Revealing the Sloth
by Robert Beaver, Museum intern for the Living Conservatory
The two-toed sloth is one of the most remarkable parts of the Living Conservatory exhibit; it can also be one of the most mysterious. Visitors will often look into his enclosure and wonder where he is. Some visitors want to know, “What in the world is a sloth?” They gaze at him in amazement as I tell them that a sloth is just a sloth, and its closest relations are actually anteaters and armadillos, not something that would be found climbing around in trees like primates. In fact, the two-toed sloth’s evolutionary ancestors were not arboreal, but a variety of massive ground-dwelling mammals that roamed the Americas tens of thousands of years ago. The story of tree-dwelling sloths represents a peculiar and interesting history of mammalian evolution that occurred relatively close to home in North Carolina.
Sloths are a part of the superorder Xenarthra that also contains armadillos and anteaters. Sloths and anteaters belong to the order Pilosa, while armadillos branch off to form their own order called Cingulata. Xenarthrans share characteristics such as strong forelimbs and curved claws, to either hang from branches or dig in the ground for food as an armadillo or anteater would. The sloth in the Living Conservatory is of the two-toed variety, genus Choloepus. Three-toed sloths belong to genus Bradypus. One would suspect hanging mammals with a differing number of toes are pretty closely related, but these sloths are more closely related to their giant ground-dwelling ancestors than to each other.
What are known as giant ground sloths are a variety of extinct sloths in the order Pilosa that inhabited the Americas. Many believe they became extinct with the close of the last Ice Age about 11,000 years ago. Giant sloths roamed on the ground and, using their back legs for support, would sit on their haunches and use their powerful forelimbs to extract branches and leaves. They might have used their claws for digging as well, which is supported by the fact that many remains were found in caves. In fact, Thomas Jefferson coined the term for a variety of ground sloth “Megalonyx” that was found in a cave in West Virginia. (He misidentified it as a giant lion.) As a part of the famous journey of Lewis and Clark to find access to the Pacific Ocean, President Jefferson also tasked the crew to unearth evidence of America’s mythical megafauna like giant ground sloths and mammoths that supposedly inhabited the West. Sloths are a relevant area of study for the NC Museum of Natural Sciences because fossil remains of several extinct ground sloths were found near the North Carolina coast, and excavated by the Museum. The skeleton of one of these individuals (species Eremotherium eomigrans) was reconstructed, and is on display in the Museum’s Prehistoric North Carolina exhibition.
Giant ground sloths were a vital part of North America’s ecosystem during the last Ice Age and left a legacy of xenarthrans that still inhabit parts of the Americas. It may seem peculiar that giant terrestrial herbivores were the precursors of the small arboreal mammals people know and love today. But according to DNA and fossil evidence, two-toed and three-toed sloths are more closely related to their extinct, ground-dwelling forebears than to each other – the two-toed is closely related to Megalonyx while the three-toed is related to the Megatherium variety.
The emergence of Choloepus and Bradypus represents an intriguing example of convergent evolution. (Convergent evolution is the developing of similar structures in organisms of different lines of descent.) Modern sloths’ predecessors are thought to have diverged from each other about 35 million years ago, with each modern genus developing its suspensory and arboreal nature separately through evolution. It seems each genus, with its own lines of ancestors, evolved from giant ground sloths to modern tree sloths. According to the fossil record, two-toed and three-toed sloths each had their own ground sloth predecessor and converged on similar anatomy and behavior: members of both genuses hang in trees, slowly moving from branch to branch with their hooked claws and powerful forelimbs.
The evolution of sloths and their closest xenarthran relatives is a little easier to understand if the anatomy is examined. Giant ground sloths had powerful forelimbs for ripping and tearing at branches, as well as potentially digging in the ground. All modern xenarthrans have strong front claws and shoulder flexors. Armadillos and anteaters can dig in the ground for food while sloths can use their strong arms and curved claws for propelling themselves while hanging, all using similar anatomy to their ancestors.
Why do sloths spend most of their time upside-down? It is suggested that an inverted lifestyle is energy-saving in arboreal arenas. The suspended posture negates the need to use energy to balance above branches, which is consistent with the low metabolic rates of sloths.
There is no fossil evidence of a tree-dwelling forebear to the modern sloth, but more recent ancestors could have been semi-arboreal. Silky anteaters are quite capable of balancing on top of branches as well as occasionally hanging and walking on the ground, suggesting one path evolution might have taken from the ground to the trees.
Sloths are misunderstood and puzzling animals, but they are also among the most interesting if one takes into account the evolutionary history of the Xenarthra. From prehistoric giants to modest, modern-sized sloths, they represent a stunning example of evolution at work, directly tied to North Carolina’s history and the history of the nation.