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When Rodents Rafted Across the Ocean

When Rodents Rafted Across the Ocean

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? Leave me a comment, and don’t forget to
go to and subscribe.

100 thoughts on “When Rodents Rafted Across the Ocean”

  1. People really need to stop poaching the world's top predators. They control populations of pest animals like rodents and deer

  2. Great video! Could you please do a video of how the fauna of New Zealand birds got to new Zealand. And similarly which animals of New Zealand are the original inhabitants from the time it was connected to Gondwana.

  3. I like how you used a Fijian Crested Iguana to illustrate the Green Iguana. At first I thought this was a mistake, but it's cool, considering the Fijian Iguanas (Brachylophus sp.) are believed to have reached Fiji on similar rafts as the Caviomorphs in this video. Just a neat coincidence!

  4. My dad used to talk to me about tuco-tucos. According to him, they would put their heads out of their caves and bark 'tuco-tuco'. That's where the name came from, just like with Pokemons.

  5. It didn't even have to be one straight voyage from coast to coast. If a chain of islands did exist then the migration could have been gradual using the islands like stepping stones. Either way it's very interesting.

  6. So how about the idea that something intelligent was traveling back and forth between the 2 continents in vessels with cargo, in which these rodents stowed away… Nah.

  7. I don’t see why anyone would doubt rafting as an almost certain explanation – as noted we see it happening now within our own short timeline and back then we’re talking hundreds of thousands of years.

  8. Rodents hitch rides with things/people all the time, thats how they spread all over the world. They return the favor when theyre big enough. 🙂

  9. En Anguila existió un Roedor de 180 kg, llamado Amblyrhiza. Nada podía detener el tamaño de ese Roedor, salvo la Geografía.
    ¿Cómo una isla de 91 Km2 podría soportar un Roedor de ese tamaño?
    Se ha especulado que se refiere a la incredulidad de Cope de que un roedor tan grande podría evolucionar en una región aislada como las islas del norte de las Antillas Menores. Ahora se acepta que Cope pensó que el animal era de naturaleza cosmopolita, moviéndose libremente desde la isla. A la isla debido a los bajos niveles del mar del intervalo, lo que permitió a Amblyrhiza desarrollar un tamaño corporal tan impresionante.
    Los restos fosilizados de este roedor notablemente grande se encuentran únicamente dentro de depósitos de cuevas del Cuaternario, todos los cuales se encuentran en las islas del norte de las Antillas Menores, específicamente Anguila y San Martín.

  10. The fact that these Capybaras are much more compassionate than us "intelligent" humans is… wow

  11. Request for a video… how did penguins become a thing? And why are there not any other animals in Antarctica (except seals, whales) when there are other animals like polar bears in the North Pole?

  12. Can you do an episode on elephants and what caused the Asian elephants to diversify so much from the African counterpart that they can't even inter-breed today despite anatomical similarities.

  13. this video's viewer numbers is a trial in natural selection in itself… WAT!?!?! videos about plague carrying shiprats don't get the clicks compared to videos about cats and dogs???…sigh… mortals…

  14. Folks…. do you REALLY believe rodents rafted?! Don’t believe this “prevailing thinking”. There’s ample evidence that there were advanced civilizations who did things we can’t do today. They had maps, showing their travels. Out of place animals and artifacts in our narrative exist because our narrative is the one that’s wrong.

  15. Great. Though the presentation makes it seem like there was intention to rafting rather than a tragic accident for the animals that turned out to be fortunate.

  16. Rats have proliferated by travelling with humans. Hominids with brains like ours existed 40 million years ago. Boats are low tech.

  17. fun fact,
    there are a couple of species of iguana native to Fiji,
    they could only have got there by rafting all the way across the Pacific.
    that's a very long way.

  18. Its not hard to figure out that they ended up on ships made by humans and got moved around, just like rabbits mice ect

  19. thanks for your video. the rodents could have drank fresh water during and after rainfall. also, if the temperature was very cold and there was ice between the continents, clearly the rodents could also have walked across the ice to find new land.

  20. Kallie is my favorite host. She can make clear and interesting the most complicated thing. And she's gorgeous too.

  21. I think the chain of islands and shallow water is the only theory that even remotely plausible. Not only would it explain the other "sweepstakes colonizers" it far more plausible than a raft of random debris that somehow managed to cross the open ocean in 11-8 days without breaking apart and without the passengers dieing of dehydration and possibly malnutrition. Also how can we be sure that the ocean that is half size of our modern ocean between Africa and south America is the same? I would think that it would be at least significantly different If not completely.

    I understand that this is not fully understood and that PBS Eons is trying to give all the information they can

  22. So Christopher Columbus or Amerigo Vespucci, did not discover sea route to Americas?

    A rat discovered sea route to Americas long ago?
    Americas should be celeberating "Rodent Jerry day" every year then.

  23. Horse sized rodents! Wow… please do an episode about them. And please do an episode about the smallest horses too!

  24. Tsunamis probably did this all the time. When you look at the crap that came to the west coast from Japan you can see that stuff can move much further than these would've had to float.

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