Natural History of Cottonmouth Moccasin, Agkistrodon piscovorus (Reptilia) Part 3

[Ill.u.s.tration: FIG. 6. Head length () and head width () expressed as a percentage of snout-vent length of living and preserved cottonmouths.

Head length was measured from the tip of the snout to the posterior end of the mandible. Head width was measured across the supraocular scales, since accuracy was greater than if measured at the posterior edge of the jaw. No s.e.xual dimorphism or geographical variation occurs in these characters.]

Proportions of various parts of the body vary considerably depending on age, size and, in some instances, s.e.x. Heads are proportionately larger in young than in adults (Fig. 6), as is true of vertebrates in general.

This larger head has survival value for the cottonmouth in permitting more venom to be produced and in permitting it to be injected deeper than would be the case if the proportions were the same as in adults.

Relative to the remainder of the snake the head is considerably larger than in the copperhead (Fitch, 1960:108) and slightly larger than in the rattlesnake, _Crotalus ruber_ (Klauber, 1956:152).

[Ill.u.s.tration: FIG. 7. Tail length expressed as a percentage of snout-vent length of living and preserved cottonmouths (--males; --females).]

In general, tails are relatively longer in males than in females of the same size (Fig. 7), except that there is little or no difference at birth. Growth of the tail in males proceeds at a more rapid rate. In certain individuals s.e.x cannot be recognized from length of the tail relative to snout-vent length because overlapping occurs, especially in medium-sized individuals. Similar changes of proportions with increase in age occur in copperheads (Fitch, 1960:106) and rattlesnakes (Klauber, 1956:158-159), but the tail of the cottonmouth is relatively much longer.

SHEDDING

The Shedding Operation

Shedding of the skin is necessary to provide for growth and wear in snakes. The milkiness or bluing of the eyes, which causes partial blindness, marks the initial stage of shedding and is caused by a discharge of the exuvial glands that loosens the old _stratum corneum_ from the layer below. In four to seven days the opaqueness disappears, and the snake sheds after an additional three to six days (Table 12).

Young snakes first shed within a few days after birth and generally shed more frequently than adults, but the interval is variable. The eyes of three young cottonmouths observed by Wharton (1960:126) became milky on the fourth day but cleared on the seventh day, and the skin was shed on the eighth day. The eyes of three young kept by me became milky two to three days after birth, cleared on the seventh to tenth days, and the skin was shed on the thirteenth day. Possibly the relatively long interval in this instance resulted from low relative humidity in the room where the snakes were kept. According to Fitch (1960:134), litters of young copperheads usually shed within three to ten days after birth; but under unusually dry conditions shedding did not occur for several weeks.

TABLE 12.--Duration of Preparatory Period (in days) to Shedding in 11 Cottonmouths.

============================== |Duration | Time |Time from | | of |between |beginning | |cloudiness|clearing| of | | | and |cloudiness| |of eyes |shedding| until | | | | shedding | |----------+--------+----------| | 5 | 6 | 11 | | 7 | 3 | 10 | | - | - | 6 | | - | - | 6 | | 5 | 3 | 8 | | 4 | 6 | 10 | | 7 | 3 | 10 | | 5 | 6 | 11 | | 5 | 3 | 8 | | 7 | - | - | | 7 | 3 | 10 | | ---- | ----| ---- | |[=X] 5.4 |[=X] 3.8| [=X] 9.0 | ------------------------------

Cottonmouths as well as other snakes usually do not feed until after the skin is shed and are generally quiescent during the period preceding shedding, except that immediately before shedding they become active and rub their snouts on some rough object and may yawn several times seemingly in an attempt to loosen the skin along the edges of the lips.

After the skin is loosened from the head, more rubbing against rough surfaces and writhing serves to pull the old skin off, turning it inside out. Once the old skin has pa.s.sed over the thick mid-body, the snake often crawls forward using rectilinear locomotion until the skin is completely shed. It normally comes off in one piece; but, if the snake is unhealthy or has not had sufficient food or water, the skin may come off in patches. Frequently one or both of the lens coverings are not shed immediately and impair the sight. Bathing or swimming ordinarily causes dried skin to peel off; and, because of the cottonmouth's aquatic habits, its chances of shedding successfully are much greater than those of less aquatic snakes. Cottonmouths that have recently shed have bright and glossy patterns, in contrast to the dull and dark appearance of those that are preparing to shed.

