Field of Science

Why is body temperature 36 degrees celsius?

While I was doing an unrelated search on the Nature website, I came across this intriguing debate about why body temperature is maintained around 36 degrees and not some other value. (Nature, Vol 324, December 4, 1986, p. 418)

The discussion was initiated by a letter from John Paul, a scientist in Australia who contended that the specific heat capacity of water is lowest at 36 degrees, and therefore heat loss would be minimal at that temperature

But he neglected a fundamental principle of physical chemistry; the rate of heat loss is proportional to the difference between the temperature of the body and that of the surroundings and is independent of the specific heat capacity (remember high school and Newton's Law of Cooling?). More importantly, the specific heat capacity of a body can be thought of as a measure of how well the body offers "resistance" to fluctuations in temperature. The reason why water works so well as an essential life fluid for example is because its specific heat is so high; there is minimal fluctuation in the temperature of water when heat is injected or taken away from it.

Thus, an optimal substance for maintaining a given temperature would be one whose specific heat capacity is as high as possible under the given circumstances, not one whose specific heat capacity is minimum at the given temperature.

These facts were pointed out by William Calder from the University of Arizona and by Steven Benner and Jack Dunitz at the ETH, Zurich. Dunitz as is known is an extremely versatile scientist, a veteran researcher and one of the greatest structural chemists and technical writers of the last century.

Dunitz and Benner make their objections to Paul's explanation clear and offer an alternative partial explanation; that 36 degrees is the optimum compromise between viscosity and hydrophobicity. It's high enough for the viscosity to not become so low as to impede diffusion-limited processes, and low enough that hydrophobic molecules do not "dissolve" too easily.

Natural selection must have taken a remarkable number of factors into account in optimizing this property.


  1. This argument makes it sound that the human body "choose" 36. But isn't it also reasonable to consider that the human body adapted to survive at the most easily maintained temperature (which happens to apparently be 36).

    I guess this is a chicken vs. egg argument.

    This also raises a question that I have never thought of before, what is the normal body temperatures for other mammals? Is it also 36°C? I've never thought of that before.

  2. High Specific heat of water originates from its hydrogen bonding I guess..So most of the energy given to the system is stored as Potential energy instead of increasing the kinetic energy (Temp..)

  3. I think this is a fascinating question and it does resemble the chicken and egg problem. I am reminded of James Lovelock's theory of Gaia where he contends that not only do organisms adapt to the planet but the planet also adapts to organisms. I don't know why body temperature is 36 degrees, but it seems that there could have probably been a tug of war between optimizing enzyme and other processes and optimizing metabolic activity including respiration. In some ways evolution must have experimented with different temperatures and selected for that which maximized reproductive success. I wonder if this is also one of the best temperatures for reproduction itself, although eggs do thrive at higher temperatures. As for other mammals, it's an interesting fact worth finding out...I also don't know if most mammals sustain body temp around 36 degrees.

  4. Quick Google search:

    Animal Normal Temperature °C
    Cattle 38.5
    Calf 39.5
    Buffalo 38.2
    Goat 39.5
    Sheep 39.0
    Camel 34.5-41.0
    Llama, alpaca 38.0
    Horse 38.0
    Donkey 38.2
    Pig 39.0
    Chicken 42.0
    Piglet 39.8

  5. Before anyone starts doing any actual calculations, I should point out that it's 36.8 Celsius, which is closer to 37, right?

  6. True. Rounding off is a lost art.

  7. Is it possible that animals with less brain activity have higher temperatures? Do active brains need more cooling, for example?

  8. simply body temperature depends more on individual's metabolism rate, that's why smaller animals have higher body temp.


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