Researchers have long been trying to discover more about dinosaurs, and new research now suggests an intriguing fact: dinosaurs might have actually been warm-blooded. Determining whether dinosaurs were warm-blooded, i.e. endotherms (making their own body heat) or cold-blooded, i.e. ectotherms (relying on outside sources of warmth) could shed some light upon how they lived, grew, and evolved.
An animal's warmth has an impact on its metabolism, consequently it affects how quickly it grows and procreates. The researchers studied the "growth lines" on animal bones, which are similar to the growth rings found in tree trunks. During slow-growth period such as the winter season, these growth lines are darker and narrower, while in fast-growing periods the bones feature lighter, wider bands.
Scientists had previously thought that such growth lines showed up only on the bones of cold-blooded animals, since these animals grow in stages. Warm-blooded animals presumably grow continuously, because they maintain their temperatures up and have high metabolic rates, thus continually producing energy to grow. Based on this theory, researchers considered the growth lines on dinosaur bones as evidence that they were cold-blooded creatures.
The new study, however, brings intriguing new insight. Researchers looked at bone lines from the leg bones of more than 100 wild ruminants, i.e. warm-blooded mammals such as sheep and cows that have multiple stomachs. They compared these bone lines with seasonal rainfall and temperature cycles, as well as with the animal's core body temperature and resting metabolic rate. The researchers found that these warm-blooded ruminants have bone growth lines as well, indicating fast, but interrupted annual growth that depended on how long the "unfavorable" season lasted.
The growth lines researchers found on these warm-blooded animals were similar to those showed in previous studies of dinosaur growth. This indicated that dinosaurs, like ruminants, had periods of high growth interrupted by "unfavorable" seasons. This finding may indicate that dinosaurs were 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 tissue that indicates very high growth rates," researcher Meike Köhler of the Autonomous University of Barcelona told LiveScience. "This is typical in dinosaurs and very different from reptiles, which have slow growth between the rest lines."
Sauropods were the only type of dinosaurs where researchers have not seen such growth lines resembling those of ruminants. Previous studies of their teeth, however, suggest they would have had high body temperatures as well, but their size might have been big enough to generate that heat from their mass - researchers call this a "gigantotherm." No animals alive today are gigantotherms, so researchers cannot determine what their growth lines would have looked like.
The new study's findings indicate that "dinosaurs also had very fast growth rates and needed to eat a lot and maintain high generation of heat internally," added Köhler, so it is very likely they were warm-blooded after all. The study was published Wednesday, June 27, in the journal Nature.
