What the ears reveal about warm-blooded animals

Since when do warm-blooded animals exist? Researchers have now approached this question in an unusual way: by examining the inner ear canals. These are narrower in warm-blooded animals than in cold-blooded animals. According to the results, the first cold-blooded animals appeared about 233 million years ago – about 33 million years before the appearance of mammals. At about the same time, fur was also developed, which, in combination with appropriate metabolic pathways, supports the regulation of body temperature.

Unlike all other animals in the world, mammals and birds have evolved a system for regulating their body temperature through their own metabolism and keeping it largely constant, without having to rely on heat sources in their environment. They are endothermic, that is, of the same temperature. This allows them to live in different environments and move farther and faster than heat, that is, cold-blooded animals. However, it is still difficult to answer the question of when the endothermic first appeared in the evolutionary history of mammals. So far, fossils have only been able to help on a very limited scale, since the body temperature of animals, which became extinct thousands of years ago, can no longer be measured and skeletons at best only indirectly indicate the degree of activity.

believe your ears

A team led by Ricardo Araujo at the University of Lisbon in Portugal took a new approach to answer this question: The researchers examined the semicircular canals in the inner ear of 341 living and extinct animals, including mammals, birds, reptiles, amphibians and dinosaurs. “Until now, semi-circular canals have generally been used to infer the movement of fossil organisms,” explains co-author Roman David of the Natural History Museum in London. “However, by carefully examining their biomechanics, we have found that we can also use them to determine body temperature.”

The idea behind it: There is a fluid in the auditory canals, the endolymph, which provides signals to the organ of balance through its movement. Similar to honey, the endolymph is thinner at higher temperatures than at colder ones. “Thus, during the transition to endothermic, morphological modifications were required to maintain optimal performance,” says David: narrowing of the ear canals in warm-blooded animals. “We were able to trace this adaptation in mammalian ancestors.”

Simultaneous development with fur

The researchers found that the architecture of the inner ear canals changed in some animals about 233 million years ago – a clear indication that their body temperatures were getting warmer. According to the authors, the changes correspond to an increase in average body temperature by about five to nine degrees Celsius. This change cannot be attributed to the long-term rise in external temperatures: at that time, in the late Triassic period, unstable climatic conditions with somewhat cooler temperatures prevailed.

Previous research assumed that the first steps on the path of warm-blooded people began about 252 million years ago, while other studies have suggested that warm-blooded animals were the first mammals about 200 million years ago. The current study shows that heat absorption appeared in mammalian ancestors at about the same time as fur – a plausible link because fur helped store body temperature resulting from increased metabolism.

rapid development

From a geological point of view, the new feature developed in a surprisingly short time: “Contrary to conventional scientific opinion, our work surprisingly shows that endothermic evolution appears to have occurred in less than a million years,” says Araujo. “It was not a slow, gradual process over tens of millions of years, as previously thought, but probably a rapid process caused by new mammalian-like metabolic pathways and the formation of fur.”

Source: Ricardo Araujo (University of Lisbon, Portugal) et al, Nature, doi: 10.1038/s41586-022-04963-z

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