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The Debunker: Did Some Dinosaurs Have a Second Brain in Their Butt?

by Ken Jennings

It's been a long time—66 million years!—since the Cretaceous Period ended in explosive fashion, so there's a lot we don't know about our predecessors atop the food chain, the dinosaurs. Were they hot-blooded or cold-blooded, fast or slow, pack animals or lone hunters? What color were they, and what did they sound like? Could you really use one to make a record player, like the Flintstones did? Luckily, our Jeopardy! correspondent Ken Jennings has just published his seventh Junior Genius Guide, this one all about the dinosaurs! He's here all month to straighten us out on all the Mesozoic misinformation we thought we knew.

The Debunker: Did Some Dinosaurs Have a Second Brain in Their Butt?

Poor Stegosaurus. He hasn't walked the Earth for 150 million years, and people are still talking about how dumb he was. That's pretty much his whole reputation. He's the Dan Quayle of dinosaurs.

dino noise

Don't get me wrong, this reputation is pretty much deserved. Nobody was around in the Jurassic Period to give IQ tests to dinosaurs, but scientists use something called the "Encephalization Quotient" to estimate animal intelligence—that's a ratio of a species's actual brain size to the brain size you'd expect given its body mass. Human beings dominate the brain game with an EQ of 8, while dolphins are a 4, dogs a 1.2, and rabbits a 0.8. Since Stegosaurus was the size of a school bus, but his brain was (famously!) the size of a golf ball, he had an EQ of 0.1, which is dumb even by reptile standards.

For over a century, paleontologists explained away the tiny size of Stegosaurus's brain by noting that they may have a relied upon additional secondary brain, something like a junction box, controlling their back legs and tails. O. C. Marsh first hypothesized this "posterior braincase" in 1881, when he noticed that Stegosaurus's spine widened at its hips. But many birds today have a similar space in their hips, and it's not a "braincase": it holds for an energy-storing organ called the "glycogen body." True, dinosaurs were so big that lots of their movements were controlled by the spinal cord and not the brain—but that's not unusual. Lots of human locomotion is regulated by your spinal cord as well. For example, you could keep up your regular walking rhythm even if your cerebral cortex were to check out completely. And, just like Stegosaurus, you wouldn't need a special butt-brain to do it.

Quick Quiz: What human hormone, made in the pancreas, tells your body to store sugar in big chains of glycogen?

Ken Jennings is the author of eleven books, most recently his Junior Genius Guides, Because I Said So!, and Maphead. He's also the proud owner of an underwhelming Bag o' Crap. Follow him at ken-jennings.com or on Twitter as @KenJennings.