Fetal gender and aggression in pregnant stumptail monkeys (Macaca arctoides).

Agonistic behaviors of pregnant female stumptail monkeys, members of a large, well-integrated group continuously living together, were studied during 480 observation hours (over 32 months). A total of 28 pregnancies of 20 females occurred, resulting in 12 male and 16 female live infants. The main finding was a significant decline in aggression, both received and performed, during the course of pregnancy. There was no clear unequivocal effect of fetal gender on aggressive behavior of the mothers-to-be.