Ear canals may reveal when mammals became warm-blooded

Warm-bloodedness, or endothermy, is one of the most fascinating traits that arose sometime during the evolution of vertebrates. Early vertebrates, including fish and amphibians, were all cold-blooded, or ectothermic, meaning that they did not regulate their body temperatures but rather reflected the ambient temperature of their surroundings. But more modern vertebrates (birds and mammals) use metabolic heat to keep their temperatures constant. When, in evolutionary time, this change occurred, is a topic of great interest for scientists. 

There are several different forms of evidence for endothermy in dinosaurs – including their bone structure and largish brains, their distribution in colder regions, and their ability to move relatively fast. However, none of this evidence is conclusive. Now, in a new study published in the journal Nature, scientists suggest an interesting relationship between metabolic rate and the size of vertebrate ear canals that may help shed light on the time when ancient mammal ancestors became warm-blooded.

Evidence of endothermy does not fossilize easily. Most dinosaur fossils don’t record the skin covering, which might be hair or feathers if the animal were warm blooded. The fossil record also does not indicate whether most dinosaurs behaved in a way that matched an active, warm-blooded metabolism or a slow, cold-blooded one. 

A team of researchers, led by scientists from London’s Natural History Museum, the University of Lisbon’s Instituto Superior Técnico, and the Field Museum in Chicago have discovered that animals’ inner ears may provide an indirect clue about their body temperatures.

The inner ears of all vertebrate animals contain tiny canals, filled with fluid, that help them balance. The fluid may be viscose or runny, depending on the temperature of the animal. Ectothermic animals have more viscous fluid because their body temperatures are lower – and associated with this increased viscosity is the fact that the ear canals need to be wider in order for the thicker fluid to move appropriately. Endotherms are warmer and so their ear canal fluid is runnier and requires only narrow ear canals through which to travel.  

“Until now, semicircular canals were generally used to predict locomotion of fossil organisms. However, by carefully looking at their biomechanics, we figured that we could also use them to infer body temperatures,” said study co-lead author Romain David. “This is because, 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 compared the sizes of the inner ear canals of 341 animals, including 243 living species and 64 extinct ones. They found that mammal ancestors didn’t develop the kinds of inner ear structures ideal for warm-blooded animals until 233 million years ago, which is nearly 20 million years later than scientists had previously proposed for the evolution of warm-bloodedness. 

In addition, they found that when those smaller semicircular canals did show up in the fossil record, they were accompanied by other, rather rapid changes like the evolution of whiskers, fur and specialized backbones in proto-mammals. And these changes all appeared in the fossil record rather rapidly, in evolutionary terms. The evolution of fur and warm-bloodedness at about the same time gives support to the ear canal hypothesis because fur traps the body heat generated by a higher metabolism, helping to keep the body at the high temperature it needs in order to thrive.

“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,” said study co-lead author Ricardo Araújo. “It was not a gradual, slow process over tens of millions of years as previously thought, but maybe was attained quickly when triggered by novel mammal-like metabolic pathways and origin of fur.”

Study senior author Ken Angielczyk said that he is excited by how the study helps answer one of the longest-standing questions about the evolution of mammals.

“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,” said Angielczyk. “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.”

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By Alison Bosman, Earth.com Staff Writer