Most "warm blooded" animals maintain their internal body temperature in a specific temperature ranging from 35 to 40oC. Humans keep it at 37. Camels and elephants at 36. Cows and cats at 38. Rabbits at 39 and Chickens at almost 42.
This heating is performed at the expense of much energy. For example, when air temperature is around 20oC, mammals spend 30 times more energy than reptiles to support their metabolic rate.
There are many reasons for maintaining the body temperature constant and elevated (Life Ascending - chapter 8), but why not having a wide range of bodies temperatures? For example, to live in a temperate condition of around 20oC, keeping the body at 20-25 would be very appropriate and economic. In the tropics conditions of around 30oC, 30-35 sounds good.
But why 35-40? Why 37? The answer may be in the water specific heat. To
Change the the temperature of 1g of water 1oC, from 15oC to 16oC, it is necessary 1 calorie (4.2 joules). But why from 15 to 16? Because this amount of energy varies according to the temperature. From 16 to 17 it is necessary less than 1 calorie and this amount reduces until 35oC. From 35 on the amount of energy required restarts increasing. This means that, around 35oC, the amount of energy to heat up the body (mostly water) is smaller than at any other temperature. This may be a small amount, but adding all the little bits in a life time may pay off (Survival of the Fittest)