Natural History of Cottonmouth Moccasin, Agkistrodon piscovorus (Reptilia) Part 1

Natural History of Cottonmouth Moccasin, Agkistrodon piscovorus (Reptilia).

by Ray D. Burkett.

INTRODUCTION

Objectives of the study here reported on were to: (1) learn as much as possible concerning the natural history and economic importance of the cottonmouth; (2) determine what factors limit its geographic distribution; (3) determine the role of the cottonmouth in its ecological community; and (4) compare the cottonmouth's life history with that of other crotalid snakes, especially the kinds that are most closely related to it.

Twenty-five live cottonmouths were kept in the laboratory for the purpose of studying behavior and fang shedding and for comparison of measurements with those of preserved specimens. Live snakes were obtained in Brazoria and Nacogdoches counties, Texas, from Hermann Park Zoo, Houston, Texas, and from the late Paul Anderson of Independence, Missouri. Preserved western cottonmouths were examined for the purpose of determining variation, distribution, food habits, body proportions, embryonic development, and reproductive cycles. The cottonmouths examined include: 221 from Texas; 33 from Arkansas; 22 from Louisiana; 2 from Illinois; and 1 each from Kansas, Mississippi, and Oklahoma.

In the preparation of this report I have examined all available literature pertaining to the cottonmouth and have drawn from these sources for comparative or additional material. Some of the more noteworthy contributions to knowledge of the cottonmouth are the general accounts of the life history by Allen and Swindell (1948), Barbour (1956), and Wright and Wright (1957); the publications by Gloyd and Conant (1943) concerning taxonomy; Klimstra (1959) concerning food habits; and Allen (1937), Parrish and Pollard (1959), Swanson (1946), and Wolff and Githens (1939b) concerning the venom. Numerous other publications, although brief, contain worthwhile contributions. Also of special interest as a source of material for comparison of cottonmouths with other crotalids are the works of Fitch (1960) on the copperhead and of Klauber (1956) on the rattlesnakes.

The cottonmouth has been well known for nearly 200 years. Wright and Wright (1957) listed the following vernacular names that are applied to the cottonmouth: black moccasin, black snake, blunt-tail moccasin, congo, copperhead, cottonmouth water moccasin, cotton-mouthed snake, gapper, highland moccasin, lowland moccasin, mangrove rattler, moccasin, North American cottonmouth snake, North American water moccasin, North American water viper, pilot, rusty moccasin, salt-water rattler, stubtail, stump (-tail) moccasin, stump-tail viper, swamp lion, Texas Moccasin, trapjaw, Troost's moccasin, true horn snake, true water moccasin, viper, water mokeson, water pilot, water rattlesnake, and water viper.

Some of the names listed above are based upon superst.i.tion and folklore prevailing in pioneer times, and others are based upon the behavior or appearance of the snake at various ages. Names like "stump-tail moccasin" are derived from the appearance of females which have short tails or snakes that have lost part of the tail. Names like "gapper" and "trapjaw" came to be applied because of the habit of the snake's lying with its mouth open when approached. The name "cottonmouth" also was derived from this behavior, although the lining of the mouth is whitish in most other snakes. The term "rattlesnake" may have come from the fact that the cottonmouth vibrates its tail vigorously when nervous as do many other snakes, or it may have been confused with rattlesnakes.

Because of the general public's fear of snakes and their reluctance to learn to discriminate between the poisonous and harmless species, numerous kinds of snakes seen in or near water have been called moccasins. The general appearance, pugnacious behavior, and whitish mouth of water-snakes (_Natrix_) have earned them a bad reputation. In fact, a great majority of the "cottonmouths" reported in many areas are found to be water-snakes.

The cottonmouth is economically important mainly because of the injurious or fatal effects of its bite and the psychological effect that its actual or suspected presence has upon many persons. The species eats a wide variety of prey items and helps to prevent overabundance of certain kinds of organisms. The venom has been used in the therapeutic treatment of blood clots owing to its anticoagulant properties (Didisheim and Lewis, 1956). It also is employed in the treatment of haemorrhagic conditions and rheumatoid arthritis, as well as in the production of antivenin (Allen and Swindell, _op cit._:13). None of these uses of venom has become widely accepted, and its value is questionable.

ACKNOWLEDGMENTS

For guidance in the course of my study, I am especially indebted to Professor Henry S. Fitch. For suggestions concerning the preparation of the ma.n.u.script, I thank Professor E. Raymond Hall. I am grateful to my wife, Janis, for her invaluable a.s.sistance and for typing the ma.n.u.script.

For use of specimens in their care, I thank Professors William E.

