Sex Ratios of Wild American Alligator Hatchlings in Southwest Louisiana

Abstract

The sex of American Alligator (Alligator mississippiensis) hatchlings is deter- mined by the egg temperature during the middle third of the 9-12 week incubation period. As a consequence, predictable sex ratios are possible for clutches incubated in constant temperatures in the laboratory, but naturally occurring sex ratios of American Alligator hatchlings from wild nests exposed to fluctuating temperatures are not well documented. Over a 5-year period (1995-1999), we determined the sex of American Alligator hatchlings from wild nests left in the field until after sex was irreversibly determined. A total of 6226 hatchlings from 232 naturally incubated wild nests showed a strong female bias (71.9% females, yearly range = 62.3-89.4% females). Most nests (64.2%) produced hatchlings of both sexes. Of the remaining clutches that produced exclusively one sex (83 nests), 78 nests produced all females, and 5 nests produced only male hatchlings. For the 2 years in which nest-cavity temperatures were known, higher temperatures led to production of significantly more male hatchlings (P < 0.001 for both 1997 and 1999). Knowledge of natural sex ratios of hatchlings can aid in the management and harvest of this commercially valuable species, and in understanding sex-ratio bias in American Alligator populations.