With a single foot, sloths can exert a pound-for-pound grip force far beyond what humans and other primates are known to muster with their hands and feet.
Sloths like to clasp onto the sides of your torso, says at the New York Institute of Technology, who studies the languid danglers. To free herself of one of the cat-sized creatures, often “two other researchers have to grab each leg and pry [its] hind limbs off”.
No one had ever measured this grip before. So Young and her colleagues built a contraption consisting of a broom handle bisected along its length, attached to a wooden frame and connected to a force-measuring plate.
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They measured the grip strength of each foot on five brown-throated three-toed sloths (Bradypus variegatus) at the Sloth Sanctuary in Costa Rica. The sloths hung by their clawed toes from the apparatus, squeezing each fore and hind foot with a force averaging as much as 100 per cent of their body weight. The sloths studied weigh roughly 4.5 kilograms, so accounting for the difference in weight, they have roughly twice the proportional grip strength of humans and primates. Some individual foot tests registered nearly 150 per cent.
Sloths have greatly reduced the musculature in their core compared with other mammals, says , also at the New York Institute of Technology. But the muscles near their feet are well-developed with fast-contracting fibres associated with heightened strength.
That power may be particularly important to sloths given their careful, leisurely pace. If a monkey misses a branch, for example, it may nimbly and relatively easily grip something nearby without falling, says at North Carolina State University. “Whereas a sloth might be in a situation where if a branch fails, they may only be able to get that one hand or foot on [another hold],” he says.
The conservation team that the researchers collaborated with shared anecdotes of sloths – too slow to escape attacking predators – gripping trees so tightly that the sloths’ skin was torn off by their assailant while they remained securely affixed. Because of this, the conservation team thinks the findings still greatly underestimate the true magnitude of the sloths’ strength, says Young.
It is easy to instruct a human to squeeze a sensor as hard as they can, she notes. “But we can’t really communicate this to a sloth,” she says. Such motivation challenges are partly why so few species have had their grip strength quantified. Some, like gorillas and other apes, may even be dangerous to attempt to measure, says Young.
The study also revealed a surprising finding – the sloths had a consistent and unexplained left side bias in their strength. Primates, in contrast, tend to favour their right side.
Journal of Zoology
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