Northern Cottonmouth

Appearance

This is the largest species of the genus "Agkistrodon". Adults commonly exceed 80 cm in length, females grow smaller than males. Total length per one study of adults was 65 to 90 cm. Average body mass has been found to be 292.5 to 579.6 g in males and 201.1 to 254.1 g in females. Occasionally, individuals may exceed 180 cm in length, especially in the eastern part of the range. According to Gloyd and Conant, the largest recorded specimen of "A. p. piscivorus" was 188 cm in length, based on a specimen caught in the Dismal Swamp region and given to the Philadelphia Zoological Garden. It should be noted, however, that this snake had apparently been injured during capture, died several days later and was measured when straight and relaxed. Large specimens can be extremely bulky, with the mass of a specimen of approximately 180 cm in length known to attain 4.6 kg.

The broad head is distinct from the neck, and the snout is blunt in profile with the rim of the top of the head extending forwards slightly further than the mouth. Substantial cranial plates are present, although the parietal plates are often fragmented, especially towards the rear. A loreal scale is absent. There are six to 9 supralabials and eight to 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.

Though the majority of specimens are almost or even totally black,, the color pattern may consist of a brown, gray, tan, yellowish-olive or blackish ground color, which is overlaid with a series of 10–17 dark brown to almost black crossbands. These crossbands, which usually have black edges, are sometimes broken along the dorsal midline to form a series of staggered halfbands 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 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 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. Young snakes wiggle the tip of their tail to lure prey animals.

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.

Status

This species is classified as Least Concern on the IUCN Red List of Threatened Species. Species are listed as such due to their wide distribution, presumed large population, or because they are unlikely to be declining fast enough to qualify for listing in a more threatened category. When last assessed in 2007, the population trend was stable.

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.

In Indiana, the cottonmouth is listed as an endangered species.

Behavior

The aggressiveness of these snakes has been greatly exaggerated. In tests designed to measure the various behavioral responses by wild specimens to encounters with people, 23 of 45 tried to escape, while 28 of 36 resorted to threat displays and other defensive tactics. Only when they were picked up with a mechanical hand were they likely to bite.

When sufficiently stressed or threatened, 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, often making a loud hiss while the neck and front part of the body are pulled into an S-shaped position. Many of its common names, including "cottonmouth" and "gaper", 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 flood plain 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.

Brown considered their heavy muscular bodies to be a striking characteristic, stating this made it difficult to hold them for venom extraction owing 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. They often emerge at sunset to warm themselves on warm ground and then become very active throughout the night, 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 made observations in Georgia and noted 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 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 this might have something to do with hibernating habits. In the southern parts of its range, hibernation may be short or omitted altogether.

Habitat

This is the most aquatic species of the genus "Agkistrodon", and is usually associated with bodies of water, such as creeks, streams, marshes, swamps and the shores of ponds and lakes. The U.S. Navy describes it as inhabiting swamps, shallow lakes and sluggish streams, but it is usually not found in swift, deep, cool water. Behler and King list its habitats as including lowland swamps, lakes, rivers, bayheads, sloughs, irrigation ditches, canals, rice fields and small clear rocky mountain streams.

It is also found in brackish water habitats and is sometimes seen swimming in saltwater. 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 describes it as not being particularly salt tolerant.

The snake is not limited to aquatic habitats, however, as Gloyd and Conant mentioned large specimens have been found more than a mile from water. In various locations, the species is well-adapted to less moist environments, such as palmetto thickets, pine-palmetto forest, pine woods in East Texas, pine flatwoods in Florida, eastern deciduous dune forest, dune and beach areas, riparian forest and prairies.

Reproduction

This species is ovoviviparous, with females giving birth to one to 16 live young and possibly as many as 20. However, litters of six to eight are the most common. Neonates are 22–35 cm in length, with the largest being "A. p. conanti" and "A. p. leucostoma" the smallest. If weather conditions are favorable and food is readily available, growth is rapid and females may reproduce at less than three years of age and a total length of as little as 60 cm. The young are born in August or September, while mating may occur during any of the warmer months of the year, at least in certain parts of its range.

Regarding "A. p. piscivorus", an early account by Stejneger described 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 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 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.

