About 233 million years ago, the mammals they converted to hot blood, being able to withstand extreme temperatures and adapting to the climate of planet Earth. But how did this come about?
A recent investigation by a vertebrate paleontologist from the University of Lisbon discovered in fossils how the key is in the contraction of the ears.
The warm-blooded process is also called endothermy, which is one of the key features of mammals, allowing animals to regulate their internal body temperature by controlling their metabolic rates.
In all vertebrates, the labyrinth of semicircular canals in the inner ear contains fluid that responds to movements of the head, brushing against tiny hair cells in the ear and helping to maintain a sense of balance. That liquid can become thicker or thinner depending on body temperature.
“Mammals have very unique inner ears,” says paleontologist Ricardo Araújo. Compared to similarly sized cold-blooded vertebrates, the dimensions of mammalian semicircular canals, such as thickness, length, and radius of curvature, are particularly small, he says. “The ducts are very thin and tend to be very circular compared to other animals.”
In warm-blooded animals, the fluid becomes less viscous, and the canals may have shrunk to compensate.
The time of the supposed change from cold to hot blood happened about 233 million years ago, corresponding to a geologically brief interlude of highly unstable climate known as the Carnian Pluvial Episode. “It was a time when global temperatures were changing a lot, and it was also a very wet and humid time,” says Kenneth Angielczyk, a paleontologist at the Field Museum in Chicago. “One of the benefits of endothermy is that it stabilizes the internal environment of the body, allowing it to operate regardless of environmental conditions.”