The muriqui monkeys of the rainforests of Brazil are known for their peaceful and egalitarian nature, a nature which has earned them a reputation as "hippy" monkeys. But then primatologists observed how six young muriqui monkeys ganged up on a lone old monkey and started to savagely bite his face, body and genitals, killing him. The reason for this violent attack is believed to be pent-up sexual frustration among the male muriqui.
The muriqui of the north feed on leaves. Leaves are ubiquitous in rainforests and so the group can stay together in one place. When there's time for mating the males patiently take turns knowing the females aren't going anywhere. In the south, however, where the attack took place, the muriqui instead eat fruit. Since fruit is not as readily available as leaves, females sometimes leave the group in search for the fruit. This leaves the males without females to mate with for long periods of time. Periods of time during which they build up their sexual frustration.
The male muriqui tend to form strong bonds with their siblings and relatives; they form the animal-equivalent of gangs. Sex-crazed aggression then causes these gangs to take out their frustrations on weaker individuals.
Maybe they should consider going gay.