Although most species of colubrid snakes are oviparous, a large number of viviparous species are also known. Eggs may be laid in a hole or burrow in the ground or within rotting vegetation, where the heat generated by decomposition presumably speeds development. Viviparous species are especially prevalent at high latitudes and at high elevations, where females apparently can overcome the adverse effects of cold environmental temperatures on embryonic development by actively thermoregulating their own temperature and thus warming their embryos.
Death feign (faking death), a defensive posture, in Heterodon platirhi-nos. (Illustration by Barbara Duperron)
The size of a clutch or litter varies widely among colubrids. Small, fossorial oviparous species often lay only a few eggs (the wormsnakes, Carphophis, average about three per clutch), whereas larger terrestrial species often lay several times as many (for example, common kingsnakes, Lampro-peltis getula, average about 10). In some species clutch size varies geographically, as in the Eastern racer (Coluber constrictor). Some species lay many more, such as the Eastern hog-nosed snake (Heterodon platirhims), which averages more than 20 eggs per clutch, and the mudsnakes (Faran-cia), which average over 30. The females of several oviparous species of colubrids are reported to attend their clutches, and others are known to lay their eggs in communal ovipo-sition sites. Likewise, litter size varies greatly among viviparous species. All of the thamnophiine natricines are viviparous, with the average number of young varying from about seven in the North American earthsnakes (Virginia) to almost 30 in the Plains garter snake (Thamnophis radix) and almost 50 in the diamond-backed watersnake (Nerodia rhombifera).
Most colubrid snakes reproduce annually, even in tropical regions, and reproduction may be timed to the seasonal patterns of temperature or rainfall. The embryos of both oviparous and viviparous snakes are well provisioned with yolk, although in some viviparous species a placenta permits the transfer of additional nutrients from mother to embryo. Even in oviparous colubrids, embryonic development usually begins well before the eggs are laid, and the embryos are already about one-third of the way through development by the time of oviposition.
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