Monkeys reproduce by engaging in a sexual act between a mature male and female. Most species are able to reproduce once they reach the age of 4 years old. It is quite common for male monkeys to fight in order to become the alpha or leader of the group, since that accords them the right to mate with the females in the group.
Monkeys do not have a definite mating season and can be seen mating at any time of the year provided the conditions are right. Most mate if there is a good supply of food and the environment seems safe for rearing young ones. Once mating has taken place, the gestation period lasts between 156 to 160 days.