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.