The structure of the reproductive systems of hatchling and 1 -year-old alligators was investigated macroscopically and histologically. Sex is fully determined at the time of hatching and irreversible thereafter. Constant temperature laboratory experiments revealed that the sex of Alligator miuissippiensis is determined by the temperature of egg incubation, 30°C or below producing females, and 34°C or above, males. Experiments in which eggs were shifted from female-producing temperatures to male-producing temperatures and vice versa at weekly intervals during incubation demonstrated that the temperature-sensitive period for sex determination is around 20 and 35 days after egg laying. In the wild, temperature probes were placed in alligator nests constructed in three different habitats: wet marsh, dry marsh and levee (elevated firm ground). The position of the eggs in the nests were mapped at 60 days incubation and the sex of the enclosed hatchlings correlated with the nest temperatures at that site. Levee nests were hot (34°C) and hatched approximately 100% males. Wet marsh nests were cool (30°C) and hatched approximately 100% females. Dry marsh nests had an intermediate temperature profile, the hottest location (34°C) being the top centre of the nest. Males developed from eggs in this location and females from eggs around the periphery and base of the nest. The sex ratio in dry marsh nests was five females to one male. The natural sex ratio at hatching was determined for a large area of representative habitat for four consecutive years and averaged five (±0.7) females to one male. Females hatched from eggs incubated at 30°C weighed more than males hatched from eggs incubated at 34°C because the former contained more absorbed abdominal yolk. These extra energy reserves enable females to grow faster and become larger than males in the early years of life. This means that heavy females become sexually mature ahead of either light females or light males, and because of the reproductive lifestyle of alligators, this early maturity constitutes a selective biological advantage for the evolution of temperature-dependent sex determination in Alligator mississippiensis. The occurrence of temperature-dependent sex determination in alligators has wide-ranging implications for embryological, teratological, molecular, phylogenetic, conservation and farming studies.
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