The ancestors of modern mammals evolved the ability to regulate their own body temperatures, which may have been useful in cool climates. Credit: Luzia Soares
The reptile-like ancestors of mammals evolved to be warm-blooded – but the timing of this transition is hotly contested. Scientists have now used fossilized inner ear canals to suggest that the adaptation occurred around 230 million to 200 million years ago1. But other researchers say this evidence is unlikely to settle the debate.
Warm-blooded animals, or endotherms, can maintain a constant high body temperature due to their fast metabolism, while cold-blooded ectotherms have a low metabolic rate and rely on their environment to keep warm. But it’s hard to measure these traits in fossils, so researchers have used skeletal characteristics like height and bone structure to infer metabolic rate. Some past studies suggest that endothermy did not appear until 145 million to 66 million years ago2; other studies place it at 300 million to 250 million years ago3.
In a study published in Nature on July 20, 1 Ricardo Araujo, a paleontologist at the University of Lisbon, and his colleagues propose that the shape and size of the bony canals of the inner ear can be used as a proxy for body temperature. The movement of fluid through the channels helps the body monitor the position and movement of the head, which is essential for vision and balance. And the viscosity of the liquid changes with body temperature. The research team hypothesized that as body temperature increased and the animals became more active, the shape of the ear canals evolved to a less viscous fluid to maintain balance and movement.
To trace this adaptation, the team compared the inner ear structures and physiology of 50 living vertebrates, including reptiles, fish, birds and mammals. They developed a thermal mobility index based on the shape of the inner ear, which, adjusted for body size, allowed them to predict an animal’s body temperature.
A cooler climate
When they analyzed the inner ear canals of 56 extinct synapsids—the reptilian ancestors of mammals—and included them in the index, the authors found that the shape of the canals changed dramatically during the late Triassic period, which lasted from 237 million to 201 million years ago. Their authors suggest that synapsids then became warm-blooded. According to the index, this transition would represent an increase in body temperature of 5–9 °C and an increased metabolism.
The authors conclude that this adaptation gave early endotherms an advantage in coping with the Triassic climate, which was cooler than that of the preceding Permian period.
José Eduardo Bicudo, a comparative physiologist at the University of São Paulo in Brazil, says the team’s approach is novel and offers a new way of looking at the emergence of endothermy.
Biologist Roger Seymour of the University of Adelaide in Australia says the origin of endothermy has been hotly debated for the past 60 years and there are multiple theories – each with evidence to support it. His and Bicudo’s own research suggests that endothermy evolved during the Permian, between 300 million and 250 million years ago3.
But he questions whether endothermy may have emerged in the Late Triassic. He points out that the body temperature proposed for early endothermic synapsids – around 34 °C – is still relatively low, similar to that of living monotreme mammals such as the platypus and the echidna.
Platypus and echidnas are sluggish compared to other mammals. Seymour suspects that animals with similar body temperatures would have struggled to survive alongside other reptiles of the time, including archosaurs – the ancestors of modern birds – which walked on two legs and could move quickly. “If you have a bipedal archosaur running around and you have a physiology that’s like a modern echidna or platypus, you’re going to be dinner.”
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