Ten things you didn't know about sloths


Sloths. Were there predatory sloths? Sloths that lived in the sea? Sloths that dug immense tunnels? Sloths on Antarctica? Sloths so keen to get to the US of A that they didn't wait for the land bridge, but swam the way instead? Well, let's see...

Today I was asked a question about sloths. Sloths are among my favourite mammals, and hence I feel particularly guilty in not having blogged about them, though I did publish a review article on them fairly recently (Naish 2005). Not only does this distraction allow me to delay yet further those promised posts on blood-feeding birds and bats, gigantic feral cats, rhinogradentians, and Triassic crurotarsans, it also occurred to me that if I write a brief list of 'things you didn't know about sloths', then that would be quicker than completing a normal, essay-type post. That was the theory. Let's get on with it. If you already know a lot about sloths, then none of this will be new to you, and I apologise for insulting your intelligence [adjacent photo of a two-toed sloth borrowed from here].

1. Sloths are entirely American, right? Most of them are, but the oldest fossil sloth (dating to the Eocene) is from Seymour Island, Antarctica. This at least suggests that sloths (and probably other xenarthrans*) were originally common to both South America and Antarctica, though right now we can't say any more than that (and yes I do know about Eurotamandua and the ernanodontids).

* The placental mammal clade that includes sloths, armadillos and anteaters.

2. Sloths only invaded North America once the Panamanian land bridge formed about 2.5 million years ago. Oh really? While it seems that some sloths did get into North America this way (McDonald & Pelikan 2006), hardly ever mentioned is the fact that two sloths (Thinobadistes and Pliometanastes) appeared in North America before the land bridge formed (McDonald 2005). Presumably they swam and island-hopped to North America. Apparently they weren't the only tetrapods that did this, given recent evidence indicating that the phorusrhachid Titanis also colonised North America prior to the formation of the land bridge. More on this in a future post (for previous writings on phorusrhacids go here and here).

3. The sloths that got into North America didn't necessarily idle around in Mexico or the southern states (though some did). Some Pleistocene sloths occurred north of the Arctic Circle: Megalonyx jeffersonii (often called Jefferson's sloth, and I'm sure you know why) is known to have occurred as far north as Alaska and the Northwest Territories. At this time the climate was a few degrees warmer than it is today, but even so it would have been cool (McDonald et al. 2000). Sloths have therefore not been restricted to tropical or temperate climates for the whole of their history. Incidentally, wouldn't it be neat if they had actually managed to cross the Bering land bridge at some stage?

4. Perhaps the most startling recent discovery from the world of sloths is that there was once a group of marine, amphibious sloths. First described for the Peruvian species Thalassocnus natans (Muizon & McDonald 1995), we now know of five of these sea-going Pacific sloths. The older species probably lounged around on the beaches, and waded or swam out to feed on marine plants. The youngest and most anatomically specialised member of the group, T. yaucensis, had phenomenally stretched out, spatulate jaw tips, and limb bones shaped like those of a sealion. All Thalassocnus sloths exhibit features of the skull, hindlimbs and tail vertebrae showing that they were specialised for swimming, and for feeding in the water, but T. yaucensis may have been about as aquatic as a sirenian or pinniped (Muizon et al. 2004a, b). Err, gosh.


5. One of the biggest and best known fossil sloths is Megatherium from the Pliocene and Pleistocene of South America. Multiple species are known. Like other sloths, megatheres have generally been imagined as folivores, reaching up into trees and shrubs and using their strong arms and large claws to manipulate branches. However, after reconstructing the forelimb muscles of the best known species, M. americanum (shown in adjacent photo), Farina & Blanco (1996) argued that this animal was a predator, specialised for stabbing and lifting large herbivores. This radical idea has not been widely accepted among other sloth workers. It is mostly based on the assumption that the ability to strike rapidly with the forelimbs correlates with a predatory habit.

i-5674d895c18198934c6149bc2c74ffab-Texas Paramylodon.jpg

6. Sloths didn't just use their arms and hooked hand claws to manipulate foliage or climb with. Some mylodontid sloths have forelimb anatomy and bone strength matching that of living mammals that regularly dig (Bargo et al. 2000). Furthermore, the wide, straight and relatively flat claws of these sloths match the idea that they were diggers [adjacent photo shows the mylodontid Paramylodon, borrowed from wikipedia's entry on sloths].

