Was Tyrannosaurus Rex a fearsome predator or just another scavenger?

When we think of T. Rex, we imagine the most frightening predator that ever walked the Earth, stalking through the Cretaceous in search of unsuspecting herds of plant-eating dinosaurs. But the real story might not be quite as scary...or flattering.

A recent census of dinosaur skeletons in eastern Montana revealed a basic truth: there were just too many Tyrannosauruses for it to survive on hunting alone. Paleontologist Jack Horner explains:

"In our census, T. rex came out very high, equivalent in numbers to Edmontosaurus, which many people had thought was its primary prey. This says that T. rex is not a cheetah, it's not a lion. It's more like a hyena. This putative apex predator is as abundant in the upper layers of the Hell Creek Formation as the herbivores, its reputed primary food source. And it's even more plentiful in the other two-thirds of the formation. This supports the view that T. rex benefited from a much wider variety of food sources than live prey."

In order to support such a large T. Rex populations, the dinosaurs couldn't afford to be picky eaters, and they almost certainly would have had to scavenge for freshly dead meat in order to sate their appetites. In normal circumstances, an apex predator - which we thought the T. Rex was - can only be about a third or a fourth the population size of its prey in order to meet the species's full energy needs.

Indeed, Horner says that apex predators only really go after prey that matches their own strengths and weaknesses. T. Rex probably didn't have that luxury:

"If you count the lions and the leopards and the cheetahs in the Serengeti, the number still does not equal the number of hyenas, because hyenas have a much wider food source," Horner said. "Cheetahs, for example, only go after things that are really fast. They don't eat turtles. But a hyena will eat a turtle, or anything else that it can catch or is dead. There's no evidence that T. rex could run very fast, so it wasn't out there being a cheetah. If it could get a sick animal, it would."

If anything, this means T. Rex was even scarier than we thought - it really would hunt anything, and the science fiction premise of the dinosaurs going after humans isn't all that ridiculous after all. That said, we should be careful in going too far with the available data - this is still only one small part of the ancient world, and it's possible this particular fossil record has skewed the demographics in T. Rex's favor. But still, just to be on the safe side, be sure to stay out of a hungry T. Rex's way - it probably can't afford to be choosy about which human it eats.

PLoS ONE. Top image by Mark Hallett.