Studies of growth lines in bones cast into doubt?the belief that dinosaurs were cold-blooded,?researchers?say.?Enlarge
Dinosaurs may not have been the slow, sunbathing reptiles researchers used to think. In fact, they may have been warm-blooded, new research suggests.Skip to next paragraph
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The researchers studied the "growth lines" on animal bones, which are similar to the growth rings in tree trunks. During slow-growing times like during the winter, they are darker and narrower, while in fast-growing times the bones have lighter, wider bands.
Figuring out if dinosaurs were warm-blooded endotherms (made their own body heat) or were "cold-blooded" ectotherms that relied on outside sources of warmth could illuminate a lot about how they lived, grew and evolved. How warm an animal is has an impact on their metabolism, and therefore how quickly they can grow and have babies.
Of bones and blood
Previously, scientists had thought that growth lines showed up only on the bones of cold-blooded animals, since these animals grow in fits and starts. Warm-blooded animals, like mammals and birds, are assumed to grow continuously, because they keep their temperatures up and have high metabolic rates, continually making energy to grow.
As such, researchers took the growth lines on dinosaur bones as evidence of their cold-bloodedness. Until now.
In this study, the researchers compared the bone lines from the leg bones of more than 100 wild ruminants (warm-blooded mammals like sheep and cows that have multiple stomachs) with seasonal rainfall and temperature cycles and with the animal's core body temperature and resting metabolic rate. The researchers showed that these warm-blooded animals also have bone growth lines indicating fast, yet interrupted yearly growth that depended on how long the "unfavorable" season lasted. [Image Gallery: Dinosaur Fossils]
The growth lines they found on the ruminants were similar to those seen in previous studies of dinosaur bones ? indicating that both ruminants and dinosaurs have periods of high growth punctuated by "unfavorable" seasons with limited resources and little growth. This means that dinosaurs were likely warm-blooded like the ruminants.
"The argument we are giving in our paper, rather in favor of endothermy in dinosaurs, is that between the growth and rest lines, there's always a big region of highly vascularized [infiltrated with blood vessels] tissue that indicates very high growth rates," study researcher Meike K?hler, of the Autonomous University of Barcelona in Spain, told LiveScience. "This is typical in dinosaurs and very different from reptiles, which have slow growth between the rest lines."