Agkistrodon piscivorus is a venomous pitviper species found in the eastern United States. Adults are large and capable of delivering a painful and potentially fatal bite, but their reputation for aggression is largely undeserved. This is the world's only semiaquatic viper, usually being found in or near water, particularly in slow-moving and shallow lakes and streams. The snake is a strong swimmer and will even enter the sea, successfully colonizing islands off both the Atlantic and Gulf coasts. Many of the common names refer to the fact that, when threatened, this species will often stand its ground and gape at an intruder, exposing the white lining of its mouth.
The diet consists mainly of fish and frogs, but is otherwise highly varied and, uniquely, has even been reported to include carrion. The specific name is derived from the Latin words piscis and voro, which mean "fish" and "to eat". Three subspecies are currently recognized, including the nominate subspecies described here.
The broad head is distinct from the neck, the snout blunt in profile with the rim of the top of the head extending forwards slightly further than the mouth. The body has a heavy build and a tail that is moderately long and slender. On top of the head, a generalized pattern of nine symmetrical head plates is present, although the parietal plates are often fragmented, especially towards the rear. A loreal scale is absent. There are 6-9 supralabials and 8-12 infralabials. At midbody, there are 23-27 rows of dorsal scales. All dorsal scale rows have keels, although those on the lowermost scale rows are weak. In males/females, the ventral scales number 130-145/128-144 and the subcaudals 38-54/36-50. Many of the latter may be divided.
The color pattern consists of a brown, gray, tan, yellowish olive or blackish ground color, which is overlaid with a series of 10-17 crossbands that are dark brown to almost black. These crossbands, which usually have black edges, are sometimes broken along the dorsal midline to form a series of staggered half bands on either side of the body. These crossbands are visibly lighter in the center, almost matching the ground color, often contain irregular dark markings, and extend well down onto the ventral scales. The dorsal banding pattern fades with age, so that older individuals are an almost uniform olive brown, grayish brown or black. The belly is white, yellowish white or tan, marked with dark spots, and becomes darker posteriorly. The amount of dark pigment on the belly varies from virtually nothing to almost completely black. The head is a more or less uniform brown color, especially in A. p. piscivorus. Subadult specimens may exhibit the same kind of dark, parietal spots that are characteristic of A. contortrix, but sometimes these are still visible in adults. Eastern populations have a broad dark postocular stripe, bordered with pale pigment above and below, that is faint or absent in western populations. The underside of the head is generally whitish, cream or tan.
Juvenile and subadult specimens generally have a more contrasting color pattern, with dark crossbands on a lighter ground color. The ground color is then tan, brown or reddish brown. The tip of the tail is usually yellowish, becoming greenish yellow or greenish in subadults, and then black in adults. On some juveniles, the banding pattern can also be seen on the tail.
This species is often confused with the copperhead, A. contortrix. This is especially true for juveniles, but there are differences. A. piscivorus has broad dark stripes on the sides of its head that extend back from the eye, whereas A. contortrix has only a thin dark line that divides the pale supralabials from the somewhat darker color of the head.
The watersnakes of the genus Nerodia are also similar in appearance, being thick-bodied with large heads, but they have round pupils, no loreal pit, a single anal plate, subcaudal scales that are divided throughout and a distinctive overall color pattern.
Catesby (1743) referred to it as the "water viper", while Lacépède (1789) called it "le piscivore." The first appearance of "water moccasin" and "cotton mouth" was in Holbrook's North American Herpetology (1838, 1842).
Campbell and Lamar (2004) mentions this species as being found in Alabama, Arkansas, Florida, Georgia, Illinois, Indiana, Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Texas, and Virginia. Maps provided by Campbell and Lamar (2004) and Wright and Wright (1957) also indicate its presence in eastern Tennessee, extreme southeastern Nebraska and limit it to the western part of Kentucky.
In Georgia it is found in the southern half of the state up to a few kilometers north of the fall line with few exceptions. Its range also includes the Ohio River Valley as far north as southern Illinois, and it inhabits many barrier islands off the coasts of the states where it is found.
Constant persecution of the species and drainage of wetland habitat prior to development has taken a heavy toll on local populations. Despite this, it remains a common species in many areas.
It is also found in brackish water habitats and is sometimes seen swimming in salt water. It has been much more successful at colonizing Atlantic and Gulf coast barrier islands than the copperhead, A. contortrix. However, even on these islands it tends to favor freshwater marshes. A study by Dunson and Freda (1985) describes it as not being particularly salt tolerant.
The snake is not limited to aquatic habitats, however, as Gloyd and Conant (1990) mention that large specimens have been found more than a mile (1.6 km) from water. In various locations the species is well adapted to less moist environments, such as palmetto thickets, pine-palmetto forest, pine woods in eastern Texas, pine flatwoods in Florida, eastern deciduous dune forest, dune and beach areas, riparian forest and prairies.
