just found this ,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.