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UNIVERSITY OF KANSAS PUBLICATIONS
MUSEUM OF NATURAL HISTORY
Vol. 17, No. 9, pp. 435-491, 7 figures in text
------------ October 27, 1966 ----------------
Natural History of Cottonmouth Moccasin,
Agkistrodon piscivorus (Reptilia)
BY
RAY D. BURKETT
UNIVERSITY OF KANSAS
LAWRENCE
1966
UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY
Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Frank B. Cross
Volume 17, No. 9, pp. 435-491, 7 figures in text
Published October 27, 1966
UNIVERSITY OF KANSAS
Lawrence, Kansas
PRINTED BY
ROBERT R. (BOB) SANDERS, STATE PRINTER
TOPEKA, KANSAS
1966
[Illustration: Printer's Logo]
31-4629
Natural History of Cottonmouth Moccasin,
Agkistrodon piscivorus (Reptilia)
BY RAY D. BURKETT
CONTENTS
PAGE
INTRODUCTION 439
ACKNOWLEDGMENTS 440
SYSTEMATIC RELATIONSHIPS AND DISTRIBUTION 441
DESCRIPTION 444
Color and Pattern 444
Scutellation 444
Dentition 449
HABITAT AND LIMITING FACTORS 450
REPRODUCTION 452
Courtship and Mating 452
Reproductive Cycles 452
Embryonic Development 454
Birth of Young 454
Number of Young per Litter 454
Population Composition 455
Reproductive Potential 455
GROWTH AND DEVELOPMENT 456
Size at Birth and Early Growth 456
The Umbilical Scar 457
Later Growth and Bodily Proportions 457
SHEDDING 459
The Shedding Operation 459
Frequency of Shedding 460
FOOD HABITS 461
Methods of Obtaining Prey 461
Food and Food Preferences 462
MORTALITY FACTORS 465
Natural Enemies and Predators 465
Parasites and Diseases 465
Miscellaneous Causes of Death 466
BEHAVIOR 466
Annual and Diel Cycles of Activity 466
Basking 469
Coiling 469
Locomotion 470
Disposition 470
Defense and Escape 471
"Head Bobbing" 471
Combat Dance 472
THE VENOM 473
Properties of the Venom 473
Venom Yield and Toxicity 473
Susceptibility of Snakes 475
THE BITE 476
Effects of the Bite 476
Treatment 477
Case History of a Bite 479
Snakebite in the United States 480
SUMMARY 480
LITERATURE CITED 485
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 superstition 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 manuscript, I thank Professor E. Raymond Hall. I am grateful to my
wife, Janis, for her invaluable assistance and for typing the
manuscript.
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 Japan (_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 anal 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 illustrating 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 altitudes 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.
[Illustration: 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
offshore 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 altitudes 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 Pass, 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 deg. 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 sexual 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 sexes 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, supralabials, and infralabials are variable.
On either side the postoculars (three in most specimens) are reduced to
two in some specimens. The supralabials (eight in most specimens)
frequently vary (usually on one side only) from seven to nine. The
number of infralabials is somewhat more variable than the number of
supralabials, 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 supralabials and seven
combinations of infralabials 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 illustrate the variability of
this character. Gloyd and Conant (1943:168) found a variation of 6 to 11
(8) and 7 to 9 (8) supralabials and 8 to 13 (11) and 8 to 12 (10.4)
infralabials 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 supralabial and last infralabial 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 supralabials
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
Supralabial and Infralabial Scales in 102 Cottonmouths.
====================================================
| |Specimens |Specimens | | |
|Number |having |having |Total |Percentage |
|of scales |number on |number on | | |
| |both sides |one side | | |
|----------------------------------------------------|
| Supralabials |
|----------------------------------------------------|
| 7 | 11 | 24 | 35 | 25.2 |
| 8 | 64 | 27 | 91 | 65.5 |
| 9 | 0 | 3 | 3 | 2.2 |
|----------------------------------------------------|
| Infralabials |
|----------------------------------------------------|
| 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 Supralabials and Infralabials of 102
Cottonmouths.