When sufficiently stressed, this species engages in a characteristic threat display that includes vibrating its tail and throwing its head back with its mouth open to display the startling white interior, while the neck and front part of the body are pulled into an S-shape. Many of its common names, including "Cottonmouth" and "gapper", refer to this behavior, while its habit of snapping its jaws shut when anything touches its mouth has earned it the name "trap-jaw" in some areas. Other defensive responses can include flattening the body and emitting a strong, pungent secretion from the anal glands located at the base of the tail. This musk may be ejected in thin jets if the snake is sufficiently agitated or restrained. The smell has been likened to that of a billy goat, as well as to a genus of common floodplane weeds, Pluchea, that also have a penetrating odor.
Harmless watersnakes of the genus Nerodia are often mistaken for it. These are also semiaquatic, thick-bodied snakes with large heads that can be aggressive when provoked, but they behave differently. For example, watersnakes usually flee quickly into the water, while A. piscivorus often stands its ground with its threat display. In addition, watersnakes do not vibrate their tails when excited. A. piscivorus usually holds its head at an angle of about 45° when swimming or crawling. It swims with much of its body floating above the surface, while watersnakes tend to swim mostly below the surface, sometimes with their heads protruding.
Brown (1973) considered their heavy muscular bodies to be a striking characteristic, stating that this made it difficult to hold them for venom extraction due to their strength.
This species may be active during the day as well as at night. However, on bright, sunny days they are usually found coiled or stretched out somewhere in the shade. In the morning and on cool days they can often be seen basking in the sunlight. At night, however, they are at their most active, when they are usually found swimming or crawling. Contrary to popular belief, they are capable of biting while underwater.
In the north, they hibernate during the winter months. Niell (1947, 1948) made observations in Georgia and noted that they were one of the last species to seek shelter, often being found active until the first heavy frosts. At this point they moved to higher ground and could be found in rotting pine stumps by tearing away the bark. These snakes could be quite active upon discovery and would then attempt burrow more deeply into the soft wood or escape to the nearest water. In southeastern Virginia, Wood (1954) reported seeing migratory behavior in late October and early November. During a period of three or four days, as many as 50 individuals could be seen swimming across Back Bay from the bayside swamps of the barrier islands to the mainland. He suggested that this might have something to do with hibernating habits. In the southern parts of its range, hibernation may be short or omitted altogether.
Many authors have described the prey items taken under natural circumstances. Although fish and frogs are their most common prey, they will eat almost any small vertebrate. Campbell and Lamar (2004) provide an exhaustive list of species that have reportedly been preyed upon by A. piscivorus, including cicadas, caterpillars, land snails (Euglandina rosea), catfish (Ictalurus furcatus), pike (Esox ssp.), sunfishes (Lepomis ssp.), bass (Micropterus ssp.), sirens (Siren sp.), eastern newts (Notophthalmus viridescens), brook salamanders (Eurycea sp.), Ouachita dusky salamanders (Desmognathus brimleyorum), spadefoot toads (Scaphiopus), eastern narrowmouth toads (Gastrophryne carolinensis), northern cricket frogs (Acris crepitans), West Indian treefrogs (Osteopilus septentrionalis), treefrogs (Hyla sp.), true frogs (Rana sp.), green anoles (Anolis carolinensis), skinks (Eumeces sp.), eastern glass lizards (Ophisaurus ventralis), ground skinks (Scincella lateralis), mudsnakes (Farancia abacura), hog-nosed snakes (Heterodon platirhinos), kingsnakes (Lampropeltis sp.), watersnakes (Nerodia sp.), crayfish snakes (Regina sp.), brown snakes (Storeria dekayi), gartersnakes and ribbonsnakes (Thamnophis sp.), other cottonmouths (A. piscivorus), rattlesnakes (Crotalus sp.), common snapping turtles (Chelydra serpentina), mud turtles (Kinosternon sp.), common musk turtles (Sternotherus odoratus), Florida cooters (Pseudemys floridana), sliders (Trachemys scripta), eastern box turtles (Terrapene carolina), Florida softshell turtles (Apalone ferox), baby American alligators (Alligator mississippiensis), wood thrushes (Hylocichla mustelina), chickadees (Parus sp.), cardinals (Cardinalis cardinalis), unidentified passerines, small ducks, juvenile anhingas (Anhinga anhinga), common egrets (Ardea alba), egrets, glossy ibises and their eggs (Plegadis falcinellus), tricolor herons (Egretta tricolor), herons and their eggs, pied-billed grebes (Podilymbus podiceps), short-tailed shrews (Blarina brevicauda), least shrews (Cryptotis parva), southeastern shrews (Sorex longirostris), eastern moles (Scalopus aquaticus), muskrats (Ondatra zibethicus), rice rats (Oryzomys palustris), hispid pocket mice (Perognathus hispidus), black rats (Rattus rattus), squirrels (Sciurus sp.), rabbits (Sylvilagus sp.) and bats.
Fish are captured by cornering them in shallow water, usually against the bank or under logs. They take advantage when bodies of water begin to dry up in the summer or early fall and gorge themselves on the resulting high concentrations of fish and tadpoles. A study by Savitsky (1992) found that they were surprisingly unsuccessful at seizing either live or dead fish underwater.