7. Confirmation of this digging idea comes from Argentinean Pleistocene burrows apparently made by these sloths. Some of these burrows are more than 1 m tall, 2 m wide and more than 20 m long. Their sides and roofs are scored with scratch marks matching mylodontid claw anatomy.

8. Sloths are phenomenally diverse in hand anatomy. Some fossil sloths had the full compliment of five fingers (albeit sometimes with the thumb and/or fifth finger strongly reduced). Others lacked the thumb, and others lacked the thumb and digit II (so much for the theory that digits tend to be lost from the edges of the hand first). Bradypus (three-toed sloth) possesses only digits II-IV on the hand, and the megalonychid Choloepus only has digits II and III. Several sloth groups exhibit fusion of various manual phalanges, including of both phalanges in the thumb (in Eremotherium) and of the two phalanges at the base of the third digit (in Thalassocnus), as well as fusion of metacarpals to carpals.

9. Historically, ground sloths have only just become extinct: some sloth remains from mainland North America are only about 8600 years old while some of the Caribbean sloths, like Synocnes comes and Parocnus serus from Haiti, were apparently alive just 500 years ago. Ground sloth survival is rumoured in Amazonia but as yet unsupported by any good evidence.


10. While it used to be thought that the living tree sloths were closely related to one another, phylogenetic work indicates quite convincingly that the two living genera (Bradypus and Choloepus) are about as distantly related as they could possibly be. Bradypus (the three-toed sloth: adjacent image - from here - shows how Bradypus moves on the ground) is archaic and represents the most basal sloth lineage that we know of, while Choloepus (the two-toed sloth) belongs to a more recently evolved clade, Megalonychidae. This has a few really interesting ramifications. Firstly, the basal position of Bradypus means that this taxon currently has a ghost lineage extending back to the Eocene at least. Secondly, the fact that this basal sloth is arboreal raises the possibility that arboreality is primitive for sloths, rather than derived. Thirdly, the many anatomical and behavioural similarities shared by Bradypus and Choloepus are convergences, in which case 'this taxonomic arrangement ... surely presents[s] one of the most striking examples of convergent evolution known among mammals' (Gaudin 2004, p. 255). One more thing: there is no longer a clear dichotomy between 'tree sloths' and 'ground sloths' (some so-called ground sloths were actually capable climbers), nor were all fossil sloths bigger than the living species (Neocnus toupiti from Hispaniola was the smallest sloth ever).

There's a lot more I could say, but that'll do. I didn't mention the armour, their bizarre vertebral and pelvic anatomy, the stuff we know about their hair, their physiology, their highly unusual feet, the whole Cueva de Milodon story, the DNA work, the combustible dung piles, their extreme 'tenacity to life', the screams, the fangs, and the scariest goddam book I've ever seen. At least I can rest, safe in the knowledge that I have finally blogged something on xenarthrans...

PS: pdf of Naish (2005) available to anyone who asks.

Refs - -

Bargo, M. S., VizcaÃno, S. F., Archuby, F. M. & Blanco, R. E. 2000. Limb bone proportions, strength and digging in some Lujanian (Late Pleistocene-Early Holocene) mylodontid ground sloths (Mammalia, Xenarthra). Journal of Vertebrate Paleontology 20, 601-610.

Farina, R. A. & Blanco, R. E. 1996. Megatherium, the stabber. Proceedings of the Royal Society of London B 263, 1725-1729.

Gaudin, T. 2004. Phylogenetic relationships among sloths (Mammalia, Xenarthra, Tardigrada): the craniodental evidence. Zoological Journal of the Linnean Society 140, 255-305.

McDonald, H. G. 2005. Paleoecology of extinct xenarthrans and the Great American Biotic Interchange. Bulletin of the Florida Museum of Natural History 45, 313-333.

- ., Harington, C. R. & de Iuliis, G. 2000. The ground sloth Megalonyx from Pleistocene deposits of the Old Crow Basin, Yukon, Canada. Arctic 53, 213-220.

