1. among nonhuman animals, a mating system in which a female mates with more than one male but a male mates with only one female. The female mates with and forms a social relationship with multiple males during one reproductive cycle. Cooperative-breeding tamarins are thought to exhibit facultative polyandry, in which a female mates with multiple males who share in care of young when a new social group is formed, but once the group is established and has many helpers, the same female will become monogamous with one of the males.

2. marriage of a woman to more than one husband at the same time, which is an accepted custom in certain cultures. Compare monogamy; polygamy; polygynandry; polygyny. —polyandrous adj.