Tyrannosaurus rex is the most extensively studied fossil creature ever known. Every year, new research is added to the towering list of papers that try to draw new secrets from old bones and dispute what others have said before. This ongoing conversation has given us a glimpse into how T. rex lived.
Paleontologists have extracted a slew of tyrannosaur secrets from old bones: T. rex went through a teenage growth spurt, packing on almost 5 pounds a day until it reached its full stature by age 20; it started reproducing by 18 years of age; it walked at 6 mph and ran at 15 to 25 mph (far slower than the fictional speed demon chasing the Jeep in “Jurassic Park”); when it caught up to prey, it had a maximum bite force of 12,800 pounds, the most powerful of any terrestrial predator; its neck muscles were estimated to be strong enough that it could have thrown a 110-pound chunk of meat about 15 feet into the air before catching it.1 I could go on — it’s estimated that T. rex could see about six times as far as an animal with eyes just 3 feet off the ground — but let’s leave it there.
Despite all that, much remains unknown about this famous flesh-ripper. For example, paleontologists are still trying to figure out how T. rex went about getting its daily bread.
At first glance, the question “What did Tyrannosaurus rex eat?” would seem simple enough to answer. A dinosaur whose name translates to tyrant lizard king must have been able to gobble up anything it wanted, and a mouth bristling with thick, serrated teeth makes it clear that meat was always on the menu. The tri-horned Triceratops and the duck-billed Edmontosaurus are the traditional tyrannosaur favorites in artistic renderings of the past, with Hollywood throwing in a few cowardly lawyers for good measure. But regardless of what T. rex consumed, these visions of the dinosaur skirt around the edges of a persistent mystery in the Cretaceous celebrity’s day-to-day — how did a 40-foot-long, 9-ton carnivore get enough meat to fuel its hot-running body?
From the time the first known skeleton was described by paleontologist Henry Fairfield Osborn in 1905, T. rex has been considered the ultimate apex predator. Nothing was bigger or more ferocious. When the legs of the first T. rex ever found were put on display at the American Museum of Natural History in December 1906, The New York Times wrote that the owner of those fossilized gams must have been “the prize-fighter of antiquity,” adding speed to an animal “who ran with great agility on his two hind feet and could play frightful havoc with his savage canine teeth.”
And that’s the way it stayed for decades, up until 1994, when paleontologist Jack Horner took a swipe at the popular dinosaur by suggesting that T. rex was nothing but a filthy, lazy scavenger. This kicked off a long and, to some paleontologists, tiresome controversy about whether T. rex was an active predator or subsisted only on rotting flesh. Paleontologists eventually found solid evidence that the dinosaur caught live prey as well as gorged on carrion when the opportunity arose. Healed wounds from tyrannosaur bites and bones bearing frightful punctures from feeding told the gory tale. Yet the question remained as to how much of its daily life T. rex spent trying to ambush prey versus sniff out what was already dead.
Given that time travel and genetically engineered tyrannosaurs are still in the realm of science fiction, researchers have tried to find alternate routes to envision how T. rex would have behaved. That’s what led Trinity College Dublin ecology graduate student Kevin Healy and his colleagues to model just how a full-grown T. rex would have fared if the dinosaur was restricted to a diet of dead meat.
Short a living T. rex to study, Healy and his collaborators instead created a virtual landscape where about 8 pounds (4 kilograms) of meat would randomly appear. “We then let loose different-sized theropods on the landscape, ranging from 1 kg to 15,000 kg, with simple foraging rules that make them move towards food items, eat until stomach capacity is full, move away if a bigger individual arrives, and so on,” Healy said in an email. With each dinosaur’s estimated metabolic rate and daily needs in mind, Healy and his colleagues were looking to see how each would fare on a diet of easy, dead meals.
For an adult T. rex, the stakes were high. “A 6-ton T. rex,” Healy said, “would need the same daily calories as 80 people” on a diet of 2,500 calories per day. That translates to about 140 kilograms of meat, which the virtual T. rex had half a day to track down in the model. It did not go well. “It costs so much to move around the huge mass of an individual,” Healy said, “that the payoff when they do find a carcass would be pretty small in comparison.” He said the much smaller, juvenile T. rex were more in the “Goldilocks zone” where searching for meat was likelier to pay off. In general, though, grown-up T. rex were just too big to live like vultures. They certainly had the equipment to dismantle any carcass they happened across, but a T. rex the size of the Field Museum’s famous SUE couldn’t just wait for a meal. She’d have to make her own luck by ambushing thick-skinned prey.
University of Maryland paleontologist Thomas Holtz Jr., who has made a career learning everything there is to know about tyrannosaurs, cautioned that “the kind of broad-stroke ecological energy studies” that Healy and colleagues performed “have to be taken with pretty significant error bars.” It’s not as if dinosaurs were optimized only for hunting or scavenging, or even finding food. “They need to function in all walks of life,” Holtz said, “and thus we wouldn’t expect them to exactly track mathematical models that concern only a single function.” Nevertheless, Holtz said, the big-picture conclusion for T. rex seems to fit. The uncertainty of finding scattered dinosaur carcasses would prevent the evolution of a supergiant scavenger. T. rex had to hunt to survive.
There’s more that remains mysterious about T. rex than its feeding habits. For starters, Holtz said, no one knows how much T. rex snoozed. “Big hypercarnivores such as big cats do spend a lot of time sleeping,” Holtz said, so it’s possible that T. rex did the same. And what about their sense of smell? A 2011 study by paleontologist Darla Zelenitsky and colleagues tracked the proportion of the brain devoted to processing smell, finding that a significant part of tyrannosaurs’ brains was dedicated to interpreting scents, but precisely how sensitive T. rex would have been to smells is difficult to work out.
All of this helps to revive T. rex not as a Spielbergian monster, but as a real animal. The dinosaur walked, chomped, mated, pooped, breathed and more during the 2 million years its species was around. And given the popular and scientific interest in this beloved superpredator, new insights into the days of T. rex will surely keep coming. Long live the king.