Posts Tagged ‘disjunct populations’

Disjunct Populations of the Cottonmouth Water Moccasin (Agkistrodus piscivorus)

March 4, 2012

When I first started reading about disjunct populations of the cottonmouth, I thought they might be interesting examples of populations isolated due to climate-influenced habitat change, but I later learned that most of them have become isolated because man has altered the surrounding environment, rendering it unsuitable for the snakes to survive.  Horizon-to-horizon cotton and corn fields eliminated habitat for intervening populations over 100 years ago in some areas and even more recently in others.

Older adult cottonmouth.  The adults turn so black, it’s difficult to see their markings.

Young cottonmouth killing a northern watersnake.  Note the brown striping which is so colorful compared to the older individual above.

Cottonmouths are a semi-aquatic snake that forages in swamps and lowlands, but in regions where frosts occur during winter, they need adjacent forested uplands where they can den in rocks, fallen logs, and rotted tree trunks.  A disjunct population of cottonmouths exists at Fosters Bend (a notable locality of which I discuss in more detail below).  Fosters Bend is an oxbow lake bordering an intact forest in northwest Georgia.  Dr. Charles Wharton believes the population of cottonmouths became isolated here because the surrounding land was cleared for agriculture.  Other disjunct populations on the periphery of cottonmouth range were Livingston County, Missouri where they were extirpated in 1987, and Montgomery County, Kansas where they were extirpated in 1993.  A genetically isolated population still occurs on Seahorse Key in Florida, but this population is isolated by sea water.

The Agkistrodus genus is part of the pit viper family of snakes.  DNA analysis suggests copperheads (Agkistrodus contortrix) evolved first, probably before the Pleistocene began. Cottonmouths evolved from copperheads that began foraging in swamps rather than dry upland forests. The speciation was likely a result of a differentiation of habitat preference.  The cantil (Agkistrodus bislineatus) then evolved from cottonmouths that expanded their range into Mexico.  Pleistocene cottonmouth fossils (just 4 vertebrae) were excavated from the Isle of Hope site in Chatham County, Georgia. Fossil skeletel remains of copperheads were found in Kingston Saltpeter Cave, Bartow County, Georgia.  Both species have a long history in Georgia, probably dating back at least 2 million years.  Reptile evolution is particularly slow.

Cottonmouth range map color-coded to depict subspecies.  The blue region is populated by the eastern cottonmouth, A. p. piscivorus.  The Florida cottonmouth A.p. conanti inhabits the red region.  The green region is inhabited by the western cottonmouth, A.p. leucostoma.  The gray area has intermediates between all 3 subspecies.

A copperhead.  Copperheads inhabit dry upland forests, while cottonmouths live in the lowlands.  Cottonmouths probably evolved from copperheads that began foraging in lowland swamps.

Pit vipers sense their prey with the pits located near their nostrils.  The pits detect changes in temperature–both warm-blooded and cold blooded prey have body temperatures that differ from the surrounding environment.  Cottonmouth poison is of the tissue destroying kind.  After they strike cottonmouths hang on to small prey such as mice and frogs which might escape before croaking (forgive the pun), but will release larger animals to avoid injury while the animal struggles.

Cottonmouths feed on small mammals, birds, birds’ eggs, baby alligators, fish, other snakes including other cottonmouths, snails, insects, and carrion.  Algae is often accidentally injested.  They are known for congregating around shrinking water holes where they can easily catch concentrated and trapped fish.  On Seahorse Key cottonmouths specialize in feeding upon dead fish dropped or vomited by wading birds.

Cottonmouths sometimes eat king snakes, and vice-versa–it depends on the size of the snake.  Smaller cottonmouths can occasionally repel larger king snakes with body blows, using their head to punch the bigger snake and knock it away repeatedly.  They also exude a musky odor when threatened.  Other natural enemies include alligators, great blue herons, indigo snakes, and largemouth bass.  Cottonmouths return the favor and prey on fledgling herons and trapped bass.  There’s a cycle of unwitting revenge in the swamp theater, depending on the age and size of the players.  Mites can infest and kill cottonmouths, and almost all are plagued with tapeworms from living on a diet of raw fish. 

During winter animosity between cottonmouths and other snakes disappear, and they will nest communally.

I’ve only seen a wild cottonmouth on one occasion.  The snake was a large fat adult, resting on a bank next to Woodbridge Lake in Evans, Georgia.  Young cottonmouths have brown stripes but older ones turn so black that the markings become less visible.  The individual I saw was completely black.  Studies show cottonmouths are not aggressive snakes.  About the only way a person can get bitten is if they step on or handle the snake.  Doctors treat the tissue-destroying venom with Crofab antivenom.  Marlin Perkins, the late zookeeper of Wild Kingdom fame, wrote that the only good thing about getting bitten by a poisonous snake was enjoying a complete recovery, once the antivenom treatment worked.

I don’t understand why people keep reptiles as pets.  Reptiles form no bonds with people.  The part of the brain involved with emotion does not exist in reptiles or any vertebrate less advanced than a bird.

Fosters Bend

Here’s a spot that may be worth exploring, if it hasn’t been developed since Dr. Wharton described it 30 years ago.

Photo of Fosters Bend in Floyd County near the border between Alabama and Georgia.  It’s from The Natural Environments of Georgia by Dr. Charles Wharton.

Fosters Bend is a Coosa River oxbow lake located in Floyd County in northwestern Georgia.  He found a floodplain hardwood forest with species more characteristic of the coastal plain than of the surrounding mountains of north Georgia.  One end of the lake has a forest of tupelo.  A forest of willow oak, water oak, overcup oak, hickory, and persimmon grows in a dried out oxbow.  The natural levee between the oxbow and the Coosa River has a diverse forest of sycamore, silver maple, willow, mulberry, basswood, and several species of oaks.  Archaic and woodland Indians sites occur here as well.