SYSTEMATIC RELATIONSHIPS AND DISTRIBUTION
Snakes of the genus Agkistrodon are relatively primitive members of the Crotalidae, which is one of the most specialized families of snakes. A majority of the pit-vipers are found in the Americas, but close relatives are found from extreme southeastern Europe through temperate Asia to Japan (A. halys) and southeastern Asia including Indonesia (Agkistrodon and Trimeresurus). Familial characters include: vertical pupil of the eye; facial pit present between the preoculars and loreal; scales usually keeled; short, rotatable maxilla bearing a large hollow fang; toothless premaxilla; chiefly hematoxic venom; and undivided anal plate.
The genus Agkistrodon includes about nine species in the Old World and three in North and Central America. Some of the primitive characters of the genus are: head covered with nine enlarged shields or having the internasals and prefrontals broken up into small scales; subcaudals on proximal part of tail undivided; fangs relatively short; tail lacking rattles. In one species, A. rhodostoma, the scales are smooth; and the female is oviparous and guards her eggs until they hatch. Other species have keeled scales and are ovo-viviparous.
There is little paleontological evidence illustrating evolution of the cottonmouth or for that matter of crotalids in general. Brattstrom (1954) summarized the current knowledge of fossil pit-vipers in North America. The few fossils found of the cottonmouth are from Alacha, Brevard, Citrus, Levy, Pasco, and Pinellas counties, Florida (Brattstrom, op. cit.:35; Auffenberg, 1963:202). All are of late Pleistocene Age and well within the present geographic range of the cottonmouth.
Of crotalid genera only Agkistrodon occurs in both the Old World and the New World, suggesting that this genus is relatively old. Schmidt (1946: 149-150) mentioned several other closely related groups of animals found in both eastern Asia and eastern North America, including the reptilian genera: Natrix, Opheodrys, Elaphe, Ophisaurus, Leiolopisma (= Lygosoma), Eumeces, Clemmys, Emmys, and Alligator. Of the groups of animals now confined to these two regions the most important are the cryptobranchid salamanders, the genus Alligator, and the spoon-bills (Psephurus in China and Polyodon in the Mississippi drainage). Fossil evidence for these groups indicates that existing forms common to eastern Asia and eastern North America are remnants of a late Cretaceous or early Tertiary Holarctic fauna which was forced southward as the climate became gradually cooler to the north. "Other clues suggest that both Agkistrodon and Trimeresurus (Bothrops) moved from Asia to America, one of these presumably giving rise to the rattlesnakes." (Darlington, 1957:228).
The named, American kinds of Agkistrodon currently are arranged as three species: the copperhead, the cantil and the cottonmouth. The copperhead (A. contortrix) is divided into four subspecies, all of which are terrestrial. This species occurs from southern New England to eastern Kansas and along the Atlantic and Gulf Coastal plains, exclusive of peninsular Florida and the delta of the Mississippi River in Louisiana. It extends southwest from Kansas through the Edwards Plateau of west-central Texas. Isolated populations occur in the Chisos and Davis mountains of Trans-Pecos Texas. The cantil or Mexican moccasin (A. bilineatus), probably the nearest relative of the cottonmouth (A. piscivorus), is divisible into two subspecies and occupies a nearly complementary range from Mexico south to Nicaragua. The cottonmouth occurs throughout the coastal plains of the southeastern United States, usually at altitudes of 500 feet or less. Two subspecies are recognized, the eastern A. p. piscivorus and the western A. p. leucostoma. A revision of the genus is underway by Professor Howard K. Gloyd.
The basic pattern and various behavioral traits are common to all three species. The young are more nearly alike in appearance than adults, the copperhead and the cottonmouth being easily confused. Adults differ in color, size, body proportions, habitat, and habits. In range and habitat preference the cottonmouth more closely resembles the southern subspecies of the copperhead, A. c. contortrix, which is usually found in lowlands, near swamps and streams, but seldom in water.
Fig. 1.