Warm-Bloodedness Appeared in Mammalian Ancestors 233 Million Years Ago, Study Suggests

Among modern animals, only mammals and birds are warm-blooded, and the ability to keep ourselves warm has enabled mammals to survive in icy weather and make long migrations. But it’s been a mystery exactly when mammals evolved their high metabolisms. In a new study, an international research team led by scientists from the Field Museum of Natural History, the Natural History Museum, London, and the University of Lisbon’s Instituto Superior Técnico points to an unlikely source for determining when ancient mammal ancestors became warm-blooded — the size of tiny structures in their inner ears.

A mammaliamorph breathing out hot hair in a frigid night, hinting at endothermy. Image credit: Luzia Soares.

Warm-bloodedness, or endothermyis a key feature of mammals and birds, and allows them to live in diverse environmental settings by keeping a near-constant core body temperature.

Endotherms are more behaviorally active and can travel further and move faster than cold-blooded ectotherms, which are generally slower, less active and have decreased aerobic capacity.

However, understanding when endothermy first emerged in mammalian evolutionary history remains challenging, as most fossil evidence is ambiguous.

“Examining the structure of the semicircular ducts of the inner ears of mammalian ancestors could help us investigate when this endothermy transition occurred,” said Dr. Ricardo Araújo, a researcher in the Instituto Superior Técnico at the University of Lisbon, and his colleagues.

“These ear canals contain a fluid called endolymphthe viscosity of which changes depending on the temperature of the animal.”

“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,” said Dr. Ken Angielczyk, MacArthur curator of paleomammalogy at the Field Museum of Natural History.

In the study, the authors compared the sizes of the inner ear canals of 341 animals, including 243 living species and 64 extinct ones.

They found that mammal ancestors didn’t develop the kinds of inner ear structures ideal for warm-blooded animals until 233 million years ago (Late Triassic epoch) — nearly 20 million years later than scientists had previously thought warm-bloodedness evolved.

And, based on when those differently-sized semicircular canals showed up in the fossil record, it seems that when mammal ancestors did evolve warm-bloodedness, it happened much more quickly than scientists had thought, around the same time that proto-mammals started to evolve whiskers, fur, and specialized backbones.

The evolution of fur and warm-bloodedness at about the same time especially make sense because fur traps the body heat generated by a higher metabolism, helping keep the body at the high temperature it needs to thrive.

“Contrary to current scientific thinking, our paper surprisingly demonstrates that the acquisition of endothermy seems to have occurred very quickly in geological terms, in less than a million years,” Dr. Araujo said.

“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.”

the study was published in the journal Nature.

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R. Araujo et al. Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy. Nature, published online July 20, 2022; doi: 10.1038/s41586-022-04963-z

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