Ecologist Alexander Quinn of the University of Canberra in Australia and his colleagues incubated bearded lizard eggs at a range of temperatures. Much like some birds, the genetics of bearded lizards' gender are the reverse of humans: Females have two different sex chromosomes (called Z and W) and males bear two of the same (Z). Between temperatures of 22 degrees C (72 degrees F) and 32 degrees C (90 degrees F), the eggs hatched an equivalent number of males and females. But once temperatures climbed above 34 degrees C (93 degrees F), the majority of hatchlings were female.
The Australian researchers isolated a specific DNA marker for femininity in the lizard before exposing three additional clutches of eggs to varying temperatures. At high temperatures, only two males were born out of 35 eggs that hatched. Furthermore, 17 would-be males had been transformed by heat into females.
The finding suggests that the W chromosome is not crucial to determining sex in bearded lizards. Rather, "male differentiation requires two copies of a Z-borne gene, the expression or activity of which is sufficient for male development only at optimal temperatures," the researchers write in the current Science. Of course, bearded lizards are not the only reptiles to be influenced by temperature but they are the first to show that temperature trumps genetics. It remains unclear what evolutionary purpose is served by such skewed sex ratios.