The Mammuthus Moment: How to tell a sloth from another sloth


Xenarthra (sloths, armadillos and anteaters) is one of the more diverse and hard to phylogenetically place mammalian orders. For example, it is still not completely clear what other group of mammals are the closest relatives of xenarthrans. This extends to within group comparisons where sloths alone contain almost 100 distinct genera. Five species contained in two genera are all that remain of sloths since the end-Pleistocene mammalian mass extinctions. That is a lot of lost biodiversity. Both surviving groups are arboreal though many extinct sloths were not. They ranged in size from the elephant sized Mylodon darwinii found in Patagonia to bear sized megathirids such as Nothrotheriops shastensis which were once abundant in the American southwest.   For a number of reasons, including convergent evolution of traits to the sheer number of fossil groups, tracing the relationships among sloths morphologically has been extremely difficult. The questions are, which sloths are related to which other sloths? More importantly for modern biology, which extinct sloths are the few remaining extant sloths groups related to? Are sloths descended from a common arboreal ancestor or did arboreality emerge independently in different lineages? What can molecular biology bring to the table?


Several studies using ancient DNA have been applied to sloths. They have addressed a variety of questions including taxanomic but also ecological and dietary such as the determination of sloth diet from ancient dung (Poinar et al. 1998; Hofreiter et al. 2000). Braving the low copy DNA, fear of lab contamination, and the smell of very old poop, these researchers were able to identify the balls of dung they were analyzing as being sloth in origin and more importantly for their study, they could identify the plants the sloths had eaten from the DNA sequences they retrieved. Looking at dung of different ages opened a window into the diet over the thousands of years in which the climate was drastically changing and the animals such as giant ground sloths were heading towards extinction. But it still could not tell us who is who.


Ancient DNA is very limited and you can only examine remains that are preserved well enough to have a chance of containing trace DNA. In other words, forget Jurassic Park. The real science is much more difficult and far less glamorous (unless working with fossil crap is high on your list of exciting things to do). Because of these technical limitations, sloth analysis has been restricted to Mylodon and Nothrotheriops. (remember, Mylodon was big as an elephant, Nothrotheriops like a bear). Morphology suggests that Myolodon should form its own separate group and have many primitive sloth characteristics (i.e., be more like the last common ancestor of sloths) and that Nothrotheriops, the extant two toed sloths, and the extant three toed sloths should be more closely related to one another (though distantly). But what does their DNA say?


The first study to address this only included Mylodon, as no suitable remains from any other group were known at the time. The remains they used came from a cold cave in Patagonia which was like a big meat freezer. Hair and skin was even found.  [T1] The results of this study suggested that in fact, Mylodons and two toed sloths are closely related to the exclusion of three toed sloths. As Mylodons were giant ground sloths and extant three and two toed sloths are arboreal, it suggested that arboreality evolved at least twice independently, or so the authors concluded. It means that sloths were on the one hand getting small and climbing trees, and at the same time getting huge and looking like trees, independently and in very different lineages (Höss et al. 1996).


Since this first study, Nothrotheriops DNA has been extracted from dung (as noted above). Not only that, Mylodon nuclear DNA was retrieved in the interim. This is novel, as most ancient DNA studies have relied exclusively on mitochondrial DNA, which only tells you what happened to the maternal lineage of any given group (Greenwood et al. 1999). The authors managed to systematically retrieve nuclear DNA sequences from a variety of extinct mammals including mammoths, sloths and an extinct cave bear. It was a proof of principle study and did not address any phylogenetic questions however.


Back again to who is who among sloths. Adding Nothrotheriops to the mix using ancient DNA further supported the findings of the initial study (Greenwood et al. 2001). This study combined all the work of the previous phylogenetic attempts and added more DNA sequences to the mix to attempt to resolve the issue with a greater quantity of data. An analysis of where the resolution was being lost was also employed which demonstrated that some genes are evolving at different rates than others which may have complicated matters somewhat for the earlier studies. In any case, again, Mylodon clearly associated with arboreal two toed sloths, while three toed sloths clustered with Nothrotheriops, completely contradicting the expectations from morphology. It would be like finding out we are more like gibbons than chimpanzees at the DNA level…ok, that is a bit of an exaggeration since the time scale is much larger for sloths than apes, but it does illustrate the contradiction.


Since science does not just say the day is over we are finished, the work has not stopped and now nuclear DNA phylogenies are coming into play (Poinar et al. 2003). For some odd reason, the authors did not include Mylodon in the study, though in previous attempts they had. This of course severely limits the utility, but it’s a good start…and I guess they can get another paper out of it by publishing Mylodon separately.[T2] 


The conclusions from each of these studies are very tentative. Can you find a testable hypothesis in any or all of these studies, e.g., Mylodon is more closely related to…what?  (Hint, you can figure it out by reading the online abstracts.) Does all the data contradict or support the conclusions, i.e., morphologically and molecularly, do they say the same thing? If not, is there something wrong with one methodology?  Both? And what should be done next?  How would you resolve the relationship among the extant sloths, Mylodon, and Nothrotheriops? Would the scientists referenced impede the publishing of results that suggested their conclusions were wrong and reject morphological work that does not support the DNA evidence? Is science omitting some kind of other evidence that would resolve the question poof bang? Think slowly as a toed sloth moves and you might find some answers.



Greenwood AD, Capelli C, Possnert G, Paabo S.

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Free Full Text

Nuclear DNA sequences from late Pleistocene megafauna. Mol Biol Evol. 1999 Nov; 16(11):1466-73.

Greenwood AD, Castresana J, Feldmaier-Fuchs G, Paabo S.

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A molecular phylogeny of two extinct sloths. Mol Phylogenet Evol. 2001 Jan; 18(1):94-103.

Hoss M, Dilling A, Currant A, Paabo S.[T3] 

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Free in PMC

Molecular phylogeny of the extinct ground sloth Mylodon darwinii.
Proc Natl Acad Sci U S A. 1996 Jan 9;93(1):181-5.

Hofreiter M, Poinar HN, Spaulding WG, Bauer K, Martin PS, Possnert G, Paabo S.

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A molecular analysis of ground sloth diet through the last glaciation.
Mol Ecol. 2000 Dec;9(12):1975-84.

Poinar H, Kuch M, McDonald G, Martin P, Paabo S.

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Nuclear gene sequences from a late pleistocene sloth coprolite. Curr Biol. 2003 Jul 1; 13(13):1150-2.


Poinar HN, Hofreiter M, Spaulding WG, Martin PS, Stankiewicz BA, Bland H, Evershed RP, Possnert G, Paabo S.

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Molecular coproscopy: dung and diet of the extinct ground sloth Nothrotheriops shastensis. Science. 1998 Jul 17; 281(5375):402-6.


 [T1]Can you make it more clear that the DNA results contradicted initial expectations, and in what way?  I’m unfamiliar with the genera names, and I had to work pretty hard to understand.

 [T2]Doesn’t seem enough info in the column to answer the questions.  Can you bring the reader closer to the answer without actually providing the answer?  People enjoy figuring things out, and usually don’t care that it was too easy as long as the column held their interest.

 [T3]References probably should be minimal in a column.  The format takes up lots of space, at least in Word, and I assume Word is wysiwyg for HTML?  Can references be moved to end?  In fact, you can grab the readers interest by turning the references into stories, e.g., Dilling, et al, braved the dangers of  the Amazon to gather ancient Nothrotheriops dung...etc...(of course, I cam make it very dramatic since I’m making it up)