How did endothermy evolve in mammals and birds?  What's the mechanism that keeps our body temperature constant? Someone told me that the human body temperature is lower then it is in most mammals by a few degrees. Is this true?

Endothermic homeostasis is mainly a result of high resting heat production rate and insulation to retard heat loss. Basal metabolic rate in kcal (or kJ) per day seems to be a good indicator of thermoregulatory type - that is endothermic animals typically use up a lot more energy per day (and thus generally produce more heat) than ectotherms.

Body temperature varies greatly in endothermic animals - typical placental mammals at 36-39 degrees C, monotremes at 30-32, marsupials at 35, and birds at 37-42 degrees C. On average, human body temperature is at 37 degrees C so it is within norm for a placental animal although there are many mammals with higher body temperatures.

The exact evolutionary sequence or origin of endothermy is still unknown and highly debated...

Hi Susan, how something evolves is a difficult question to answer, because we can't really test an evolutionary pathway directly.

Endothermy is a name given to the observable outcome of various processes that have multiple physiological controls. Some animals have evolved simple thermoregulatory controls, such as tuna which have a mechanism called the "counter-current heat exchanger", which prevents heat built-up from muscle activity from being lost into the surrounding water when blood passes through the gills. This system simply allows the warmth from the deoxygenated blood that has passed through the muscles to be transferred to oxygenated blood before it passes into the gills that are cooled by water flow. This system allows the tuna to operate at temperatures upto 20'C warmer than the surrounding water, when most other fish are only about 2'C warmer.

This simple way of reducing heat loss (through skin rather than gills) is also used by mammals, although only as part of their thermoregulation. Mammals and birds have additional methods of maintaining their internal temperature, such as having a big liver, which produces large amounts of heat through the chemical processes that happen there. Of course this requires lots of food. To reduce the amount of food needed, heat loss needs to be minimised, so the skin has insulation (fat, fur, feathers). Getting bigger also helps maintain heat.

Losing heat then becomes a problem, usually solved by allowing water to evaporate from the body (sweating or panting), by radiating heat (big ears, bare legs) or by behavioural methods (wallowing, resting in shade).

Most of the individual mechanisms involved in thermoregulation are probably quite simply evolved (at least in vertebrates), but only having a whole suite of these mechanisms results in true endothermy.

It is important to note that endothermy is only an advantage if there is plenty of food available and if an organism needs to function in cold conditions or needs to maintain a high level of activity.