Posted by: Rob Viens | September 3, 2013

Where Did All the Sloths Go?

Sloth Week logo

On the last day of Sloth Week, I thought it would only be appropriate to talk about the fate of the sloths, glyptodonts, and other large mammals (whose fossils that Darwin has yet to discover). The answer, in a nutshell, is that they first migrated to the Caribbean and North America and then, by about 10,000 years ago, the last of the Xenarthran megafauna disappeared from the world forever.

As noted earlier last week, the Xenarthrans evolved in South America, and were unique (endemic) to that continent for many millions of years.  After South America became separated from Gondwana it remained isolated from the rest of the world’s land masses for over 100 million years (for more on the movement of South America, see The Slow Boat from Africa (Falkland Geology Part II)). This isolation caused some species to divergent (such as the New World monkeys that evolved along a slightly different path than their cousins in the Old World). And in other cases, it led to the evolution of entirely new types of organisms (such as the Xenarthra).

Eventually, however, that isolation began to come to an end as (like two ships) the North and South American continents “drifted” closer together.  Starting back in the Oligocene (about 35-30 million years ago), some species began to “island hop” – migrating northward (by swimming, rafting or flying) to a nearby islands, and gradually working their way toward North America. This was the case for the ground sloths, for example, who began showing up in the Caribbean Islands sometime between around 35 to 30 million years ago.  Some of them even island hopped all the way to North America by about 9 million years ago.

To continue using the ship analogy, about 3 million years ago the two continents were ultimately bridged by a “gangplank” – the Isthmus of Panama. Suddenly it was possible for species from both hemispheres to “jump ship” and walk north or south to a new continent. This massive migration was known as the Great American Interchange, and led to a major mixing of species in the Americas. (For more on Darwin’s thoughts about the interchange, see Bridging the Continents).

Great American Interchange (from BBC News by David Stevenson and Greg Wenzel)

Great American Interchange

The great success story for South American Mammals were the Xenarthrans – with representatives of all of the major taxa making the trip to North America (including sloths, armadillos, gyltodonts, and pamatheres). That is ultimately why Thomas Jefferson was the one who introduced ground sloths to the scientific community, and why I can find an entire fossil of a giant sloth at my local natural history museum. (it was excavated when the Seattle-Tacoma International Airport was being built.) Heck, sloths even made it as far north as Alaska.

So with all that success, what happened to the sloths? And for that mater, what happened to all the other large mammals, like the woolly mammoth, Irish elk, cave bear, giant kangaroos, toxodons, sabertooth cats, and so many more?  The fate of the sloth is linked to all of these other large mammals that disappeared at the end of the Pleistocene – roughly by about 10,000 years ago.

Megafauna of the Late Pleistocene in Spain (from the Public Library of Science):

Megafauna of the Late Pleistocene

The extinction of large mammals (over about half a ton (~500 kg)) was worst in North and South America and Australia, where more than 75% of all large mammals went extinct in the late Pleistocene. Large mammals in Sub-Saharan African, on the other hand, (and to some degree southeast Asia) largely survived the extinction.  This makes for a curious pattern and the difference may provide clues to the cause of the extinction.

There are typically two leading hypotheses that have been proposed for this extinction – (1) climate change and (2) the arrival of humans – those pesky little hominids.

Did humans eat the megafauna out of existence? An early painting by Heinrich Harder (1858-1935)
Hunting glyptodonts

It may be a bit unscientific of me to just present one side, but my gut tells me that in this case humans were the primary problem. Maybe it was not the whole problem, but it certainly was the root cause.  Climate had been changing – both cooling and warming – through numerous glacial cycles for the better part of two million years.  There is nothing to suggest that the most recent climate change that ended the last glacial period about 10,000 years ago was any worse than those that came before (which the mega fauna survived).  And it is interesting how the timing of the large mammal extinctions occurs at slightly different times across the globe – generally (by not always) seeming to match up with the arrival of humans. The main exception – Africa – is where humans evolved, so it often argued that because the large mammals there co-evolved with humans, they were able to adapt and survive.

Large mammal extinction and the arrival of humans (after Martin, 1989 via Wikipedia Commons):

Graph of extinction compared to arrival of humans

It is likely that human hunting alone would not be enough to wipe out all the large mammals, but it might have tipped the balance in populations that where also being stressed by changing climate.  And once some of the larger species started to disappear from an ecosystem (say mammoths), others that indirectly depended on them (as a keystone species in the ecosystem) might have been impacted, as well. This should not come as a surprise to us today, where every day our actions are leading to what some have called the 6th great mass extinction in the history of the Earth. We rarely kill every individual in a species, but our activity makes populations more vulnerable, so that the their “everyday” problems become much more serious.

A few sloths and other megafauna managed to hang on for a few thousand years after the main pulse of the extinction.  And to further support the “human cause” of their demise, all of these animals were isolated on islands that where not inhabited by our ancestors.  For example, sloths survived in the Antilles Islands (Cuba, Puerto Rico and Hispaniola) until about 7000 years ago, and mammoth fossils have been found from Wrangel Island in the Arctic that date to about 4000 years ago.  But in the end, even these holdouts finally shuffled off and the era of the giant ground sloth finally drew to a close. Alas, the world is a poorer place for the loss.

Megalocnus rodens, a fossil sloth from the Caribbean (from the American Museum of Natural History via Wikipedia Commons)

Megalocnus rodens

Back in 1833, Darwin seemed to be rather idle – still waiting for the next exciting leg of his overland journey.  In early September, he simple wrote:

“Returned in the evening. During the last week the weather has been very hot & dry; in consequence of this all the pools & shallow lakes, which before contained saline water, now presented a level plain of salt-petre, as white as snow. — This resemblance was the more complete from the edges of the pools appearing like drift heaps.” (Sept 1)

And even more telling the next day:

“Nothing to be done.” (Sept 2)

Alas, a phrase that also applies to the permanent loss of the ground sloths. (RJV)

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