Reproductive Biology of the Water Snake Nerodia rhombifer from Veracruz, Mexico, with Comparisons of Tropical and Temperate Snakes

We collected Nerodia rhombifer on the Papaloapan River and nearby lagoons, near Tlacotalpan, Veracruz, Mexico. Females reach maturity at a snout-vent length (SVL) of 670 mm. Reproduction is aseasonal with some females undergoing vitellogenesis throughout the year, but the majority of females ovulate in May and June. Litter sizes (x̄ = 17.1), based on vitellogenic follicles and embryos, are significantly correlated with SVL. Based on sperm in the vas deferens, males reach maturity at a SVL of approximately 475 mm. The spermatogenic cycle is prenuptial. In the majority of snakes, spermatogenesis begins in September and progresses to the spermatid stage by October. Peak spermatogenesis occurs from November to early January. Hypertrophy of the sexual segment of the kidney begins in January, and the sexual segment remains hypertrophied into early June. The staining intensity of the granules in the sexual segment of the kidney is most intense from late January through June. Clutch size in the population from Veracruz was significantly smaller than in temperate populations. To examine if this trend existed in other species, we compared clutch sizes for 17 oviparous colubrid snakes from tropical regions with 15 species of roughly equivalent temperate species. Our analysis indicated that tropical species did not differ from temperate species in mean clutch size, the mean frequency of reproduction, or the overall net annual fecundity (clutch size × frequency).