Family Viperidae: This family comprises 150+ species in 20+ genera distributed on all continents except Antarctica and Australia (Zug, 1993). Most are heavy-bodied snakes with a distinct head and vertical pupils in the eye. Vipers possess a pair of hollow fangs, one on each of the two maxilla bones located beneath the nostrils. The bones and fangs rotate from a resting position along the roof of the mouth to an erect position by the mechanical action of lowering the lower jaw. These fangs provide the functional means to inject modified saliva (venom) deep into prey. Venom glands lie behind the eyes under the masseter muscle and each is connected to the fang by a hollow duct. Vipers regulate the amount of venom injected by their control over the masseter muscle. Venom, and the means to inject it, evolved for the purpose of prey capture, but it is sometimes used as a defensive measure.

The majority of the species in this family are pit vipers (120+ species); the remaining 50 or so species are true vipers. Each species of pit viper possesses a heat-sensing pit located between the eye and nostril that is used to aid in prey location. This group is often classified in the subfamily Crotalinae, although some taxonomists refer them to full family status, the Crotalidae. The pit organ contains heat-sensitive cells that are responsive to changes in temperature of 0.001°C (Halliday and Adler, 1986). This mechanism probably evolved to allow prey capture in dark spaces, like rodent burrows. A pit viper can detect the presence of a rodent prey and determine its relative size and distance in total darkness.

The family Viperidae is represented in Virginia by only three species of pit vipers: the Timber Rattlesnake (Crotalus horridus), the Eastern Copperhead (Agkistrodon contortrix) and the Northern Cottonmouth (Agkistrodon piscivorus).

Systematics: Originally described as Crotalus piscivorus by Bernard-Germain-Etienne de la Ville-sur-Illon Lacepede in 1788-1789, based on a specimen from "Carolina." The type locality was restricted to the vicinity of Charleston, South Carolina, by Schmidt (1953). Cope (1860) first placed this species in the genus Ancistrodon, and Stejneger (1895) first used the corrected spelling, Agkistrodon. Both names were used until the late 1950s. In the Virginia literature, Ancistrodon piscivorus was used by Cope (1900), Schmidt (1953), Wood (1954a), Goodwin and Wood (1956), and Martin and Wood (1955); and Agkistrodon piscivorus was used by Dunn (1918, 1936), Werler and McCallion (1951), Burger (1958), Conant (1958, 1975), Mitchell (1974b), Linzey and Clifford (1981), and others.

Using multi-locus nuclear data, Burbrink and Guiher (2015, Zool. J. Linn. Soc. 173: 505–526) partially confirmed previous mitochondrial hypotheses (Guiher and Burbrink 2008, Mol. Phylogenet. Evol. 48: 112–125) that the two North American species (A. contortrix and A. piscivorus) each consist of multiple species-level taxa. Subspecies are not recognized.

Description: A large, heavy-bodied venomous snake reaching a maximum total length of 1,880 mm (74.0 inches) (Gloyd and Conant, 1990). In Virginia, maximum known snout-vent length (SVL) is 1,340 mm (52.8 inches) and maximum total length is 1,536 mm (60.5 inches). In this study, tail length/total length was 10.4-23.9% (ave. = 14.8 ± 1.5, n = 132).

Scutellation: Ventrals 125-136 (ave. = 131.8 ± 1.8, n = 144); subcaudals 39-50 (ave. = 45.0 ± 2.0, n = 137) and single, except for 6-35 (ave. = 19.7 ± 5.9, n = 126) paired (divided) subcaudal scales distally; ventrals + subcaudals 168-184 (ave. = 176.8 ± 2.8, n = 137); dorsal scales strongly keeled, scale rows 25 at midbody; anal plate single; infralabials 11/11 (46.2%, n = 119), 10/10 (18.5%), 11/10 (28.6%), or combinations of 9-12 (6.7%); supralabials 7/7 (25.4%, n = 118), 8/8 (42.4%), 7/8 (27.1%), or combinations of 6-9 (5.1%); loreal scale absent; preoculars 2/2; series of 2-4 small scales separating eye from supralabials and temporal scales; temporal scales variable, usually 3-5 or 4-5 on each side.

Coloration and Pattern: Dorsum of body and tail yellowish olive to black with a series of 10-23 (ave. = 13.2 ± 1.5, n = 134) black crossbands that are somewhat dumbbell-shaped, wide on sides and narrower at middorsal line; outer edges of bands darker (usually black) than centers; edges of crossbands project well onto venter; center portion of bands olive to brown, similar to body color between bands; bands darken posteriorly and are entirely black on tail; a variable number of crossbands broken at middorsal line, and some may occur as individual halves; in some snakes, bands obscured by black pigment, especially during shedding; venter cream-colored with a variable amount of black pigmentation in the form of large patches to transverse streaks; head chunky, dark brown, olive, or black dorsally, and brown laterally; chin and infralabials cream with brown patches or streaks; dorsum of head flat; head somewhat triangular and distinctly separated from narrow neck.