- . & Pelikan, S. 2006. Mammoths and mylodonts: exotic species from two different continents in North American Pleistocene faunas. Quaternary International 142-143, 229-241.

Muizon, C. de & McDonald, H. G. 1995. An aquatic sloth from the Pliocene of Peru. Nature 375, 224-227.

- ., McDonald, H. G., Salas, R. & Urbina, M. 2004a. The youngest species of the aquatic sloth Thalassocnus and a reassessment of the relationships of the nothrothere sloths (Mammalia: Xenarthra). Journal of Vertebrate Paleontology 24, 387-397.

- ., McDonald, H. G., Salas, R. & Urbina, M. 2004b. The evolution of feeding adaptations of the aquatic sloth Thalassocnus. Journal of Vertebrate Paleontology 24, 398-410.

Naish, D. 2005. Fossils explained 51: sloths. Geology Today 21 (6), 232-238.


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I just recently found your blog, and I'm loving every post.
Where can we find reconstructions of these marine sloths?

Dude, you write awesome posts. Thanks for a great post about the namesakes of my second-favourite sin.

Er... sloths are the closest thing there are to green mammals, I think (due to algae that grows in their fur).

". . . the scariest goddam book I've ever seen." ???

Wonderful Post Darren! Greg McDonald is a good friend and reconstructing sloths with his help and advice have been some of my favorite projects. I actually did a painting for Discover magazine back in the mid 1990s of a predatory sloth based on the Farina and Blanco paper. Drawing it attacking another animal just didn't seem right!

Awesome! I knew much, but nowhere near all of that...

The hypothesis of carnivory has yet to explain the complete absence of incisors and canines. A beak is an option for a placental.

Concerning the Panama land bridge... I'm getting the feeling that the bridge itself is older. There was that SVP meeting abstract about a 9 Ma old tapir from Brazil or so, concluding that the land bridge was already there, but few migrants managed to get first into the rainforest and then out of it into the fossiliferous pampa. The Great American Interchange 6 Ma later was then just a period of continuous grasslands across the land bridge.

were apparently alive just 500 years ago.

Surely you mean 5000?

By David Marjanović (not verified) on 30 Jan 2007 #permalink

Some of these burrows are more than 1 m tall, 2 m wide and more than 20 m long. Their sides and roofs are scored with scratch marks matching mylodontid claw anatomy.

For me, there's something deeply sad about that, and the knowledge that there were still living ground sloths just 500 years ago. Those burrows, made by fascinating, marvelous beasts and still bearing their last scratch marks, waiting patiently for the return of owners who will never return . . .

By Maureen Lycaon (not verified) on 30 Jan 2007 #permalink

Another great post, Darren. Here's one sloth fan who just learned a lot. I always figured the differing number of cervical vertebrae between the two modern sloth genera was pretty good evidence the two diverged a long time ago. Curious about that scary Goddamn book.

And this is a Vampire post how? How come there are no Vampire sloths.

"PS: pdf of Naish (2005) available to anyone who asks."

If it's comprehensible to a layman, I'm asking.

Fascinating stuff. Especially the aquatic sloths and the idea the giant sloth might have been predatory. I'll look forward to more on sloths, but will understand if you're slow with that. :-)

Thanks for such a great post! I thought I read somewhere (a long time ago) that some sloths are green due to the algae growing in their fur...?

I didn't mention...the combustible dung piles, their extreme 'tenacity to life', the screams, the fangs, and the scariest goddam book I've ever seen.

Aw, man, that's just cruel!

You know I'm a big fan of the end-of-post teasers, but this is too much. Burning piles of shit? Screaming? Scary books? I think you've got the making of 'Ten MORE Things You Didn't Know..." right there.

Your people are lost in the wilderness of sloth ignorance. Lead us home, Brother Darren!

Your disciple,


P.S. I always go out of my way to say sloathes. It just sounds classier. Ta ta!

