The Evolution of the Sloth

Sloths are a staple animal of the rainforest of South America. They're best known for being small, extremely slow, tree-dwelling creatures that don't do much besides hang upside down and eat leaves. However, sloths have one of the most fascinating histories of any mammal on Earth. Sloths are part of a larger group of mammals known as Xenarthra. This is an ancient lineage that evolved in the continent of South America. While fossils of Xenarthra date back to only around 60 million years ago during the Paleocene, some estimates trace its evolution back to the A common belief is that they're most closely related to the order Aprotheria, a group we've previously covered here on this channel, and that together they form the clade Atlantogenata. Other theories posit that Xenarthra could be a sister group to Ferongulata, which contains ungulates, carnivorans, as well as the pangolins, or to Uarchontoglires, an order containing rodents, lagomorphs, and primates. In most cases, however, it's assumed that Xenarthrans are more distantly related to other mammal groups. Some evidence supporting this is that these animals lack a stirrup-shaped middle ear bone, which can be found in all other mammals. Xenarthra can be further divided into two other clades. One of these is Singulata, which contains the armadillos, and the other is Pelosa, which contains the anteaters under the suborder Vermilingua, and the sloths, which are part of the suborder Folivora. Folivora contains sloths that sport an incredible diversity of morphologies and lifestyles, with the first members of the clade evolving in the early Eocene. When and where sloths evolved is still a subject of heated debate among researchers, but current data puts their evolution at around the end of the Eocene epoch. One of the earliest sloths to have been discovered, Pseudoglyptodon, is known from skull and mandible fragments and inhabited Chile and Argentina. It should be noted that while some scientists believe this to be an ancestral sloth, others put it just outside the group containing sloths as their closest relative. Another sloth, part of a group known as Megalonychidae, was found in Puerto Rico and was dated to have lived around the same time as Pseudoglyptodon at the start of the Oligocene. The vast distance between these two animals indicates an important point, that being the current earliest sloth on record may have still lived at a time where sloths were more widespread. This means that sloths could have evolved even earlier on during the Cenozoic than previously assumed. This is further supported by the fact that Megalonychidae was a more developed sloth group. A more basal family of sloths are the greater Antilles sloths of the Megalochnidae. At a glance, these animals don't resemble our existing sloths in the slightest. If anything, they look more like stout rodents walking on all fours. However, this body plan would become more prominent in the future of sloths and get taken to extreme heights. Following these early sloths came two distinct superfamilies in Folivora that diverged around 40 million years ago. The first of these superfamilies is Megatherioidea. In looking at the superfamily, we might as well start off with a group of sloths that contain some of the largest and most visually impressive mammals of all time. This group is known as Megatheriidae, which evolved during the late Oligocene and was home to the giant ground sloths. Giant here could seem like a relative term. Compared to our modern sloths, most historic sloth species are gigantic by comparison, but the sloths of this group are some of the largest of all Cenozoic animals. One such sloth found here was Megatherium, an enormous creature that weighed four tons and measured 20 feet from head to tail. Save for larger elephants and their relatives, as well as the Oligocene rhino Paraceratherium, Megatherium was the largest land mammal to have ever lived. At this size, Megatherium would likely not have to worry about any predators except for humans. Despite its immense size, Megatherium shared many other morphological features that characterized ground sloths as a whole. This includes having a long snout and tongue meant for feeding, strong forelimbs and giant claws. These claws could be used for pulling down branches to obtain leaves as well as for self-defense. The nature of these claws meant that ground sloths often had to walk on their knuckles, a behavior we also see in other large-clawed Cenozoic mammals like Calicotheres. In addition, Megatherium, as well as many other ground sloths, had the ability to balance itself on its hind legs using its long tail as a base, forming a tripod stance. While Megatherium inhabited South America, another member of Megatheridae, Aremotheria, managed to enter the southern United States after crossing the Panama Land Bridge 2.2 million years ago. It could be found in states such as Florida and Georgia. If Megatheridae contained the most physically imposing sloths, then its sister family, Nothrotheridae, contained the sloths with the most fascinating lifestyles. The first member of this group is argued to be Thalassocnus. What's notable about this animal is the various adaptations we can see in its skeleton compared to other sloths. These include having incredibly dense bones as well as having its nostrils move further up on the skull. This has led scientists to believe that this sloth led a semi-aquatic lifestyle, as these physical traits would make for the animal to be able to move along ocean floors where it would feed on aquatic plants. In order to fight against the powerful waves and currents, Nothrotheridae such as Thalassocnus could have used its long claws to help stabilize