Viviparity in Snakes: Some Ecological and Zoogeographical Considerations

Some snakes lay eggs, while others give birth to living young. The oviparous condition is the primitive one; but viviparity developed early in the history of snakes, and some of the most primitive extant ophidians are live-bearers. Among United States snakes viviparity follows phylogenetic lines; the only live-bearing snakes of this country are boas, pit vipers, and a closely related group of genera belonging to the harmless snake family Colubridae. Even when the geographic view is expanded to include all the New World, viviparity with but two exceptions follows phylogenetic lines. But when the view is broadened to include the Old World, especially Asia, it is evident that viviparity and oviparity do not necessarily follow such lines, and may even occur together in one genus. Live-bearing snakes are apt to have unusual anatomical and behavioral specializations, with which viviparity is linked. Many aquatic snakes have developed viviparity. Snakes that are exceedingly capable of self-defense, either by virtue of large size or of an elaborate venom-conducting mechanism, have often developed viviparity, and the usefulness of this development is understandable; the gravid female is relatively immune to predation as compared with an egg clutch. Other snakes, very capable of defense, have not developed viviparity but instead the habit of guarding the clutch; these species obtain one advantage of viviparity without actually having to carry the young. There are some cases in which viviparity has, apparently, not been developed by a snake species or genus but has simply been retained as an inheritance from an ancestor that profited more by the live-bearing condition. For example, the cryptozoic offshoots of a certain essentially aquatic group have retained viviparity. There is no reason to believe that dry climates hold more promise for viviparous than for oviparous snakes. The predominance of live-bearers among the elapine snakes of Australia does not reflect frequent development of viviparity in that area, but rather adaptive radiation from one or a few viviparous stocks. Among snakes, viviparity is often correlated with residence in cold climates. The live-bearing condition not only encourages invasion of cold areas but may actually develop in such places. As a result, the high altitude and latitude records are held by live-bearers. Under the present climatic regime, snakes are unable to live as far north as Bering Strait. However, past fluctuations of climate were such as to permit a few viviparous and cold-adapted oviparous snakes to reach that area, and to move from one hemisphere to another via the old Bering Bridge. Most of the movement was from Asia to North America. In the case of Asiatic groups with both egg-laying and live-bearing members, only the latter were able to reach the bridge. This explains why various groups are strictly viviparous in the New World but not in Asia. One need not postulate many such movements across the bridge; various New World species are derived through adaptive radiation from a few stocks that made the crossing. Much of this movement, through a gantlet of cold climate at Bering Bridge, probably took place in the Miocene. A Pacific sea-snake may have reached the New World at about the same period, swinging eastward around the southern margin of the bridge, then southward along the North American coast. There are, however, indications of many earlier movements in a warm period; boas, for example, had reached South America from Asia by the Eocene. There are herpetogeographic problems on which the development of viviparity throws no light; there are unexplained cases of viviparity and oviparity. But by and large, an analysis of the live-bearing condition among snakes aids considerably in understanding herpetofaunal distribution, and reveals that viviparity has developed in many lines of descent in response to various environmental demands. Reptiles exhibit transitional stages between oviparity and the mammalian type of viviparity; and it is possible that similar environmental demands were involved in the evolution of mammals from reptilian forebears. In an effort to explain the evolution of ophidian. viviparity, the known facts have been fitted into the following theoretical framework: Viviparous snakes have developed from oviparous ones. Whenever eggs in the nest are in greater danger than eggs in the oviducts, natural selection puts a premium on longer retention of the eggs by the female. The period from deposition to hatching, normally somewhat variable, will be shortened progressively if the gravid female is unusually capable of defending herself from environmental hazards, or if the eggs are egregiously exposed to danger. Thus the live-bearing condition today is most characteristic of snakes that are venomous, or large and powerful; that are restricted to some habitat, especially the aquatic one, where safe nesting sites are few; or that inhabit high altitudes and latitudes, where eggs are menaced by the likelihood of chilling. Reversion from viviparity to oviparity is deemed unlikely. Thus certain snakes-especially dwarfed, cryptozoic ones-have simply retained the live-bearing condition as an inheritance from ancestors that belonged to some one of the above three categories.