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 ones. According to the results, the first cold-blooded animals appeared about 233 million years ago – about 33 million years before the emergence of mammals. At about the same time, the fur also developed, which, in addition to the adapted metabolic pathways, supports the regulation of body temperature.
Unlike all other animals in the world, mammals and birds have developed a system to regulate their body temperature through their own metabolism and to keep it largely constant, without having to rely on heat sources in their environment. They are endothermic, i.e. of the same temperature. This allows them to live in different environments and to move further and faster than ectothermic, i.e. cold-blooded, animals. However, the question of when endothermy first appeared in the evolutionary history of mammals remains difficult to answer. So far, fossils have only been able to help to a very limited extent, since the body temperature of the animals, which have been extinct for thousands of years, can no longer be measured and the skeleton at best only indirectly indicates the degree of activity.
Believe your ears
A team led by Ricardo Araújo from the University of Lisbon in Portugal has now taken a new approach to answering 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, the semicircular canals have generally been used to infer the locomotion of fossil organisms,” explains co-author Romain David of the Natural History Museum in London. “However, by carefully examining their biomechanics, we found that we can also use them to determine body temperature.”
The idea behind this: There is a liquid in the auditory canals, the endolymph, which provides signals for the organ of equilibrium through its movement. Similar to honey, the endolymph is thinner at higher temperatures than at colder ones. “Thus, during the transition to endothermy, morphological adjustments were required to maintain optimal performance,” says David: Ear canals narrowed in warm-blooded animals. “We were able to trace this adaptation in the ancestors of mammals.”
Development contemporaneous with the fur
The researchers found that the geometry of the inner ear canals changed in some animals around 233 million years ago – a clear indication that their body temperatures were becoming warmer. According to the authors, the changes are consistent with an increase in average body temperature of around five to nine degrees Celsius. This change cannot be attributed to long-term higher outside temperatures: At that time, in the late Triassic, unstable climatic conditions prevailed with rather cooler temperatures.
Previous research assumed that the first steps on the way to warm-blooded people began around 252 million years ago, while other studies assumed that the first warm-blooded animals were the first mammals around 200 million years ago. The current study now shows that endothermy appeared in the mammalian ancestors at about the same time as fur – a plausible link since fur helped to store the body temperature generated by increased metabolism.
Rapid evolution
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 the development of endothermy seems to have taken place in less than a million years,” says Araújo. “It was not a gradual, slow process over tens of millions of years, as previously thought, but possibly a rapid process triggered by novel mammalian-like metabolic pathways and the formation of fur.”
Source: Ricardo Araújo (University of Lisbon, Portugal) et al., Nature, doi: 10.1038/s41586-022-04963-z