Cold-blooded dinos would have been ‘too weak’

By Jacqueline Outred 24 July 2013
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Cold-blooded dinosaurs wouldn’t have had enough strength and energy to dominate, a new study says.

THOUGH OUR KNOWLEDGE OF the lifestyle and physiology of dinosaurs has increased exponentially in the last few decades, one issue still being debated by experts is whether dinosaurs were warm-blooded or cold-blooded.

A new study from the University of Adelaide has questioned whether a cold-blooded physiology would have given dinosaurs sufficient muscle power, and suggests that they would have been sluggish and much weaker than mammals of the same size.

Dinosaurs were once thought to be plodding, cold-blooded, lizard-like animals. But research amassed since the 1970s has revealed many of them to be speedy, nimble creatures with more similarities to birds than reptiles.

Dinosaurs more like crocodiles or birds?

The closest living relatives of dinosaurs are birds, which are warm-blooded, and crocodiles, which are cold-blooded. “Where dinosaurs existed in the middle of that is open to debate,” says Professor Roger Seymour, an animal physiologist at the University of Adelaide.

“Some point out that a large saltwater crocodile can achieve a body temperature above 30°C by basking in the sun, and it can maintain the high temperature overnight simply by being large and slow to change temperature,” says Roger.

“They say that large, cold-blooded dinosaurs could have done the same and enjoyed a warm body temperature without the need to generate the heat in their own cells through burning food energy, like warm-blooded animals.”

But as he details in a July paper in the journal PloS One, these animals may have been warm but they wouldn’t have been active.

Roger set out to test relative energy levels between dinosaurs of both physiologies. His experiment looked at how much energy a crocodile can produce during peak muscular activity compared to mammals of the same size.

He found that crocodiles of 1kg in weight had 50 per cent less energy than mammals of the same size. As weight increases, so does the difference in energy levels; a crocodile of 200kg has just 14 per cent of the energy of a mammal of the same size, for example.

Cold-blooded dinosaurs sluggish and slow

“There are two major problems with a crocodile-like dinosaur compared to a mammal-like dinosaur,” Roger says. “The crocodile-like dinosaur, when it’s large, doesn’t have the energy at the beginning of the fight, and they fatigue relatively quickly.”

Crocodiles have long been used as physiological models for dinosaurs, but Roger’s research shows they may not be a good analogy after all.

He told Australian Geographic that the result “undermines probably one of the last arguments for cold-blooded dinosaurs.” While large dinosaurs may have been able to hang on to heat by virtue of their large size, that warmth would not have been equivalent to energy output.

Dr Steve Salisbury, a palaeontologist at the University of Queensland in Brisbane, who wasn’t involved in the study, says “trying to understand physiology in dinosaurs is tricky because we obviously don’t have a lot of animals that are really similar to them, other than birds and crocodiles.”

Nevertheless, he agrees that cold-blooded dinosaurs “probably wouldn’t have had the capacity for sustained exercise that would’ve been necessary for dinosaurs to have been as dominant as they appear to have been.”

Roger adds that the warm-blooded physiology “explains why dinosaurs out-competed mammals for 180 million years.”