Allow me to introduce you to the biggest rodent in the world: the capybara.
It’s sometimes known as the ‘water pig’, because it can be found swimming, diving,
or simply lounging in the waters throughout South America
And capybaras have also earned a reputation for being remarkably social animals
Not only do they live in large packs of their own kind, but they’ve also been known to
allow other species to hitch a ride on their backs, or even their heads, no questions
asked
But while capybaras are interesting and indisputably cute, they’re just one part of a lineage
of rodents from South America with a long history of intrigue.
These are known as the Caviomorpha
Caviomorpha originated in South America, and it’s one of the most diverse groups of mammals
in the whole Western Hemisphere.
It contains roughly 250 species across 10 families, and includes some characters you
might know, like guinea pigs, chinchillas, and porcupines.
But … here’s the problem:
The best evidence we have suggests that, while Caviomorpha originated in South America, they
came from ancestors in Africa, over 40 million years ago.
And even back then, Africa and South America were really far apart.
So … how did all of these famous (and adorable!) rodents wind up where they are now?
Well, the prevailing thinking is that Caviomorpha are a rare, but not unheard of!, example of
a group that got to where they are by just … going with the flow.
Caviomorphs like our friend the capybara belong to an infraorder of rodents known as Hystricognathi,
whose species are found in the Americas, but also Africa and parts of Asia.
And even though they’re separated by an ocean, both the Old World and New World hystricognaths
share some traits that show that they’re closely related.
For one thing, they all have huge infraorbital foramina, which are openings on each side
of the skull just in front of the eyes.
Part of the jaw muscle that these rodents use for chewing passes through this opening,
which gives their molars extra strong bite force for grinding their food.
And the teeth of Old and New World hystricognaths also share some similarities, which has helped
scientists make the connection between the two groups.
But the story of these rodents gets even more interesting when you look at some of their
fossils.
The earliest known caviomorph fossils in South America are 41 million years old, dating to
the Middle Eocene Epoch.
They were discovered in the 2000s from a clay deposit in Peru, and right away, researchers
knew there was something special about them.
These fossil rodents were pretty small compared to later caviomorphs, around the size of voles
and small rats.
But they all shared key traits in their teeth -- traits that were also found among fossils
of Old World Hystricognaths of about the same age.
For example, their teeth all had low crowns and well-developed roots, as well as rounded
cusps linked by ridges, and upper molars that had 5 distinct ridges on them.
Which is interesting...because again, the Peruvian fossils were found in clay that was
41 million years old.
Which was a long time after South America and Africa separated, during the breakup of
the supercontinent Gondwana between 90 and 100 million years ago.
What’s more, the dates of these fossils have been corroborated by genetic evidence,
too!
Using that method known as the molecular clock, scientists used well-dated fossils, along
with average mutation rates, to estimate how long ago the New World hystricognaths diverged
from the Old World ones.
And the results came in at just about the same age as those Peruvian fossils, as early
as 45 million years ago, again in the Middle Eocene.
So, caviomorphs must have originated in South America tens of millions of years after South
America separated from Gondwana.
Which means that some Old World rodents must have migrated to South America, by some route,
to give rise to the caviomorphs.
The question then is: How?
Well, a couple of hypotheses proposed that the rodents actually originated from ancestors
in Asia, and their descendents migrated through North America, or Australia and Antarctica.
But there’s no fossil evidence of any likely caviomorph ancestors along either of these
routes.
So the prevailing theory these days?
These animals arrived in South America from Africa by a trans-Atlantic dispersal that
involved riding on floating masses of plant debris.
In other words...
THEY RAFTED THERE!
This hypothesis stems from the work of paleontologist René Lavocat and taxonomist Robert Hoffstetter
who published a series of studies on these rodents from 1969 to 1975.
And they argued that an African origin for South America’s hystricognaths was the best
explanation for the similarities we see between Old and New World species today.
And that, combined with the more recent fossil evidence, suggests that the South American
lineage must have been founded by African rodents that traveled across the Atlantic
… somehow.
From there, the simplest and most logical explanation seemed to be that they just … floated
there.
And decades of research into this idea has found that it’s at least plausible!
