Mammals are warming up. The ability to maintain a high and constant body temperature, known scientifically as endothermia and colloquially as warm blood, is a key part of what has allowed these beasts to adapt to many different habitats, from polar seas to deserts. scorching. However, exactly when mammals evolved this ability, it has long been a mystery.
Finding direct evidence of how the physiology of ancient organisms worked is a difficult task. Soft tissues that would be more informative about hot or cold blood are rarely preserved in fossils, and paleontologists obviously cannot directly measure the body temperature of an extinct animal. But looking at the inner ears of fossil mammals and their relatives, paleontologists have discovered that mammals began to warm up just as the Age of Reptiles was in full swing.
The new study by paleontologist Romain David of the Museum of Natural History in London and colleagues focuses on the system of semicircular ducts, or SDS, in the inner ears of mammals. As researchers reported Wednesday in Nature, these small loops inside vertebrate ears regulate critical functions such as balance, vision, and motor control. The evolution of these structures was influenced by body temperature: a relatively high temperature makes the liquid inside the loops less viscous, so that the ducts and channels of the inner ear have a different shape and are often smaller in warm-blooded species than in cold-blooded ones. -those of blood. David and colleagues used the shape and size of the inner ear as an indicator of an animal’s body temperature and behavior to determine when mammals began to become endotherms.
“I was amazed that the inner ear could help us assess body temperature and warm blood so well,” David says. The whole project began with a post-conference discussion between him and his co-author Ricardo Araújo, a paleontologist at the University of Lisbon, on why fish have much larger inner ears for their size than terrestrial animals. The researchers thought that body temperature could explain the difference, and that this difference could also appear in fossils. The researchers set about accumulating a data set of 56 fossil synapsids, the group containing mammals and their relatives, to determine when mammalian ancestors crossed this critical physiological threshold.
Size differences between the inner ears (in red) of warm-blooded mammals (left) and the anterior cold-blooded synapses (right). The inner ears are compared to animals of similar body sizes. Credits: Romain David and Ricardo Araújo
Researchers propose that mammalian precursors began to warm up about 233 million years ago, during the Triassic period. At that time, dinosaurs were just beginning to proliferate, reptiles were the most prominent and diverse creatures on earth, and the predecessors of mammals were relatively small creatures, somewhat similar to the weasel. Technically known as mammals, the latter experienced a large jump in body temperature, between five and nine degrees Celsius, during this period. This moment means that warm blood is a very old heritage that evolved before the first true mammals.
“Researchers have been trying to unravel the trajectory of mammalian characteristics such as endothermia for decades,” says Oxford University paleontologist Elsa Panciroli, who did not participate in the new study. Each of the above methods gave different answers about time and what types of mammalian relatives were warmed up. But new findings based on the anatomy of the inner ear appear to be “robust,” Panciroli says, mostly because the same relationship between SDS and body temperature can be observed in living species.
The results recalibrate what some previous studies had proposed about the origins of endothermia in mammals. Using other lines of evidence that include bone chemistry and cell structure, they had hinted that endothermia evolved even earlier among protomammals that lived more than 252 million years ago, such as carnivores with saber-toothed gorgonopsids and beak-and-tooth herbivores called dicinodonts. Despite these previous investigations, however, new evidence indicates that warm blood became an important feature of mammalian history millions of years later. The date of the new study about 233 million years ago was closer to when small, furry creatures (more like what we think of as true mammals) began to evolve. Given that the origins of traits such as sensitive whiskers and changes in the inner ear bones that drive sound occurred during the Triassic, notes Field Museum co-author and paleontologist Kenneth Angielczyk, ” it doesn’t end up being so surprising that endothermia is emerging over time, with everything else. “
There are some caveats to the findings of the new study. As in previous research, some of the protomammals examined by the researchers appear to be closer to warm-blooded, and some early mammals appear to have been closer to cold-blooded. “Researchers have long recognized that the characteristics that distinguish mammals, such as warm blood, skin, milk production, and increased activity, probably evolved scattered among different groups at different times.” , says Panciroli. The new study identifies the general change in mammalian body temperature, but there were always some individual species that broke the rules.
The evolution of warm body temperatures and the fact that they are much more active contributed to the spread and success of mammals during the era of reptiles. In the last two decades, paleontologists have dispelled the old notion that mammals during the Triassic, Jurassic, and Cretaceous were tiny creatures that stung insects that ran in the dark. Recent findings have also established that early mammals and their relatives swam, dug, glided from tree to tree, and even ate dinosaurs. Bodies that warmed and fed much more active and complex behaviors endorsed the success of mammals even when dinosaurs rose above them, a critical part of the deep history of ourselves and the magnificent beasts we see. around us today.