Sophie the Stegosaurus in the Museum's Earth Hall

Stegosaurus may have had a warm-blooded ancestor, and then later became cold-blooded. Image © The Trustees of The Natural History Museum, London, All Rights Reserved.

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Dinosaurs may have evolved from warm-blooded ancestor

The question of whether dinosaurs were warm or cold-blooded has been debated for decades, with conflicting views and evidence for both.

A new study suggests the ancestor of the dinosaurs may have been warm-blooded, but not all dinosaurs stayed that way. 

Some dinosaurs, such as stegosaurs, may have needed to bask in the sun to keep their body temperature constant, while others like sauropods were able to be active all day. 

Debate over whether dinosaurs were warm or cold-blooded has been going on for over 60 years after scientists realised the ancient reptiles were not as sluggish as had previously been thought. 

A new study has added further fuel to the metabolic fire by looking for signs of the 'waste products' of activity. Their results suggests that the ancestor of all birds, dinosaurs and pterosaurs was warm-blooded, but that some such as Triceratops and Stegosaurus later lost this ability.

This may have influenced how these dinosaurs behaved as they tried to maintain their body temperature.

Lead author Dr Jasmina Wiemann says, 'Dinosaurs with lower metabolic rates would have been, to some extent, dependent on external temperatures. Lizards and turtles sit in the sun and bask, and we may have to consider similar "behavioural" thermoregulation in ornithischian, or bird-hipped, dinosaurs with exceptionally low metabolic rates. 

'Cold-blooded dinosaurs also might have had to migrate to warmer climates during the cold season, and climate may have been a selective factor for where some of these dinosaurs could live. Meanwhile, the hot-blooded giant sauropods were herbivores, which probably faced more of a problem cooling down than heating up.'

Prof Paul Barrett, a dinosaur expert at the Museum, says that the study raises interesting new questions for researchers.

'This is a really intriguing study which throws up a lot of questions about dinosaur physiology and its consequences for dinosaur ecology and behaviour, and it forms a nice framework for future research.

'For instance, the idea that giant bird-hipped dinosaurs, like Triceratops, lost their warm-bloodedness is brand new and interesting. It’s becoming clearer that the biology of ornithischian dinosaurs like these differed markedly from that seen in members of the other major dinosaur groups, and this research underscores just how unusual they were.

'None of the dinosaurs suggested to lose their warm-blooded status had feathers or fuzz, and this absence could, perhaps, be linked to their lack of endothermy. It would be very interesting to explore this aspect further with regard to feather evolution.'

The findings of the study were published in Nature

Dippy the Diplodocus in the Museum's Hintze Hall

The size of Diplodocus may have meant it was a gigantotherm - trapping enough heat to maintain its temperature without being warm-blooded. Image © The Trustees of The Natural History Museum, London, All Rights Reserved.

What does being warm-blooded mean?

Being warm-blooded, technically known as 'endothermy', means that an animal maintains a constant body temperature by releasing energy as a by-product of the metabolic processes taking place within their cells. 

While this allows an animal to be more active and grow more quickly, it is very costly. Endotherms need to eat more food than their cold-blooded relatives, and their faster metabolic processes produce molecules that can damage cellular components.

Cold-blooded animals, or 'ectotherms', don't manage their body temperature in this way. They instead rely on energy from the environment, such as sunlight, to maintain their body temperature and so need less food to survive.

However, this trade off means that cold-blooded animals can't be active all day. Significant periods of the day are given over to activities such as basking that raise their body temperature, and it means that they are unable to do energy-intensive behaviours such as flying.

When the biology of dinosaurs was first discussed in the 1840s, they were compared to living lizards. This led to the assumption that they were all cold-blooded, and so would have been slow-moving animals lumbering around during the Mesozoic.

From the 1960s, however, scientists began to reassess this view. Dinosaurs are now universally considered to have been highly active animals, which has been used as evidence they were warm-blooded.

However, the sheer size of dinosaurs meant they still could have been cold-blooded and active. Larger animals have a relatively lower surface area than smaller animals, giving a lower area for heat to escape over. The concept of gigantothermy suggests that large dinosaurs would have been able to maintain their body temperature by trapping more heat for longer.

Paul explains why it's difficult to separate these competing ideas.

'There are no direct ways of assessing the metabolism of an extinct animal,' Paul says. 'We can infer certain characteristics, such as body temperature, from looking at chemical isotopes but this could show that they're warm-blooded because they live in a warm area. We can also investigate growth rate, with high rates often found in warm-blooded animals, but it's not a perfect fit.

'There's been an emerging consensus among palaeontologists that the metabolism of dinosaurs was probably different from any living animal, and that would have grown faster than other reptiles. However, it's not known whether this is due to dinosaurs having higher metabolic rates or to other factors.'

The scientists behind the new technique proposed by the study hope that its way of estimating metabolism will stand the test of time. 

Microscopic view of extracted soft tissues from the bones of one Allosaurus

Bones from dinosaurs such as Allosaurus were analysed for levels of metabolic byproducts. Image ©  J. Wiemann

Were dinosaurs warm-blooded?

The researchers examined fossils for compounds known as advanced lipoxidation end-products, or ALEs, which are signs of metabolic stress in living animals. These molecules are extremely stable and don't dissolve in water, making it likely evidence of ALEs will be preserved in a fossilised animal.

The levels of ALEs in the thigh bones of warm and cold-blooded animals, including mammals, reptiles and birds, were used to assess the thresholds between endothermy and ectothermy in different groups. Unlike some methods of assessing endothermy, the method is non-destructive and so preserves fossils for future use.

The level of ALEs in fossils were then analysed and used to infer whether a particular species was warm-blooded or not. The researchers found that all fossil mammals, pterosaurs and plesiosaurs were within similar ranges to living endotherms.

The dinosaurs were split, with theropods such as Tyrannosaurus rex, Deinonychus and Allosaurus likely to be warm-blooded, as well as other animals such as sauropods. Other dinosaurs, including Triceratops, Stegosaurus and hadrosaurs, were within the range of ectothermy.

The researchers interpreted this as evidence that the ancestors of all mammals, plesiosaurs and ornithodirans (the group containing the dinosaurs and pterosaurs) each evolved endothermy separately, and that some of their descendants later lost this ability and become ectotherms.

'Some of the results are surprising,' Paul says. 'Sauropods are thought to have been so big that they would have had high temperatures without being warm-blooded. Plesiosaurs were also found to be possibly warm blooded, which is something that's not been seriously considered before.'

'As only one species of each of these big groups was included in the analysis, I’d like to see more examples from each analysed to confirm these ideas. Also, extinct monitor lizards appear as warm-blooded in their analysis, whereas no living lizard is an endotherm, so it may be that their high levels of activity were enough to make them appear as endotherms using this technique.'

The researchers behind the study have said they are working to expand their current data set to include more specimens, including more marine reptiles and Pleistocene megafauna such as the woolly mammoth, to improve the resolution of their findings. They also hope to expand the technique to non-amniote groups such as amphibians and fish.

Without living individuals to test (and short of a time machine) it will never be entirely certain whether dinosaurs, pterosaurs and other extinct animals were warm-blooded or not. However, studies like this offer new ways that fossils can tell us about how they would once have lived.