and brace itself. This habit of feeding at the bottom of the seas is one we see in other animals such as Sirenians, as well as extinct marine mammals like Desmostelians. As Nothrotheridae progressed in its evolution, even more evidence of an increasingly aquatic lifestyle emerged. This can be evident by looking at the remains of their teeth, for example. Successive members of the group showed less and less wear on their teeth, indicating a departure in a diet of mostly harder ground-based plants to one of mostly aquatic plants. Megalonychidae is also an interesting group for including sloths that are present in North America long before the Isthmus of Panama formed during the Pliocene. One of these animals was the aforementioned disputed Megalonychid from Puerto Rico from the early Oligocene. The earliest confirmed Megalonychid from North America, however, was Pliometanastes, which evolved in the late Miocene. This genus led to the most iconic member of this group, Megalonychs, which evolved during the Pliocene. These sloths can be found from Alaska to Central America. They have an interesting history of discovery. When their claws were first discovered, the President of the United States at the time, Thomas Jefferson, believed them to belong to an extremely large lion. This was part of the reason he sent Lewis and Clark out west. He aimed to try and find these giant lions, as well as other New World megafauna such as Mastodons. This was his attempt to prove to his European contemporaries at the time that New World animals were just as cool as Old World animals. The final group of Megatherioidea is probably one you're all familiar with. These are the members of Bradypotidae, the three-toed sloths. Recent phylogeny shows that these sloths are sister to the group containing ground sloths such as Megalonychs, but these two animals could not be more different. For one, the three-toed sloth is a nearly fully arboreal animal, only coming down to go to the bathroom. Moreover, these animals are far smaller than their ground-dwelling counterparts. Whereas sloths in Megatherioidea could reach elephantine sizes, tree sloths clock in at a maximum weight of 10 pounds. There are some similarities between these two sloths, however, if you look close enough. Both tree and ground sloths share their long claws that curve inwards, but while ground sloths use these claws for purposes such as pulling down foliage, tree sloths use them in order to grab onto branches as they hang upside down. By doing this, the sloths manage to conserve as much energy as possible. The main sloth, Bradypis torquatus, was the first to split off from this group 12 million years ago, with the pygmy three-toed sloth, B. pygmaeus, splitting off next. The last two species to split off from one another were the pale-throated sloth, B. tridactylus, and the brown-throated sloth, B. variegatus. You might think that these three-toed sloths are close relatives of the other extant group of sloths, the two-toed sloths, but in reality, these two tree sloths are separated by 40 million years of evolution. For reference, that's about the same level of evolutionary distance that animals such as dogs and bears or cats and hyenas share, showing just how old Folivora really is as a group. To look into the two-toed sloths, we'll have to delve into the second major superfamily of Folivora, the Mylodontoidea. This superfamily is very similar to Megatherioidea in that it contained both large ground sloths, such as those in the families of Skeletotheridae and Mylodontidae, as well as smaller tree sloths, such as those in the families Colopodae. There isn't a whole lot I can cover about the first two ground sloth families that except for the fact that recent studies indicate that Mylodontidae is more closely related to the two-toed sloths, with Skeletotheridae being a more basal group inside the superfamily. There are currently two species of two-toed sloths alive today, part of the genus Colopus. These are C. didactilis, Linnaeus' two-toed sloth, as well as C. hoffmani, Hoffman's two-toed sloth. The incredible similarities between three and two-toed sloths are a great example of convergent evolution. It also gives an indication as to what kind of sloths managed to survive past the massive mammalian extinction at the end of the Pleistocene. Despite these two groups of sloths being the smallest and least imposing, it was the tree sloths that outlasted their massive cousins, with sloths such as Megatherium going extinct around 10,000 years ago. This is likely due to the fact that being as small and high up as they were, they didn't have to worry as much about human predation. In addition, they were better suited to habitat change. The grasslands present throughout the Americas disappeared after the last ice age for the most part, but that didn't affect these tree-dwelling sloths who were perfectly content with forest lifestyles. In addition, their slow metabolisms and lifestyles made them need a lot less food to survive. That being said, while most sloths today are listed under least concern when it comes to vulnerability, two species are under the endangered species list. These are the maned sloth, which is listed as vulnerable, as well as the pygmy three-toed sloth, which is critically endangered. The major threats to these animals are habitat loss, as much of their forest ecosystem is being torn down for uses such as cattle pastures. With proper conservation efforts, however, these animals can make a recovery and ensure that this ancient lineage can continue to live long into the future. Hey guys, thanks for watching my video. If you liked it, make sure to give it a like and make sure to subscribe. I'll see you guys next time.