Habitat

Colubrids occupy a wide array of habitats, and independent lineages have repeatedly entered the same habitat. It is impossible to know the ancestral habitat of the Colubridae. Although the earliest fossils have been allied with living terrestrial lineages, the vertebrae of most colubrids do not unambiguously reflect the species' habitat. Terrestrial species occur on all occupied continents and in all subfamilies except the Homalopsinae. Likewise, aquatic species abound in most subfamilies, and their morphologies range from minimally specialized to highly modified. Examples of the former include many members of the Natricinae, including the North American watersnakes (Nerodia) and Eurasian watersnakes (Natrix), as well as the African watersnakes (Lycodonomorphus) of the Lamprophiinae. The Xenodontinae include many aquatic taxa, from the relatively generalized Neotropical wa-tersnakes (Helicops) to the more highly modified Neotropical swampsnake (Tretanorhinus), which has dorsally placed eyes and valvular nostrils. Some aquatic xenodontines, such as mudsnakes (Farancia) of North America and Hydrops and Pseu-doeryx of South America, have rounded heads and smooth, shiny scales, and seem to be adapted for burrowing in aquatic habitats. The entire subfamily Homalopsinae is aquatic, and its members range from moderately specialized forms such as Enhydris and the puff-faced watersnake (Homalopsis) to extreme specialists such as the keel-bellied watersnake (Bitia) and the bizarre tentacled snake (Erpeton), which rarely leave the water. Homalopsines have valvular nostrils and mouths similar to those of seasnakes.

Some primarily terrestrial colubrids occasionally climb small shrubs, and even some relatively unmodified taxa, such as the ratsnakes (Elaphe) of the Colubrinae, are adept climbers of trees. More specialized arboreal species include the various vine-snakes of the Colubrinae, Xenodontinae, and Lamprophiinae, described above, and the nearly as specialized rough greensnake (Opheodrys aestivus), a North American colubrine. The Pareati-nae are primarily arboreal, and the Colubrinae and Xenodon-tinae include many arboreal species in the tropics. Such species usually exhibit slender bodies, which may be laterally compressed. The most extreme condition occurs in the blunt-headed vinesnakes (Imantodes), Neotropical xenodontines that have extremely slender bodies and chunky heads. The Southeast Asian flyingsnakes (Chrysopelea) are arboreal colubrines that can flatten their bodies and glide from tree to tree.

Likewise, cryptozoic (hiding) and fossorial (burrowing) forms abound in most subfamilies. Aside from being relatively small, cryptozoic species are sometimes little modified morphologically, as in the colubrine North American ground-snakes (Sonora) and the xenodontine coffeesnakes (Ninia). Others are highly modified for burrowing, including the col-ubrine shovel-nosed snakes (Chionactis) of North America and shovel-snouted snakes (Prosymna) of Africa. Fossorial species abound in the Xenodontinae, including the Neotropical burrowing snakes (Geophis) of Central America and Apostolepis of South America. Such species often have pointed snouts, fused head scales, compact cranial bones, and smooth, shiny scales on the body. Even a few members of the Natricinae exhibit fossorial adaptations, such as the lined snake (Tropidoclonion) and rough earthsnake (Virginia striatula) of North America.

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