Was Tyrannosaurus rex a fearsome hunter or a scavenger? The answer is "both."
In the early 1990s, the paleontologist Jack Horner popularized the idea that Tyrannosaurus fed entirely on carrion. The idea that this dinosaur—the "prize fighter of antiquity"—could not catch or kill other dinosaurs was shocking. Reporters and documentary-makers ate it up, but other paleontologists were quick to respond with evidence that Tyrannosaurus truly was the apex predator of its time. The academic debate over whether Tyrannosaurus was capable of bringing down live prey has been over for years now, and a study published today in the Proceedings of the Royal Society B finds new support for Tyrannosaurus as one of prehistory's super-predators.
In order for Tyrannosaurus to have made a living as an obligate scavenger, tons of dinosaur carcasses would have to have been scattered over the Cretaceous landscape. If there were enough dead dinosaurs, Tyrannosaurus could have hypothetically gotten by through scavenging, but the trouble is that it was not the only carnivore around. Smaller, more numerous carnivores would have seriously limited its feeding opportunities.
As tabulated by paleontologists Chris Carbone, Samuel Turvey and Jon Bielby in their new study, there were as many as nine other species of meat-eating dinosaurs alongside Tyrannosaurus during the Late Cretaceous of North America. They ranged in size from the large tyrannosaur Albertosaurus down to the six-foot-long "raptor" Dromaeosaurus. (The authors count the supposed "pygmy tyrant" Nanotyrannus on their list, but these specimens are probably juvenile Tyrannosaurus and do not belong to a distinct genus.) Altogether, there was an entire guild of meat-eating dinosaurs that would have competed for carcasses, just as we see mammals of different sizes competing for carcasses on the African savanna today. In order to subsist on carcasses alone, adult Tyrannosaurus would have been in intense competition with multiple, smaller predators, including their own offspring.
After compiling a list of carnivorous species and prey species, Carbone and colleagues used information about the ecology of modern ecosystems to estimate the number of available carcasses on the landscape and the ability of the carnivores to detect them. The carcasses of small herbivorous dinosaurs would have been relatively abundant, but an adult Tyrannosaurus would have had to walk for days to reach a large carcass. In fact, the researchers estimate that an individual Tyrannosaurus would have had to search for nearly a year before finding a five-ton carcass, and it would have had to rely upon more frequent and less-filling meals.
Unfortunately for Tyrannosaurus, more abundant carnivorous dinosaurs probably would have arrived at the carcasses first. Many small mouths can destroy a body faster than one big one. For example, let's say that a Triceratops weighing about 8,500 kilograms keels over and dies. Based upon the estimates of search time and carnivore abundance used in the new study, about 1,000 Dromaeosaurus-level carnivores could have reached the carcass in the same amount of time that it would take one Tyrannosaurus to find it. There were simply more of them spread over the landscape.
Overall, the best bet for a scavenging Tyrannosaurus would be to find smaller carcasses more frequently, but even these were probably consumed before it could reach them. As the authors of the new study state, "it is extremely unlikely that an adult T. rex could use scavenging as a long-term sustainable foraging strategy."
Tyrannosaurus was the biggest meat-eating dinosaur within its ecosystem and certainly would have dominated any carcass it came across, but the likelihood of it reaching a carcass before its destruction at the jaws of smaller, faster dinosaurs was low. We know from fossil evidence that Tyrannosaurus cannibalized carcasses of its own species, and that its cousin Tarbosaurus wasn't above scavenging, but in order to survive the tyrant king had to hunt. That it did so is clear from its anatomy—Tyrannosaurus was well-adapted for delivering devastating bites that would have felled the large herbivorous dinosaurs of its time. The hunting method of this dinosaur, how often it had to hunt, whether it hunted in groups, and other questions remain, but there can be no doubt that Tyrannosaurus was a formidable predator.
Carbone, C., Turvey, S., & Bielby, J. (2011). Intra-guild competition and its implications for one of the biggest terrestrial predators, Tyrannosaurus rex Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2010.2497