New research adds to the debate about whether dinosaurs were slow, sluggish and cold-blooded or active and warm-blooded like mammals. Scientists looked at holes in dinosaurs' bones for the answer.

Roger Seymour from the University of Adelaide compared dinosaur bones to mammals and reptiles to get an idea of how active the dinos were. The research, the results of which will be published this month in the Proceedings of the Royal Society B "Biological Sciences," is full of holes — bone holes.

Most bones, both fossilized and fresh, have a single opening where an artery and a vein enter. The hole, called the nutrient foramen, is bigger in more active animals. The more active the animal, the more oxygen the bones need, and hence the bigger the artery that is needed to supply that oxygen.

"Far from being lifeless, bone cells have a relatively high metabolic rate and they therefore require a large blood supply to deliver oxygen,” said Seymour in a University of Adelaide press release.

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"My aim was to see whether we could use fossil bones of dinosaurs to indicate the level of bone metabolic rate and possibly extend it to the whole body's metabolic rate," he said. "One of the big controversies among paleobiologists is whether dinosaurs were cold-blooded and sluggish or warm-blooded and active. Could the size of the foramen be a possible gauge for dinosaur metabolic rate?”

Seymour and one of his students, Sarah Smith, looked at the hole left by the blood vessels in the thighbones of mammals and reptiles. He found that dinosaurs had larger nutrient foramen than both reptiles and mammals.

"The results were unequivocal. The sizes of the holes were related closely to the maximum metabolic rates during peak movement in mammals and reptiles," Professor Seymour said. "The holes found in mammals were about 10 times larger than those in reptiles."

Don Henderson, Curator of Dinosaurs from the Royal Tyrrell Museum in Alberta, Canada, and Daniela Schwarz-Wings from the Museum für Naturkunde and Humboldt University in Berlin, Germany, collaborated with Seymour in this research. They measured the nutrient foramen in 10 species of dinosaur from five different groups, including two-legged and four-legged carnivores and herbivores, weighing from 50 kilograms to 20,000 kilograms (110 pounds to 44,100 pounds).

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When the dino bones were compared to the mammal and reptile bones, the researchers found something surprising.

"On a relative comparison to eliminate the differences in body size, all of the dinosaurs had holes in their thigh bones larger than those of mammals," Seymour said.

"The dinosaurs appeared to be even more active than the mammals. We certainly didn't expect to see that. These results provide additional weight to theories that dinosaurs were warm-blooded and highly active creatures, rather than cold-blooded and sluggish."

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IMAGE 1: Tyrannosaurus fossil at the National Museum of Natural History in Washington D.C. (Wikimedia Commons)

IMAGE 2: A hole in the femur of Centrosaurus apertus, a ceratopsian dinosaur. (Credit: Photo by Dr Donald Henderson, Curator of Dinosaurs, Royal Tyrrell Museum, Alberta, Canada.)

IMAGE 3: The y-axis is an index of the amount of blood flow through the foramen in relation to the body size of mammals (red), reptiles (blue) and dinosaurs (orange-red). Credit: Image courtesy of Professor Roger Seymour, School of Earth & Environmental Sciences, University of Adelaide.