The idea that dinosaurs were warm-blooded is rather at odds with the popular image of them as large, lumbering reptiles. But that’s what’s suggested by recent evidence of their anatomy, behaviour and metabolism.
Despite the fact that the name dinosaur actually means ‘terrible lizard’ (from the Greek deinos and sauros), their closest modern descendants are birds, which are definitely warm-blooded, or endothermic.
And indeed, many fossils show dinosaurs with feathers, which they possibly used for insulation rather than flight. There’s also evidence they lived in polar areas, which would have been rather challenging if they couldn’t maintain a suitable body heat.
But there is a complication: dinosaurs’ other surviving relatives are crocodiles, and they’re cold-blooded, or ectothermic. So what gives?
Professor Roger Seymour from the University of Adelaide recently took a closer look at crocodile metabolism and found further support for the idea that dinosaurs are warm-blooded (Seymour RS 2013, “Maximal aerobic and anaerobic power generation in large crocodiles versus mammals: implications for dinosaur gigantothermy”, PLoS ONE 8(7): e69361. doi:10.1371/journal.pone.0069361).
The key was to look at muscle performance, which is related to the number of mitochondria in the cells. Mitochondria are the cellular power sources, and they not only work better at certain temperatures, but they also produce heat when they burn energy.
Warm-blooded animals – like mammals – tend to have more mitochondria and hence better muscle performance than cold-blooded reptiles.
Of course, to quantify this Professor Seymour had to actually go out and measure the physical fitness of big salt-water crocodiles in northern Australia. He and his colleagues captured them with loops, then let them thrash around till they were exhausted – that way he could determine how much energy their muscles produced.
He found that the larger a crocodile was, the weaker were its muscles – particularly compared to mammals. A 200 kg crocodile produced only 14% of the energy that an equivalent-sized mammal would have.
This isn’t normally a problem for crocodiles, because they hunt by just lying in wait, which doesn’t require much energy. But dinosaurs appear to have had many different styles of feeding and probably led much more active lives. Plus, they would have had to compete with early mammals, which were evolving at the same time.
As Professor Seymour put it, “If you imagine a fight between a crocodile-like dinosaur and a mammal-like dinosaur, it’s clear that the mammal-like dinosaur would win.”
Of course, there are still some unanswered questions. For one, aren’t humans mammals? Despite the calculations, I’d be reluctant to fight a crocodile my size.
But there’s another question: why are crocodiles cold-blooded if they’re related to warm-blooded dinosaurs?
Interestingly, this may be a recently evolved trait. Crocodiles actually have many physiological features that are similar to warm-blooded creatures, like a four-chambered heart (most reptiles have three-chambered hearts). Professor Seymour suggests that maybe the crocodile’s ancestors were warm-blooded, but as they evolved into their current, lie-in-wait niche, they needed less energy and went back to being cold-blooded.
Then there’s the question about whether all dinosaurs were the same. According to some calculations, the really big dinosaurs – which were mostly herbivores like
Brontosaurus Apatosaurus – would have had to be cold-blooded. Huge bodies like theirs would take a long time to cool down, so if they weren’t cold-blooded they probably would have overheated.
But if crocodiles were able to adapt their metabolism to their lifestyle, it’s possible that the big sauropods did too, and so there may be more than one way that dinosaurs operated.
Then again, there may have been some big dinosaurs had other ways of using the excess heat…
(This story first aired on 22 August 2013 – you can listen to the podcast.)