Facultative polyandry is a mating pattern where in a female animal has more than one mate due to environmental pressures. For instance, female Saddle-Back Tamarind monkeys, which often give birth to twins, mate with two males. This is advantageous for them since their small body frame cannot carry two babies while feeding; hence, the monkeys’ mate sharing system allow the females to eat while the males carry their young.
Furthermore, a study on Tasmanian native hens found out that the male birds learned to overcome their instinct to fight with each other for the sake of the survival of their chicks which are more protected by tall shrubs and ready access to fresh water. Since there is a scarcity of good territory in their environment, it is more beneficial to share a mate and better defend their shelter for the progress of their species. Regarding humans, the same system is practiced by Tibetan tribespeople who find it more beneficial for a woman to have two husbands (usually brothers) to prevent family land from being split up.