Who can get enough of dinosaurs? We’re curious about what color they were, how fast they moved, and whether they could really spit venom at Newman from Seinfeld. One of the most fundamental questions is whether they were cold-blooded or warm-blooded. Just because some of them looked like fearsome, giant lizards doesn’t mean they had to bask to raise their body temperature. After all, birds, likely their lone surviving descendants, are warm-blooded.
Researchers have looked at this question from many different angles (including temperature measurements from teeth, as we reported last year). One intriguing line of evidence has come from the microscale structure of their bones. Cross sections through fossils from most groups of dinosaurs (except sauropods) reveal cycles in growth, including dark lines where growth temporarily ceased.
This has long been cited as strong evidence in favor of cold-bloodedness, as the bones of modern cold-blooded species also show annual cycles. Since their body temperature is at the whim of the seasons, their growth slows during non-ideal conditions. Warm-blooded animals, on the other hand, keep their body temperature constant, and so their bone growth, too, remains constant. Or so the story went.
Some argued that the dinosaur bones actually showed signs of very high rates of growth in between the cyclical lulls. The high metabolism required to do so is more characteristic of warm-blooded animals, they said. But the cold-blooded camp maintained that only cold-blooded animals showed the alternating growth patterns.
As it turns out, that well-ordered house was built on sand. The evidence for constant bone growth in warm-blooded organisms was lacking. A paper published in the journal Nature describes a large review of ruminants (mammals that chew cud) and comes to the opposite conclusion—dinosaur bone growth looks more like warm-blooded organisms than cold-blooded ones.
The researchers examined femurs from over 100 African and European ruminants spanning climate zones from the tropics to the arctic. They found patterns of high bone growth rates that correlated with the growing season and hiatuses in growth during the dry or cold season.
To dig into the mechanisms driving this pattern, they used physiological data collected from Svalbard reindeer and alpine red deer. These studies measured changes in things like hormones and body temperature throughout the year.
The data showed that these species save energy by slowing their metabolism (and growth) when food is scarce, even reducing body temperature by a small amount (less than 1°C). During the best part of the growing season, metabolic activity kicks into high gear. The progress comes in tying that physiological strategy to the fine-scale bone structure, and showing that it’s pervasive across such a large group of warm-blooded animals.
The researchers argue this work not only "debunks the key argument from bone histology in support of" cold-bloodedness, it also corrals the bone patterns under the umbrella of evidence for warm-bloodedness. They think there’s a good chance that dinosaurs had similar metabolic schemes as these modern ruminants, staying in tune with the seasonal availability of food.
We’ll see if the rest of the paleontology community agrees. Odds are, some will have a bone to pick.