Frequency of Shedding

Most of our knowledge concerning the frequency of shedding is based upon observations of captives. It is known that the intervals between exuviations are largely dependent upon the amount of food taken and the rate of growth. Unless laboratory conditions closely resemble those in the field, shedding frequencies in captives probably differ much from those of free-living snakes.

Only two of my captives shed twice. The intervals between exuviations in the two snakes were eight and five months, lasting from August to April and from December to May, respectively. Ten other snakes shed once in the period from January through July. Stabler (1951:91) presented data concerning shedding of two cottonmouths kept 12 and 14 years in captivity. One shed 25 times in 12 years and the other shed 37 times in 14 years, giving an average of 2.1 and 2.6 per year, respectively.

Neither of the snakes shed from December through March, but the period of shedding corresponded to the period of greatest activity and growth.

In Florida, cottonmouths shed four to six times a year, according to rate of growth (Allen and Swindell, 1948:7).

FOOD HABITS

Methods of Obtaining Prey

Food is obtained by a variety of methods depending on the type of food, age of the cottonmouth, and possibly other factors. Some captives lie in ambush and others crawl slowly in active search. At the first cue of possible prey, either by sight, scent, or differential temperature detection by the pit, the snake appears to become alert and flicks its tongue out at fairly rapid intervals.

By means of the facial or loreal pit found in all crotalids, the snake is able to detect objects having temperatures different from that of the surroundings of the objects. In detecting prey the tongue acts to sharpen the sense of "smell" by conveying particles to Jacobson's organs in the roof of the mouth. On many occasions cottonmouths appeared to rely solely on sight; they pa.s.sed within a few inches of prey, apparently unaware of its presence until it moved. When pools of water begin to dry up toward the end of summer, cottonmouths often congregate and feed on dying fish. In these instances the fish are usually taken as they come to the surface. In laboratory observations moccasins seize live fish and some moccasins carry the fish until they have received lethal doses of venom; afterward the fish are swallowed. But grasping and manipulation of the prey occurs without the fangs' being employed, especially in the case of dead fish. On one occasion a cottonmouth was observed to grasp the edge of a gla.s.s dish that had contained fish and apparently retained the odor. On another occasion I placed several fish in a bowl, rubbed a stick on the fish, and then touched each snake lightly on the nose with the stick. The snakes crawled directly to the bowl and began feeding. At other times these same snakes crawled around the cage in an apparent attempt to locate the food but paid little attention to fish held in front of them. If the catching of prey under natural conditions were as uncoordinated as it sometimes is in captivity, the snakes probably would not be able to survive.

Wharton (1960:127-129) described tail-luring in one individual of a 76-day-old brood of cottonmouths. The snake lay loosely coiled with the tail held about six centimeters from the ground; a constant waving motion pa.s.sed posteriorly through the terminal inch of the tail. These movements ceased at 7:20 p.m. but were resumed at 7:40 a.m. the following day. All observations were under artificial light. The "caudal lure" as a means of obtaining prey has been described in other species and related genera by Neill (1960:194) and Ditmars (1915:424).

Various authors have suggested that the method of capture differs according to the kind of prey. Allen and Swindell (1948:5) stated that cottonmouths retain their hold after striking fish or frogs but will release a mouse after delivering a bite and are timid in striking at larger rodents. Neill (1947:203) noted that a cottonmouth always waited several minutes after biting a large rat before approaching its prey.

This same type of behavior has been reported for copperheads (Fitch, 1960:194) and rattlesnakes (Klauber, 1956:618). Cottonmouths observed by me retained a strong hold on fish, frogs, and sometimes mice, but almost always released large mice and baby chicks, which were not eaten until after death.

Different behavior according to type of prey is correlated with ability of prey to retaliate, although some animals may not be released because they could easily escape. For instance, a frog could hop far enough to escape in a matter of seconds if released. A 73-millimeter _Rana pipiens_ that I observed was bitten twice within one and a quarter hours and died 45 minutes after the last bite. Its movement was uncoordinated by the time of the second bite, but it could have escaped had the frog not been confined. Although it is doubtful that normal, healthy fish are frequently captured by cottonmouths, Allen (1932:17) reported that a cottonmouth was seen pushing a small, dead pike about on the surface of a stream. A wound on the belly of the fish indicated that it had been bitten. A 17-gram creek chub (_Semotilus_) and a 13.7-gram ba.s.s (_Micropterus_) were injected by me with one-fourth cubic centimeter of fresh venom near the base of the tail in order to determine whether the fish could escape after being bitten and released. The creek chub flipped onto its back after a minute and 45 seconds and gill movements stopped in eight minutes and 35 seconds; the ba.s.s flipped over after 50 seconds and died in two minutes and 10 seconds. The venom immediately affected both fish, and it is unlikely that either could have swum more than a few feet.

