Sex ratios of American alligators (Crocodylidae): male or female biased?

Recent theoretical papers on temperature-dependent sex determination in rep tiles are based on the assumption that crocodilian populations, particularl y adults, are markedly female biased. While there is evidence that some cro codile populations may conform to this expectation, there is no compelling evidence that American alligator populations are female biased anywhere in the species' range. Previous data had indicated that some populations of ju venile and/or adult alligators were significantly male biased. However, the se studies were criticized for sampling errors. Adult males typically occup y a different habitat from that of females, and males frequent areas where they are more likely to be caught. In contrast, in juveniles, both sexes oc cupy the same habitat. We determined the sex ratios of c. 3000 juvenile all igators collected from 11 sites over 6 years in south Louisiana. Our result s indicate a significant sex bias (58% male), but variation was evident amo ng samples. Sex ratios varied by year and site, and the interaction of thes e factors was significant. At one site in one year, there was a female bias (71%), but in no year was there an overall female bias across sites. From this study and earlier reports, we conclude that there is no evidence at pr esent of female-biased sex ratios in the juvenile and/or adult alligator po pulations, and consequently, that certain models of temperature-dependent s ex determination in reptiles require reevaluation. Furthermore, our data su ggest a pattern of differential mortality of females vs males during the fi rst years of life, a pattern consistent with a key prediction of several di fferential fitness models for the adaptive significance of temperature-depe ndent sex determination in reptiles.