Temperature of egg incubation determines sex in Alligator mississippiensis

Abstract

The factors controlling sexual differentiation in crocodilians are unknown, but heteromorphic sex chromosomes are absent from all species¹. Nichols and Chabreck² speculated that the sex of Alligator mississippiensis was not rigidly determined at the time of hatching but could be influenced by the post-hatching environment. They presented little evidence to support their hypothesis³ (no histological sections of hatchling gonads, no indication of the sex ratio at hatching), and their study failed to take account of habitat preferences of adult male and female alligators⁴. Here we demonstrate by laboratory and field experiments, that in A. mississippiensis: (1) Sex is fully determined at the time of hatching and naturally irreversible thereafter, and depends on the temperature of egg incubation, temperatures ⩽30 °C producing all females, ⩾34 °C yielding all males. (2) The temperature-sensitive period is between 7 and 21 days of incubation. (3) Natural nests constructed on levees are hotter (34 °C) than those constructed on wet marsh (30 °C), thus the former hatch males and the latter females. (4) The natural sex ratio at hatching is five females to 1 male. (5) Females hatched from eggs incubated at 30 °C weigh significantly more than males hatched from eggs incubated at 34 °C. This weight difference constitutes a possible selective evolutionary advantage of temperature-dependent sex determination (TSD) in alligators in that females become large and sexually mature as early as possible. The occurrence of TSD in alligators has wide-ranging implications for embryological, teratological, molecular, evolutionary, conservation and farming studies as well as for theories relating to the extinction of other Archosaurs.