May 31 2022

Were Dinosaurs Warm or Cold-Blooded?

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If you haven’t seen the new series, Prehistoric Planet, hosted by David Attenborough, you should see it. The visuals are stunning, the science is updated, and it provides a compelling look into the world of dinosaurs and other animals contemporary to the dinosaurs. While watching an episode last night, depicting a velociraptor leaping around energetically, my wife asked, “Were dinosaurs cold-blooded?” The classic concept of dinosaurs is of large lumbering and slow animals, cold-blooded (ectothermic) like other reptiles. However, scientists have long suspected that some if not all dinosaurs may have been warm-blooded (endothermic) – hence the updated vision of dinosaurs as energetic animals. (The picture is of a baby T-Rex.)

Birds, which are dinosaurs, are very endothermic, even more so than mammals. So the real question is not if dinosaurs were endothermic, but when in their evolution did they become so. One reasonable hypothesis is that the bird clade evolved endothermic metabolism in order to fuel their very high energy lifestyle of flying, so it may be a later development within the bird subgroup. In any case, I gave a short version of that answer, continued to watch the show and vowed to update my knowledge on where the question of dinosaur metabolism lies. By coincidence, a recent study sheds considerable light on this question, possibly settling it, in fact.

Ectothermic vs endothermic metabolism is mostly about how efficiently oxygen is metabolized with fuel in the body to produce energy, which also produces heat as a byproduct. Ectothermic creatures, like modern reptiles, burn oxygen slowly, so that can eat less, breath less, but also are less active. Further, their metabolism does not produce that much heat, so that cannot regulate their own body temperature. They have to use the environment to do so, like basking in the sun. We have a skink as a pet, and you have to provide a warm and cool side to their environment, so that they can use external temperature to regulate their internal temperature.

Endothermic animals have a higher rate of oxygen metabolism, which produces a lot of heat as a byproduct. This allows them to regulate their own temperature, increasing their metabolism to raise their temperature, and shedding heat (sweating, panting, etc.) to cool down. Oxygen metabolism also produces waste products, which happen to be very stable and insoluble. This is where the new study comes in – the researchers used “in situ Raman and Fourier-transform infrared spectroscopy” in order to identify specific molecules in dinosaur bones that were well-preserved enough to retain some organic material. They used extant animals with known metabolisms to calibrate this process, and then applied it to extinct species.

They found that the answer to the question of whether or not dinosaurs were warm or cold-blooded is yes – they were both. Of course, the real question was, when and in which lines did dinosaurs become warm-blooded. There are two basic groups of dinosaurs, the saurischians (lizard-hipped) and the ornithischians (bird-hipped). The saurischians include the theropod dinosaurs, such as T-rex, and agile velociraptor, and related species, which are warm-blooded. In fact they are “hot-blooded” like birds, with higher metabolisms than even mammals. This group also gave rise to the bird, so this makes perfect sense. It also includes the sauropods – the giant long-necked dinosaurs. The ornithischians include groups like triceratops and stegosaurus, and they turned out to be cold-blooded. This also makes sense as many of these dinosaurs, like the two mentioned, have large plates or frills that seem to be designed for environmental temperature regulation, as would be needed in a cold-blooded animal.

The researchers also were able to address some related questions. The concentration of oxygen in the atmosphere was a bit higher 65 million years ago than today, so they had to figure out if the higher oxygen lead to higher oxygen metabolism. They found that it did not – there was no correlation between atmospheric oxygen and metabolism.

They also addressed an interesting question – were sauropod and other really big dinosaurs “gigantothermic”? In other words, their incredible size would mean that they would shed relatively less heat. They might be able to warm up even with a slower metabolism, and so be metabolically cold-blooded but effectively warm-blooded just because of their size. However, the researchers found that this is not true. The sauropods are “true endotherms”.

This was a pretty powerful study, which seems to definitely answer many questions about the metabolic rates of various groups of dinosaurs. We also now know that many theropod dinosaurs, and even some other related groups, had feathers, some just as young “chicks” but many into adulthood. It’s not surprising that birds evolved out of a group of hot-blooded feathered dinosaurs, including many small agile species.

It’s amazing how much our vision of dinosaurs has evolved over the course of my lifetime, fueled by clever and powerful scientific research. Prehistoric Planet is a good way to update your knowledge of dinosaurs, and they also have a companion website with links to the relevant scientific research.

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