A team of researchers from Spain and Norway studied bone "growth lines" of more than 100 wild ruminants (warm-blooded mammals like sheep and cows that have multiple stomachs) which are similar to the growth rings in tree trunks.
Called lines of arrested growth or LAGs, the markers show fast, yet interrupted yearly growth that depended on how long the "unfavourable" season lasted.
These growth lines were similar to those seen in dinosaur bones previously studied, indicating that both ruminants and dinosaurs have periods of high growth marred by "unfavourable" seasons with limited resources and little growth.
This means that dinosaurs were likely warm-blooded like the ruminants, the researchers said. "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 vascularised (infiltrated with blood vessels) tissue that indicates very high growth rates," study author Meike Kohler of Autonomous University of Barcelona told LiveScience.
"This is typical in dinosaurs and very different from reptiles, which have slow growth between the rest lines." Lags have also been found in the bones of reptiles and amphibians and have until now been assumed to be limited to ectotherms -- cold-blooded animals -- that are more subject to the whims of harsh environments. And Lags on dinosaur bones has so far been taken as evidence of their cold-bloodedness.
According to the researchers, who published their work in the journal Nature, sauropods were the only dinosaurs where they haven't seen growth lines similar to those of ruminants.
Past studies of their teeth indicate they would have had high body temperatures as well, though they might have been big enough for their mass to generate that heat -- what the researchers call a "gigantotherm."
Researchers don't know what their growth lines would have looked like, since no animals alive today are gigantotherms. This indicates that "dinosaursalso had very fast growth rates and needed to eat a lot and maintain high generation of heat internally," Kohler said, so they were most likely warm-blooded.
The theory that dinosaurs were warm has been gaining traction in the last few years in multiple fields, but the researchers admit that other, non-bone-based arguments for cold-bloodedness still stand.
Endotherms should have the physical ability to move quickly, and lung volume to pump oxygen to muscles needed for running, which researchers can't be sure dinosaurs had. "There are a lot of arguments in favour and against endothermy in dinosaurs," Kohler said.
"It could be that they have some traits that are clearly endothermic," but others may be muddled.