Sexual Dimorphism: Adult male body size averaged considerably larger (976.9 ± 142.2 mm, 755-1,340, n = 51) than females (772.8 ± 74.1 mm, 660-940, n = 43), and males reached a larger total length (to 1,536 mm) than females (to 1,100 mm). Sexual dimorphism index was -0.26. Tail length/total length was similar in both sexes (males 11.7-23.9%, ave. = 15.0 ± 1.7, n = 70; females 10.4-16.4%, ave. = 14.3 ± 1.1, n = 55). Average body mass for adult males (1,035.0 ± 463.7 g, 384-1,700, n = 8) was nearly twice that for adult females (560.3 ± 109.8 g, 435-700, n = 8). Blem (1981b) found that average body mass of males (857.2 ± 499.7 g) was twice that of nonreproductive females (402.6 ± 107.4 g) in a population in Chesterfield County.

The average number of ventrals was similar in both males (131.9 ± 2.0, 125-136, n = 73) and females (131.6 ± 1.5, 128-135, n = 62), but the average number of subcaudals was slightly higher in males (46.3 ± 1.6, 41-50, n = 70) than in females (43.8 ± 1.4, 39-46, n = 60). A similar level of difference was seen in ventrals + subcaudals (males 178.1 ± 2.7, 168-184, n = 70; females 175.4 + 1.9, 170-181, n = 60). Both sexes had a similar average number of body bands (males 13.3 ± 1.6, 11-23, n = 70; females 12.9 ± 1.3, 10-16, n = 58) but females had a higher average number of divided subcaudals (21.2 ± 6.0, 8-35, n = 58) than males (18.3 ± 5.6, 6-31, n = 66). There are no sexual differences in color or pattern.

Juveniles: Juveniles are patterned as adults but are brighter, with more-conspicuous crossbands. The background body color may be pinkish and the bright, sulfur-yellow tail tip is always present. Neonates were 221-237 mm SVL (ave. = 227.9 ± 5.2, n = 9), 268-289 mm total length (ave. = 275.9 ± 7.2), and 14.5-18.0 g body mass (ave. = 16.3 ± 1.0). Neonates in a Chesterfield County population averaged 211.3 mm SVL and 17.3 g body mass (Blem, 1981b).

Confusing Species: This species is very often confused with the harmless Northern Watersnake, Nerodia sipedon, as well as several other aquatic and semi-aquatic snakes. Nerodia sipedon has anterior cross-bands that do not widen on the sides, a series of alternating dorsal and lateral blotches on the rest of the body, paired subcaudal scales along the entire tail, no facial pit, no vertical pupil, and no fangs. Other aquatic snakes are either uniform in color and pattern or have alternating dorsal and lateral blotches, as in Nerodia taxispilota.

Geographic Variation: There is no apparent geographic variation in pattern, color, or scutellation in Virginia populations. Merkle (1985) examined genetic (protein) variation in six populations of this species in Virginia with starch-gel electrophoresis and found that all exhibited a high degree of similarity.

Biology: Agkistrodon piscivorus is a semiaquatic snake inhabiting lowland habitats in southeastern Virginia. These snakes have been found in swamps, freshwater and brackish marshes, ditches, streams, rivers, and forested and grassland habitats adjacent to wet areas, as well as around permanent and semipermanent ponds. When out of the water, Northern Cottonmouths often lie under vegetation, in grasses, or under boards and other shelters. Some individuals bask on logs and horizontal limbs overhanging water. Adults will not venture far from water, but juveniles may disperse over long distances (Martin and Wood, 1955). I have found them to be both diurnal and nocturnal, especially in warm months. The seasonal activity period based on museum records is 28 February to 13 November, but most records are from April through September. Wood (1954a) noted a late season activity date of 4 December, and observed that Northern Cottonmouths migrated from bayside marshes on Currituck Spit to hibernation sites on the mainland in late October and early November. Body temperatures of active snakes were 22.7-32.6°C (ave. = 27.1 ± 4.2, n = 5); ambient temperatures were 21.5-29.5°C (ave. = 25.3 ± 3.5) for these captures.

Northern Cottonmouths eat fish, reptiles, amphibians, small mammals, and invertebrates. The following species have been recorded from Virginia specimens: Coastal Plains Leopard Frogs (Lithobates sphenocephalus utricularius), juvenile Eastern Ratsnakes (Pantherophis alleghaniensis), Ring-Necked Snakes (Diadophis punctatus), juvenile Northern Watersnakes (Nerodia sipedon), and Northern Brownsnakes (Storeria dekayi). Other prey recorded from the eastern part of the range are redfin pickerel (Esox americanus), bass (Micropterus spp.), catfish (lctalurus spp.), Green Treefrogs (Hyla cinerea), Eastern Narrow-mouthed Toads (Gastrophryne carolinensis), hatchling Snapping Turtles (Chelydra serpentina), Southeastern Mud Turtles (Kinosternon subrubrum), Eastern Hog-Nosed Snakes (Heterodon platirhinos), and least shrews (Cryptotis parva) (Hamilton and Pollack, 1955; Bothner, 1974; Brown, 1979; Collins, 1980). Bothner (1974) observed Georgia Northern Cottonmouth biting and swallowing fish under water, although one carried its prey out of the water to finish swallowing it. Juveniles may elevate and wag their tails to lure small prey to them (Wharton, 1969). Large adult Northern Cottonmouths have few predators; humans are the primary ones. Known predators of juveniles found in Virginia include longnose gar (Lepisosteus osseus), bullhead catfishes (lctalurus spp.), large-mouth bass (Micropterus salmoides), Snapping Turtles (Chelydra serpentina), Eastern Kingsnakes (Lampropeltis getula), egrets (Ardea, Egretta), horned owls (Bubo spp.), hawks (Buteo spp.), eagles (Haliaeetus), raccoons (Procyon lotor), otters (Lutra canadensis), dogs, and free-ranging domestic cats (Ernst and Barbour, 1989b; Ernst, 1992).

