Critical periods and pivotal temperatures for sexual differentiation in loggerhead sea turtles

Eggs of loggerhead sea turtles, Caretta caretta, were incubated at constant temperatures ranging from 24 to 34 °C. At temperatures of 32 °C or above all embryos developed into females as judged by histology of the gonads. At 28 °C or below all developed into males. At 30 °C there were approximately equal numbers of both sexes; 30 °C therefore is the pivotal temperature for the thermal effects on sexual differentiation in this population of turtles. The direction of sexual differentiation is not determined by temperature throughout incubation but by the levels prevailing during a critical period. Experiments with temporary alterations of temperature, either upwards or downwards from the pivotal value, localized the critical period to somewhere between stages 12 and 22 of embryonic development. Definition of the critical period in sea turtles should prove useful both in further work on the theoretical aspects of the phenomenon and in conservation programmes for sea turtles.