Animal life is often violent, starting with getting enough to eat. There are exceptions. Nectar and fruit eaters live off the bounty of plants without harming their food source. Their consumption benefits plants by facilitating reproduction and seed dispersal. Herbivores, while damaging plants, tend not to chew their meal ticket out of existence.
For meat eaters, sustenance necessitates killing. Success may spark celebration. Many predators toy with their victims. Orca are well known for flipping their next meal about before killing it. Cats with a full belly torment mice with no intention of eating them. Foxes sometimes fiddle with their food before eating it. To a natural predator, violence is a form of play.
Surplus killing is common among predatory species. A fox might kill a flock of geese before making off with 1. Hyenas attack a herd of gazelles at night and kill dozens; far more than the pack can eat.
Predators who kill beyond need sometimes store food to eat later. Foxes and weasels cache part of their catch. But surplus killing is often gratuitous.
In many animals, from spider mites to men, males are more aggressive than females. These behaviors are regulated by pheromones.
Much of male aggression owes to mating and attendant territoriality, as males compete for females. Male chimpanzees and men raid and kill members of other groups for territory and for fertile females.
Community living itself can be a source of male aggression in species with dominance hierarchies. Violence can sweep a broad swath, including against prospective sex partners, offspring, and to maintain cooperative behavior by intimidation.
For females in communities with mixed-sex social groups, those that form strong social bonds live longer and have higher offspring survival rates. When gregariousness matters, a fit mother is an adroit socializer.
In mammals where mothers are primary caregivers, females tend to compete with each other, sometimes more than males. This is especially true in cooperative breeding, where a single female monopolizes reproduction. Where males contribute to offspring, or in precocial species, competition plays less of a role in female existence.
Female aggression tends to be more limited than in males. Females risk reproductive success from physical disability. For males, dominance may be the only route to procreation, whatever the cost.
Besides the so-called ‘civilized’ primates called people, several animal species wage war. The most disciplined large-scale warriors are ants.