Being warm-blooded, as opposed to cold-blooded animals like lizards that must bask in the sun, means that one of the characteristics that distinguish mammals from other animals is that our bodies have high metabolisms that enable us to regulate our body temperatures without being dependent on our environment.

Only mammals and birds are warm-blooded species in the current animal kingdom, and our capacity to stay warm has allowed mammals to endure frigid conditions and undertake lengthy migrations.

However, the precise time when mammals developed their fast metabolisms has remained unknown.

In a recent study published in Nature, researchers use an unexpected source for dating the evolution of warm-blooded mammals: the dimensions of the inner ear components.

It’s difficult to determine whether a fossilized animal had warm blood because we can’t monitor its temperature and can only speculate on whether its activity was consistent with a warm-blooded or cold-blooded metabolism based on how active or slow its metabolism appeared to be.

However, a group of scientists led by the Natural History Museum in London, the Instituto Superior Técnico at the University of Lisbon, and the Field Museum in Chicago discovered that animal ears give a subtly accurate indication of their body temperatures.

All vertebrate animals have small canals in their ears that are filled with fluid to help with balance.

Our inner ears have developed varied diameters so that the fluid can flow properly since the viscosity, or runniness, of that fluid changes depending on temperature.

Warm-blooded species have runnier ear fluid, whereas cold-blooded animals’ ear fluid is cooler and thicker, requiring bigger passageways for it to pass through. As a result, our semicircular canals don’t need to be as large.

“Until now, semicircular canals were generally used to predict locomotion of fossil organisms,” says Romain David, one of the study’s lead authors. But, “by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures.” 

“This is because,” David explains, “like honey, the fluid contained inside semicircular canals gets less viscous when temperature increases, impacting function. Hence, during the transition to endothermy, morphological adaptations were required to keep optimal performances, and we could track them in mammal ancestors.”

The researchers analyzed the inner ear canal widths of 341 animals, including 243 current species and 64 extinct ones, to chart these evolutionary variations.

They discovered that warm-bloodedness did not evolve until 233 million years ago, which is over 20 million years later than previously thought. Instead, mammal ancestors took far longer to acquire the types of inner ear components suited for warm-blooded creatures.

And, based on the appearance of different-sized semicircular canals in the fossil record, it appears that when mammal ancestors did evolve warm-bloodedness, it happened much faster than scientists had thought, around the same time that proto-mammals began to evolve whiskers, fur, and specialized backbones.

Given that fur traps the body heat produced by a higher metabolism and helps maintain the body at the high temperature it need to flourish, the evolution of fur and warm-bloodedness at roughly the same period makes the most sense.

According to Ricardo Arajo, one of the paper’s primary authors, “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.”. 

“It was not a gradual, slow process over tens of millions of years as previously thought,” adds Arajo, “but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.”  

Ken Angielczyk, who is the lead author of the study, says he is excited that the study helps answer one of the oldest questions about how mammals evolved.

“The origin of mammalian endothermy is one of the great unsolved mysteries of paleontology. Many different approaches have been used to try to predict when it first evolved, but they have often given vague or conflicting results,” adds Angielczyk, the Field Museum’s MacArthur Curator of Paleomammalogy. “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.”

Image Credit: Getty

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