Duellman, University of Kansas; Robert L. Packard, formerly of Stephen F. Austin State College; W. Frank Blair, University of Texas; and William B. Davis and Richard J. Baldauf, Texas Agricultural and Mechanical College. Mr. John E. Werler of the Hermann Park Zoo, Houston, Texas, contributed live individuals; Mr. Richard S. Funk contributed information on the birth of a brood of cottonmouths; and Dr. Henry M.

Parrish contributed information on the incidence of snakebite. To numerous other persons at leading museums throughout the United States for information on the cottonmouths in their collections, to all who helped with the field work in various ways, and to others at the University of Kansas for their help and suggestions I am grateful.

SYSTEMATIC RELATIONSHIPS AND DISTRIBUTION

Snakes of the genus _Agkistrodon_ are relatively primitive members of the Crotalidae, which is one of the most specialized families of snakes.

A majority of the pit-vipers are found in the Americas, but close relatives are found from extreme southeastern Europe through temperate Asia to j.a.pan (_A. halys_) and southeastern Asia including Indonesia (_Agkistrodon_ and _Trimeresurus_). Familial characters include: vertical pupil of the eye; facial pit present between the preoculars and loreal; scales usually keeled; short, rotatable maxilla bearing a large hollow fang; toothless premaxilla; chiefly hematoxic venom; and undivided a.n.a.l plate.

The genus _Agkistrodon_ includes about nine species in the Old World and three in North and Central America. Some of the primitive characters of the genus are: head covered with nine enlarged shields or having the internasals and prefrontals broken up into small scales; subcaudals on proximal part of tail undivided; fangs relatively short; tail lacking rattles. In one species, _A. rhodostoma_, the scales are smooth; and the female is oviparous and guards her eggs until they hatch. Other species have keeled scales and are ovo-viviparous.

There is little paleontological evidence ill.u.s.trating evolution of the cottonmouth or for that matter of crotalids in general. Brattstrom (1954) summarized the current knowledge of fossil pit-vipers in North America. The few fossils found of the cottonmouth are from Alacha, Brevard, Citrus, Levy, Pasco, and Pinellas counties, Florida (Brattstrom, _op. cit._:35; Auffenberg, 1963:202). All are of late Pleistocene Age and well within the present geographic range of the cottonmouth.

Of crotalid genera only _Agkistrodon_ occurs in both the Old World and the New World, suggesting that this genus is relatively old. Schmidt (1946: 149-150) mentioned several other closely related groups of animals found in both eastern Asia and eastern North America, including the reptilian genera: _Natrix_, _Opheodrys_, _Elaphe_, _Ophisaurus_, _Leiolopisma_ (= _Lygosoma_), _Eumeces_, _Clemmys_, _Emmys_, and _Alligator_. Of the groups of animals now confined to these two regions the most important are the cryptobranchid salamanders, the genus _Alligator_, and the spoon-bills (_Psephurus_ in China and _Polyodon_ in the Mississippi drainage). Fossil evidence for these groups indicates that existing forms common to eastern Asia and eastern North America are remnants of a late Cretaceous or early Tertiary Holarctic fauna which was forced southward as the climate became gradually cooler to the north. "Other clues suggest that both _Agkistrodon_ and _Trimeresurus_ (_Bothrops_) moved from Asia to America, one of these presumably giving rise to the rattlesnakes." (Darlington, 1957:228).

The named, American kinds of _Agkistrodon_ currently are arranged as three species: the copperhead, the cantil and the cottonmouth. The copperhead (_A. contortrix_) is divided into four subspecies, all of which are terrestrial. This species occurs from southern New England to eastern Kansas and along the Atlantic and Gulf Coastal plains, exclusive of peninsular Florida and the delta of the Mississippi River in Louisiana. It extends southwest from Kansas through the Edwards Plateau of west-central Texas. Isolated populations occur in the Chisos and Davis mountains of Trans-Pecos Texas. The cantil or Mexican moccasin (_A. bilineatus_), probably the nearest relative of the cottonmouth (_A.

piscivorus_), is divisible into two subspecies and occupies a nearly complementary range from Mexico south to Nicaragua. The cottonmouth occurs throughout the coastal plains of the southeastern United States, usually at alt.i.tudes of 500 feet or less. Two subspecies are recognized, the eastern _A. p. piscivorus_ and the western _A. p. leucostoma_. A revision of the genus is underway by Professor Howard K. Gloyd.

The basic pattern and various behavioral traits are common to all three species. The young are more nearly alike in appearance than adults, the copperhead and the cottonmouth being easily confused. Adults differ in color, size, body proportions, habitat, and habits. In range and habitat preference the cottonmouth more closely resembles the southern subspecies of the copperhead, _A. c. contortrix_, which is usually found in lowlands, near swamps and streams, but seldom in water.

