Hello Makarand; The resting temperature set point among endothermic animals actually does vary. Marine mammals, marsupials, monotremes and birds all have different set points. The ecology surrounding these differences is delightfully complex. Look on Google Scholar and you will find an avalanche of papers. Best regards, Jim Des Lauriers

Citation: "an organism’s fitness as a function of body temperature... Fitness represents the balance between the benefit of fighting of fungal pathogens and the energy costs of maintaining a higher body temperature."

A. Bergman and A. Casadevall. Mammalian endothermy optimally restricts fungi and metabolic costs. mBio, Vol. 1:e00212-10. doi: 10.1128/mBio.00212-10

in fact mammals have body temperature from +30 to +39.4. C. Ladd Prosser, ed. Environmental and Metabolic Animal Physiology, 4th ed. Wiley-Liss, 1991

Hello; Here is another twist to the issue. Male Eutherians that have body temperatures above about 35 C have a scrotum...at least seasonally. Sperm aren't produced at higher temperatures. So, to adds to what Alex has said there must be compelling selection for those higher body temperatures. Carrying your testes outside the body wall makes for some unpleasant experiences! Best regards, Jim Des Lauriers

OK Thanks.

I was wondering if there was an answer wrt Thermodynamics involved in Human or mammal respiration in general.

Makarand Phadke There are many factors involved in the choice of optimal temperatute beside the cited above. Environment plays some role too. Most enzymes have a limited range of temperatures for optimal activity. As a result, mammals lower their body temperature during hibernation and reptiles and amphibians reduce their activity at low ambient temperatures.

It is clear that a stable high body temperature increases an organism's fitness in different locations and seasons. The optimal temperature (30-40 degrees Celsius) was probably close to the average environmental temperature at the time when these organisms with thermostable bodies evolved.