Reproductive Cycle of the American Alligator

Abstract

The reproductive cycle of the American alligator, Alligator mississippiensis , was studied using blood and tissue samples from wild alligators, and blood samples drawn from a captive breeding stock at the Rockefeller Wildlife Refuge in Louisiana. Generally the cycle commences in March as air and water temperatures increase. By early April the seminiferous tubules are full of actively dividing spermatocytes and maturing spermatids. Testis mass is greatest at this time and plasma testosterone is at a peak (>50 ng/ml), but mature spermatozoa do not appear until late April and mid May when mating takes place. Plasma testosterone levels decline rapidly in June after spermiation, and are low to nondetectable in July and August when the testes are fully regressed. A second small peak in testosterone occurs in September without any obvious changes in testicular histology. In the female a similar temperature dependent initiation of the ovarian cycle occurs in late March. Plasma estradiol levels reach a peak (>600 pg/ml) in mid April, and ovarian follicles increase from 5 mm to 45 mm in diameter at ovulation in May. Plasma testosterone is also high in preovulatory females (ca. 1.5 ng/ml). Plasma progesterone shows a periovulatory surge to levels as high as 16 ng/ml, but declines rapidly during the 3½ wk between ovulation and oviposition. During egg incubation (66 days) when the female remains close to the nest, ovarian steroids remain undetectable.