Hibernating bats, fungal infections, warm-bodied mammals, the mass extinction of the dinosaurs – one controversial theory from 2005 connects them all.
White-nose syndrome, the disease now believed to have killed about a million North American bats, confounded scientists after it was first documented in 2006. In a sense it was merely a sort of athlete's foot: a fungal infection that attacked the skin. So how did it kill?
Researchers have found a clue in Arturo Casadevall's theory based on the ability of mammals' bodies to control, and elevate, their temperatures.
In 2005, Casadevall, chairman of the department of microbiology and immunology at Albert Einstein College of Medicine in New York, first proposed that, since many potential fungal attackers can't handle temperatures much higher than those of the environment, this ability gave mammals an advantage over other animals.
Researchers exploring the pathology of white-nose syndrome have found the idea helpful, because such a protection would disappear during hibernation, when bats' body temperatures drop.
Casadevall says he is following the white-nose research with interest. "It's very important because it provides indirect evidence that this theory had legs," Casadevall told LiveScience.
Casadevall extended this idea to speculate that, given evidence of an intense flurry of fungal growth around the time of the dinosaurs' extinction, the success of mammals after the extinction of the dinosaurs may have had a body heat connection. That suggestion has not been well received by paleontologists.
In the cold
Of the more than 1.5 million estimated species of fungus, only about 150 cause disease in mammals, and only a few of them are common pathogens, Casadevall wrote in a 2005 issue of the journal Fungal Genetics and Biology.
"This was one of the papers I read early on in the white-nose disease investigation, when a lot of people were making the argument that this can't be a fungal disease, because fungal diseases don't kill mammals like this," said David Blehert, a microbiologist with the U.S. Geological Survey's National Wildlife Health Center who studies the disease.
The key for the fatalities among bats, it turns out, is hibernation. Hibernation allows animals to survive lean times by slowing their metabolism dramatically and becoming inactive. However, their immune defenses also weaken. (One study found that hibernating bats with infected wings showed no signs of inflammation, an immune response expected to occur in injured tissues.)
In the lab, the white-nose fungus, which inhabits soil, grows in temperatures from 34 to 59 degrees Fahrenheit. That is well below the body temperature of a bat — when it's not hibernating.
"All of a sudden this cold-loving fungus can get into the tissues of bats," said Paul Cryan, a bat ecologist with the USGS.
An expensive strategy
Mammals' ability to regulate body temperature is somewhat mysterious, simply because it requires so much food to maintain.
"We live in an era when people are worried about being fat, but most of evolution you had to go out and look for 2,000 calories in a prairie or something like that," Casadevall said. "It's a lot of work."
So there is a trade-off. He and Aviv Bergman, also from Albert Einstein College of Medicine, calculated that a body temperature of around 98.1 F provided the best balance between metabolic demands and fungal resistance. This ideal closely approximates mammalian body temperatures. Nonhibernating bats have a body temperature that ranges from 95 to 102 F.
In another study, involving 4,082 fungal strains, Casadevall and another collaborator found that each 1 degree Celsius (1.8 F) increase in body temperature in a certain range excluded 6 percent of the strains from growing.
Looking further back
Through the lens of this theory, Casadevall looks back 65 million years, to a time before bats appear to have even been present in North America, when the world was run by dinosaurs.
Small mammals also were around, and had been for millions of years, but "it was an experiment that wasn't going anywhere," Casadevall said.
Then came a mass extinction of the dinosaurs, most likely prompted by an asteroid slamming into the Gulf of Mexico, and in time, mammals came to replace them.
Fossils found in New Zealand show that the conifers and flowering plants that had dominated the landscape suddenly disappeared at the end of the Cretaceous Period, and they were followed by a layer of fungal spores and threads coinciding with evidence for the asteroid impact.
In the Fungal Genetics and Biology article, Casadevall writes that in the dark and decaying world at the time, fungi would have flourished, their spores filling the air. This would have created an environment where warm-blooded creatures had an advantage.
"Perhaps fungi provided the filter that allowed the mammals to become dominant life forms," he said.
A controversial theory
However, several paleontologists were, at best, skeptical of extending it to explain mammals' success since the end-Cretaceous and the extinction of dinosaurs and many other living things.
"It was a bad day for everybody," said Kirk Johnson, chief curator and vice president for research and collections at the Denver Museum of Nature and Science.
The fossil record in North America for life before the extinction is limited to basins in the Rocky Mountain West, primarily in eastern Montana, and it shows that while about half of the mammal species disappeared with the dinosaurs, some cold-blooded creatures, like amphibians and snakes, fared much better than their mammal contemporaries. Evidence of fungal proliferation is limited to New Zealand, where there is no sign of mammals, Johnson said.
Even highly virulent infectious disease does not cause extinctions – because as population density decreases, so does transmission, and the remaining individuals are more resistant. In addition, at the end of the Cretaceous, dinosaurs weren't the only ones to be decimated. Marine animals were affected, as were many species of flowering plant, according to Douglas Robertson of the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.
"It is not even vaguely plausible that all these extinctions, let alone just the various dinosaur species extinctions, were all caused by some pathogen," Robertson wrote in an e-mail.
Casadevall dismissed these and other criticisms, saying that reptiles suffered heavier losses than mammals, and that amphibian extinctions attributed to the chytrid fungus have invalidated the argument that infectious disease cannot wipe out species.
"What remains unexplained is why reptiles did not recapture the world to launch an 'Age of Reptiles II,'" he wrote in an e-mail, adding: "There is the fungal selection hypothesis."
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