===========================================
| Number of | Number of | Number of |
| individuals | supralabials | infralabials |
| 37 | 8 | 11 |
| 15 | 8 | 10-11 |
| 12 | 7-8 | 11 |
| 6 | 7-8 | 10-11 |
| 5 | 8 | 10 |
| 5 | 8 | 9-10 |
| 4 | 7 | 11 |
| 3 | 7 | 9-10 |
| 3 | 7-8 | 10 |
| 2 | 7 | 9 |
| 2 | 7 | 10 |
| 2 | 8 | 10-12 |
| 2 | 8-9 | 10 |
| 2 | 7-8 | 8-9 |
| 1 | 7-8 | 9 |
| 1 | 8-9 | 10-11 |
-------------------------------------------
The dorsal scales of cottonmouths are strongly keeled except that those
of the two lower scale-rows on each side are weakly keeled. Also they
are slightly larger than the others. Two apical pits are present on each
dorsal scale. The shape of the scales and number of scale rows vary
depending upon the position on the body. Scales on the neck are
considerably smaller than those elsewhere on the body and are arranged
in two or three more rows than those at mid-body. The skin in the region
of the throat, neck, and fore-body is especially elastic and allows the
swallowing of large prey. Posteriorly from the mid-body the scales
decrease in size and become more angular, those on the tail tending to
be rhomboidal and wider than long. In the region of the anus the number
of scale rows diminishes rapidly, leaving only 12 to 14 rows at the base
of the tail and only three rows immediately ahead of the tail tip. The
tail ends in a spine composed of two scales: one scale covers the
bottom, lower parts of the sides, and tip of the spine; and a shorter
dorsal scale covers the top and upper parts of the sides of the basal
two-thirds of the spine. The spine of embryos and young cottonmouths is
blunt, but is pointed in most adults.
TABLE 3.--Variation in Numbers of Supralabials and Infralabials
in a Brood of Seven Cottonmouths.
===========================================
| Number of | Number of | Number of |
| individuals | supralabials | infralabials |
| 1 | 7 | 9 |
| 1 | 7 | 9-10 |
| 2 | 7-8 | 8-9 |
| 1 | 7-8 | 9 |
| 1 | 8 | 9-10 |
| 1 | 8-9 | 10 |
-------------------------------------------
TABLE 4.--Analysis of Number of Scale Rows at Three Parts of
the Body in 81 Cottonmouths.
==============================================================
| | Neck | Mid-body | Anterior to anus|
| |--------+--------+--------+--------+--------+--------|
| Number | Number | Per- | Number | Per- | Number | Per- |
| of | of |centage | of |centage | of |centage |
| scales |indivi- | |indivi- | |indivi- | |
|per row | duals | | duals | | duals | |
|--------+--------+--------+--------+--------+--------+--------|
| 29 | 1 | 1.2 | ... | ... | ... | ... |
| 28 | 3 | 3.7 | ... | ... | ... | ... |
| 27 | 52 | 64.2 | ... | ... | ... | ... |
| 26 | 16 | 18.0 | 2 | 2.5 | ... | ... |
| 25 | 8 | 9.9 | 67 | 82.7 | ... | ... |
| 24 | 1 | 1.2 | 4 | 4.9 | ... | ... |
| 23 | ... | ... | 8 | 9.9 | 4 | 4.9 |
| 22 | ... | ... | ... | ... | 4 | 4.9 |
| 21 | ... | ... | ... | ... | 68 | 84.0 |
| 20 | ... | ... | ... | ... | 5 | 6.2 |
--------------------------------------------------------------
The number of scale rows on the neck, at mid-body, and just anterior to
the anus is relatively constant at 27-25-21, respectively; but some
individual variation is evident (Table 4). Since the rows are diagonally
arranged, it is necessary in counting scales to proceed either
anteriorly or posteriorly across the back; or the row may be counted in
either direction up to the center of the back and then reversed on the
other side of the snake. In order to count the scale rows in a position
where no scale reduction or addition was occurring and to avoid as much
error as possible, I counted from anterior to center and back on the
neck, in any direction at mid-body, and from posterior to center and
back near the anus. Because females generally are the larger in
circumference posteriorly, they could have more scale rows than males
just anterior to the anus. The few snakes having more than 21 scale rows
in the posterior region offer no conclusive evidence as to tendencies,
but in both instances in which this occurred the females outnumbered the
males three to one. An odd, rather than an even, number of scale rows
occurs on most of the length of the snakes examined, because there is a
mid-dorsal row and scale rows tend to be lost on both sides at about the
same level. An example of scale reduction of one snake was as follows:
6+7 (13) 6+7 (96)
27 -------- 25 -------- 24 -------- 23 --------- 22 ---------
5+6 (13) 5+6 (90) 7+8 (111) 7+8 (114)
6+7 (122) +7, -5 (125)
23 -------- 22 -------- 23 --------- 21 -------- 22 ------------
-6 (118) +6 (119) 6+7 (121) +6 (123)
-6 (126)
22 -------- 21 (130).