I reckon the olecranon process on the ulna of the biggest sloths (your Megatherium's and Eremotherium's) shows some adaptations for weight bearing. Unlike fossorial sloths, the olecranon in these big taxa is angled perpendicular to the long axis of the ulna, which is the condition seen in elephants. This enables the forelimb to be fully extended into a columnar position, which is, of course, dead useful for an animal that may have weighed 4000 - 6000 kg (although sloth morphology makes working out their weight more problematic than other mammals).

"Scariest goddam book I've ever seen".

? Come on, Darren, give us that at least!

By Steve Bodio (not verified) on 31 Jan 2007 #permalink

I didn't mention the armour,....

You are a wicked tease. I am sure you are doing this just to keep us coming back day after day.

It's working.

Very interesting! I think I read for the first time about those aquatic mammals in "Dinosaur Encyclopedia: From Dinosaurs to the Dawn of Man" (Just why?) or at a german website about prehistoric whales. But there was no desciption of their actual anatomy. Their skulls look really strange, the anterior part of the mandible looks nearly like the beak of a spoonbill. Sometimes I think (okay, I changed my mind about this thing in the last months) that nothing can surprise me anymore, and then things like this one appear. Sloths are really fantastic, surely among the most bizarre mammals on earth. It is really a shame that they are now extinct, especially if you keep in mind that animals like Megalocnus were so recently extinct.
I can well imagine that some giant sloth took carrion on occasion, but I highly doubt that they really hunted. Many mammals possess huge and strong claws which would be great to kill other animals, but they are only used for burrowing or defense. Even if ground sloths were more agile than their modern relatives, they were surely not able to walk very fast and able to kill even comparably slow big mammals. But to keep on the style of their locomotion, I read in one book that they walked for longer distances on their hind legs, what would be really unusual for such a big beast.

I recently read a Stephen Jay Gould essay about sloths, where he makes some similar points to yours. It's always important to be reminded how small a percentage of history's diversity is represented by the branches that are extant today.

Having powerful, clawed forelimbs and being able to use them with fearsome effect and precision does not necessarily imply carnivorous habits. Look at the ratel or the giant anteater for example.
Even lions have great respect for a ratel.

By Tommy Tyrberg (not verified) on 31 Jan 2007 #permalink

That's a lot of commments there, thanks to all for their thoughts. For me, the 'take home' message is that 'Ten more things you didn't know about sloths' is definitely going to be a future post.

Select responses..

Yes, the two living sloths both have algae growing on their fur. But in keeping with the amazing convergence they display, they both grow algae in different places on the hairs (in Choloepus the algae grows in special longitudinal grooves; in Bradypus the algae grows in transverse cracks).

And David: no, not 5000. It really was 500.

As for the scariest book I've ever seen, one day all will be explained.. (and, yes, these dirty little tricks are included to make you visit again)...

Darren, I missed that reference to the burning dung the first time. Well, if You're going to write about ramparts maybe you should mention the last hope as well?

By Tommy Tyrberg (not verified) on 31 Jan 2007 #permalink

Given that modern sloths are quite capable swimmers while I'm surprised they made it to North America they are one of the groups you might expect to island hop. Elephants are good at swimming and crop up as island endemics fairly often.

I find the idea of Phorusrachids doing this rather more unusual. It does lend credence to the idea that the land bridge might be older than previously thought.

I'd be very interested in your review article if that's ok.

Great post, looking forward to more!
Correct me if I'm wrong, but wasn't the "predatory sloth" assumed to have been a glyptodont specialist? I think the reasoning was that it could tip the slow animals over and stab them in the underbelly.
That said, I don't think sloths were more than occasional scavengers.

I wrote...

A beak is an option for a placental.

GAH! Of course it's not an option (due to the complete loss of the caruncle).

500 years? Now that is interesting.

By David Marjanovi? (not verified) on 01 Feb 2007 #permalink

I can't even imagine them scavenging without incisors or canines (or a beak).

On another note... sloths and koalas are sorta kinda analogs, aren't they? Now why haven't the koalas evolved a crazy diversity? Maybe that's the right question to ask. Just speculating.

By David Marjanović (not verified) on 01 Feb 2007 #permalink

Great post!!

P.S. I'd certainly be interested in a PDF of your review article.

