Colubrids

(Colubridae)

Class: Reptilia

Order: Squamata

Suborder: Serpentes

Family: Colubridae

Thumbnail description
Highly variable in size, body form, and color pattern; ranges from short, stout, drab species to large, slender, boldly marked forms

Size
6 in–12 ft (160 mm–3.7 m)

Number of genera, species
Approximately 300 genera; approximately 1,700 species

Habitat
The family includes terrestrial, fossorial, arboreal, and aquatic species

Conservation status
Extinct: 1 species; Critically Endangered: 6 species; Endangered: 7 species; Vulnerable: 8 species; Lower Risk/Near Threatened: 4 species; Data Deficient: 10 species

Distribution
Worldwide except Antarctica, extremely high latitudes of Eurasia and North America, and central and western Australia

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(5–25 million years ago), and by the end of that period a fauna previously dominated by boa-like species had largely been replaced by colubrids, elapids, and viperids.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 (Erpeton tentaculatus), rarely leave the water. Several occupy mud flats and mangrove forests, including the dog-faced watersnake (Cerberus rynchops) and the white-bellied mangrove snake (Fordonia 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 monophyly 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 fossorial 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 fossorial 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 (Spilotes), 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, although 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 (Diadophis), 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 fossorial 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 (Lystrophis) 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 hognosed 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 Helicops 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 (Micrurus), 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.

Physical characteristics

Again, generalizations are difficult in light of the extraordinary diversity encompassed within this family. Compared with basal families and with boas and pythons, colubrids are characterized by the loss or simplification of many features. Notably, all vestiges of the hind limbs are absent in colubrids, and the coronoid bones of the lower jaws have been lost. Externally, most colubrids have relatively wider ventral scales and fewer rows of dorsal scales than boas and pythons. In addition, most colubrids possess a standard complement of enlarged scales on the head, including nine scales on the top of the head. That feature, like other external attributes, is shared with members of the Atractaspididae and Elapidae and even with some Viperidae, and at the same time is not seen in all members of the Colubridae. The vertebrae of colubrids are, in general, more slender and lightly built than those of basal snakes, boas, and pythons. In addition, the interconnecting chains of muscles that effect locomotion in snakes are longer in colubrids than in those other groups.

One important feature that distinguishes most colubrids is the presence of Duvernoy's glands, a pair of glands located on either side of the head behind the eye. These are the homologues (the evolutionary counterparts) of the venom glands of vipers, elapids, and Atractaspis. Although they are often described as modified salivary glands, they in fact are very different in both their tissue characteristics and their origin. The Duvernoy's glands are associated with the rear teeth of the maxillary bones, the major toothed bones of the upper jaws, and they secrete a complex mix of chemicals whose composition is still poorly understood. In some colubrids the secretion is known to serve as a slow-acting venom or as a digestive adjunct. The rear pair of maxillary teeth are often modified to assist with the delivery of secretion from Duvernoy's glands, and may be either enlarged, grooved, or both. That is generally known as the rear-fanged, or ophisthoglyphous, condition, although a variety of terms has been applied to specific conditions of the rear teeth. In most cases the secretion of Duvernoy's gland has no apparent effect on humans, but in some species it can cause local swelling. A few taxa, including the boomslang (Dispholidus) and twigsnake (Theletornis) of Africa and the Yamakagashi (Rhabdophis) of Asia, are capable of delivering a lethal bite to humans. The xenodontine genera Apostolepis and Elapomorphus resemble some aparallactine atractaspidids in approaching the front-fanged condition of elapid snakes, with large Duvernoy's glands and few teeth preceding the greatly enlarged rear fangs. Although Duvernoy's gland is found, to some degree, in most colubrids, it is not universally present. In some colubrids it may have been lost as a consequence of the evolution of constricting behavior, which constitutes an alternative mechanism for immobilizing prey. In other colubrids the enlarged rear maxillary teeth may serve a purely mechanical function. In some toad-eating snakes, such as hog-nosed snakes (Heterodon), those teeth may be used to deflate the prey, which in others, such as kukrisnakes (Oligodon), they are used to slice open the eggs of lizards and snakes.

Distribution

Members of the Colubridae occur worldwide, except Antarctica, the highest latitudes of North America and Eurasia, and the central and western regions of Australia. Only vipers extend farther north, in Scandanavia, than do colubrids. In Australia, unlike other regions, colubrids account for a minority of the snake fauna, with members of the Elapidae predominating.

