Biologists have known for some time that among certain egg-laying reptilescrocodiles, for examplethe sex of the embryos depends on the temperature at which they develop. No one suspected that the phenomenon could occur in reptiles that bear live young because the mothers maintain fairly stable body temperatures. Findings reported today in the journal Nature, however, reveal that, in fact, a female lizard can determine the sex of her offspring through thermoregulationa mechanism that helps the creatures to balance sex ratios in the wild.

Kylie A. Robert and Michael B. Thompson of the University of Sydney studied a captive female population of the Australian skink, Eulamprus tympanum, which normally dwells in high-elevation habitats in southeastern Australia. They found that, in the laboratory, the females all maintained body temperatures of 32 degrees Celsius and produced only male offspring. Observations of the skinks in the wild, on the other hand, reveal equal sex ratios.

The mechanism by which the mothers select body temperature to balance the population sex ratio remains a mystery. But the fact that these skinks are subject to temperature-dependent sex determination may explain why the species occupies only alpine regions: warmer areas could result in the production of males only, which would be a death sentence for the population. Indeed, the team's findings may portend the effects of global warming on these animals. "For alpine species, there can be no retreat to cooler climates, so a rise in environmental temperature would result in increased production of males," the authors note. "Models predict a temperature rise of [4 degrees C] by 2100, which could seriously alter the sex ratio and lead to extinction of species such as E. tympanum."