  The thermoregulation system in animals removes body heat in hot temperatures
and retains body heat in cold temperatures. The better the animal removes heat,
the worse the animal retains heat and visa versa. It is the balance between
these two conflicting goals that determines the mammal's size, heart rate and
amount of hair. The rat's loss of tail hair and human's loss of its body hair
are responses to these conflicting thermoregulation needs as these animals
evolved to larger size over time.
