Some female animals go into "heat" (not sure how often) which is basically them ovulating. It also makes them incredibly lustfull.

This is all layman's terms. Any vets want to chime in?

EDIT: Thanks everybody! Glad it was all cleared up! Also, changed a word. 

EDIT 2: my highest comment is now about bitches in heat. Thanks reddit! 

EDIT 3: as /u/myth1n pointed out,  i can't spell.