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 watersnakes (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 Pseudoeryx 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 Pareatinae are primarily arboreal, and the Colubrinae and Xenodontinae 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 colubrine 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.

Behavior

As expected, behaviors are extremely diverse among Colubridae and generally poorly known, with the exception of a few common or unusual species. The behavior of a species often reflects its environment, and arboreal, cryptozoic, and fossorial species are especially difficult to study behaviorally. Defensive behavior, in particular, seems to reflect both coloration and habitat. Many species are cryptically colored (camouflaged) and, in the various vinesnakes, similarity to the snake's surroundings is enhanced by the narrow body and pointed head. More active defenses include threat displays, which may involve flattening the body, as in many North American watersnakes (Nerodia); spreading of a horizontal hood reminiscent of cobras, as seen in the xenodontine false water-cobras (Hydrodynastes); or vertical inflation of the neck, as in the colubrine tiger ratsnake (Spilotes). In general, horizontal displays characterize terrestrial species, whereas arboreal ones have vertical displays. Tail displays also occur in some species, such as the xendontine ring-necked snakes (Diadophis), in which the tail is upturned to show the bright ventral coloration and is moved in a distinctive spiral action. Other species gape when confronted by a predator and many bite, although others simply attempt to escape. Asian natricine snakes of the genera Rhabdophis and Balanophis have glands on the dorsal surface of their neck or along their entire back that exude a noxious compound when the snakes are threatened. Tail loss is rare among snakes, but a few taxa apparently lose the tail freely to escape predators, including the neck-banded snake (Scaphiodontophis), a Neotropical colubrine. Unlike the tails of some lizards, those of snakes do not regenerate if lost. Mimicry is common among snakes. Some species, such as the false pitvipers (Xenodon), mimic vipers, while in the Neotropics many species of xenodontines and a few colubrines mimic the elapid American coral snakes (Micrurus).

Colubrid snakes do not appear to be territorial. Instead, the males of some species, especially among the colubrines, are known to engage in ritual combat to obtain access to mates. Such combat generally resembles that of vipers and elapids, with the individuals intertwining their bodies and attempting to topple each other. That behavior may have served as the inspiration for the caduceus, the staff with entwining snakes that was said to have been carried by the god Hermes and that now is emblematic of the medical profession. In some species biting may also occur during combat. Species with male combat generally exhibit sexual size dimorphism in which males are larger than females, reflecting the fact that larger males generally are more successful in combat and therefore are favored to reproduce. Some other species exhibit the opposite pattern of sexual size dimorphism, in which females are larger than males. That condition reflects selection in which larger female size is associated with greater numbers of offspring.

Courtship behavior is known in a number of colubrid species, and it generally involves the male tongue-flicking and pressing his head along the back of the female. If the female is receptive, the male inserts one of his paired hemipenes into the female's cloaca to inseminate her. Males often locate females by following pheromone trails, a behavior mediated by the vomeronasal organ located in the roof of the mouth. That organ receives cues from the environment by means of the tongue tips. In some species courting males aggregate around females and compete for matings, as in certain Canadian populations of the common gartersnake (Thamnophis sirtalis).

Feeding ecology and diet

Colubrids feed on a wide variety of prey. Some species are prey generalists, feeding on virtually any animals within an appropriate size range. The racer (Coluber constrictor), for example, is known to feed on a wide variety of prey, including mammals, lizards, frogs, baby turtles, and insects. Some wide-ranging species, such as the ring-necked snake (Diadophis punctatus), consume different prey in different parts of their range, while in others with varied diets, such as the common garter snake (Thamnophis sirtalis), prey preference is influenced by early experience with a particular prey. Location of prey frequently involves the use of chemical cues, which are picked up by the tongue and delivered to the vomeronasal organ. However, many terrestrial and arboreal snakes are also strongly dependent upon vision to locate prey.

