Hyenas, mongooses, and chimpanzees wage group warfare upon each other, ostensibly for territory, though the violence that individuals inflict often far exceeds necessity to that purpose. Defeated victims are often killed or maimed rather than allowed to escape without serious injury. Herbivorous mammals are not known for such cruelty.
In an instance of evolutionary irony, gratuitous violence is especially common among certain gregarious mammals. Chimpanzees and humans are notable for their violent tendencies, including ganging up on helpless victims. Their proclivity to violence is as much a cultural characteristic as biological. Pygmy chimpanzees – bonobos – are close relatives who do not readily engage in violence.
Sexual violence among species is not bound by territorial desire, as it is driven by an altogether different reward system. Forcible rape has been observed in birds, seals, dolphins, coatimundis, bighorn sheep, wild horses, and orangutans, as well as being unexceptional among men.
Cruelty is often conspecific: violence against social competitors. Scapegoating – singling out a victim for group aggression – has been seen in various social mammals, especially captive animals.
In the wild, an ostracized animal may leave a group; but not always. A wolf might not only be ousted from its pack, but also viciously attacked by other wolves.
Jealousy has been observed among various social animals, including parrots and orcas. Wild female chimpanzees are known to furtively leave their group to copulate with low-ranking males, to avoid jealous violence from males with higher social status.