Sloths are surprisingly stronger than their adorably silly expressions suggest, which makes sense considering they live their lives hanging from trees literally by their bony fingernails.
Curiously, despite using all four limbs to cling to branches, sloths seem to be stronger on their left side than on their right. Melody Young, an anatomist at the New York Institute of Technology, and her colleagues discovered it during the very first attempts to accurately measure the strength of a brown-throated three-toed sloth.
Very few mammals are shaped for a life spent almost exclusively in the canopy, especially those that exist only on leaves. Frankly, while you’re not starving for food, it’s hard to extract enough nutrients from fibrous plant matter.
Many folivorous (leaf-eating) mammals, such as giraffes and moose, get around this problem by using an extended digestive system. However, not all mammals can afford to grow in size to help process tough, nutrient-poor plant matter. Other mammals, like the koala, are content with what they can get and avoid wasting precious energy lounging among the branches.
Likewise, the sloth’s elongated bone claws allow them to stay strong without wasting mass and energy on muscle power. According to this new study, the grip strength behind this slow swing is exceptional for work.
It takes two searchers to snatch a sloth that hugs a third, Young said. new scientist; one seeker for each leg. And there are reports of sloths continuing to cling to trees even in death.
Young and his colleagues built a custom rig to measure this powerful hold in five brown-throated sloths (Bradypus variegatus). They found that relative to body weight, sloths, with an average weight of 3.8 kilograms (8.4 pounds), have about twice as much strength in their digital flexor muscles as humans and other primates.
Languid hairballs could easily support more than 100% of their body weight with a single hand or foot, with no measurable difference between forelimbs and hindlimbs. Primates, on the other hand, are strongest in their hind limbs, which gain about 50-70% of their weight even when climbing. It also happens that sloths distribute their weight more evenly.
But there was up to 16% difference between their left and right grip strength, especially in their hands. In contrast, primates like us tend to be stronger on their right side.
“The consistent left-sided tendency in the individuals studied was unexpected, and future work should explore the potential ecological and anatomical correlates of such a finding,” the team writes.
Young and his colleagues suspect that they even underestimated the sloth’s grip, given the limitations of their experimental setup.
“Anecdotal reports from members of the conservation team have described B. variegatus to cling so tightly to their substrates to prevent predators from pushing them away that the skin on their backs would sooner be torn off,” the researchers explain.
That’s a hell of a force by any animal’s standard, let alone such a slow-moving beast with a ridiculously low metabolism.
This research was published in the zoology journal.