Why 98.6º F

The constantly high temperatures maintained by mammals may be a way to fight fungal infections

Over the past few years, fears of the mass extinction of many species of frogs have proved alarmingly realistic. A fungal pathogen—chytridiomycosis—is responsible. In a 2007 paper, Australian researcher Lee Berger and colleagues noted, ‘The impact of chytridiomycosis on frogs is the most spectacular loss of vertebrate biodiversity due to disease in recorded history.’

Fungal infections are deadly for cold-blooded animals. Of the 1.5 million fungal species, as many as 50,000 infect insects, but only a few hundred infect people. The difference lies in human blood. According to a Yeshiva University release, Arturo Casadevall, doctor and professor of microbiology & immunology ‘investigated how 4,082 different fungal strains… grew in temperatures ranging from chilly—4º C or 39º F— to desert hot—45º C or 113º F. They found that nearly all of them grew well in temperatures up to 30º C. Beyond that, though, the number of successful species declined by 6 per cent for every one degree centigrade increase’.

“Our study makes the argument that our warm temperatures may have evolved to protect us against fungal diseases,” says Casadevall. But warm temperatures come at a cost. Reptiles can survive on one meal for several days, humans cannot. According to the release, ‘Casadevall and his team devised a mathematical model that analysed the benefits gained by body temperatures that protect against fungi versus the costs (in terms of extra food consumption) required to maintain body temperatures between 30º and 40º C. The optimal temperature for maximising benefits while minimising costs was found to be 36.7º C, which closely approximates normal body temperature.’

Says Casadevall, summarising the result of a recently published paper in mBio, “One of the mysteries about humans and other advanced mammals has been why they are so hot compared with other animals. This study helps to explain why mammalian temperatures are all around 37º C.” And why we need to eat three meals a day.

body temperaturefungal infectionsmammal