The Trouble with Scavenging

Here's the thing about Jack Horner's much-talked-about T.rex scavenging hypothesis: It's not so much an actual theory as a Creationist-like argument about why T.rex could not have been a predator. The king has puny arms, tiny eyes, a big nose, couldn't run very fast, and was "ugly," which apparently means that Tyrannosaurus rex was just plain incapable of killing its own dinner, and was thusly forced to walk the Earth a lowly scavenger, picking across any food it stumbled over. Horner's theory is more personal preference than anything else. Just because he can't see it any other way must mean that he's right. Well, I was so happy to see a paper in my new T.rex book by Thomas Holtz, Jr. that sets the record straight. And by "sets the record straight" I mean "mowed down the obligate scavenger hypothesis."

The claims bombarded by Dr. Holtz's silly "evidence" are as follows: T.rex has small eyes relative to skull size; the tibia and femur are about the same length; the forelimbs are really short; the teeth are wide and not bladelike. These passive-aggressive questions really mean the following: T.rex couldn't see very well; T.rex couldn't run very fast; T.rex had puny little worthless arms; T.rex's teeth were not very strong and would have shattered if they hit bone. I've called bullshit on those claims for years. It's good to see somebody actually throw some hard evidence at the problem, and maybe this will finally shut Horner up.

T.rex had "beady little eyes."
I wish I'd made that up, but it comes right out of Horner's mouth. T.rex's eyes are unlike those of, say, Velociraptor, which has large eyes. The first question here is "so what?" But also, how do T.rex's eyes compare to other theropods? It turns out that as theropod dinosaurs get larger, their eyes get smaller. And the eyes don't actually shrink--the skull just grows faster than the eyes as skull size increases. In fact, there are other theropods with smaller-than-average eyes including Giganotosaurus and Dilophosaurus. They must have been scavengers! T.rex was just about average given its size. Besides, in terms of absolute size, T.rex had really big eyes! The largest measured specimen's orbit diameter is 120 mm. That's pretty big. And keep in mind that unlike allosauroids, T.rex had pretty respectable binocular vision. If you're scavenging all day, why do you need binocular vision?

T.rex couldn't run very fast.
That's true, but it ultimately doesn't matter as long as T.rex could run faster than its prey, Einstein. When you plot the measurements, it turns out that T.rex's hind limbs were quite fast, and one step taken by a T.rex was longer than that of an allosauroid or ceratosaur of the same femur length. This means that--gasp--the tibia and metatarsals were elongated relative to the femur. Thus, tyrannosaurs were faster than other similarly-sized theropods. And you're never going to believe this, but bear with me here: tyrannosaurs were faster than contemporary herbivorous dinosaurs! SHOCK AND AWE! Better yet, tyrannosaurs developed arcometatarsus, or a "pinched" metatarsus which suggests that T.rex could turn faster on its narrow feet than non-arcometatarsus theropods. And wouldn't you know it? Duckbills, ceratopsids, and ankylosaurs lack the arcometatarsal condition. So T.rex could run faster and turn quicker than his prey. Go figure.

T.rex had useless little forelimbs that were totally useless. Useless.
Several papers in this book deal with T.rex's pectoral girdle and arms specifically. Muscular reconstructions, pathologies of the arms and furcula (Rothschild & Molnar), and range of motion of the arms have shown that--guess what--T.rex actively used its arms. The king probably got its forelimb workout by clinging to struggling prey (Lipkin & Carpenter) and/or helping right itself while getting up from a sitting position (Stevens, Wills, Larson & Anderson). Even besides that, there are plenty of modern (and extinct) carnivores that hunt prey without the help of their arms, including orcas, modern raptors, terror birds, Diatryma, and crocodilians.

T.rex had stout, deeply-rooted teeth that would have shattered on impact with bone. Wait...seriously?
For me, this is the dumbest argument. It's been demonstrated multiple times that Tyrannosaurus rex and its immediate relatives had ridiculous, astronomical bite forces and that their teeth and jaws were specialized for crushing bone. I imagine that their teeth were modified to allow such an activity. There are at least three instances in the fossil record of a tyrannosaur tearing through bone in the pusuit of food: an Edmontosaurus tail with several neural spines lobbed off, a Triceratops pelvis with several deep scraping tooth marks in it, and a Triceratops skull with a bitten-off horn core (Happ, this book). While the Triceratops pelvis may have been so marred by scavenging activity, the Edmontosaurus lived another day, and the bones healed. Thus, it escaped its pursuer. And one can only imagine that T.rex did not pick its teeth with the orbital horn cores of dead ceratopsians, so that particular exchange probably took place during either an attack by the tyrannosaur or an attack by the Triceratops, perhaps defending its brood from the marauding predator.

So, while it's difficult to prove that Tyrannosaurus rex was a predatory animal, it turns out that all of Horner's claims which would force the king into a purely scavenging role are wrong. It's silly to say that Tyrannosaurus never scavenged--like any other wild carnivore alive today, it probably would've happily feasted on a fresh corpse if it found one. However, it's clear that Tyrannosaurus rex was perfectly capable of finding and taking down its own meals. And circumstantial evidence (the Edmontosaurus tail in particular) suggests that it did. It's also worth mentioning that not a single modern animal that scavenges exclusively. Hyenas actually kill a lot of their meat, and turkey vultures are not above eating hares, lizards, and other small vertebrates.

There is another classic Horner claim that I would like to address, though it is not addressed by Dr. Holtz. The fact that Tyrannosaurus rex had a great sense of smell is apparently indicative of a scavenging lifestyle. Like the tiny arms, I don't see the connection. Dinosaurs in general had above-average olfactory senses (except the more avian ones). Ankylosaurs had large, complex sinus passages. Does that mean they scavenged carcasses? No! Modern animals with great senses of smell, like dogs, do not use their noses exclusively to find dead meat. Often, they use their noses to find live meat, or marks left by other dogs, or other dogs. Perhaps dinosaurs had a complex marking system. Maybe they just had a good sense of smell! If Tyrannosaurus rex lived in a family unit, a good sense of smell would be advantageous to finding your chicks, your mate, and rival families. Great sense of smell does not equal obligate scavenger.

In fact, in every Horner claim made above, the man makes a very basic logical error: the fallacy of equivocation. Although his claims are patently false, even if they were true, that doesn't mean that Tyrannosaurus rex was an obligate scavenger. He fails to make any real connection between, say, short arms and scavenging. Allosaurus didn't have arms as long as Velociraptor either. Does that mean it was a scavenger? In fact, Allosaurus had fairly short arms, very little binocular vision, a shorter femur/tibia ratio than Velociraptor (or T.rex), and blade-like teeth that certainly couldn't damage bone. According to Dr. Horner, it must have scavenged, too! In fact, virtually all large carnivorous dinosaurs must have been scavengers! Those big sauropods in Gondwanaland had nothing to worry about.

I'm so thankful that Dr. Holtz shot the obligate scavenger hypothesis down. I have never liked it, and I'm glad somebody finally threw some facts at it. Also, if you have any interest in the Tyrant Lizard King, this book is really awesome.