the red-eared slider turtlelike many turtles, alligators
and crocodiles, and some lizardsthe 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.
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.
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.
additional information about the basic mechanisms of temperature-dependent
sex control in turtles, see:
D. 1996. Temperature-dependent sex determination: the interplay
of steroid hormones and temperature. Zoological Science
information about the impact of PCBs on turtle sex determination,
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.