After its prey has been killed, a cottonmouth examines the body from end to end by touching it with the tongue. Then the animal is grasped in the mouth without the use of the fangs and is slowly manipulated until one end (usually the head) is held in the mouth. The lengthy process of swallowing then takes place, the fangs and lower jaws alternately pushing the prey down the throat.

Food and Food Preferences

The cottonmouth seems to be an opportunistic omni-carnivore, because it eats almost any type of flesh that is available, including carrion. It feeds primarily upon vertebrates found in or near water; but invertebrates and eggs have also been found in the diet. The only potential prey items that seem not to be normally eaten are bufonid toads and tadpoles. I have occasionally offered tadpoles and frogs to cottonmouths, but only the frogs were accepted. But, Stanley Roth kept a cottonmouth in captivity that ate both toads and tadpoles. If tadpoles are commonly eaten, their probable rapid digestion would make identification almost impossible.

Following is a list of known foods of the cottonmouth:

Captivity: "... rattlesnake.... The same moccasin also killed and ate a smaller snake of its own species...." (Conant, 1934:382.)

Florida: "3 heron feathers, bird bone, _Eumeces inexpectatus_, 3 fish all under one inch in length, 1 heron egg sh.e.l.l" (Carr, 1936:89). According to Allen and Swindell (1948:5), "the food included other moccasins, prairie rattlesnakes, king-snakes, black snakes, water snakes, garter snakes, ribbon snakes, and horn snakes ... most of the species of frogs, baby alligators, mice, rats, guinea pigs, young rabbits, birds, bats, squirrels, and lizards ... a mud turtle ... a case of a four footer eating ten to twelve chicken eggs. The most common food appears to be fish and frogs. Catfish are included on this list...." Yerger (1953:115) mentions "an adult yellow bullhead, _Ameiurus natalis_ ... 306 mm. in standard length [from a 63-inch cottonmouth]."

Georgia: "... full grown _Rana catesbeiana_, several foot-long pickerel ... dead fish if placed in a pan of water.... _Natrix sipedon fasciata_ and _Masticophis flagellum_ ... rats....

Toads and large _Eumeces laticeps_ were always ignored."

(Neill, 1947:203.) "_Natrix_, _Heterodon_, _Kinosternon_, _Rana_, _Hyla cinerea_, _Microhyla_, Microtine [_Pitymys pinetorum_]." (Hamilton and Pollack, 1955:3.)

Mississippi: "... _Hyla gratiosa_.... In captivity specimens have eaten frogs, mice, birds, dead fish, pigmy rattlers and copperheads. Toads ... were refused" (Allen, 1932:17). One moccasin "disgorged a smaller decapitated moccasin ... killed the day before by boys" (Smith and List, 1955:123).

Tennessee: "Beetles in one stomach; lizard (_Eumeces_) in another stomach; small snake (_Natrix_) in one intestine, and hair in another intestine. One stomach contained numerous bits of wood, up to four inches in length...." (Goodman, 1958:149.)

Kentucky: "_Siren intermedia_ was the most abundant food item in both volume and occurrence. Frogs of the genus _Rana_ ranked second. Together, these two items comprised almost 2/3 of the food of the snakes. The other food items were distributed among the fishes, reptiles, and other amphibians [one _Rana_ tadpole included]." (Based on 42 samples--Barbour, 1956:37.)

Illinois: (Based on 84 samples--Klimstra, 1959:5.)

_Per cent Frequency_ _Per cent_ _Food Item_ _of Occurrence_ _Volume_

Pisces 39.3 31.9 Amphibia 36.9 26.0 Reptilia 25.0 18.2 Mammalia 30.9 17.9 Gastropoda 17.8 1.0 Miscellaneous 25.0 5.0 (Algae, Arachnida, Aves, Insecta)

Louisiana: Penn (1943:59) mentions that a "female had just eaten two young cottonmouths...." Clark (1949:259) mentions "100 specimens--34 fish; 25 _Rana pipiens_; 16 _Rana clamitans_; 7 _Acris_; 4 _Natrix sipedon confluens_; 8 birds; 5 squirrels ... catfish thirteen and one-half inches in length ... small-mouth black ba.s.s [eleven inches]."

