[JennyGracie (OP)]

Dan wants to know,

How does animal hair grow?  My cat got a spot shaved and it grew back, naturally.  But I don’t understand how it knew to grow there and when to stop growing.  Same with baboons and other primates, immaculate hair, probably no baboon barbers.

Sadly no jungle hairdressers, but do you know the answer?

[evan_au]

Mammalian hair has a growth cycle, where it grow for a while, then falls out, rests for a while and then starts growing again.

It is the duration of the growth phase (anagen phase) that determines the length of the hair.

The duration of  each of these phases is determined genetically, and can differ on different parts of the body (eg whiskers are longer than fur), and in different seasons (eg some animals have longer fur in winter).
 
See: https://en.wikipedia.org/wiki/Human_hair_growth#Growth_cycle