Rodents of unusual size

My writer friend Vivienne Tuffnell, of the lovely blog Zen and the Art of Tightrope Walking, frequently amuses her friends with photos of the antics of her pet guinea pigs.  Her love for her little pals is undoubtedly what prompted her to post a somewhat more alarming photo a couple of days ago:

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.

****************************************

Danger down under

Following hard on the heels of Saturday's post, which started with a description of the cassowary -- Australia's killer bird -- today we follow up with a paper in Nature Communications just yesterday that falls under the "You think things are bad now?" department.

It's not like modern Australian wildlife is anything to trifle with.  The country is where you can find some of the world's most dangerous snakes, including the taipan, the brown snake, and the tiger snake.  The north coast is home to the enormous and aggressive saltwater crocodile, while the south coast has a sizable population of great white sharks.  The eastern coast, not to be outdone, is where you can run into the harmless-looking box jellyfish, which is in contention for winner of the most-potent venom contest; it injects its victim with a substance that has an LD-50 of 0.04 milligrams per kilogram of body weight, and can kill in under five minutes if an antidote isn't administered.  Even the plants bear watching.  The north coast has the beach spinifex grass, which reinforces the pointed tips of its leaves with silica drawn from the soil, essentially turning the plant into a cluster of tiny glass shards.  Worst of all is the gympie-gympie, which is like the humongous nettle from hell, inflicting an excruciating sting that can last for years.

(The Wikipedia article I linked says that the fruit of the gympie-gympie is "edible if the stinging hairs are removed first."  To which I respond, "Do I look like a fucking lunatic to you?"  I'll stick with fruit that's not attempting to murder me, thanks.)

But the paper "Extinction of Eastern Sahul Megafauna Coincides with Sustained Environmental Deterioration," by a team led by Scott Hocknull of the University of Melbourne, gives you a good feeling for how much worse it could be.  It describes a treasure-trove of fossils from Walker Creek in northeastern Australia that had the remains of hitherto-unknown species of fauna, including:

• a thus-far unclassified kangaroo that was four meters tall and weighed just shy of three hundred kilograms
• a new species of the genus Diprotodon, which was basically a wombat on steroids -- it's estimated to have been two meters tall at the shoulder and had a mass of 2,500 kilograms
• a new species of the horrific carnivorous marsupial Thylacoleo, which was slightly smaller than your average African lion, but is estimated to have had the most powerful bite of any known mammal, living or extinct
• a six-meter-long goanna and two never-before-seen species of monitor lizards
• a land-dwelling crocodile, because apparently the water-dwelling ones weren't bad enough

Artist's reconstruction of Thylacoleo carnifex [Image licensed under the Creative Commons Nobu Tamura (http://spinops.blogspot.com), Thylacoleo BW, CC BY 3.0]

The kicker is that these things were around after the colonization of Australia by humans, and in fact, by some estimates there was a fifteen thousand year overlap where the ancestors of today's Native Australians had to contend with a nightmarish megafauna.  Me, I wonder why they stuck around, you know?  If I was one of them, and landed in my boat on the shores of Australia, and saw land crocodiles and six-meter-long lizards and a lion-sized Tasmanian devil, I would have used the words of the inimitable Eric Cartman: "Screw you guys, I'm goin' home."

Of course, home was Papua-New Guinea, which honestly wasn't all that much better.

It's an interesting question as to what finally did in these formidable critters.  Hocknull et al. write the following, in an article in The Conversation:

Why did these megafauna become extinct?  It has been argued that the extinctions were due to over-hunting by humans, and occurred shortly after people arrived in Australia. 

However, this theory is not supported by our finding that a diverse collection of these ancient giants still survived 40,000 years ago, after humans had spread around the continent. 

The extinctions of these tropical megafauna occurred sometime after our youngest fossil site formed, around 40,000 years ago.  The timeframe of their disappearance coincided with sustained regional changes in available water and vegetation, as well as increased fire frequency.  This combination of factors may have proven fatal to the giant land and aquatic species.

As magnificent as these creatures undoubtedly were, it's probably better that they're gone.  I've heard Australia is a pretty cool place, even considering its dangerous flora and fauna, but if the animals of Walker Creek were still around, it'd be hard to understand how anyone could manage to live there.  Just taking a short walk to the grocery store would be risking getting dismembered by enormous carnivorous marsupials.

Makes today's snakes and crocodiles and whatnot seem tame by comparison, doesn't it?

**********************************

This week's Skeptophilia book of the week is six years old, but more important today than it was when it was written; Richard Alley's The Two-Mile Time Machine: Ice Cores, Abrupt Climate Change, and Our Future.  Alley tackles the subject of proxy records -- indirect ways we can understand things we weren't around to see, such as the climate thousands of years ago.

The one he focuses on is the characteristics of glacial ice, deposited as snow one winter at a time, leaving behind layers much like the rings in tree trunks.  The chemistry of the ice gives us a clear picture of the global average temperature; the presence (or absence) of contaminants like pollen, windblown dust, volcanic ash, and so on tell us what else might have contributed to the climate at the time.  From that, we can develop a remarkably consistent picture of what the Earth was like, year by year, for the past ten thousand years.

What it tells us as well, though, is a little terrifying; that the climate is not immune to sudden changes.  In recent memory things have been relatively benevolent, at least on a planet-wide view, but that hasn't always been the case.  And the effect of our frantic burning of fossil fuels is leading us toward a climate precipice that there may be no way to turn back from.

The Two-Mile Time Machine should be mandatory reading for the people who are setting our climate policy -- but because that's probably a forlorn hope, it should be mandatory reading for voters.  Because the long-term habitability of the planet is what is at stake here, and we cannot afford to make a mistake.

As Richard Branson put it, "There is no Planet B."

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]