Oklahoma: Force (1930:37) remarks that the moccasin "eats bullfrogs ... but refuses leopard frogs." Trowbridge (1937:299) writes: "several sun perch.... Another had eaten six catfish six to ten inches long ... a water snake (_Natrix s.

transversa_) about 18 inches long ... frogs, mostly _Rana sphenocephala_." Carpenter (1958:115) mentions "a juvenile woodthrush.... Seven last instar cicadas ... a young cottontail." According to Laughlin (1959:84), one moccasin "contained the following items: 18 contour feathers of a duck, probably a teal; one juvenile cooter turtle, _Pseudemys floridana_; and a large ma.s.s of odd-looking unidentifiable material. The other cottonmouth contained one juvenile pond turtle, _Pseudemys scripta_...."

Texas: "... several ... feeding on frogs.... One ... found DOR was found to contain a large catfish." (Guidry, 1953:54.)

Of 246 cottonmouths that I examined for food items, only 46 contained prey in their digestive tracts. Almost all of the snakes examined were museum specimens that had been collected at many places over a period of about 40 years. It was not known how long each had been kept alive before being preserved. Therefore it was impossible to determine what proportion of any population of cottonmouths could be expected to contain food. The food items were not a.n.a.lyzed numerically because the scales and hair, by means of which many food items in the intestine were identified, yielded no clue as to the number of individuals actually present unless several distinct kinds were found. Each occurrence of scales or hair was thus recorded as a single individual, although some such occurrences may have represented more than one animal. The contents of some stomachs were so well digested that it was difficult to determine the number of items present. As a rule only one food item was present in a digestive tract, but a few tracts contained several items of the same or different species. Three frogs (_Acris crepitans_) were in one snake and three hylas (_Hyla versicolor_) in another. Still another individual captured beside a drying pond contained six individuals of _Lepomis_ each about three inches long and two pikes (_Esox_) about six inches long.

TABLE 13.--a.n.a.lysis of Food Items of 46 Cottonmouths Collected in Arkansas, Louisiana, and Texas (1922-1962).

===========================+========+==========+=========+========== | | Number | Percent |Estimated|Estimated | | | of | | | | | |samples |frequency | weight |percentage| | | in | | | | | FOOD ITEMS | which | of | in | by | | | item | | | | | |occurred|occurrence| grams | bulk | |---------------------------+--------+----------+---------+----------| |Fish | (7) | 13.2 | 20 | 18.4 | | _Esox_ sp. | 1 | | | | | _Lepomis_ sp. | 2 | | 15 | | | Unidentified | 4 | | | | |Amphibians | (12) | 23.0 | | 20.4 | | _Scaphiopus hurteri_ | 1 | | 13 | | | _Acris crepitans_ | 2 | | 4 | | | _Hyla cinerea_ | 2 | | 12 | | | _Hyla versicolor_ | 1 | | 12 | | | _Rana catesbeiana_ | 1 | | 20 | | | _Rana pipiens_ | 3 | | 15 | | | Unidentified | 2 | | | | |Reptiles | (15) | 28.4 | | 29.9 | | _Pseudemys scripta_ | 2 | | 15 | | | _Anolis carolinensis_ | 1 | | 6 | | | _Eumeces fasciatus_ | 1 | | 7 | | | _Lygosoma laterale_ | 2 | | 5 | | | _Natrix_ sp. | 1 | | 10 | | | _Natrix erythrogaster_ | 2 | | 10 | | | _Agkistrodon piscivorus_ | 2 | | 20 | | | _Crotalus_ sp. | 1 | | 30 | | | Unidentified snakes | 3 | | | | |Birds | (4) | 7.6 | | 18.6 | | _Anhinga anhinga_ (juv.) | 1 | | 60 | | | Egret (head and neck) | 1 | | 20 | | | Pa.s.seriformes | 2 | | 20 | | |Mammals | (6) | 11.3 | | 12.7 | | _Blarina brevicauda_ | 1 | | 12 | | | Cricetinae | 5 | | 18 | | |Unidentified | (9) | 17.0 | | | ---------------------------+--------+----------+---------+-----------