In the red-eared slider turtle—like many turtles, alligators and crocodiles, and some lizards—the sex of a growing embryo is by the temperature at which it is incubated. Cooler temperatures cause an embryo to grow up as a male. For example, if eggs are incubated at 26.6°C, all eggs will be male. In contrast, if a clutch is incubated at 31°C, all eggs will be female. A 1:1 ratio is found at about 29.2°C.

This effect is seen, it is thought, because higher temperatures speed the conversion of testosterone to estradiol. Embryos incubated at a warmer temperature are thus exposed to more estradiol during the critical period for sex determination, and they become female.

It has also been found that certain synthetic estrogens can override the temperature effect. For example, administering certain PCBs to eggs incubated at all-male or male-biased temperatures produces more females than expected on the basis of temperature alone.

For additional information about the basic mechanisms of temperature-dependent sex control in turtles, see:

Crews, D. 1996. Temperature-dependent sex determination: the interplay of steroid hormones and temperature. Zoological Science 13:1-13.

For information about the impact of PCBs on turtle sex determination, see:

Bergeron, JM, D Crews, and JA McLachlan. PCBs as environmental estrogens: turtle sex determination as a biomarker of environmental contamination. Environmental Health Perspectives 102:780-781.