Were dinosaurs warm-blooded?

The short answer to whether dinosaurs were warm-blooded or not is “Probably”. However, the long answer starts in the 1970s when palaeontologists realised that dinosaurs weren’t like sluggish giant lizards at all, as we thought, but active like birds.

‘Warm-blooded’ really means that the body is maintained at a constant, high temperature and doesn’t rely on the sun for warmth. Most warm-blooded animals are capable of being highly active; mammals are warm-blooded, and birds typically run even hotter. But obviously we don’t have live dinosaurs whose body temperature and respiration rate can be measured, and we can’t ever be certain how active they were.

On the other hand, we do now know that, like birds, dinosaurs had many features linked with warm-bloodedness. They had hollow bones showing signs of really fast growth; a system of air-sacs connected to the lungs; many of them cared for their young in nests; and many of them had feathers.

In that case, how could giants like Brachiosaurus or Diplodocus be warm-blooded without overheating? It may help to picture them as massive, four-legged, flightless chickens – bird-like internal air-sacs in contact with internal organs could have meant easy air-cooling even for the biggest land animals ever to have lived.

But the question remains open. Much of the hard evidence comes from studies of bone growth. It used to be thought that ‘cold-blooded’ animals left telltale lines, like tree rings, indicating slow seasonal growth. Some primitive, extinct birds have these ‘lines of arrested growth’ (LAGs), but modern birds do not. Does this mean that ancient birds were cold-blooded after all?

Not necessarily – the bone in modern birds is heavily modified in life, possibly expunging any lines. And recently it was found that many living, hot-blooded mammals routinely have LAGs. The debate continues, and could run and run.