Dinosaurs hot or cold-blooded? The debate continues

A team at the University of Adelaide, led by Professor Roger Seymour, has applied the latest theories of human and animal anatomy and physiology to provide insight into the lives of dinosaurs.

Human thigh bones have tiny holes -- known as the "nutrient foramen" -- on the shaft that supply blood to living bone cells inside.

The new research has shown that the size of those holes is related to the maximum rate that a person can be active during aerobic exercise. Researchers have used this principle to evaluate the activity levels of dinosaurs.

"Far from being lifeless, bone cells have a relatively high metabolic rate and they therefore require a large blood supply to deliver oxygen. On the inside of the bone, the blood supply comes usually from a single artery and vein that pass through a hole on the shaft -- the nutrient foramen.

The researchers wondered whether the size of the nutrient foramen might indicate how much blood was necessary to keep the bones in good repair. For example, highly active animals might cause more bone "microfractures", requiring more frequent repairs by bone cells and thus greater blood supply.

"The 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.

"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 said.

Comparisons were made with the sizes of the holes in living mammals and reptiles, and their metabolic rates. Measuring mammals ranging from mice to elephants, and reptiles from lizards to crocodiles, the team combed the collections of Australian museums, photographing and measuring hundreds of tiny holes in thigh bones.

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

These holes were compared to those of fossil dinosaurs. "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," Professor 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," he said.

The findings are to be published in an upcoming edition of the 'Proceedings of the Royal Society B' journal.