Pathogens Led to Rise of Warm-blooded Animals

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The ability to mount a rapid fever response to invading pathogens may explain the emergence of warm-blooded animals six hundred thousand years ago, suggests a new study.

At the time, nearly all animals were cold-blooded and unable to regulate their own body temperature internally.

Being cold-blooded has numerous advantages. Warm-blooded animals—which include mammals and birds—can maintain a body temperature than their environment, but use a lot of calories doing so. On the other hand, cold-blooded animals, such as reptiles and amphibians, just have to find a warm spot to increase their internal temperature—a process that uses 30 times less energy than warm-blooded creates of a similar size.

And yet, despite the extra energy required, some cold-blooded animals evolved into warm-blooded creatures.

Researchers have proposed numerous theories as to why this occurred—that it aids physiological processes or helps animals maintain activity over long periods of time or even that it helps parents take better care of their offspring—but none have found strong support among the scientific community.

"One of the 'dirty secrets' of modern  evolutionary biology is that scientists still don't agree on how or why warm-bloodedness first evolved in the ancestors of mammals and birds. It is such an important, distinct trait that humans share with our closely related animal kin, and yet we still don't really know how natural selection could have favored such an energetically costly strategy to begin with. In other words, given the energetic savings that cold-bloodedness affords, why aren't we all still cold-blooded?" Michael Logan, author of the study and a Tupper Fellow at the Smithsonian Tropical Research Institute in Panama, told Laboratory Equipment.

Pathogens may be the reason warm-blooded animals first emerged, new research published in Ideas in Ecology and Evolution says.

"Warm-blooded animals can ramp their body temperatures up to fever-temperatures quickly, and unlike their cold-blooded ancestors, they can do it in almost any environmental circumstance. This would seem to give warm-blooded animals, relative to cold-blooded creates, a huge advantage in the fight against pathogens," Logan said.

Fever serve as a key weapon in fighting infection, giving warm-blooded an evolutionary advantage over other animals, even though it requires a higher energy expenditure. The higher body temperatures of warm-blooded animals serve to optimize the immune system to withstand infection, helping more animals survive and reproduce.

Cold-blooded creatures have to sit in warm places for extended periods of time in order to achieve fever-like temperatures, which can be severely limited by variations in surrounding temperatures and environmental conditions and can even expose the animal to predators.

"I want to emphasize that 'warm-bloodeness' is a complex trait that has taken many forms in the millions of species of mammals and birds that have existed over the last hundred plus million years. Like any complex trait, it was likely shaped by multiple agents of natural selection. I am not claiming that pathogens represent the only possible source of selection favoring the rise of endothermy, only that they may represent one of those agents, and possibly one of the more important ones," Logan said.