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 Volsøe (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 length | Number of gravid females | Total number in size class | Percentage gravid |
| 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).
Fig. 4. Length of testes in cottonmouths of various sizes
( ·—left; °—right ). The right testis is always longer than the left.
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.