David Marjanovic wrote:

'On another note... sloths and koalas are sorta kinda analogs, aren't they? Now why haven't the koalas evolved a crazy diversity? Maybe that's the right question to ask. Just speculating.'

Well, koalas (analogs of surviving tree-sloths) are one of two surviving groups of Vombatomorphians, and it's this clade as a whole that is comparable in diversity to the sloths sensu lato. As well as wombats (large burrowing grasers, which included giant Pleistocene forms) there were rhino-sized diprotodontids and the somewhat ground-sloth-like palorchestids (with tapir-like trunk, much longer arms than legs, and well-clawed hands for ripping roots up and branches and enemies down). Plus a few other less extreme groups that went extinct in the Miocene; diversity not quite as crazy as the sloths, but maybe marsupials have an excuse for relative morphological conservatism in their (very crazy) rites of birth passage.

By John Scanlon (not verified) on 01 Feb 2007 #permalink

I would be interested in a pdf of your review article as well.

By António Matos (not verified) on 02 Feb 2007 #permalink

The mention of the burrowing ground sloths is highly interesting, if you keep in mind that the south american Guaranà indios know a large ground sloth-like animal, which is said to be in hidden in self-buried holes, and comes only out at night...

This information was very interesting it will be a great start for my project thanx.

How is it possible for a giant flightless bird like Titanis to swim or drift towards ancient America ?

By Amongthylacines (not verified) on 08 Apr 2008 #permalink

How is it possible for a giant flightless bird like Titanis to swim or drift towards ancient America ?

Not at all. It walked across the landbridge as soon as the landbridge was established.

By David Marjanović (not verified) on 09 Apr 2008 #permalink

yes but Darren wrote: "....given recent evidence indicating that the phorusrhachid Titanis also colonised North America prior to the formation of the land bridge..."
I find that hard to believe.

Hello, I discovered this very interesting blog some weeks ago and I have been going through past posting and this one on sloths caught my attention (one of the many that did!!). There was indeed information on sloths that was totally new to me. But the part on the Caribbean sloths alive until 500 years ago caught me by surprise. As far as I know, the youngest dates for Caribbean sloths are 4,190 yr BP obtained through carbon dating from a tooth of Megalocnus rodens, followed 4,391 yr BP obtained from a humerus and ulna from Neocnus (=Synocnus) comes from Haiti (MacPhee et al., 2007; Steadman et al. 2005). Now if the 500 year mark comes from early 1900s literature, then this might not be reliable. The reason is that in the Caribbean region, people used to mine guano for agricultural purposes, resulting in cave deposits that are mixed and therefore misleading. I have personally seen some of these mixed cave deposits in the island of Puerto Rico. With no stratigraphic control left in some of these fossiliferous deposits, we can only try to find undisturbed pockets within the cave or look for caves that are less accessible and therefore undisturbed.

MacPhee, R. D. E., M. A. Iturralde-Vinent and O. Jiménez Vázquez. 2007. Prehistoric sloth extinction in Cuba: implications of a new last appearance date. Caribbean Journal of Science 43(1):94-95.
Steadman, D. W., P. S. Martin, R. D. E. MacPhee, A. J. T. Jull, H. G. McDonald, C. A. Woods, M. Iturralde-Vinent and G. W. L. Hodgins. 2005. Asynchronous extinction of late Quaternary sloths on continents and islands. Proceedings of the National Academy of Sciences 102(33):11763-11768.

By Jorge Velez-Juarbe (not verified) on 12 May 2008 #permalink

Your blog is wonderful and the sloth post was just fascinating. I would appreciate receiving the pdf of your 2005 review article.

By Katherine Magwene (not verified) on 22 Aug 2009 #permalink

Belatedly found this blog...and am fascinated!
1. pdf of Naish (2005), please.
2. scariest book? please, share!

Hi everybody:
If you want to take a look inside the homes of giant burrowing sloths and giant armadillos, take a look on the pictures at www.ufrgs.br/paleotocas. Homepage is in portuguese, sorry, it´s a part of our media program.
We are doing routinely research inside the tunnels dug by these animals. Darren: correct your blog, the tunnels may be more than FORTY meters long.
Cheers from Brazil.