This scale reduction follows the method proposed by Dowling (1951b: 133)
in which the numbers on the mid-line represent the number of scale rows,
upper figures refer to the right side of the snake, and figures in
parentheses indicate the number of the ventral scale (counted from the
anterior end of the series), thus marking the position of the addition
or reduction. Addition of a row is shown by a plus sign and the number
of the row, whereas reductions are shown by a minus sign and the number
of the row that is lost or by a plus sign between the number of two rows
that join. According to Dowling, variation in number of dorsal scales
characterizes the few genera and species of snakes in which it has been
studied. The time and difficulty involved in ascertaining the number of
scales explain why it has not been widely used in classification.
[Illustration: FIG. 2. Number of ventral scales in 48 female and 34
male _A. p. leucostoma_.]
Ventral scales on 34 males averaged 134.4 (128 to 139), and on 48
females 133.5 (128 to 137) (Fig. 2.). Barbour (1956:34) found an average
of 135.3 ventral scales on 64 males and 44 females, and Gloyd and Conant
(_loc. cit._) found an average of 134 for both males and females. The
average for the eastern cottonmouth obtained by Gloyd and Conant,
however, was 137 ventrals in both sexes. Some of my counts were made
before I knew of the standard system of counting ventrals proposed by
Dowling (1951a:97-99), in which the first ventral plate is defined as
the most anterior one bordered on both sides by the first row of
dorsals. Therefore, some inconsistencies may exist in my counts. Where
differences occur, Dowling's method probably will indicate the presence
of an additional scale, since it appears to begin farther anteriorly on
the average, than I began counting.
[Illustration: FIG. 3. Number of caudal scales in 44 female and 34
male _A. p. leucostoma_.]
TABLE 5.--Caudal Scale Combinations in 95 Cottonmouths.
U = Undivided; D = Divided.
=====================================================================
| Number of scales
|-------------------------------------------------------------
Number | | | | | | | | | | | | | | | | |
of | | | | | | | | | | | | | | | | |
samples| D | U | D | U | D | U | D | U | D |U| D|U| D|U|D|U|D
-------+---+-----+-----+---+----+---+----+---+-----+-+--+-+--+-+-+-+-
25 | |13-35|10-32| | | | | | | | | | | | | |
11 |1-2|12-33|14-28| | | | | | | | | | | | | |
20 | |16-39| 1-9 |1-3|3-24| | | | | | | | | | | |
20 |1-4| 3-37| 1-21|1-5|1-29| | | | | | | | | | | |
4 | |14-30| 1-8 |1-7|1-8 |1-4|2-10| | | | | | | | | |
3 | 1 |18-23| 1-2 |1-2|6-11|1-3|6-9 | | | | | | | | | |
4 | | 1-17| 1 |1-3|1-8 |1-4|1-3 |1-4|13-22| | | | | | | |
2 |1-2| 4-16| 1 |1-4| 2 | 1 |1-4 | 1 |18-21| | | | | | | |
1 | | 20 | 1 | 1 | 1 | 1 | 6 | 1 | 3 |1|11| | | | | |
1 | | 10 | 2 | 3 | 2 |10 | 1 | 2 | 2 |1| 4|4| | | | |
1 | | 20 | 1 | 1 | 2 | 1 | 1 | 4 | 4 |2| 4|1| 3| | | |
1 | 1 | 13 | 1 | 1 | 1 | 3 | 1 | 1 | 1 |4| 2|4|13| | | |
1 | | 17 | 1 | 1 | 2 | 1 | 1 | 6 | 2 |1| 2|3| 2|7| | |
1 | | 9 | 1 | 1 | 8 | 1 | 3 | 1 | 1 |3| 1|1| 2|1|1|1|6
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Analysis of caudal scales revealed sexual dimorphism. In the six
specimens from Tennessee, Blanchard (1922:16) found the same thing.
