A mammaliamorph breathing out hot hair in a frigid night, hinting at endothermy (warm-bloodedness). Credit: Luzia Soares

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New paper sheds light on origin of warm-bloodedness in mammals

An international team of scientists led by researchers from the Natural History Museum, University of Lisbon’s Instituto Superior Técnico, and the Field Museum of Natural History, USA, have revealed the origins of endothermy in mammals, tracing the characteristic’s origins to the Late Triassic period, around 233 million years ago.

The scientists based their method on the observation that body temperature affects the viscosity, or runniness, of the endolymph, a fluid contained in the tiny semicircular canals of the inner ear, which detect head rotation and aid balance. Animals with sustained high body temperatures, such as mammals, have had to change the shape of their ear canals to maintain proper function, meaning the structure of the ear can be used as an accurate guide to when endothermy evolved.

Until now the general expectation was that endothermy was achieved around 252 million years ago near the origin of Cynodontia, a group of mammalian ancestors. The new results suggest that endothermy appeared much later and that many early cynodonts, often pictured as warm-blooded with fur, were actually quite different from mammals.

The findings show that endothermy evolved later than previously hypothesized, but around the same time as when other observably mammalian traits evolved, such as the different parts of the backbone taking on different functions, and the evolution of whiskers and/or fur.

Dr Romain David, Post-Doctoral Researcher at the Natural History Museum and Lead Author on the paper, said: “Until now, semicircular canals were generally used to predict locomotion of fossil organisms. However, by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures. This is because, like honey, the fluid contained inside semicircular canals gets less viscous when temperature increases, impacting function. Hence, during the transition to endothermy, morphological adaptations were required to keep optimal performances, and we could track them in mammal ancestors.”

Dr Ricardo Araújo, Junior Researcher at Instituto de Plasmas e Fusão Nuclear, Instituto Superior Técnico, University of Lisbon and Lead Author on the paper, said: 

“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seem to have occurred very quickly in geological terms, in less than a million years. It was not a gradual, slow process over tens of millions of years as previously thought, but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.”

Dr Kenneth D. Angielczyk, MacArthur Curator of Paleomammalogy at Field Museum of Natural History, USA and Senior Author on the paper, said:

“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results. We think our method shows real promise because it has been validated using a very large number of modern species, and it suggests that endothermy evolved at a time when many other features of the mammalian body plan were also falling into place.”

The study Inner ear biomechanics reveals Late Triassic origin of mammalian endothermy is published in Nature. It can be accessed here.

Notes to editors

Natural History Media contact: Tel: +44 (0)20 7942 5654 / 07799690151 Email: press@nhm.ac.uk

Images and paper available to download here.

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