Author’s note: Between interviews, guest posts, book proposals, academic papers, and an extensive essay I’m working on, I have not had very much time to feed this blog. It is all I can do to keep up with my day-to-day writing duties, but I don’t intend to leave you hanging. Included below is one of my favorite posts from this past May. If you are looking for something new, though, head on over to the Scientific American guest blog to see my essay “Breaking Our Link to the March of Progress” or to Smithsonian magazine for an excerpt from Written in Stone about the search for early whales. With any luck I will be back on track by the beginning of next week.
There is something fantastically weird about giant ground sloths. They are creatures from a not-too-distant past – close enough to us in time that their hair and hides are sometimes found in circumstances of exceptional preservation – but they have no living equivalent. Their arboreal cousins still live in the tropics of the western hemisphere, but they can hardly be considered proxies for the ground sloths of the Pleistocene.
The most famous of these ancient beasts was Megatherium. Among the first giant ground sloths ever found, this imposing beast has been fascinating paleontologists and the public for over 200 years. What many people don’t know, however, is that there were many different species of ground sloth. Megatherium was not a lone aberration but a part of a highly successful family, one of the few types of weird South American mammal that flourished in North America when the two continents came into contact a few million years ago. Not all of them were the same. While some made their living grazing in open habitats others preferred to browse among most forested environs, and a recent study published in the Journal of Morphology provides a way to tell which kind of lifestyle particular sloths might have had.
Everybody knows that teeth can tell you quite a bit about what an animal eats, but they are not the only informative parts of the skull when it comes to diet. To ascertain the range of feeding habits in giant ground sloths researchers Susana Bargo, Nestor Toledo, and Sergio Vizcaino looked at the muzzles of the species Megatherium americanum, Glossotherium robustum, Lestodon armatus, Mylodon darwini, and Scelidotherium leptocephalum. This swath of ground sloth diversity exhibited a variety of skull shapes. Two of them, Glossotherium and Lestodon, had squared-off muzzles, while the rest had more narrow snouts. These disparities were almost certainly indicated differences in diet, but in order to resolve what giant sloths ate and how they ate it the scientists went about reconstructing the soft tissue anatomy of each species.
Figuring out the muzzle shape of each sloth was a multi-step process. The first task was determining the extent of cartilage which capped the nasal bones in life. Together the bone and cartilage provided the framework for the various muscles involved in lip movement, and once the muscles were properly arranged the entire head could be fleshed out. So restored, the scientists could ascertain whether an individual species had the skull of a grazer or a browser. Whereas grazers would have had wide, squared-off muzzles suited to taking in large amounts of low-quality foods (e.g. grass), the browsers had narrower muzzles best suited to selective feeding on leaves, fruits, and other relatively high-quality foods.
The results were fairly clear cut. Lestodon armatus and Glossotherium robustum both had comparatively wide, spoon-shaped muzzles, while Scelidotherium leptocephalum, Mylodon darwini, and Megatherium americanum had narrow muzzles (the latter species having the narrowest of all). Overlain on top of each other, there is a wide gap between the muzzle shapes of the grazers and browsers; the sloths selected are not grades along a continuum but occupy opposite, well-defined ends of the spectrum. Additionally, the researchers behind the analysis proposed that Megatherium americanum may have been such a specialized feeder that it had a prehensile upper lip akin to what is seen in the black rhinoceros. The grazing sloths Lestodon and Glossotherium, on the other hand, would have had lips more like that of the white rhinoceros – squared off and better suited to bulk feeding.
The hypothesized feeding habits of the sloths are in accord with the skull shapes of modern grazing and browsing mammals, and this disparity may have been a result of competitive exclusion. During the course of their evolution of giant ground sloths multiple groups undoubtedly ended up vying for the same food resources. In such circumstances variations which would allow sloths to more efficiently feed on unexploited resources – such as grass – would be selected for and generate the disparate muzzle shapes identified in the new study. (This phenomenon – called niche partitioning – is a common part of modern day ecology.) Competition between sloth species did not necessarily lead to the survival of one at the expense of the other – sometimes these interactions led to the origin of new varieties of giant sloth, further adding to the diversity of the group. Taken another way, however, this makes the recent disappearance of the giant sloths all the more mysterious. Megatherium and its kin were no evolutionary sluggards, and, as bizarre as they were, it is even stranger that there are no giant ground sloths trundling about the landscape today.
Post script: The kind of niche partitioning seen among the giant ground sloths was not just restricted to interactions between species. It could also occur within species as organisms grew up. A recently-described juvenile Diplodocus skull suggests that young individuals were browsers while adult Diplodocus were better suited to grazing.
Top image: The skeleton of Megatherium, as figured in William Buckland’s Geology and Mineralogy Considered With Reference to Natural Theology Vol, 2.
Bargo, M., Toledo, N., & Vizca√≠no, S. (2006). Muzzle of South American Pleistocene ground sloths (Xenarthra, Tardigrada) Journal of Morphology, 267 (2), 248-263 DOI: 10.1002/jmor.10399