Caudals averaged 45.4 (41 to 50) on 34 males and 42.6 (39 to 49) on 44
females (Fig. 3). Barbour (_loc. cit._) found an average of 45.7 (30 to
54) caudals in males and 43 (17 to 56) in females. Caudal scale counts
by Gloyd and Conant (_loc. cit._) averaged 44 (38 to 49) in males and 42
(37 to 48) in females of _leucostoma_; in _piscivorus_ they averaged 48
(42 to 53) in males and 44 (41 to 49) in females. Another
seldom-mentioned, unusual characteristic of the caudal scales of
copperheads and cottonmouths is that some are single (usually those at
the base of the tail) and others divided (Table 5). To my knowledge, all
other species have either single or divided scales the entire length of
the tail. See Klauber (1941:73) and Fox (1948:252) concerning
correlation of few scales with warm environment.
Dentition
Cottonmouths, like other pit-vipers, have their teeth reduced in number
and have enlarged, highly specialized fangs. Small teeth occur on the
palatine and the pterygoid in the upper jaw and on the dentary in the
lower jaw. The dentary bone bears 17 curved teeth that decrease in size
posteriorly. The palatine bears five small, strongly curved teeth, and
the pterygoid bears 16 to 18 strongly curved teeth decreasing in size
posteriorly. The numbers of teeth mentioned above in each instance refer
to the number of sockets rather than the actual number of teeth, because
teeth are frequently shed, leaving some of the sockets empty at any one
time.
The maxillary bone has two sockets side by side which bear the poison
fangs, usually one at a time. During the period shortly before a fang is
to be shed, however, its replacement becomes attached in the alternate
socket; and both fangs may be functional for a short time. The old fang
then becomes weakened at its base, eventually breaks off, and is
swallowed. At any one time four or five replacement fangs in various
stages of development are found in the gum behind the functional fang.
These replacement fangs, which are arranged in alternate rows, gradually
enlarge as they move forward in their development and, in juveniles, are
generally slightly longer than the fangs that they replace.
In 1963 I examined the fangs of 14 cottonmouths at four- to seven-day
intervals for a period of six weeks. The fang-shedding cycle was found
to be highly irregular, with a double condition (on one or both sides)
occurring one-third of the time. Approximately the same proportion of
double fangs was found in preserved individuals. A replacement period of
at least five days was observed in one snake. One-half the cycle (from
replacement on one side to replacement on the other) varied from five to
twenty days, indicating that the cycles for each fang are independent of
one another. Bogert (1943:324) found that young rattlesnakes are born
with functional fangs in the two inner sockets. Nonsynchronous use of
the sockets on opposite sides of the head in rattlesnakes is a later
development which results from accidents or other conditions leading to
a longer retention of the fang on one side than on the other (Klauber,
1956:723). I found a double set of fangs in cottonmouths only twice in
the six-week period. A complete cycle was recorded in ten instances in a
period of 19 to 23 days and in two instances in 32 days. One cottonmouth
was examined periodically over a 34-day period by Allen and Swindell
(1948:12), but a complete fang-shedding cycle was not observed. Fitch
(1960:110) reported a 33-day cycle in copperheads; Klauber (1956:726)
estimated the normal active life of each fang of an adult rattlesnake to
be from six to ten weeks, but he made no observations to confirm his
estimation.
Fangs measured from the tip of the notch of the basal lumen to the end
of the fang vary from about 1.3 per cent of the snout-vent length in
juveniles to about 1.0 per cent in large adults (Table 6). The fangs are
longer than those of copperheads (Fitch, 1960:111). Klauber's (1956:736)
figures on fang-lengths in all species of rattlesnakes are percentages
of total length rather than of the snout-vent length. The fangs of
various species of rattlesnakes range from nearly the same proportionate
length as those of cottonmouths to some much longer.
