Temperature-sensitive periods of sex determination in Emydid turtles.

In many turtles sex differentiation is controlled by the incubation temperature of the embryo, with low temperatures producing males, high temperatures producing females. This study investigates the developmental period of temperature-sensitivity in two species of emydid turtles, using different combinations of incubation at a male-determining temperature (25 degrees C) and at a female-determining temperature (31 degrees C). The sensitive period extends throughout much of the middle third of development. Sex is more readily influenced by 25 degrees than by 31 degrees, however, so that maleness can be determined much earlier in development than can femaleness. Comparison of these results with a previous study of snapping turtles indicates that the sensitive period occupies somewhat the same developmental interval in these different turtles. However, in snapping turtles, the female-determining temperature used (30 degrees) is more influential than the male-determining temperature (26 degrees), in contrast with these results from emydids.