Mammals’ ears reveal the surprising evolution of warm-bloodedness

This trait was likely key in the spread of mammals and birds across the globe.

A mammaliamorph breathing out hot hair in a frigid night, hinting at endothermy (warm-bloodedness).
Credit: Luzia Soares

Warm-bloodedness is a key trait of mammals, but it was long a mystery when proto-mammals evolved this feature to stand out from their more reptilian, cold-blooded ancestors. Now, in a new study published in the July 21 issue of the journal Nature, scientists find the ancestors of mammals may have evolved warm-bloodedness more recently, and abruptly, than previously thought.

HERE'S THE BACKGROUND — Animals that are warm-blooded, or endothermic, can keep their body temperature stable despite surrounding temperatures. Warm-bloodedness not only helps mammals and birds live in a variety of environments, but the high metabolisms that drive endothermy help warm-blooded animals prove more active than their cold-blooded, or ectothermic, rivals. All in all, warm-bloodedness was likely key in the spread of mammals and birds across the globe.

"To me, it is really puzzling to imagine our world before endothermy," study co-author Ricardo Araújo, a paleontologist at the University of Lisbon, tells Inverse. "How were these animals attacking their prey? Was everyone sluggish? Both carnivores and herbivores? Was it kind of a world in slow motion? Or were these animals doing intermittent locomotion, as lizards do today, moving and stopping, then moving and stopping again?"

It remains uncertain precisely when mammals first evolved warm-bloodedness, as researchers long lacked clear ways of deducing a species' metabolic rate from its fossils. One strategy looks at growth rates, focusing on a series of lines in bones that, like tree rings, correspond to years of growth. However, this often involves cutting into and permanently damaging rare fossils, which scientists are loathed to do, resulting in limited data from this method. Another technique analyzes the minerals in fossils to estimate the temperatures at which they might have formed, but scientists have found it hard to draw conclusions from this research as well, as it's still unclear how the process of fossilization might affect this mineral data.

Warm-bloodedness was likely key in the spread of mammals and birds across the globe.

WHAT DID THE SCIENTISTS DO? — In the new study, the researchers focused on an unlikely source for answers regarding this mystery — the ear.

The inner ears of mammals each possess three canals filled with a fluid called endolymph. Sensory hair cells within these "semicircular canals" detect any sloshing of this liquid. The canals are positioned at different angles so that each senses the movement of the head along a different axis. All in all, the inner ear helps mammals keep their balance and orient themselves in three dimensions.

The viscosity or runniness of endolymph changes based on body temperature, and semicircular canals have evolved different sizes so the fluid can flow correctly. Cold-blooded animals' endolymph fluid is cooler and thicker, so it needs wider spaces in which to move, whereas warm-blooded animals' endolymph is less viscous, so the semicircular canals do not have to be as big. As such, analyzing fossil semicircular canals may shed light on the metabolisms of those animals.

"This work was really facilitated by the recent rapid increase in the availability of CT scan data of fossil and extant animals," study co-author Kenneth Angielczyk, a vertebrate paleontologist at the Field Museum of Natural History in Chicago, tells Inverse.

However, collecting this data wasn't easy. "We have a long list of fossils that we scanned but retrieved no more than a grey blob on our computer screens," Araújo says.

All in all, the researchers examined the semicircular canals of 341 animals, including 234 living species and 64 extinct ones. They also examined the relationships between the soft tissues and the bony structures within the inner ears of more than 50 living species to help infer the nature of the inner ears of extinct species, study co-author Romain David, a vertebrate paleontologist at London's Natural History Museum, tells Inverse.

"A lot of previous work on the topic had involved a lot of arguing back and forth about other, older proxies for body temperature that can give ambiguous, conflicting results," Angielczyk says. "Our method is a really new and different way to approach the problem."

WHAT DID THEY FIND? — The scientists discovered the ancestors of mammals did not develop the kinds of inner ear structures ideal for warm-bloodedness until 233 million years ago, during a time of climatic instability. In contrast, previous research suggested warm-bloodedness evolved in mammal ancestors about 252 million to 260 million years ago, Angielczyk says.

"The origin of endothermy in the mammalian lineage actually precedes the origin of mammals," Araújo says.

In addition, the evolution of mammal-like warm-bloodedness in these extinct species did not appear to happen slowly and gradually over tens of millions of years as previously thought. Instead, it apparently occurred rapidly, in less than a million years, reflecting a rise in body temperature of between 5 to 9 degrees C.

Size differences between inner ears (in grey) of warm-blooded mammaliamorphs (on the left) and cold-blooded, earlier synapsids (on the right). Inner ears are compared for animals of similar body sizes.

Credit: Romain David and Ricardo Araújo

The new findings suggest this warm-bloodedness apparently evolved about the same time proto-mammals, which are similar to mammals but don’t yet have a mammal’s defining characteristics, started to evolve whiskers and fur, key traits unique to mammals. This makes sense, because hair would trap the heat that a higher metabolism would generate, helping keep the body at the high temperatures needed to thrive.

The abrupt nature of this change might prove surprising, but there are older species preceding this shift that this new method suggests may have had body temperatures higher than most modern cold-blooded animals but lower than most modern mammals, Angielczyk says.

"So there may have been a little evolutionary experimentation with higher metabolic rates and higher body temperatures earlier on," he says, before mammal ancestors finally crossed over to significantly higher body temperatures. "The transition to endothermy likely was a little more complicated than just flipping an on-off switch."

WHAT'S NEXT? — Future research should "increase our sample of specimens even further, especially at the time and place on the family tree where we estimated endothermy evolved," Angielczyk says. "It would be nice to have a really detailed view of what was happening there — for example, to see if several different groups independently got over the 'endotherm line,' or if it only happened once."

Scientists may also apply this new technique on dinosaurs and other extinct reptiles, many of which recent work suggested might have been warm-blooded. Although mammals and their ancestors apparently changed the structure of their semicircular canals to handle warm-bloodedness, there is some evidence that dinosaurs and birds may have instead changed the chemical composition and viscosity of their endolymph when they became warm-blooded, Angielczyk says.

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