Just Warm Enough

Mammals' body temperatures may represent balance between warding off fungi and limiting food needs.

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By Tina Hesman Saey, Science News

Fungi may be to thank for mammals’ warm blood, a new theory suggests. But exactly how hot-blooded an animal is may depend on balancing fungal protection with food consumption.

The optimum body temperature for organisms to ward off fungal infections without burning too much energy is 36.7° Celsius—close to the core body temperatures of mammals, including humans, researchers at Albert Einstein College of Medicine in New York City reported online November 9 in mBio. The finding is the latest piece of evidence for a theory that fungi may have been a driving force in the evolution of mammalian body temperatures. The new mathematical analysis also helps explain why mammals aren’t even hotter.

“Mammals don’t make any sense,” says Arturo Casadevall, a microbiologist at Einstein who devised the theory. “We have to eat all the time. Our reproduction rate is low.” In fact, until catastrophic events caused the extinction of the dinosaurs, “mammals were an experiment that wasn’t going anywhere,” he says.

Casadevall wondered why reptiles didn’t retake control of the Earth once environmental conditions had stabilized again.

A couple of pieces of evidence led him to develop the new theory. First, a massive fungal bloom swept the Earth about the time of the dinosaur extinction. “The world became a huge a compost pile,” he says.

Second, fungi plague plants, insects and other cold-blooded creatures far more often than they do mammals or birds. Putting two and two together, he formulated a theory that the warm body temperatures of mammals and birds might have protected them from fungal pathogens, while diseases caused by fungi might have been a factor keeping the reptiles from rising again.

“We are cautiously suggesting that fungi may have been responsible for the success of the mammals,” he says.

To test the theory, Casadevall and Vincent Robert of the CBS Fungal Biodiversity Center in Utrecht, the Netherlands, measured the thermal tolerance of 4,802 types of fungi. For every degree Celsius the researchers raised the temperature above 30° C, 6 percent fewer fungal species could grow, the team reported last year in the Journal of Infectious Diseases.

Most mammals have body temperatures of about 37° C (98.6 ° Fahrenheit). But if higher temperatures ward off more fungi, why don’t mammals run even hotter?

In the new paper, Casadevall and coauthor Aviv Bergman, an evolutionary systems biologist also at Einstein, attempted to answer the question with a mathematical model. Mammalian body temperature is a trade-off between fighting fungi and burning too much fuel, they found.

“If you were to go higher, you’d have more protection, but then you’d have to eat a lot more,” Casadevall says.

Their model doesn’t address all the biological questions related to mammalian body temperature, says Bergman, but it does suggest that threats from fungi could impose constraints on some aspects of mammalian evolution.

“I think it’s a really cool idea,” says Leah Cowen, a medical mycologist at the University of Toronto. What’s striking about the new study is that the model is simple, “but the vision is large,” potentially answering a huge question in evolutionary biology. “This is a big picture question addressed by a simple mathematical model.”

The real mark of a good model is whether it can make predictions, says Joseph Heitman, a microbiologist and geneticist at Duke University. This model is “really creative and a bit out there,” he says, but “one of the beauties of it is that it is fairly straightforward.”

One might predict from this model that raising an animal’s temperature would lead to greater resistance to fungi. Lowering body temperature would then be expected to make animals more vulnerable to fungal infections.

Frogs and other amphibians in decline around the world—in part because of infections with a chytrid fungus—may provide some evidence that the theory is correct. Warming up infected frogs can help clear them of the fungus, Heitman says.