Why 98.6 Degrees? Our Body Temperature Strikes a Perfect Balance

By Andrew Moseman | December 28, 2010 3:42 pm

Standard human body temperature is 98.6 degrees Fahrenheit: It’s one of those numbers from grade school science textbooks—like 65 million years since the dinosaur extinction or nine eight planets in the solar system—that just gets stuck in your head. But why should it be that balmy temperature and no other?

According to a study by researchers at Albert Einstein College of Medicine, the 98-degree range is in perfect balance.

Every one degree Celsius rise in body temperature wards off about 6 percent more fungal species. So tens of thousands of fungi can infect reptiles and amphibians, but we can only be invaded by a few hundred fungi. In the new work, the researchers created a mathematical model that weighed the fungal protection benefits versus the metabolic cost of high body temperature. And the optimal temperature was 98.1, quite close to what evolution figured out. [Scientific American]

The reason for mammals’ hot body temperatures had been an open question, the scientists say. According to study coauthor Aviv Bergman and colleagues, while most mammals keep an inside temperature of about 98 Fahrenheit [37 Celsius], most other animals have considerably lower body temperatures. As a result, the researchers say, mammals must spend more time eating to bring in energy. However, there may be thousands of species of fungus that can infect amphibians, for instance, but can’t tolerate the hot temperatures inside our bodies.

“This study is a good example of how mammalian evolution has been driven by both external biological factors and internal physiological constraints,” said Dr. Bergman. [Press release]

Their study appears in the journal mBio.

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CATEGORIZED UNDER: Health & Medicine
MORE ABOUT: fungus, metabolism
  • http://reallymagazine.com Martin g

    On the other hand (again from schooldays), a ten degree (C) rise in temperature roughly doubles the rate of many chemical reactions – and thus it speeds up bacterial reproduction no end. But the word ‘bacteria‘ is mysteriously missing from the press-release. The fungal-growth versus temperature standpoint is interesting – but what of the bacterial elephant in the room? Together they’re perhaps our greatest predators, and most of them just love a bit of warmth.

  • http://biologyfiles.wordpress.com/ Emily Willingham, PhD

    If this were true, one might expect that women (and other mammals) in the proliferative phase of their cycles might be more susceptible to fungal infections (as they run at about 97 F or so basally), while less so in the luteal phase, when they are warmer. Yet, apparently, women are most likely to develop these infx at the end of the luteal phase: http://health.howstuffworks.com/wellness/women/yeast-infection/the-basics-of-yeast-infection.htm. Sugar is allegedly to blame.

    Also curious about how very young children–who have higher body temps in general–are more susceptible to fungal infections.

    Finally, the above question is a good one. Given the microbial wars going on, wouldn’t defeating the fungi make room–nice warm room–for all those bacteria trying to kill us?

    Reptiles appear to have had similar body temps at least at some point (e.g., these possibly endothermic marine reptiles http://www.sciencemag.org/content/328/5984/1379.abstract) and certainly warm up to those temps in many of their habitats (iguanids can sit at 110 F), so…I’m just thinking of so many counter-examples to this hypothesis. If heat were an adaptive defense against fungi, it seems like equatorial ectotherms would be especially resistant (?). There also are, of course, relatively large fluctuations in ectotherm temperatures, but the idea that heat would be worse for fungi than cold seems counterintuitive, unless there is also a cold-stress-related immunosuppression at work. Again, not something that would be applicable in tropical reptiles or amphibians.

    How susceptible are crocodilians to fungal infections? Slow metabolism, low body temps, watery habitat–seems like they’d be overrun…

  • Phil

    We need the bacteria because they help us digest our food. And if we catch a bad one, or a virus we get a fever.

  • Larry

    Ah…bacteria…the human body has a sybiotic relationship with many bacterium strains, which will destroy invading bacteria, all the while providing other services, as in digestion. The body benefits, and provides a home for the bacteria.

