Re: Why do some baboons have swollen butts?

The condition you describe is typical of females in estrus, or "in heat" as 
you put it. Most female primates have some type of visual clue that 
indicates when they are sexually receptive and about to ovulate. The 
notable exception to this pattern is the human female. It is one of the 
great mysteries of anthropological science to explain 
the human's lack of a visible estrus display in an evolutionary context.

For reference read:
The Evolution of Primate Behavior. by Alison Jolly 1985 Macmillan 
Publishing Co.: New York