Evolution and systematics

The Colubridae comprise by far the largest and most diverse family of snakes, containing about 70% of all snake species. For that reason, few useful generalizations apply to this expansive family. Indeed, it is the ability of colubrids to adapt to widely different habitats, diets, and life history modes that above all characterizes this extraordinary lineage. Although some colubrids are generalists, many exhibit strong specialization for a particular environment and/or specific prey.

Fossils attributed to the Colubridae first appear in the Lower Oligocene, about 35 million years ago. However, the group seems to have radiated rapidly during the Miocene (5-25 million years ago), and by the end of that period a fauna previously dominated by boa-like species had largely been replaced by colubrids, elapids, and viperids. However, our knowledge of fossil snake faunas is drawn primarily from temperate remains, so inferences for tropical regions should be drawn with caution.

The relationships among genera of colubrids remain poorly understood and highly controversial, despite numerous attempts to bring order to this complex group. Early classifications were based largely upon similarities in scale characteristics, dentition, and the form of the hemipenes (the paired copulatory organs of male squamates). In some cases these features yielded groupings that have stood the test of time, but in many instances the failure to distinguish between ancestral and derived conditions resulted in unnatural groups.

The application of both phylogenetic systematic (cladistic) and molecular methods has helped to clarify the relationships within many groups of colubrid snakes. However, even when well-defined clusters of genera are confirmed, the relationships among those clusters often remain unclear, and widely accepted phylogenetic hypotheses concerning colubrid relationships have remained elusive. Furthermore, the relationship between colubrids and two other colubroid families, the Elapidae and Atractaspididae, is unclear, and it is possible that the Colubridae itself is paraphyletic relative to one or both of those families. That is, some colubrids may be more closely related to members of one of those families than they are to certain other colubrids.

That said, several subfamilies of Colubridae are widely recognized, if not universally accepted. Seven are recognized here. The Xenodermatinae is a small group of six genera and about 15 species from southern and eastern Asia. Little is known of the biology of these strange colubrids, many of which have unusual, protuberant scales on the body or head. Most occupy terrestrial habitats in moist tropical forests, and some are known to prey on amphibians. The Pareatinae is a small but well-defined lineage of three genera and about 18 species of Southeast Asian snakes that are highly specialized to prey on terrestrial mollusks (snails and/or slugs). A slightly larger but still well-defined lineage of Asian colubrids is the Homalopsinae, with about 10 genera and 35 species. All are strongly aquatic and some, such as the tentacled snake (Er-peton tentaculatus), rarely leave the water. Several occupy mud flats and mangrove forests, including the dog-faced watersnake

Dipsas Gaigeae

A sample of the wide variety of scale pattern and color in the milksnake subspecies of Lampropeltis triangulum. From left to right: L. t. blanchardi; L. t. campbelli; L. t. triangulum; L. t. elapsoides; L. t. gaigeae; L. t. hondurensis (Tangarine morph); L. t. taylori. (Illustration by Barbara Duperron)

A sample of the wide variety of scale pattern and color in the milksnake subspecies of Lampropeltis triangulum. From left to right: L. t. blanchardi; L. t. campbelli; L. t. triangulum; L. t. elapsoides; L. t. gaigeae; L. t. hondurensis (Tangarine morph); L. t. taylori. (Illustration by Barbara Duperron)

(Cerberus rynchops) and the white-bellied mangrove snake (For-donia leucobalia), while the keel-bellied watersnake (Bitia hydroides) occupies coastal marine waters. Many feed on fishes, but some prey on frogs and Fordonia feeds on crustaceans. All are rear-fanged.

Four other subfamilies are much larger, and their mono-phyly is uncertain. The Natricinae is the most cohesive of these, including many species with aquatic tendencies, including the familiar North American gartersnakes (Thamnophis), North American watersnakes (Nerodia), and the grass snake (Natrix natrix) of Europe. The subfamily includes about 38 genera and almost 200 species distributed throughout Europe, Asia, Africa, and North and Central America. Especially large radiations occur in southern and eastern Asia and in North America. The American members constitute a well-defined tribe, the Thamnophiini, all of which are viviparous, in contrast to most of the Old World forms. In addition to the many aquatic species of natricines, some are small cryptozoic or fos-sorial forms.