For one thing, around 40 million years ago, the distance between Africa and South America
was a lot shorter than it is now -- around 1,000 to 1,500 kilometers instead of 2600
kilometers today.
And the rodents may have been able to make stops along the way.
There’s evidence that, back in the middle Eocene, there may have been islands and shallow
waters that spanned parts of the Atlantic.
You can still see traces of this today on maps of the ocean floor, where two underwater
ridges almost meet at the bottom of the Atlantic.
Running east from South America, there’s a ridge called from the Rio Grande Rise.
And drill samples from that ridge contain rocks that formed above water, as well as
fossils of shallow-water animals, and red algae, which need light to survive.
Meanwhile, running west from Africa, there’s the Walvis Ridge, and drilling there has revealed
Middle Eocene volcanic rocks that were probably released above the ocean’s surface.
So, all of these findings point to there having been a strip of islands and shallow seas in
the South Atlantic about 40 million years ago.
But, how would these little mammals have crossed the ocean, whether by island-hopping or some
other way?
Well, there’s no evidence of any actual raft.
But it may have been a big mass of plant material, like branches and logs.
That’s partly because those sorts of materials would be buoyant enough to carry the animals’
weight.
But it’s also supported by some modern, albeit very rare, observations of animals
floating from place to place.
For example, about a month after a hurricane struck the Caribbean in 1995, fishermen witnessed
more than 15 green iguanas arriving on the shore of the island of Anguilla -- where the
iguanas had never been seen before.
And the vessel these reptiles used was a mat of logs and trees more than 9 meters long
in some places.
Biologists believe that these iguanas floated hundreds of kilometers from their native population
on the island of Guadeloupe to reach Anguilla.
So, it’s at least possible.
And as for how long this voyage might have taken for the ancient caviomorphs, one study
actually did the math, using modern currents and wind velocities, to estimate that a trans-Atlantic
float could have taken 11 days, if the journey took place 40 million years ago.
Or, if it happened 50 million years ago, when the continents were even closer, it might
have been possible in as little as 8 days!
Now, a week and a half on the open ocean with no fresh water would definitely have been
… unpleasant.
But experts think that small mammals like the ancestors of the caviomorphs could have
survived the journey.
Now, the unlikely way in which these wayward rodents came to be where they are is an example
of sweepstakes colonization, a concept originally proposed by paleontologist George Gaylord
Simpson in 1940.
Through this process, organisms cross through unsuitable territory to arrive in a new environment
by chance.
While uncommon, sweepstakes colonization is not completely unheard of.
It’s actually thought that the ancestors of the New World monkeys made an ocean voyage
very similar to that of the rodents around 10 million years later.
And when the rodent colonists finally got to their new home, they had indeed hit the
jackpot.
Eocene South America had lots of resources for them, as well as new niches to fill, and
few species to compete with.
As a result, caviomorphs colonized the island continent and diversified rapidly, in a number
of bursts between the Oligocene and Miocene.
This is known as adaptive radiation, when a lineage of organisms undergoes an exceptionally
high amount of diversification, to fill in a wide array of ecological niches.
Descendents of these bursts of rodent diversification went on to fill all sorts of roles.
Some wound up being semiaquatic like the capybara.
Others became arboreal like the spiny tree rat.
And some even took to burrowing underground like the cute little tuco-tuco.
And they also covered an impressively broad range of sizes.
The smallest members of Caviomorpha weigh between just 50 and 80 grams!
But by 4 million years ago, South America was also home to the biggest rodent that ever
lived, which was about the size of a horse.
So, caviomorphs seem to have an especially strange origin story: They exist because their
ancestors defied the odds by surviving a voyage across the Atlantic Ocean.
And when they arrived in their new world, the rodents rapidly diversified into the forms
we know today, from the capybara to the guinea pig.
And some members have migrated beyond South America, into the Caribbean and parts of North
America!
So, rafting across the ocean on a mat of plants is by no means the preferred mode of migration
for any terrestrial animal.
But is it possible?
The evidence in both fossil and modern caviomorphs says: Almost certainly.
It just goes to show you that, sometimes things that we think are impossible turn out to be
the best explanation.
Thanks for joining me for today.
And BIG thanks to our Eontologists: Jake Hart, Jon Ivy and of course STEVE!
Now, what do you want to learn about?
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