Ecology/Cottonmouth Snake

Introduction

The Cottonmouth snake, also known as a water moccasin (Agkistrodon piscivorus) is found near bodies of water all over the Southeastern United States. It is best known for the white lining of its mouth, which it exposes when threatened. They are the world’s only semi-aquatic vipers with the capacity to deliver a painful, if not fatal, bite (5). There are three subspecies of A. piscivorus; A. p. pscivorus, A. p. leucostoma, and A. p. conanti (1). Of these three subspecies, A. p. conanti is dominant in Florida.

Physical Characteristics

Cottonmouth snakes are usually 30 to 48 inches in length, but can reach lengths of up to 74 inches (3). Its body is a heavy, thick build and thins quickly at the tail. Color patterns range from black or gray to tan and yellowish. A series of dark cross bands run the length of the snake and can be either brown or black, but are typically reserved for the juveniles of the species. The pattern fades with age, leaving adult cottonmouth snakes with a uniform brown, gray or black coloring. The underbelly is usually pale or white (1). The snakes are a member of the viperidae family, possessing hollow fangs to inject venom deep into its prey. It belongs to the subclass crotalinae, or pit-viper (1). Pit vipers have developed special organs found in pits, hence the name, located right behind the nostrils. These pits, covered with a temperature sensitive membrant, allow the cottonmouth to sense warm blooded animals. It is an adaptation which proves especially useful at night, when most cottonmouths prefer to hunt. Some cottonmouths may also use this adaptation to find cool places to escape inhospitable temperatures (6).

Habitat

The geographic range of A. piscivorus differs among the subspecies of the snakes. A. p. piscivorus is dominant along the Atlantic coast, as far north as Southern Virginia. The smaller subspecies, A. p. leucostoma is found as far west as Texas and Oklahoma and as far north as Southern Illinois. A. p. conanti, the Florida Cottonmouth dominates all of Florida (1). The cottonmouth snake is most commonly found around swamps, lakes, slow moving streams, marshes and even drainage ditches. They are most active at night when they hunt, but will spend the early part of the day lying on a tree branch, basking in the sun. They are known to gather around dying water holes in order to feast on the trapped fish and frogs (1). The snakes are very strong swimmers, spending most of their time in the water. They have been known to enter the sea and successfully colonize offshore islands. They occur in particularly large numbers on some of the islands on the Gulf Coast of Florida. Their success has much to do with their opportunistic feeding.

Feeding

The snake’s diet mainly consists of small mammals, birds, fish, frogs and tadpoles. Fish are caught by cornering them underwater. Contrary to popular belief, the cottonmouth snake has the ability to bite and deliver its venom underwater, making it a very effective hunter (1). Even though it has the ability to catch its own prey, the main reason the snake has become so successful is due to their opportunistic feeding habits, especially when colonizing offshore islands in the gulf. The snake will scavenge for food, seeking out carrion that washes up onto the shore, or detritus that falls from the nests of birds. Cottonmouth populations are reflected by bird populations of offshore islands, with the higher concentration of snakes existing where there is a higher concentration of birds (bioone). When attacking, the snake utilizes a hemotoxin which affects the blood and its ability to clot (6). The broken down blood cells travel to the heart and coagulate in the pulmonary artery. The pain is intense, and as blood cells begin to break down there is an immediate swelling and discoloration. Severe hemorrhage occurs at any point in contact with the venom (2). The venom is produced in glands near the back of the mouth, at the junction of the upper and lower jaws. As the snake strikes, the muscles around the venom glands contract and force the toxin out through the hollow teeth and into the prey. Whereas in normal conditions the venom supply of the cottonmouth is never fully depleted; full venom replacement can take up to three weeks (2).

Behavior

The snake has been given a bad reputation over the years as a very aggressive snake that will attack anything nearby. When tested to measure the different responses of the cottonmouth to encounters with humans, 23 of 45 (51%) tried to escape while 28 of 36 (78%) resorted to threat displays and other defensive tactics. It was only when they were picked up with a mechanical hand were they likely to bite (4). The characteristic threat display that has earned the cottonmouth its name includes throwing its head back with its jaws open to show the white lining of the mouth. The tail vibrates, much like other vipers like rattlesnakes, and the head and body are curled into an S-shape, appearing as though the snake is waiting to strike. The snake is most likely to stand its ground and resort to these most common threat displays which has often been mistaken for aggressive behavior (1). The cottonmouth can be active during the day or the night. Usually during the night. They bask during the day. They are also stretched out or coiled up in the shade. On cooler days they have been known to sun themselves on rocks, tree branches or logs lying close to shore. They are most active at night. Northern species of A. piscivorus have been known to become inactive during the winter, however they are typically the last species to seek shelter. They do not enter full hibernation, though, as they are known to be very active upon discovery and will immediately try to burrow deeper or escape to nearby water. Shelter for the cottonmouth during these inactive periods usually consists of rotting trees, crayfish burrows or the burrows of other mammals – muskrat, beaver, etc. (4).

