According to the latest installment in the ongoing saga over the king of dinosaurs' dietary habits, Tyrannosaurus rex was not simply an oversize scavenger. By the time T. rex was lucky enough to find a carcass, smaller animals would have stripped it to the bone first, claims a new study of the giant dino's fierce competition for rotting flesh.

The Jan. 26 study in Proceedings of the Royal Society B doesn't explore what T. rex hunted, but it counters earlier research that suggested the creature could survive only on carrion when it roamed North America some 65 million years ago.

“This has been a discussion for as long as we’ve known about [T. rex's] existence,” said study co-author and ecologist Chris Carbone of the Institute of Zoology in London. “Some people say it’s a settled issue, that it both scavenged and hunted, but evidence is scarce and we see both extremes in behavior in large carnivores today.”

Adult tyrannosaurs were about 40 feet long, 15 feet high and weighed between 5 and 7 tons, making them one of the largest land-based carnivores ever. However, they shared features common to both full-time scavengers and predators, causing debate about how they subsisted.

Powerful jaws, impact-resistant teeth and huge size are all associated with predatory behavior. An enhanced sense of smell, small eyes and puny forelimbs are associated with scavenging. Previous studies tallied up T. rex’s energy costs, determining that a Serengeti-like environment could support the 7-ton toothy beasts on carcasses alone.

But that research took too narrow a view of dinosaur life, counting how many calories T. rex needed but not the competition it faced in getting them, said Carbone.

“In that study they essentially fed T. rex carrion hamburgers at a regular interval, which resolved the question of energy intake,” he said. “The approach was good, but it was in isolation. We found this argument breaks down quickly in an ecological context.”

To see if T. rex could survive on a carrion diet, Carbone's team developed a computer model that merged dinosaur abundance, based on fossils found in the same formations as T. rex, with modern Serengeti scavenging data. Because predator-to-prey ratios in the fossil record are similar to ratio in Africa’s scavenger-filled savannas, Carbone said the Serengeti was a good model to bring all of the dinosaur abundance and territory data into perspective.

They concluded that T. rex would have faced such fierce competition from smaller scavenging dinos, that carrion alone wouldn't suffice as a primary food source.

“I never really bought the ‘it was only a scavenger’ arguments. They treated T. rex in an ecological vacuum with no other animals around,” said evolutionary biologist John Hutchinson of the Royal Veterinary College, who was not involved in the study. “This paper puts [T. rex] in the realistic context of a complex environment.”

Assuming T. rex could cover about one square mile of territory every day, it would find a 25-ton sauropod carcass once every 5 years, a triceratops-sized carcass once a year and a horse-sized carcass once every 2 months. Even an adult-human–sized carcass would be found only every 6 days.

“Just about any carnivore will eat carrion if it encounters it, so there could not have possibly been enough around for a tyrannosaur to subsist on,” said paleontologist and evolutionary biologist Martin Sander of Germany's University of Bonn, who was not involved in the study. “They would have been outnumbered by smaller, more abundant scavengers.”

If T. rex did compete with tiny-yet-formidable scavengers, it would need to sniff carcasses from about 2 miles away and traverse at least 80 miles per day. Applying the dinosaur's most generous top speed estimate (about 45 mph), that would add up to nearly 2 hours of sprinting to cover that distance.

However, paleontologist Jack Horner of Montana State University, who was not involved in the study and has supported the idea of T. rex as a full-time scavenger, says Carbone's study is interesting but flawed.

Horner said the dinosaurs included in the study cover a time span far beyond when T. rex was certain to have lived, and that his unpublished data contradict assumptions on the abundance of T. rex's competitors. “If their numbers were correct, I would agree with the paper. But I have numbers to suggest they’re not,” he said.

Horner could not yet share his specific results, as he didn't want to compromise his team's forthcoming publication in the journal PLoS One.

In whatever way Horner's work proves to impact Carbone's interpretations, Carbone said it will all be part of the process of science.

“We made our best argument, which I think firmly rejects the idea that T. rex was a scavenger. If there’s new evidence to suggest otherwise, I’d love to take a look at it,” Carbone said. “We’ve just got to wait and see what [Horner] has got.”

In the meantime, Carbone said he'd like to go after a "million-dollar question" surrounding T. rex.

"What did it hunt, and how?" he said. "There are different answers in different fields. We need to bring these groups together, I think, to understand what *T. rex *was like as a hunter."

*Images: 1) Fossilized skeleton of Tyrannosaurus rex./Flickr/Chealion. 2) The predicted relative abundance of predators, by mass, that may have competed with T. rex for food (top) and the predicted relative abundances of herbivores that would have made up the majority of carcasses (bottom)./Proceedings B/C. Carbone.

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