More often, however, colubrids are prey specialists, preferring either a general class of prey, such as fishes, frogs, or mammals, or a very specific diet, such as lizard eggs or skinks. Unlike lizards, few snakes feed regularly on insects. Among colubrids, only a few lineages take large numbers of insects or other terrestrial arthropods, of which the sonorines are the largest. Even within that group, some (such as Sonora) take a variety of arthropods, whereas others specialize on particular arthropods, such as centipedes (Tantilla). Stenorrhina preys primarily upon scorpions, tarantulas, and grasshoppers. Some snakes specialize on prey of a certain shape. Often these are fossorial snakes, whose narrow heads limit them to relatively elongate prey. For smaller species these may be worms, whereas large species may feed on snakes or elongate lizards. Within the aquatic genus Farancia, the rainbow snake (F. erytrogramma) feeds mainly on eels, whereas the red-bellied mudsnake (F. abacura) feeds primarily on elongate aquatic amphibians, such as sirens (Siren). Eggs are a rich, if seasonal, source of food for many snakes. The arboreal cat-eyed snakes (Leptodeira), of the Neotropics, feed both on frogs and on frog eggs that are laid on vegetation overhanging water, such as those of the red-eyed treefrog (Agalychnis callidryas). The soft-shelled eggs of lizards and snakes are eaten by a variety of snakes, including the scarletsnake, Cemophora, a North American colubrine, the Asian kukrisnakes (Oligodon), and several Neotropical xenodontine genera, such as Umbrivaga and Enulius. An extreme case of specialization involves the egg-eaters (Dasypeltis), African colubrines that feed only on bird eggs. They consume eggs that may be several times the diameter of their own heads, forcing their nearly toothless jaws around the egg. Once in the esophagus, the eggs are cracked on ventral projections of the vertebrae; the shell is regurgitated and the liquid contents are swallowed. A few snakes possess hinged teeth, which fold back when their hard-bodied prey are consumed. Among these are several lineages of skink-eating snakes, including the neck-banded snake (Scaphiodontophis), a Neotropical colubrine, and some that feed on hard-shelled crayfish, including the striped crayfish snake (Regina alleni), a natricine from southeastern North America.

Reproductive biology

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.

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, Lampropeltis 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 platirhinos), which averages more than 20 eggs per clutch, and the mudsnakes (Farancia), 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 oviposition 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.

Conservation status

The 2002 IUCN Red List includes 36 colubrid species. One species, Alsophis sanctaecrucis, is categorized as Extinct. In addition, 6 species are listed as Critically Endangered; 7 as Endangered; 8 as Vulnerable; 4 as Lower Risk/Near Threatened; and 10 as Data Deficient.

Very few colubrids are legally protected at either the national or international levels, although more regional regulations protect a few more. No colubrid species appear on Appendix I of CITES, only four appear on Appendix II, and three appear on Appendix III. As with most organisms, habitat destruction is probably the leading threat to colubrid snakes throughout most of their range. In addition, however, collection for the pet trade has contributed to the decline of some species, such as the Eastern indigo snake (Drymarchon corais couperi), and some species may be at risk due to their use in the skin trade and other commerce.

Overwhelmingly, it is humans who pose an ongoing threat to many colubrid snakes. However, one species, the brown treesnake (Boiga irregularis), has itself become a severe threat to the native fauna of the Pacific island of Guam. The accidental introduction of this adaptable predator to the island following World War II, apparently as stowaways on military transports, has resulted in the decimation of the native lizards and birds of the island and has resulted in the collapse of the natural food web. Efforts are under way to control this species and to recover the populations of those birds that have not already been completely extirpated.

Significance to humans

Colubrid snakes appear to figure in human commerce in two primary ways. First, many species are sold in the pet trade, including large numbers of wild-caught individuals of many species. Several groups of colubrids have become sufficiently popular that they now are bred in captivity in large numbers. Among the latter are the cornsnake (Elaphe guttata) and several species of kingsnakes (Lampropeltis). Although most species are too small to be useful as leather, a few colubrids are exploited in the skin trade, including the large Asian ratsnakes (Ptyas). Recently a large trade in aquatic colubrids of the subfamily Homalopsinae has been documented in Cambodia, and much of that trade involves the preparation of skins for leather. In addition, however, snakes are used as food for humans and are also fed to farm-raised crocodiles that are in turn used for leather. The magnitude of the trade was enormous, with an estimated 4,000–8,500 snakes per day traded at one port during peak periods. That catch is likely to be unsustainable.

Species accounts

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[Article by: Alan H. Savitzky, PhD]