Three-toed sloths walk differently from their cousins

Abstract

Life is difficult when you live in the canopy, but even more so when you spend much of your day hanging suspended from the branches. Yet, there are a number of animals for which this is common, including tree sloths. Both two-toed and three-toed sloths spend nearly their entire lives hanging from branches and have therefore evolved some remarkable tricks for thriving in this unusual way of life, including their slow movements. But just how these sloths walk on the underside of branches is not fully understood. This led A. J. McKamy and Michael Butcher of Youngstown State University, USA, to ask how does the brown-throated three-toed tree sloth (Bradypus variegatus) move? And does it do so similarly to its previously studied cousin, Linnaeus's two-toed sloth (Choloepus didactylus)?Working alongside Melody Young and Michael Granatosky from New York Institute of Technology College of Osteopathic Medicine, USA, and Angela Mossor and Jesse Young of Kent State University USA, the team travelled to the Sloth Sanctuary in Costa Rica to meet up with Judy Avey-Arroyo. There, the researchers used a pressure sensor to measure how much force the sloths applied as they traversed a beam made of a local bamboo-like woody grass. They also videoed the movements to see how the sloths were gripping the woody beam and whether they were doing this differently from the two-toed sloths.Interestingly, the researchers found that as the three-toed sloths were walking, they were pulling down on the beam more strongly with their hindlimbs than with their front limbs. This is unique to the three-toed sloths, as their two-toed cousins pull down equally hard with their front and back limbs. This suggests that the three-toed sloths are actually using their back limbs to support more of their body weight than their front limbs, whereas the two-toed sloths spread their weight out evenly. McKamy suggests that this may be because three-toed sloths are very deliberate about where they place their front feet. They are frequently seen reaching out with their front limbs to test the branch for stability while most of their weight is on their hindlimbs.However, the back limbs are doing more than just supporting their body weight. These limbs are also used for slowing down while the animal is moving about, while the front limbs are used to propel the sloths forward. Additionally, the hindlimbs in three-toed sloths press inward on the beam with more pressure than the front limbs do, while their front limbs are pulling the beam out to the side. This pulling motion away from the beam is minimal in the two-toed sloths, but the researchers suggest that this is a result of how three-toed sloths grip tree branches. Instead of just hanging on with their claws from underneath, the three-toed sloths wrap their feet over the top of the branch, further gripping the branch with their claws to keep stable. So, although sloths may look similar and even live life in a similar way, at least two species have come up with different ways of moving in a life spent mostly hanging from the branches.