Reproduction and Growth

)The cottonmouth snake is ovoviviparous , meaning they develop in eggs protected inside the mothers body until they hatch, or are about to hatch. This type of reproduction is found in many aquatic life forms. Nourishment is provided by the egg yolk instead of the mother; however the mother’s body does provide gas exchange (8). In the case of the cottonmouth, the eggs hatch inside the mother and the young, typically 15 per litter, are birthed alive. There is typically one litter birthed every other year.

Breeding occurs during the warmer months, and most births occur after a three month gestation period. However, differences in the physiology of the individual female snake cause the actual gestation period to vary from season to season (9). Cottonmouths are not known to give parental care after birth, however there have been cases reported of defensive behavior of a mother for her litter. One report details a female positioned at the mouth of a burrow surrounded by her young. When one of the young was moved a distance away from the mother, she became agitated and faced the intruder. Once the offspring was returned, she remained at the mouth of the burrow, ready to strike (1).

Natural Predators

The cottonmouth snake is preyed upon by larger organisms, including large birds, and other reptiles like the snapping turtle and American alligator. There have also been widespread reports of cannibalism by other cottonmouths (1). Humans are also a threat to the cottonmouth. Seeing as it gained a reputation as a very aggressive snake, many people take it upon themselves to kill the cottonmouth, and even other snakes that resemble cottonmouths. The cottonmouth is listed as Least Concern (3).

Cladogram

Snakes in the genus Agkistrodon are closely related to those in the genus of Crotalus and Sistrurus. Agkistrodon and Sistrurus have nine large head plates, whereas Crotalus as well as all other vipers have a head covered in smaller scales. Both Crotalus and sistrurus have rattles on the end of their tails. Crotalus is best known as the rattlesnake. Sistrurus lives farther north in Canada and has a much smaller rattle (9)(10). What separates Agkistrodon is their adaptation to water. Neither Crotalus or Sistrurus have adapted to life in lakes or streams. In fact, Eastern Diamondback Rattlesnakes are known to inhabit dry, arid areas of the U.S.

Resources

1. (2009, April 2). Agkistrodon piscivorus. Retrieved April 26, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Agkistrodon_piscivorus 2. (2009). Smithsonian National Zoological Park. Retrieved April 26, 2009, from Cottonmouth Fact Sheet Web site: http://nationalzoo.si.edu/Animals/ReptilesAmphibians/Facts/FactSheets/Cottonmouth.cfm 3. (2008). International Union for Conservation of Nature and Natural Resources. Retrieved April 27, 2009, from ICUN 2008 Red List Web site: http://www.iucnredlist.org/details/64298 4. Gibbons, J. Whitfield, & Dorcas, Michael E. (2002). Defensive Behavior of Cottonmouths (Agkistrodon piscivorus) toward Humans. Copeia. 1, 195-198. 5. University of Georgia, (2008). Snakes of Georgia and South Carolina. Retrieved April 27, 2009, from Cottonmouth (water moccasin) Web site: http://www.uga.edu/srelherp/snakes/agkpis.htm 6. (2009, April 2). Snake Venom. Retrieved April 28, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Snake_venom#Vipers 7. Ovoviviparlty. Retrieved April 28, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Ovoviviparity 8. (2009). Cottonmouth (water Moccasin) Reference Library. Retrieved April 28, 2009, from Red Orbit Web site: http://www.redorbit.com/education/reference_library/reptiles/cottonmouth_water_moccasin/2557/index.html 9. (2007). Crotalus. Retrieved April 28, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Crotalus 10. (2007). Sistrurus. Retrieved April 28, 2009, from Wikipedia Web site: http://en.wikipedia.org/wiki/Sistrurus 11. Cundall, David, & Deufel, Alexandra (2006). Influence of Venom Delivery System on Intraoral Prey Transport in Snakes. Zoologicher Anzeiger. 245, 193-210.