University in Jena, Germany, lead author of the study appears this month in Zoology. “What differs is the way muscles are arranged, and “the attachment sites of muscles are much more variable in evolution and can completely alter the functionality of limbs.”

Sloths don’t grasp branches but instead suspend themselves from tree limbs with long, hook-like claws on their sideways-oriented hands and feet. Given how weird sloths are, it seemed possible they may have evolved a different way of moving from other mammals. To investigate whether strange anatomy translated to strange locomotion Nyakatura and colleagues used video and x-rays to see inside sloths as they moved along a wooden pole and a motorized “treadpole” they were trained to move along for a snack.

They found that the sloths didn’t simply lumber along the branches at a slow-and-steady pace. Not only did their velocity vary from 0.02 meters per second to 0.47 meters per second, but they also used several different gaits while moving. In the “exploratory gait,” the sloths assumed an inverted crouch position in which they held their nose close to the pole, while in the “traveling gait” they extended their arms further to increase their stride.

The range of movements in sloths overlaps with those of other twee-dwelling mammals that either run over the tops of branches or grasp them for support. In particular, the sloths used what is called a diagonal couplet gait in which the front limb on one side and the hind limb on the opposite side move together. Researchers have suggested this kind of gait allows animals to securely touch down on branches as they move.

While sloths don’t have to worry about falling off the top of a branch, the scientists suspect they use this gait while moving quickly to allow them to draw back if a branch cracks. Even though sloths moved upside-down, they used some of the same locomotor strategies of animals that move along the tops of tree limbs. Though sloths evolved the strange habit of traveling along the underside of branches, “the way the legs themselves are moved remained very similar to other mammals,” Nyakatura.

Strangely, this repertoire of upside-down locomotion may have evolved twice in sloths. Although not specifically studied in the new research, Nyakatura’s team proposes three-toed sloths might move using the same strategies. And if this is correct, it would be a case of evolutionary convergence.

Superficial appearances to the contrary, two-toed and three-toed sloths are not very closely related to each other and last shared a common ancestor over 21 million years ago. Because both lineages became adapted in similar ways to living in trees, however, it is likely both types of sloths co-opted some of the anatomy of their ancestors to allow them to make that move into the trees.

Based upon the close relationships of sloths to anteaters and armadillos, as well as some peculiar anatomical traits in their postcranial skeletons, Nyakatura suspects that the last common ancestor of each of these lineages was a digger. This means that some of the specializations which allowed sloths to move into trees evolved first as adaptations to digging.

It may be that life on the ground caused sloths to evolve the anatomical specializations which allow them to suspend themselves from the forest canopy today.

See Also:

• 'Megatheria' muzzles provide clues to giant ground sloth diets …
• When Pseudo-Crocs Walked Tall
• Video: How Leaping Fish Species Left the Water — For Good
• Newly-discovered bones answer questions about fossil primate …

Authors: Brian Switek

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