She captioned it, "Guinea pigs and guinea biggers?"
What's most amazing about this is that the smaller animal isn't even a guinea pig; it's a capybara, the largest living rodent species. On this scale, your typical guinea pig would about fit in the space underneath the capybara's belly.
So, who's the big guy?
That's Josephoartigasia, a Pliocene (five million to one million years ago) animal from southeastern South America that is thought to be the largest rodent species ever. From skulls found in Uruguay, a full-grown Josephoartigasia weighed something like five hundred kilograms -- heavier than an adult grizzly bear.
It's hard to talk about this thing without lapsing into superlatives. The one that blew me away was a calculation of its bite force, putting it at an estimated 950 Newtons, which is right around what an adult jaguar can exert. Because of this, its skull was heavily reinforced; the more powerful the muscle contraction, the stronger the bones have to be (given that bones provide anchorage and leverage for the pull of the muscles).
What it needed this kind of bite force for is a matter of conjecture. It's possible it was just for gnawing things. Like granite outcrops, or something. On the other hand, South America during the Pliocene was replete with huge predators including Xenosmilus, a sabre-toothed cat, and phorusrhacids -- the aptly-named "terror birds" that looked like a cross between an ostrich and a velociraptor. So it's possible its fearsome bite was defensive.
When food is abundant and there are lots of large carnivores around, there is a significant evolutionary pressure favoring large body size, and that seems to be what happened here. Back then, South America's fauna resembled what now lives in southern Africa -- abundant wildlife and lots of very big animals. Josephoartigasia would have shared the habitat with giant ground sloths, glyptodonts (think "an armadillo on steroids"), toxodonts (the Pliocene answer to hippos), and the giant peccary Platygonus.
So it was a world of megafauna, and Josephoartigasia fit right in.
But we're used to thinking of large ungulates; even giant ground sloths are familiar to anyone who's seen a kid's book on prehistoric animals.
But rodents the size of an adult longhorn steer are a little hard to imagine.
So thanks to Vivienne, who has provided cool topics for Skeptophilia before and definitely didn't fail me this time. Me, I'm just as glad Josephoartigasia is not around any more. I have enough of a hard time keeping squirrels out of the birdfeeder.
Having a cow-sized squirrel that could eat the birdfeeder would be another thing entirely.