From patterns of bites of venomous snakes, Pope and Perkins
(1944:333-335) attempted to correlate number, size, and patterns of
tooth marks with size and generic identity of the snake responsible for
the bite. Distance between fangs is relatively constant for snakes of a
particular size (Table 6) regardless of genus, but the fangs of a
cottonmouth are directed outward to variable degrees, and puncture
wounds could easily resemble those of a much larger snake (Table 7).
Also there is no direct relationship between size of snake and toxicity
or amount of venom injected. Consequently information of this kind is of
little or no value from a medical standpoint.
TABLE 6.--Correlation of Relative Fang-length and Distance
Between Fangs at Base with Snout-vent Length of Cottonmouths.
=======================================================
|Snout-vent |Number |Average |Number |Average |
|length |in |ratio of |in |ratio of |
|(millimeters) |sample |fang-length |sample |distance |
| | |to | |between |
| | |snout-vent | |fangs to |
| | |length | |snout-vent |
| | |(percent) | |length |
| | | | |(percent) |
|--------------+-------+------------+-------+-----------|
| 200-299 | 3 | 1.33 | 3 | 2.57 |
| 300-399 | 7 | 1.30 | 5 | 2.48 |
| 400-499 | 13 | 1.21 | 9 | 2.21 |
| 500-599 | 12 | 1.22 | 8 | 2.19 |
| 600-699 | 7 | 1.17 | 1 | 2.10 |
| 700-799 | 5 | 1.07 | 4 | 1.65 |
| 800-899 | 1 | 1.00 | 1 | 2.00 |
-------------------------------------------------------
TABLE 7.--Contrast in Measurements Between the Base of the
Fangs and Between Fang Punctures of Nine Cottonmouths (in
millimeters).
==================================================
| Distance between | Distance between | Snout-vent |
| base of fangs | fang punctures | length |
|------------------+------------------+------------|
| 7.7 | 13.0 | 400 |
| 8.7 | 14.0 | 575 |
| 10.0 | 22.5 | 526 |
| 11.0 | 18.0-19.0 | 590 |
| 12.0 | 18.0 | 793 |
| 13.0 | 17.0, 20.0 | 558, 612 |
| 15.5 | 23.5 | 800 |
| 16.0 | 24.0 | 800 |
--------------------------------------------------
HABITAT AND LIMITING FACTORS
Although usually associated with swamps and lowlands along river
bottoms, the cottonmouth lives in a variety of habitats ranging from
salt marshes to cool, clear streams and from sea level to an altitude of
2300 feet. Shaded, moist areas either in or beside shallow waters are
preferred, but cottonmouths occasionally wander as far as a mile from
water.
In the pine-oak forests of Nacogdoches County in eastern Texas
cottonmouths and copperheads are probably the most abundant species of
snakes. Specimens have been collected near Nacogdoches in ponds, swamps,
clear and fast-running streams with rock bottoms, and sluggish muddy
streams. On the Stephen F. Austin Experimental Forest numerous
cottonmouths live in a swamp until around mid-July, when it becomes dry.
A small stream west of the swamp seems to be used as a migration route
to and from the swamp. Slightly more than a mile downstream cottonmouths
are common in a bottomland area. The ground is always moist and no
undergrowth occurs; a few small clear springs produce shallow trickles
that run into a swamp. Cottonmouths can often be found here, lying in or
beside the small trickles.
I have seen cottonmouths in various types of aquatic habitats in
Brazoria County. In most places in this area, cottonmouths are found in
association with one or more species of water-snakes (including _Natrix
cyclopion_, _N. erythrogaster_, _N. rhombifera_, and _N. confluens_),
which greatly outnumber the cottonmouth. Interspecific competition may
be reduced somewhat by cottonmouths sometimes feeding on water-snakes.