[Ill.u.s.tration: FIG. 1.]

FIG. 1. Geographic range of the cottonmouth, showing marginal and near-marginal records, based largely upon maps by Gloyd and Conant (1943:165) and Conant (1958:336) but including additional records. The more important of these records (from east to west) are discussed in the following paragraphs. Crosshatching indicates the area of intergradation between the eastern and western subspecies. Old records, indicated by dates, and their sources are as follows: 1850's and 1891--U. S. National Museum numbers 4263 and 32753 respectively; 1897--Hurter (1897); and 1895--Stejneger (1895:408).

The northernmost record for the eastern subspecies is in the Petersburg area, Prince George County, Virginia (Anon., 1953:24). A sight record (Hickman, 1922:39) near Bristol, West Virginia, probably was based on a water-snake (_Natrix_ sp.), since the stream in which the snake was seen flows north into the Ohio River rather than southeast through Virginia.

In North Carolina the most inland record is from the Neuse River, six to eight miles east of Raleigh (Stejneger, 1895:408). Neill (1947:205) reported a population in the vicinity of Dry Fork Creek on the boundary line of Wilkes and Oglethorpe counties, Georgia. Distribution of cottonmouths in Florida is statewide, including the Keys and other offsh.o.r.e islands.

The ranges of the two subspecies, _piscivorus_ and _leucostoma_, meet near the eastern border of Mississippi. _A. p. piscivorus_ has been reported from Tishomingo County to the Gulf and east of the Loess Bluff area in central Mississippi, and _A. p. leucostoma_ has been reported from this area westward. A few specimens from along the Coast indicate intergradation (Cook, 1962:33) between the two subspecies.

Barbour (1956:33) reported one specimen from Cypress Creek, in the Green River drainage, Muhlenberg County, Kentucky, and stated that suitable habitat can be found in several areas east of Kentucky Lake. Hence, cottonmouths may have entered this area via the Ohio River. Stejneger (_loc. cit._) reported the species in the Wabash River at Mount Carmel, Wabash County, Illinois, and mentioned a former occurrence at Vincennes, Knox County, Indiana; but there are no recent records at these localities. Hurter (1897) reported having seen cottonmouths in Illinois, opposite St. Louis; Smith (1961:265) believes that this and a population in Monroe County, Illinois, are isolated relicts, since no specimens have been found within 50 miles to the south of Monroe County. The specimens reported by Anderson (1941:178; 1945:274) near Chillicothe (three miles southwest and seven miles northwest, respectively), Livingston County, Missouri, also are thought to represent a relict population. Hall and Smith (1947:453) reported one specimen from Jasper County, Missouri, in the Spring River which flows through extreme southeastern Kansas and into Oklahoma and another in the Neosho River at Chetopa, Kansas. Both of these specimens were taken after a flood, and no additional specimens have been taken in this region. Nevertheless, sufficient habitat is probably available along the Neosho and Verdigris rivers in the southeastern part of Kansas.

In Texas the cottonmouth has penetrated marginal habitat perhaps farther than anywhere else in its range. Formerly it was thought to be limited to the country east of the Balcones Escarpment (Smith and Buechner, 1947:8), but semiarid areas of the state have been invaded primarily via the Colorado and Brazos River systems up to alt.i.tudes of 2300 feet. Two additional specimens are said to have been collected along the Rio Grande. Dr. Howard K. Gloyd (_in litt._) stated that the specimen reported from Eagle Pa.s.s, Maverick County, is believed to have been taken in the 1850's; and the one said to have come from the mouth of the Devil's River is actually marked "near Santa Rosa, Cameron County, September 30, 1891." No additional specimens have been taken in that area; and the range now probably extends no farther south than Corpus Christi, Texas. Brown's (1903:554) knowledge of the extension of the range of the cottonmouth west of longitude 98 is probably based upon the records along the Rio Grande reported in the nineteenth century.

Three extensions of the known range in Texas are reported herein. One specimen was captured by Mr. Harry Green (HWG 346) along the San Saba River, 8.1 miles west of Menard, Menard County. The other two specimens (KU 84375 and 84376) were taken by the late Paul Anderson one and one-half miles north of Pecan Crossing, South Concho River, Tom Green County, and one mile west of Mertzon, Irion County.

In the hypsithermal period following Pleistocene glaciation, cottonmouths gradually moved northward occupying areas beyond their present range. The distributional records since the 1850's and the apparent relict populations now in existence indicate that the range of this species has since receded.

