Hanging Out

       Remember hanging from the monkey-bars at recess as a kid? The strain on your shoulders as you held on tightly swinging from bar to bar. Your legs kept moving you forward as each bar was seemingly drawn to your hand as you moved like Tarzan. Now imagine having to do this without swinging to each new bar. Also, think about hanging there for hours at a time, even while you nap. This is just a glimpse into the lifestyle of tree sloths. Whereas human and non-human great apes, and monkeys are able to swing their bodies while hanging to move, tree-sloths take a different approach. Instead of arm swinging, they very slowly and steadily pull themselves up tree trunks and below tree branches to navigate their rainforest environment.

Several researchers have looked into the benefits of this slow, steady movement, in an attempt to answer the question of “why do sloths move in such a way?” However, few have sought to answer the question of “how do sloths move in such a way?” That question is what inspired our team to study these extraordinary creatures. We were particularly interested in three-toed sloths because data for their movement patterns were unavailable. Given that they are related to two-toed sloths, it provided an opportunity to learn more about the similarities and differences among tree-dwelling animals. Three-toed sloths have muscles in their front limbs (called forelimbs) that efficiently work to help them hang from and move beneath tree branches. Our team also previously learned about the relative strength of their grip and use of their forelimbs for propulsion below branches. This is very similar to how upright mammals, for example horses, use their back limbs (called hindlimbs) to propel themselves forward overground. However, there is still a gap in knowledge about the function of hindlimb muscles in sloths. Knowing how their hindlimb muscles work could help us solve the puzzle of how three-toed sloths move in their special slow and steady way all while supporting their body weight. 

With this in mind, we set off to Costa Rica to collect data from the limbs of three-toed sloths to understand how they control their movement. The study required a customized beam device which would simulate a tree branch to measure their movements. As animals walked across the beam and gripped onto a handle in the center, we could measure the amount of force being applied by the sloth on the “branch”. This would allow us to know how much pulling and pushing force sloths generate while moving. We also used an instrument to measure when muscles turn on and off, and how much of a muscle is being activated. This information indicates how a muscle is being used while the sloth is actually in motion. We used this device inside specific muscles of the hindlimbs so that we could analyze the activity of different muscles that we believe are important to certain movements. 

The muscles analyzed were split into two primary groups: flexors and extensors. Flexor muscles move the body forward and support the body weight while upside down. These muscles are considered “pulling” muscles, whereas extensor muscles are considered “pushing” muscles in sloths. Using the two devices at the same time lets us see when a muscle activates and whether it is being used to push or pull the body forward or backward. We also used video cameras to sync up all of the instruments so that we can know the exact time of when the feet of the sloths are on or off the beam. All of these methods gave us a complete picture of how three-toed sloths specifically use their hindlimbs while slowly moving beneath branches.

Additionally, we were able to examine similarities and differences among each individual sloth used in the study, along with comparing the differences between two-toed and three-toed species. What we found was somewhat unexpected and different from previous studies that examined how two-toed sloths use their limbs. All of the new data combined with our previous findings gave us a better understanding of how nature provides several solutions to a single problem. 

Throughout our test trials, we found that the hindlimbs primarily act as a brake to control the forward movement of the body. They also act as an anchor, making sure the bodyweight of sloths is supported from the branch. Comparing the new data with our past studies, it is now clear that all four limbs initially act to propel the body forward, with the forelimbs providing the greatest source of forward movement. Then, the hindlimbs act to slow the body down for the rest of each step. This suggests that the hindlimbs play an important role in ensuring that the sloth is able to slow down the body to precisely control movement. They also secure the body to the branch to limit unexpected falls from the tall canopies of the rainforest. 

Interestingly, two-toed sloths balance their bodyweight equally between their fore- and hindlimbs. Conversely, three-toed sloths support more body weight with their hindlimbs. This may be a result of the three-toed sloths having shorter hindlimbs, whereas the limbs of two-toed sloths are equal lengths. This characteristic of three-toed sloths is not common among tree-dwelling mammals, like monkeys and lemurs. However, it may free the forelimbs of three-toed sloths to reach forward along the branch or even grab food at the ends of branches. That said, the pattern of body weight support observed in three-toed sloths is uncommon. These results revealed a new strategy by which tree-dwelling animals can traverse their tricky environments upside-down.

This study was one that combined several ideas for measuring the way an animal moves. We used multiple methods to analyze sloth movement and were able to deduce the ways in which three-toed sloths evolved to navigate the canopies of their rainforest homes. In summary, we found that three-toed sloths are able to use their hind limbs like car brakes. Several strong hindlimb muscles act to control the forward movement of the body while those and other muscles simultaneously support the majority of the body weight suspended from the branch. This pattern differs from primates and even species of two-toed sloths, and it allows for efficient and safe movements below branches and vines high above the ground. 

The study benefits from careful planning to ensure that multiple types of data were collected and could be analyzed together to provide the big picture of sloth movement. That is not to say there were not any difficulties during the study. One difficulty included sedating the sloths before walking along the force beam, which can impact their willingness to walk. Also, the custom force beam was made of a plant material that is not identical to those of the tree branches that sloths prefer. In the future, our plans are to further reveal what muscles activate together in the forelimbs and hind limbs. For now, we can show that not every tree-dwelling animal has to behave the same to perform the required task of moving throughout trees to find food and shelter and avoid predators. Any given problem has a number of solutions, and nature has chosen a rather unique solution for tree sloths.

Written By: Dr. Michael Butcher & A.J. McKamy

Academic Editor: Medical Scientist 

Non-Academic Editor: Retiree - Experience in the Healthcare Industry

Original Paper

• Title: Pump the brakes! The hindlimbs of three-toed sloths decelerate and support suspensory locomotion

• Journal: Journal of Experimental Biology

• Date Published: 19 August 2023

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