Food

Raymond Ditmars described this species as "omnicarnivorous". Its diet includes mammals, birds, amphibians, fish, snakes, small turtles and small alligators. Cannibalism has also been reported. Normally, though, the bulk of its diet consists of fish and frogs. On occasion, juvenile specimens feed on invertebrates. Catfish are often eaten, although the sharp spines sometimes cause injuries. Toads of the genus "Bufo" are apparently avoided.

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 provided an exhaustive list of species that have reportedly been preyed upon by "A. piscivorus", including cicadas, caterpillars, land snails, catfish, pike, sunfishes, bass, sirens, eastern newts, brook salamanders, Ouachita dusky salamanders, spadefoot toads, eastern narrowmouth toads, northern cricket frogs, West Indian treefrogs, treefrogs, true frogs, green anoles, skinks, eastern glass lizards, ground skinks, mudsnakes, hog-nosed snakes, kingsnakes, watersnakes, crayfish snakes, brown snakes, gartersnakes and ribbonsnakes, other cottonmouths, rattlesnakes, common snapping turtles, mud turtles, common musk turtles, Florida cooters, sliders, eastern box turtles, Florida softshell turtles, baby American alligators, wood thrushes, chickadees, cardinals, unidentified passerines, small ducks, juvenile anhingas, common egrets, egrets, glossy ibises and their eggs, tricolor herons, herons and their eggs, pied-billed grebes, short-tailed shrews, least shrews, southeastern shrews, eastern moles, muskrats, rice rats, hispid pocket mice, black rats, squirrels, rabbits 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 found they were surprisingly unsuccessful at seizing either live or dead fish underwater.

They are opportunistic feeders and will sometimes eat carrion. Campbell and Lamar described having seen them feeding on fish heads and viscera that had been thrown into the water from a dock. Heinrich and Studenroth reported an occasion in which an individual was seen feeding on the butchered remains of a feral hog that had been thrown into Cypress Creek.

Conant gave 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 described a captive specimen that ate copperheads, 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 greenish tail tips and engage in caudal luring. The tail tip is wriggled to lure prey, such as frogs and lizards, within striking distance. Wharton observed captive specimens exhibiting this behavior between 07:20 and 19:40 hours, which suggests it is a daytime activity.

Predators

These snakes are preyed upon by snapping turtles, American alligators, horned owls, eagles, hawks, loggerhead shrikes, and large wading birds, such as herons, cranes and egrets.

They are also preyed upon by ophiophagous snakes, including their own species. Humphreys described how a 34-inch specimen was killed and eaten by a 42-inch captive kingsnake. On the other hand, Neill reported captive kingsnakes were loath 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 species by Klauber, 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.

Defense

"A. piscivorus" venom is more toxic than that of "A. contortrix" and is rich with tissue-destructive enzymes. It is a powerful cytotoxic venom that destroys tissue. Although deaths are rare, the bite could leave scars and, on occasion, require amputation. Absent an anaphylactic reaction in a bitten individual, however, the venom does not cause systemic reactions in victims and does not contain neurotoxic components that are present in numerous rattlesnake species. Bites can be effectively treated with CroFab antivenom; this serum is derived using venom components from four species of American pit vipers.

Bites from the cottonmouth are relatively frequent in the lower Mississippi River Valley and along the coast of the Gulf of Mexico, although fatalities are rare. Allen and Swindell compiled a record of "A. piscivorus" bites in the state of Florida from newspaper accounts and data from the Bureau of Vital Statistics: 1934, eight bites and three fatalities; 1935, 10; 1936, 16; 1937, 7; 1938, 6; 1939, 5; 1940, 3; 1941, 6; 1942, 3; 1943, 1; 1944, 3, 1998; 1. Wright and Wright report having encountered these snakes on countless occasions, often almost stepping on them, but never being bitten. In addition, they heard of no reports of any bites among 400 cypress cutters in the Okefenokee Swamp during the entire summer of 1921. These accounts suggest that the species is not particularly aggressive.

Brown gave an average venom yield 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 described a 152 cm specimen that yielded 3.5 ml of venom during the first extraction and 4.0 ml five weeks later.

Symptoms commonly include ecchymosis and swelling. The pain is generally more severe than bites from the copperhead but less so than those from rattlesnakes. 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 US Navy states the venom has strong proteolytic activity that can lead to severe tissue destruction.