DESCRIPTION

Color and Pattern

Color predominantly brown, ranging through pale reddish-brown or dark reddish-brown, brownish-green, to almost black; 10 to 17 irregular dark brown bands on paler brown ground color; young paler (some nearly salmon pink), retaining a vivid pattern throughout first year; pattern of most individuals nearly obliterated by third year; brilliance and dullness of predominant color correlated with molting cycle (skin especially bright and shiny immediately following shedding); tip of tail yellowish in juveniles; posterior part of venter and tail uniformly black in some adult individuals, especially females; secondary s.e.xual differences in dorsal coloration, such as found in copperhead by Fitch (1960:102), not noted.

The eastern subspecies, _A. p. piscivorus_, has the more brilliant pattern in which the centers of the dark cross-bands are invaded by the ground color. The cross-bands are slightly constricted in the mid-line and may or may not be bilaterally symmetrical. One-half of the cross-band may be displaced anteriorly or posteriorly to a slight degree or may even be completely absent. From one to several dark spots may be present within the cross-bands.

The western subspecies, _A. p. leucostoma_, has a comparatively dull pattern in which the ground color does not invade the center of the cross-bands. In many instances the bands are outlined by white scales, as in the Mexican moccasin (this character is not so prominent in _A. p.

piscivorus_ because of the paler ground color). A large, dark blotch usually occurs at the base of the cross-band and may completely cross the ventral scales. The characteristic variations found in _piscivorus_ are also present in _leucostoma_.

The number of bands is often difficult to count because of the dark color of some specimens. Gloyd and Conant (1943:168) reported averages of 12.5 (11 to 16) and 12.2 (10 to 16) in males and females, respectively, of _leucostoma_ and ranges of 10 to 17 for males and 10 to 16 for females with averages of 13 in both s.e.xes of _piscivorus_. On 20 specimens of _leucostoma_ from Texas the average number of bands was 12.7 (11 to 15). If the number of bands differed on the two sides of an animal, the total number of the two sides was divided by two.

Scutellation

The scutellation of the cottonmouth closely resembles that of the other species of _Agkistrodon_. For example, the nine cephalic shields are characteristic of most species of _Agkistrodon_, as well as most other primitive crotalids and viperids, and most colubrids. Most individuals have an additional pair of large scales behind the parietals.

The numbers of postoculars, supral.a.b.i.als, and infral.a.b.i.als are variable.

On either side the postoculars (three in most specimens) are reduced to two in some specimens. The supral.a.b.i.als (eight in most specimens) frequently vary (usually on one side only) from seven to nine. The number of infral.a.b.i.als is somewhat more variable than the number of supral.a.b.i.als, the usual number being 11, but 10 is also common; 8, 9, and 12 are more rare (Table 1). In 102 snakes in which these characters were examined, four different combinations of supral.a.b.i.als and seven combinations of infral.a.b.i.als were found. Both characters together yielded 16 combinations, considering only the actual number of scales and not taking into account the side of the head on which they occurred (Table 2). The combinations found in a brood of seven young from Houston, Texas, are shown in Table 3 to ill.u.s.trate the variability of this character. Gloyd and Conant (1943:168) found a variation of 6 to 11 (8) and 7 to 9 (8) supral.a.b.i.als and 8 to 13 (11) and 8 to 12 (10.4) infral.a.b.i.als in samples of 301 _leucostoma_ and 119 _piscivorus_, respectively (numbers in parentheses represent average). Also of interest is the variability of the scales themselves. In one instance a scale was found that had not completely divided. In another specimen the last supral.a.b.i.al and last infral.a.b.i.al were one scale that completely lined the angle of the jaw. Instances of one scale almost crowding out another were common. In still other instances one or two supral.a.b.i.als were divided horizontally into two scales. Individual variation rather than geographical variation occurs in these characters.

TABLE 1.--Frequency of Occurrence of Various Numbers of Supral.a.b.i.al and Infral.a.b.i.al Scales in 102 Cottonmouths.

==================================================== | |Specimens |Specimens | | | |Number |having |having |Total |Percentage | |of scales |number on |number on | | | | |both sides |one side | | | |----------------------------------------------------| | Supral.a.b.i.als | |----------------------------------------------------| | 7 | 11 | 24 | 35 | 25.2 | | 8 | 64 | 27 | 91 | 65.5 | | 9 | 0 | 3 | 3 | 2.2 | |----------------------------------------------------| | Infral.a.b.i.als | |----------------------------------------------------| | 8 | 0 | 2 | 2 | 1.5 | | 9 | 3 | 10 | 13 | 9.6 | | 10 | 12 | 32 | 44 | 32.4 | | 11 | 53 | 22 | 75 | 55.1 | | 12 | 0 | 2 | 2 | 1.5 | ----------------------------------------------------

TABLE 2.--Numbers of Supral.a.b.i.als and Infral.a.b.i.als of 102 Cottonmouths.