Later Growth and Bodily Proportions

The only records of growth increments in a natural population of cottonmouths are those in Table 11. The period of growth is mostly the period of activity, and differences are expected between northern and southern populations. As size increases, determination of growth rate becomes more difficult because age classes overlap in size. Growth of any individual depends not only on climate and food but also on disease and parasitism and the innate size potential. Stabler (1951:91) showed weight and length relationships in two cottonmouths for a period of six and one-half years.

TABLE 11.—Growth Increments in Cottonmouths (Barbour, 1956:38-39).

Number of
individuals		 Total length
in millimeters		 Estimated age
in months		 Estimated growth
from preceding year
in millimeters
19 260-298 7-8 25
11 312-337 19-20 45
40 355-485 31-32 95±
83 500-1000 43-44+ ?

My study failed to reveal any secondary sexual difference in growth rate and maximum size. Of the 306 cottonmouths measured by me, 16 males and five females exceeded 700 millimeters in snout-vent length. Two males were more than 850 millimeters long. One cottonmouth lived in captivity for 18 years and 11 months (Perkins, 1955:262). The maximum total lengths were reported by Conant (1958:186-187) to be 74 inches (1876 mm.) in A. p. piscivorus and 54 inches (1370 mm.) in A. p. leucostoma.

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 sexual 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, sex. 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).

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 sex 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.