The numerous statements in the literature concerning the habitat of the
cottonmouth can be summarized most easily by the following short
quotations:
_Agkistrodon piscivorus piscivorus_--"Marshes and lakes; ponds
and streams with wooded shores; low country near water;
roadside ponds; drainage ditches; coastal 'banks'; keys; some
Gulf coast islands; mangrove swamps." (Wright and Wright,
1957:919.)
_Agkistrodon piscivorus leucostoma_--"Cypress, gum, river
swamps; alluvial swamps wooded or not wooded; water courses of
the south such as rivers, bayous, backwaters of small branches;
hill streams in the north; ... marshy places in prairies ...
rice fields, bottomland pools; margins of above habitats,
pools, shallow lakes, swampy places, temporary flood lands....
In, under, or on fallen timber, in holes in banks, rocky
bluffs, crayfish burrows. In short it is very aquatic." (Wright
and Wright, _op. cit._:923.)
Geographically cottonmouths differ somewhat in their ecological
requirements, but are basically much alike in most respects. The areas
of greatest abundance are those having 40 inches or more of annual
rainfall. The northern edge of the range has a mean temperature of
approximately 38 deg. F. in January in Virginia and 30 deg. F. in Missouri,
although the lowest temperature reached in these areas is more important
as a limiting factor. The annual rainfall in both Virginia and Missouri
amounts to approximately 40 inches. Moisture, as well as temperature,
may play an important role in the northward distribution of the species.
The eastern cottonmouth seems to be less tolerant of low temperatures
than the western subspecies. Mean January temperatures equal to those
along the northern limits of the western cottonmouth's distribution are
reached in the vicinity of Connecticut, which is north of the geographic
range of the eastern subspecies.
The depths to which cottonmouths penetrate into their dens may have a
limiting influence upon the geographic range, especially in the northern
extremes. Bailey (1948:215) discussed the possibility that populations
of snakes may be significantly depressed because of winter kill of
individuals that "hibernate" at shallow depths. He speculated also that
the short growing season does not allow enough time for the essentials
of existence to be carried out, and the prolonged period of inactivity
overtaxes the energy reserve of the species.
Available food does not seem to be of much importance as a limiting
factor, for the cottonmouth is remarkably indiscriminate in its choice
of prey, feeding upon almost any vertebrate animal that happens to come
within reach. Competition for food, however, may play an important role.
REPRODUCTION
Courtship and Mating
A review of available literature indicates no records of courtship of
the cottonmouth other than statements that breeding occurs in early
spring. In a close relative, the copperhead (see Fitch, 1960:159-160),
mating occurs almost any time in the season of activity but is mainly
concentrated in the few weeks after spring emergence, at about the time
when females are ovulating. Klauber (1956:692) concluded that along the
southern border of the United States rattlesnakes normally mate in
spring soon after coming out of their winter retreats; but farther north
where broods are produced biennially, the mating times may be more
widely dispersed, and summer and fall matings may even predominate.
The only record of copulation in the cottonmouth was reported by Allen
and Swindell (1948:11), who observed a pair copulating for three hours
on October 19, 1946, at the Ross Allen Reptile Institute. Davis
(1936:267-268) stated that courtship in cottonmouths is violent and
prolonged but did not note any nervous, jerky motions or nudging of the
female along her back and sides as had been observed in other genera of
snakes. Carr (1936:90) saw a male cottonmouth seize a female in his
mouth and hold her, but no courtship followed.
Reproductive Cycles
Many persons have assumed that gestation periods in snakes are the
intervals between mating and parturition, and that mating and ovulation
occur at approximately the same time. However, retention of spermatozoa
and delayed fertilization indicate that copulation is not a stimulus for
ovulation.
A biennial reproductive cycle was found for the copperhead in Kansas
(Fitch, 1960:162), the prairie rattler in Wyoming (Rahn, 1942:239) and
in South Dakota (Klauber, 1956:688), the great basin rattler in Utah
(Glissmeyer, 1951:24), and the western diamondback rattler in
northwestern Texas (Tinkle, 1962:309). Klauber's (1956:687) belief that
the reproductive cycle of rattlesnakes varies with climate, being
biennial in the north and annual in the south, is supported by similar
climatic variation in the reproductive cycle of the European viper which
was discussed by Volsoe (1944:18, 149).
