You are what you eat, even if you're a dinosaur.

University of Alberta scientists are learning more about the lives of the ancient creatures by studying their teeth to see how they used them — and what on.

"If we're to fully understand how these animals were living, we need to understand what they were eating and how they were eating," said Ryan Wilkinson, an undergraduate and co-author of a paper published Thursday in Current Biology.

Wilkinson and his colleagues studied scratches left by struggling prey on the teeth of three different raptors and read them like grooves on a record to determine how the dinosaurs tore into lunch. They then used techniques developed to test the strength of bridges to suggest what prey was preferred.

"The phrase we use in the paper is 'puncture and pull,'" Wilkinson said.

The scientists looked at three similar dinosaurs — Dromaeosaurus, Saurornitholestes and Troodon.

Each were meat-eating, stood on their hind legs, were about two metres long and weighed between 15 and 25 kilograms. They lived at about the same time in the same environments and are often found together in fossil beds. All three had serrations like steak knives on the back of their teeth to help slice through flesh.

'Vertical plunging bite'

The team first examined grooves on the teeth.

The scratches ran in two directions: up and down and angled laterally. That's evidence that dinosaurs ate by chomping down into their prey then tearing the flesh off the carcass, said Wilkinson.

"There's a vertical plunging bite, then an oblique cut as the animal closes its mouth as it draws its head backwards."

This may be the way all meat-eating dinosaurs ate. Teeth from Gorgosaurus, a gigantic, nine-metre cousin of T. rex, show the same pattern.

But the teeth had more to say than that. Troodon's serrations, called denticles, were much larger than those of the others.

"They have these large, hooked denticles, really bizarre," Wilkinson said.

Trying to understand what those denticles might reveal about Troodon's diet, the team built a computer model of the teeth and subjected them to stress tests.

"We use this technique to test the stresses exerted on the teeth during the biting. We applied forces at different bite angles."

'Uncontrollable force'

When the bite lined up with the most common angle of the scratches, the teeth worked fine. When the angle drifted away from the scratches, the models suggested Troodon's teeth began to break.

"Struggling prey can really exert a lot of uncontrollable force on the teeth at angles that aren't really ideal," Wilkinson said.

If prey that fights back is likely to break your teeth, you'll probably hunt something else. The teeth suggest Troodon went after smaller, slower prey — even carrion — and left the feistier meals to Dromaeosaurus and Saurornitholestes.

The study goes a long way to explaining how different dinosaurs filled different niches in their ancient ecosystems. It's one of the first times such techniques have been applied to meat-eating dinosaurs, Wilkinson said.

"There's a lot more room to look at this in more detail."