Agkistrodon piscivorus is viviparous. Mating presumably occurs in the spring, but it may occur sporadically in other months, notably in fall. Male Northern Cottonmouths are known to perform the combat dance described for Agkistrodon contortrix (Gloyd and Conant, 1990). The smallest mature male in the Virginia sample was 755 mm SVL and the smallest mature female was 660 mm SVL. The smallest mature female from the Chesterfield County population measured by Blem (1981b) was 732 mm total length (about 629 mm SVL). Ovulation occurs in late May, and growth of embryos continues until birth in September (Blem, 1981b). Known birth dates are 1-15 September. Litter size for eight females was 5-9 (ave. = 6.1 ± 1.6). Average litter size for the Chesterfield County population was 7.7 ± 1.8 (n = 24), with larger females bearing larger litters and larger young than smaller females (Blem, 1981b, 1982). Female Northern Cottonmouths appear to have a biennial-to-triennial cycle of litter production (Gloyd and Conant, 1990).

Werler and McCallion (1951) found Northern Cottonmouths to be common in freshwater creeks in Virginia Beach and in the area of Back Bay. They are uncommon in Dismal Swamp National Wildlife Refuge, despite the perceptions of most people. The population in Seashore State Park appeared to be smaller in the 1980s than in previous years, in contrast to the contrary statement in Linzey and Clifford (1981). Blem (1981b) thought that unusually cold winter temperatures reduced juvenile survivorship and caused low recruitment in some years in the Chesterfield County population. He reported an adult sex ratio of 1.8:1.0 (males:females) and observed that the population was skewed to large adults. Environmental variation affecting survivorship and procurement of resources for reproduction, as well as loss of individuals due to killing and overcollecting, has apparently caused population declines in Virginia.

Northern Cottonmouths are not aggressive but will often remain in place when approached by humans, unlike other snakes. They will flatten their bodies, vibrate their tails, elevate their heads about 45°, and open their mouths, exposing the white interior (this is the origin of the common name). They will not hesitate to bite if molested, however. Snakes will usually perform this behavior in warm months, but not necessarily in other months. Musk is emitted from glands at the base of the tail whether or not cottonmouths are captured, although much more is excreted then. They can bite whether coiled or not and whether or not they are underwater.

Remarks: Other common names in Virginia are cotton-mouth water moccasin (Dunn, 1918, 1936); water moccasin and cotton-mouth (Carroll, 1950); and lowland moccasin and stub-tailed moccasin (Linzey and Clifford, 1981). The term "cotton mouth" was first used in the scientific literature by Holbrook (1838a, 1842). I concur with Gloyd and Conant (1990), who strongly criticize the use of "moccasin" and "water moccasin" for A. piscivorus. Most people cannot identify Northern Cottonmouths from several species of harmless water snakes (Nerodia), and their use of these vernacular names, implying that the snake is venomous, has unnecessarily caused the deaths of many harmless, nonvenomous snakes.

The historical reference, Lacepede (1788-1789), in which the specific name piscivorus was first used, was ruled unavailable as a binomial publication by the International Commission on Zoological Nomenclature (1987). In this ruling, however, the ICZN also placed the name piscivorus on the Official List of Specific Names in Zoology, "notwithstanding that it was published in an unavailable work," and thus keeping it available for the cottonmouth (Agkistrodon piscivorus).

The largest individual A. piscivorus known (1,880 mm total length) was said to have been caught in the Dismal Swamp region near the Virginia-North Carolina line (Gloyd and Conant, 1990). The largest specimens from Virginia were males and were collected in the Dismal Swamp (1,535 mm total length) and in the upper end of Newport News City Park (1,536 mm).

The venom of this snake is more toxic than that of the Eastern Copperhead (Agkistrodon contortrix); thus a bite from this snake is of more concern and is potentially lethal. Considerations of snakebite and snakebite patterns are in the section "Venomous Snakebite." No deaths have been recorded from A. piscivorus in Virginia since the Bureau of Vital Statistics has been keeping records.

Conservation and Management: Agkistrodon piscivorus is not considered a species of special concern at this time. However, its habitat continues to be lost and some populations are declining. Populations in some areas apparently no longer exist. The conservation status of the cottonmouth should be reinterpreted with information on the status of historically known populations. Maintenance of this species in Virginia requires the protection of all forms of wetlands and their surrounding forests.

U.S. Range

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