborg’s team carried the electrodes up into the canopy, six of those hours went away.

The authors declined to overclaim. Juvenile mammals sleep longer than adults, they noted, so the inclusion of juveniles may have pushed up the captive average on its own. Either way, the 15-hour number remains an accurate measurement of something, and that something is not a sloth in a forest. Whale sleep has the same gap, only wider: every brain recording of it was made in captivity, on five toothed species, and no sperm or baleen whale has had its sleeping brain recorded at all.

A sloth crossing a branch is doing it on a retina that stopped resolving colour around the end of the Cretaceous, powered by a stomach still fermenting last week’s leaves. Moving slowly is the only strategy that equipment supports, and the animal is wide awake for nearly all of it. Strictly FYI.