Another subfamily with many northern representatives is the Colubrinae, with over 100 genera and roughly 650 species. Also included in this subfamily are many tropical species and several clusters of genera that are sometimes recognized as tribes or even subfamilies in their own right. Worldwide in distribution, the Colubrinae include significant radiations in North America, Eurasia, Africa, and Southeast Asia. Many are fast-moving terrestrial species, although arboreal and fossor-ial members are numerous. Many are rear-fanged, although others lack enlarged rear maxillary teeth and some have evolved constricting behavior. Among the more familiar groups are the North American racers (Coluber), kingsnakes (Lampropeltis), and ratsnakes (Elaphe). Other important members of the Colubrinae include a large group of tropical Asian burrowing snakes, including the reedsnakes (Calamaria) and related genera, and a group of African genera that include the dangerously venomous boomslang (Dispholidus) and twigsnake (Theletornis). Few alethinophidian snakes prey on arthropods, but the colubrines include the largest radiation of arthropod-eating snakes, the sonorines, with about a dozen genera, including the North American groundsnakes (Sonora), black-headed snakes (Tantilla), and shovel-nosed snakes (Chionactis). Sonorines are broadly distributed throughout the Americas, but they have their greatest diversity in the dry regions of southwestern North America. Another important cluster of genera includes the diverse African sandsnakes (Psammophis) and related species, including the Montpellier snake (Malpolon monspessulanus) of the Mediterranean region. Several genera of arboreal colubrines have independently evolved the so-called vinesnake morphology, with very narrow heads, slender bodies, and even behavioral modifications that allow them to blend into the surrounding vegetation. These include genera in the Neotropics (Oxybelis), Africa (Theletornis), and Southeast Asia (Ahaetulla). (Other vinesnakes have evolved in the subfamilies Xenodontinae and Lamprophiinae.) In addition to many small, burrowing species, the Colubrinae includes some giants among colubrids, such as the indigo snake (Drymarchon), tiger ratsnake (Spi-lotes), and bird snake (Pseustes) of the Americas, and the banded ratsnake (Ptyas) of Asia, which approaches or exceeds 10 ft (about 3 m) in length.

The Xenodontinae comprise another large colubrid subfamily, with a distribution limited to the New World. With about 90 genera and over 500 species, this group dominates the colubrid fauna of the Neotropics, where most of its diversity occurs. Two major groups have been identified, one centered in Central America and one in South America, al though the two groups broadly overlap geographically, and a number of genera cannot be assigned to either of those two major clades. Some familiar North American species belong to the Xenodontinae, including the ring-necked snakes (Di-adophis), wormsnakes (Carphophis), and mudsnakes (Farancia). Even more than the colubrines, the Xenodontinae encompass an extraordinary range of natural histories. Among the major themes are repeated invasions of aquatic, arboreal, and fos-sorial habitats, as well as specialization on any of a wide range of prey, including fishes, amphibians, earthworms, and terrestrial mollusks. A number of well-defined clades are recognized within the Xenodontinae. Among those in the South American group are the false pitvipers (Xenodon) and such related genera as the Neotropical hog-nosed snakes (Ly-strophis) and Liophis. Many members of this group feed on frogs, including such highly toxic species as toads (Bufo) and poison frogs (Dendrobatidae). The North American hog-nosed snakes (Heterodon) are also xenodontines, but apparently represent an independent evolution of toad-eating habits. Other primarily South American groups include the pseudoboines, which include the mussurana (Clelia) and related genera, the diverse terrestrial to arboreal species of Philodryas, and the highly aquatic members of the genera He-licops and Hydrops. A substantial radiation from the South American clade occurs in the West Indies, and a few species are found on the Galápagos Islands. Some have evolved the vinesnake morphology, including Uromacer and Xenoxybelis. The Central American clade includes a very important radiation of predators on worms or terrestrial mollusks. Among them are worm-eaters, such as Geophis and Atractus, and the so-called snail-suckers, such as Dipsas and Sibon, which prey on snails and/or slugs. The snail-eating xenodontines strongly resemble the pareatines of Asia, both externally and in details of their skulls and jaw muscles, and they apparently use similar feeding mechanics to extract snails from their shells. Many xenodontines of both major clades are rear-fanged. A number of xenodontine species mimic venomous coralsnakes (Mi-crurus), as do several colubrines.

The final subfamily of colubrids is the Lamprophiinae, a more modest radiation in both numbers and distribution. The 44 genera and roughly 200 species of lamprophiines are limited to sub-Saharan Africa. Most are small to moderate-sized snakes, and many prey upon reptiles, including other snakes. Some are more specialized, however, including aquatic specialists such as Lycodonomorphus and Grayia, and prey specialists such as the slug-eaters (Duberria). An important radiation occurs on Madagascar, where members of this group dominate the snake fauna. Among that distinctive fauna are the Madagascan hog-nosed snakes (Lioheterodon) and the bizarre arboreal Madagascan vinesnakes (Langaha), which have a scaly proboscis on their snout.

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