If data for a large number of females were arranged as are those in
Table 8, they might reveal whether the breeding cycle is annual or
biennial. The figures presented in Table 8 are misleading if viewed
separately because of the small number of individuals included in some
of the size classes.
The smallest reproductive female found measured 455 millimeters in
snout-vent length. Conant (1933:43) reported that a female raised in
captivity gave birth to two young at an age of two years and ten months.
The size classes represented by gravid females found by Barbour
(1956:38) in Kentucky indicate that breeding occurs at least by the
third year.
The ovaries of female cottonmouths examined revealed ova in various
stages of development. In individuals less than 300 millimeters in
snout-vent length the ovaries are almost completely undeveloped; in
immature individuals from 300 to 450 millimeters in length the follicles
are from one to two millimeters in length; in post-post females
follicles vary in size, the largest being about seven millimeters.
Reproductive females also contain follicles of various sizes. One or two
sets are less than three millimeters in length, and large ova that soon
are to be ovulated are present. Ovarian ova found in April ranged in
length from 23 to 35 millimeters. No embryonic development was observed
in most individuals until June or later.
TABLE 8.--Percentage of Gravid Females of _A. p. leucostoma_ in
50 Millimeter Size Classes.
=====================================
|Snout-vent |Number |Total |Percentage|
| | of |number| |
| length |gravid | in | gravid |
| |females| size | |
| | |class | |
|-----------+-------+------+----------|
| 450-499 | 3 | 14 | 21.4 |
| 500-549 | 7 | 17 | 41.2 |
| 550-599 | 8 | 17 | 47.1 |
| 600-649 | 5 | 7 | 71.4 |
| 650-699 | 2 | 9 | 22.2 |
| 700-749 | 2 | 3 | 66.7 |
| 750-799 | 1 | 1 | 100.0 |
| 850-899 | 1 | 1 | 100.0 |
| Totals | 29 | 69 | 42.0 |
-------------------------------------
Increase in length of testes appears to be correlated with length of the
individual rather than cyclic reproductive periods (Fig. 4).
The reproductive cycle in cottonmouths resembles that illustrated by
Rahn (_op. cit._:237), in which the ovarian follicles of post-partum
females begin to enlarge in late summer and autumn, with ovulation
occurring the following spring. By means of retaining sperm successive
broods possibly are produced after only one mating. In captivity, at
least, some females may not follow this biennial cycle; Stanley Roth
(M.S.), biology teacher in high school at Lawrence, Kansas, had a female
of _A. p. piscivorus_, from Florida, that produced broods of 14 and 12
young in two consecutive years.
[Illustration: FIG. 4. Length of testes in cottonmouths of various
sizes (.--left; º--right). The right testis is always longer than
the left.]
Embryonic Development
After ova are fertilized a three and one-half to four-month period of
development begins which varies somewhat depending on the temperature.
In almost every instance the ova in the right uterus outnumber those in
the left. Embryos usually assume the serpentine form in the latter part
of June and are coiled in a counterclockwise spiral with the head on the
outside of the coil. At this time the head is relatively large and
birdlike in appearance with conspicuous protruding eyes. Sex is easily
noted because the hemipenes of males are everted. By late July scales
are well developed and the embryo is more snakelike in appearance, but
pigmentation is still absent. By mid-August the color and pattern are
well developed, the egg tooth is present, the snake shows a considerable
increase in size over that of the previous month, and much of the yolk
has been consumed. Some females that contain well developed embryos also
contain eggs that fail to develop. Sizes of ova vary irrespective of
size of female and stage of embryonic development. Lengths of ova ranged
from 22 to 51 millimeters in May to 35 to 49 millimeters in July and
August. A two-yolked egg was found in one female.
Birth of Young
Accounts in the literature of 15 litters of cottonmouths fix the time of
birth as August and September. Conant (1933:43) reported the birth of a
litter in mid-July by a female that had been raised in captivity, and
one female that I had kept in captivity for two months gave birth to a
litter between October 19 and October 25. The conditions of captivity
undoubtedly affected the time of birth in both instances.