  • Arun

    The body is a delicate mechanism. It is endowed with natural intelligence to maintain optimum balance. It is capable of factoring in all the mulititude of symbiotic dynamics that exist among innumerable living organisms in every cell and space in the body. So, fever is naturally a balancing act to deal with changes in the internal and external factors. Unfortunately, the antibiotics and fever reducing chemicals that are prescibed and consumed wreak havoc on this delicate internal eco-system. Hopefully, researchers, scientists, medical practitioners and patients learn to tamper with the system in a not too obtrusive way.

  • Jim from Queens

    Somehow I question this rationale. Do the great apes share our temp? It also leds me to question what IS the minimum regular body temperture possible if this bateria/fungi factor were thrown out? Could we biochem minimize/rachet down body temp to a “safe normal” low for surgery or oxygen conservation for space flight or a better remedy for hypothermia?

  • http://biologyfiles.wordpress.com/ Emily Willingham, PhD

    Posted a comment yesterday but it included links to papers, so maybe didn’t get through?

    [Moderator’s note: That prior comment was stuck in the spam filter because it had too many links, but I rescued it.]

    Several things strike me as not a fit for this hypothesis. Examples: Women in the luteal phase have higher body temps than in the proliferative phase but are more susceptible to fungal overgrowth in the luteal phase. Some reptiles run very hot–e.g., some iguanids–and ancient reptiles may have done so, as well, possibly even as endotherms. If this tradeoff was a good one, why did it not persist? Another question that comes to mind concerns crocodilians, with their very slow metabolisms and relatively lower temps. They don’t seem to be particularly susceptible to fungal overgrowths. And then there is the bacteria/fungi question and the microbiota balance between the two. Finally, children run very hot comparatively speaking…yet can have greater susceptibility to fungal overgrowth. So…while this is an interesting possibility for exploration, there are a number of counter examples that come quickly to mind.

  • http://ppppana.tumblr.com Pana

    I don’t quite understand then why is that when we get sick our body response/defense is to get a fever… Oh OK, to sweat off the virus right? Well… wouldn’t that be a bad idea?
    I’m a little confused.

    I read here (I think) that we humans are a complex evolved virus.

  • MT-LA

    Emily, I’d like to explore some of those questions, but I have to say right off that bat that I know NOTHING about the answers. But if you care to engage:
    1) The higher temperature in children and women in the second half of their cycle seem to be the same issue, so A) Can we discuss both as the same issue? B) Is this population more susceptible to ALL fungal infections, or only a select few that seem to fare better under the higher-body-temp conditions?

    2) Do the select reptiles that you mention live in habitats with better access to food than their lower-temp counterparts? As mentioned in the article, higher body temps (and therefore higher metabolism) will need more fuel to keep running. Perhaps the species with higher body temps have better access to food resources, thus easing evolutionary pressure to keep their temps low. Easier food sources would explain why this trade-off wasn’t available to all species, but it’s just a guess.

  • http://biologyfiles.wordpress.com/ Emily Willingham, PhD

    1. Let’s assume an acceptance of the fungus-temp-metabolism hypothesis. It may simply be that the fungi that have co-evolved with us evade the temperature defense. That simple. But…these two examples do stand out as countering the overall idea. I’m not saying that they alone stand as counterpoints–they just came immediately to mind as standing outside the fungus-high-temp defense concept.

    2. The desert iguanids likely do not reside in habitats with greater food abundance than, say, reptiles in a rainforest. So for them, that doesn’t work. And that still leaves the crocodilian question.

  • Jeremy Berryman

    Why is it that the U.S. and UK standard body temperature is different? 98.4 in the UK. Is it because of the easier conversion to Centigrade? Or do American’s just run a bit hotter than the British?

  • gurpreet

    helo, according my view body temp is mainly related with enzymatic activities or chemical reaction, on the hand related with fungal growth they changes with evolution because if they are sensitive to that temperature they produce new strain in this long time of evolution.

  • Rajendra Raj Sharma

    If the body temperature were other than 98.6, we still have wondered why that value.

  • Glen

    This makes me wonder if inhaling hot air, but not too hot, could be used as a treatment for people with fungal infections of the lung.


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