Love your sloth info. I work as a volunteer educator in Amersfoort Zoo, The Netherlands, where we have two couples of sloths in a night building, where they move freely amongst the visitors. Both couples reproduce well, which is remarkable since only a few couples in European Zoos manage to reproduce (apparently, they really have to like each other). Could I please receive the pdf of your 2005 review article? Thanks!

I thought I knew my sloths, but your post still managed to give me new interesting information (especially the burrowing sloths!). Thanks Darren!

I'd also like to receive your 2005 sloth review, please.

PS. Further posts on extinct xenarthrans would be much appreciated!

By Tuomas Jokela (not verified) on 01 Jul 2010 #permalink

I am awed, again, beyond adjectives. Now I want more sloths!!!

May I see the pdf please?

God, I wish I was rich, so I could pay you for the worth (wonder and joy) of these facts you bring.

1) Xenarthran status of Antarctic fossil was doubtful, and was reinterpretated.

Reinterpretation of a Middle Eocene record of Tardigrada (Pilosa, Xenarthra, Mammalia) from La Meseta Formation, Seymour Island, West Antarctica. (American Museum novitates, no. 3689)MacPhee, R. D. E.; Reguero, Marcelo.

2) Ghost lineages in South American tropical groups are very common, since almost all fossil sites came from Patagonia. There are few sites in Northern South America (Amazonia included) - Paleocene Cerrejon Formation in Colombia has been explored (a giant boa, two crocs and a turtle until now) and sounds a nice source for future findings. Oldest Paleogene fossil site in Brazil is ItaboraÃ, near Rio de Janeiro, in SE Brazil, closer to Argentina than to Amazonia, where we will surely find plausible anteaters' and sloths' ghost ancestors.

3) Aquatic sloths as island-hoppers sounds plausible, but a running bird like Titanis doesn't. Panamanian is older, or there was some alternative land bridges through Caribbean before the isthmus was formed.

By J.S. Lopes (not verified) on 29 Apr 2011 #permalink

Thanks for adding new comments to this rather old article. As you note, it includes several mistakes: the Caribbean sloths are definitely not as young as 500 years old, and the Seymour Island sloth tooth was later re-interpreted as that of an indeterminate mammal. For more on Titanis and phorusrhacid distribution in North and South America, go here. I really should update this piece at some stage.

Arboreality in sloths and pygmy anteaters would imply an scansorial/arboreal origin for Pilosa (Tardigrada+Vermilingua). I wonder what North American Cretaceous/Paleocene mammals would closer to Xenarthrans: some of the "condylarts", pantodonts or "proteutherian-cimolestan"? A cluster Xenarthra-Pantodonta would provide a nice name for the clade: Amblytheria. Some of the South American ungulates could be xenarthran cousins, maybe. Unfortunately until now, Argentinian sites didn't show any clue to explain the origin of xenarthrans. Armadillos seem to get to reach higher latitudes earlier, being present in Argentina and Brazil since Paleocene, but Pilosa groups made their debut in the South later, sugesting a Northern craddle.

By J.S. Lopes (not verified) on 29 Apr 2011 #permalink

the last bit is quite a tease indeed - screams, fangs and a scary book ? I wanna know more

as for the ground sloth being predatory, the idea doesn't click for me. It would only get to take slow prey like glyptodont. I would imagine a great anteater to be faster (aren't their limbs built in similar size ?) and they are not even that fast

and as for the aquatic sloths... can't wait til there is more known about them, sounds so interesting

Re: J.S. Lopes (#44)
Genetic evidence suggests that Xenarthrans are closer to Afrotheria than to any extant Boreutherians, so (unless the claim that some North American "condylarths" are actually Afrotheres turns out to be right), they may not be closely related to ANY North American late Cretaceous or Paleocene types.
As for the relationship to South American ungulates... some of them lived into the Pleistocene. I would dearly love to see an attempt to extract DNA from their bones: it would be nice to have some indication whether they are closer to the southern (Afrothere,Xenarthran) or Northern branches of extant placentals!

By Allen Hazen (not verified) on 01 May 2011 #permalink