They are opportunistic feeders and will sometimes eat carrion. Campbell and Lamar (2004) describe having seen them feeding on fish heads and viscera that had been thrown into the water from a dock. Heinrich and Studenroth (1996) report an occasion in which an individual was seen feeding on the butchered remains of a wild hog (Sus scrofa) that had been thrown into Cypress Creek.
Conant (1929) gives a detailed account of the feeding behavior of a captive specimen from South Carolina. When prey was introduced, the snake quickly became attentive and made an attack. Frogs and small birds were seized and held until movement stopped. Larger prey was approached in a more cautious manner; a rapid strike was executed after which the snake would withdraw. In 2.5 years the snake had accepted three species of frogs, including a large bullfrog, a spotted salamander, water snakes, garter snakes, sparrows, young rats and three species of mice. Brimley (1944) described a captive specimen that ate copperheads (Agkistrodon contortrix), as well as members of its own species, keeping its fangs embedded in its victims until they had been immobilized.
Young individuals have yellowish or greening tail tips and engage in caudal luring. The tail tip is wriggled to lure prey, such as frogs and lizards, within striking distance. Wharton (1960) observed captive specimens exhibiting this behavior between 07:20 and 19:40 hours, which suggests that it is a daytime activity.
They are also preyed upon by ophiophagous snakes, including their own species (cannibalism). Humphreys (1881) described how a 34-inch (864 mm) specimen that was killed and eaten by a 42-inch (1067 mm) captive kingsnake. On the other hand, Neill (1947) reported that captive kingsnakes (Lampropeltis getula) were loathe to attack them, being successfully repelled with "body blows." Also called body-bridging, this is a specific defensive behavior against ophiophagous snakes, first observed in certain rattlesnake (Crotalus) species by Klauber (1927), that involves raising a section of the middle of the body above the ground to varying heights. This raised loop may then be held in this position for varying amounts of time, shifted in position, or moved towards the attacker. In the latter case, it is often flipped or thrown vigorously in the direction of the assailant. In A. piscivorus, the loop is raised laterally, with the belly facing towards the attacker.
Regarding A. p. piscivorus, an early account by Stejneger (1895) describes a pair in the Berlin Zoological Garden that mated on January 21, 1873, after which eight neonates were discovered in the cage on July 16 of that year. The young were each 26 cm in length and 1.5 cm thick. They shed for the first time within two weeks, after which they accepted small frogs, but not fish.
Combat behavior between males has been reported on a number of occasions and is very similar in form to that seen in many other viperid species. An important factor in sexual selection, it allows for the establishment and recognition of dominance as males compete for access to sexually active females.
A few accounts exist that describe females defending their newly born litters. Wharten (1960, 1966) reported several cases where females found near their young stood their ground and considered these to be examples of guarding behavior. Another case was described by Walters and Card (1996) in which a female was found at the entrance of a chamber with seven neonates crawling on or around her. When one of the young was moved a short distance from the chamber, she seemed to be agitated and faced the intruder. Eventually, all of her offspring retreated into the chamber, but the female remained at the entrance, ready to strike.
Brown (1973) gives an average venom yield (dried) of 125 mg, with a range of 80-237 mg, along with LD50 values of 4.0, 2.2, 2.7, 3.5, 2.0 mg/kg IV, 4.8, 5.1, 4.0, 5.5, 3.8, 6.8 mg/kg IP and 25.8 mg/kg SC for toxicity. Wolff and Githens (1939) describe a 152 cm (60 inches) specimen that yielded 3.5 ml of venom during the first extraction and 4.0 ml five weeks later (1.094 grams of dried venom).
Symptoms commonly include ecchymosis and swelling. The pain is generally more severe than bites from the copperhead (A. contortrix), but less so than those from rattlesnakes (Crotalus sp.). The formation of vesicles and bullae is less common than with rattlesnake bites, although necrosis can occur. Myokymia is sometimes reported. On the other hand, the U.S. Navy (1991) states that the venom has strong proteolytic activity that can lead to severe tissue destruction.
CroFab polyvalent antivenin is used to treat bites from this species.
|Subspecies||Taxon author||Common name||Geographic range|
|A. p. conanti||Gloyd, 1969||Florida cottonmouth||The United States, in extreme southern Georgia and virtually all of the state of Florida, including many of the islands off the coast.|
|A. p. leucostoma||(Troost, 1836)||Western cottonmouth||The United States, from southern Alabama along coast of the Gulf of Mexico, including many offshore islands, to southeastern and central Texas, and north to Oklahoma, Missouri, Illinois and Indiana.|
|A. p. piscivorus||(Lacépède, 1789)||Eastern cottonmouth||The United States in southeastern Virginia, the Atlantic Coastal Plain and lower Piedmont of North and South Carolina, including the banks, peninsulas and islands along the Atlantic coast, and west across Georgia.|