vivipare

Vivipare, or viviparity, is the reproductive mode in which the embryo develops inside the parent and is born as a live young, rather than hatching from an egg laid externally. The term is used across animal groups and is contrasted with oviparity, in which eggs are laid and hatch outside the body. In viviparous reproduction, the developing young may receive nourishment directly from the mother through a placental or placenta-like connection, or may rely on yolk with varying amounts of maternal input.

There are several forms. True placental viviparity, also called matrotrophy, involves sustained maternal provisioning of the embryo via a placental connection or equivalent structure. Ovoviviparity retains eggs inside the female until they hatch, with little or no ongoing maternal nourishment beyond the yolk. Some species display intermediate or mixed modes, such as lecithotrophic or matrotrophic development where nourishment is supplied during gestation by yolk or by maternal tissues.

Viviparity is widespread across animal groups but is most prominent in mammals. Beyond mammals, some reptiles (notably certain snakes and lizards) and several sharks and other fishes exhibit viviparity with placental or placental-like nourishment. The evolution of viviparity has occurred repeatedly and is influenced by ecological factors such as predation, climate, and parental investment strategies. While it can enhance offspring survival, it often reduces the number of offspring per reproductive event and increases energetic demands on the mother.

See also: reproduction, placentation, oviparity, lecithotrophy, matrotrophy.