Dinosaurs didn’t hibernate, says study

By Branwen Morgan 26 September 2011
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Dinosaurs that lived near the poles of the Earth didn’t necessarily hibernate in winter, say experts.

FOSSIL BONES UNEARTHED IN Victoria have revealed that dinosaurs which once lived in the Antarctic Circle were little different to those living in other climes when it came to staying active year-round.

During the Cretaceous Period (145-65 million years ago) Australia was much further South than today, and parts of it sat inside the Antarctic Circle. This meant that it would have experienced total darkness for many months of the year and perhaps frigid temperatures too. 

A team of palaeontologists in Australia and the US, have disproved their own theory which proposed some Australian dinosaurs hibernated to adapt to the extreme conditions of their near-polar habitat.

The ‘hibernation hypothesis’ was based on the presence or absence of tree-ring-like growth markings, called lines of arrested growth (LAGs), in cross sections of fossilised bones. LAGs can be used to determine the age of an animal; they form as a result of an animal’s slowed metabolic processes, such as those experienced during hibernation.

All but the smallest dinosaurs were found to have LAGs, the researchers revealed in the study published in the online journal PLoS One

Dinosaurs stayed active

The physiology of these “polar” dinosaurs “strongly resembled” that of their warmer-clime cousins, says study co-author Professor Patricia Vickers-Rich at Monash University.

“Based on bone microstructure alone, we can say that the dinosaurs living near the South Pole were not physiologically different from dinosaurs living anywhere else in the world during that time,” she says. “This tells us something very interesting: that basically from the very start, early dinosaurs, or even the ancestors of dinosaurs, evolved a physiology that allowed an entire group of animals to successfully exploit a multitude of environmental conditions.”

Co-author Dr Thomas Rich of Museum Victoria in Melbourne, told Australian Geographic that being able to study additional fossils led to a “change in ideas”. He explains that their earlier investigations, published 13 years ago, relied on observations from just two species. The researchers had been trying to figure out when animals developed the ability to hibernate to cope with extreme cold periods.

For the new study, the researchers analysed bones from 18 species of dinosaur, mostly potoroo-sized hypsilophodontids (two-legged, plant-eating dinosaurs) that lived 112-100 million years ago, in the Early Cretaceous.

University of Queensland paleontologist Dr Steve Salisbury, who was not involved in the study, is not surprised by these findings. He explains that LAGs are not exclusive to hibernators. “Most exothermic animals – those that depend on environment for regulation of body temperature, such as crocodiles, turtles, various lizards – go through periods of faster growth and slower growth,” he says.

Dinosaur bone growth linked to seasons

Slower bone growth (and therefore, LAGs) is likely to occur during cooler seasons and when resources are in short supply.  “Gradually evidence is emerging, and a better understanding of dinosaur growth and physiology is showing that [the hibernating dinosaur theory] was a good story at the time, but it probably doesn’t hold up today,” says Steve.

He adds that while the many months of darkness would have been an issue, climatic conditions were probably not as harsh as found near the poles today. “These dinosaurs existed when Australia was still connected to Antarctica and there was no Southern Ocean encircling Antarctica, which meant that warm tropical currents could circulate down to very high latitudes and that would have kept the continent much warmer than it is now.”

While the revelations discount the hibernation theory, they still don’t give the whole story about the differences in dinosaurs that lived around the poles and those that didn’t. “The new study doesn’t mean there was nothing unique about polar dinosaurs, but those qualities aren’t apparent in bone tissue,” says US co-author Holly Woodward at Montana State University.