Pair formation occurs in most species, although the duration of the pair bond is extremely variable. Species such as many birds of prey, large parrots, geese, swans, geese and swift foxes mate for life. Others maintain the pair bond only so long as necessary to copulate or to raise the young (weasels, most small migratory songbirds). Most amphibians, reptiles, and fish do not form established pairs at all but only come together for copulation or fertilization of the eggs.

Pair of parrots Calary Zoo

Harems are common strategies for many mammals as is transient pair formation for mating only (bears, most cats, bats, lagomorphs, mink). Some species have pairs or harems that last only the season (elk, fur seals, and elephant seals) others hold permanent harems (sperm whales, horses, and pronghorns). During the nonbreeding season many species form herds that are all male or all female with young (bighorn sheep, walrus, and elk).

Elk buck with several does in his harem, Banff National Park.