Most viperids have an annual or biennial breeding cycle. A biennial cycle normally is encountered in females in temperate regions, as in Europe, but in some species (e.g., Crotalus hor-ridus) females may reproduce much less frequently. Males always have annual reproduction, which means that reproductive males always outnumber reproductive females in the local population. In Vipera berus the production of sperm takes place during the fall and the following spring, whereas in V. aspis ripe sperm is ready in late fall. In the former species mating is a spring event, triggered by the first molt for the season in males; in the latter species mating can take place during both fall and spring, even if spring mating is most common. In this case the spring molt is not involved in the start of mating activities.
In some genera, such as Vipera, Bitis, Agkistrodon, and Cro-talus, males engage in ritualized fights that once were interpreted falsely as mating ceremonies. They lift their heads and approach each other in this vertical position, wrap their bodies around each other, and push with their fore bodies, head to head. In Vipera berus a male hierarchy becomes established, and the dominant male is the first to copulate with a female. By activating a sphincter muscle in the female genital tract, a copulatory plug effect is created temporarily. This lessens the possibility that additional males will fertilize the eggs. In some tropical genera a more lengthy breeding season is reported.
Most viperids are ovoviviparous, bearing live young that hatch from membranous eggs. Pregnancy time, meaning the time from ovulation to birth, is normally about 2.5 months for European vipers. Some viperids (e.g., Lachesis and Deinagk-istrodon) lay eggs. The night adders (Causinae) of Africa and the rare Fea's viper (Azemiopinae) of Asia also lay eggs. Incubation has not been noted in any viperine species, nor has any kind of parental care, but egg or neonate attendance has been documented in various pitviper species.
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