Eggs of the common snapping turtle, Chelydra serpentina selpentina were incubated at 30°C and at 20°C. The incubation period at the higher temperature was about 63 days. At the lower temperature, the period was estimated to be 140 days. Lengths of the embryos at various times of development were recorded.A series of 26 stages is described. The staging is based on timed intervals at a constant temperature, 20°C.
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.
Eggs of Chelydra serpentina were shifted during incubation between the female producing temperatures of 20°C or 30°C and the male producing temperature of 26°C. In the 20°C and 26°C combination, the stages during which incubation temperature determined sex were stage 14 through stage 16 (stages of normal series, Yntema, '68). In the 30°C and 26°C combination, the temperature sensitive stages for sex determination were stage 14 through stage 19. Incubation at 26°C throughout this period was needed to produce all males. Incubation at 30°C during either the first or second half of the period produced nearly all females; shorter periods of incubation at 30°C were more effective in producing females during the second half of the sensitive period. In the 20°C and 26°C combination, incubation at 20°C or 26°C for parts of the sensitive period produced both males and females. In three of the 57 clutches of eggs used in the experiments, incidence of females was atypically high.
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