Certain Female Lizards Choose Sex of Offspring

By John Roach
for National Geographic News
August 15, 2001

Male lizards of a species known as southern water skinks, which live high in the mountains of southeastern Australia, better hope the threat of global warming is a farce. If Earth does warm up, they may find themselves left with no females to mate with, and the species pushed to extinction.

The concern arises because scientists have found that females of the lizard species Eulamprus tympanum control the sex of offspring by controlling their own adult body temperature.

This is the first time that temperature-dependent sex determination has been reported in a species that gives birth to live young, according to the researchers who described their findings in the August 16 issue of Nature.

Preliminary data indicate that when temperatures are warm, the female lizards give birth exclusively to males. That doesn't bode well for the future of the species if global warming models are correct, said Kylie Robert of the University of Sydney, a co-author of the report.

"This species is already restricted to mountain tops," she said. "With a 4-degree Celsius [7.2-degree Fahrenheit] rise as predicted by global warming models, they cannot retreat to cooler regions and will, in turn, produce entirely male offspring and eventually become extinct."

Unusual Trait Among Reptiles

The sex of an advanced organism is determined through a well-orchestrated cascade of biological events. The reproductive organs begin undifferentiated, then follow a pre-programmed pattern of development to become either female or male.

In vertebrates, sex is determined by genetics at the time of fertilization, as in humans, or by environmental factors, such as temperature, that occur after fertilization.

Temperature-dependent sex is common among many egg-laying reptiles, such as crocodiles and turtles. In a nest, incubation temperatures can vary widely. Eggs at the top of a nest, for example, incubate at different temperatures than eggs in the middle or at the bottom, Robert explained.

Finding the same kind of trait in a species that gives birth to live young, such as E. tympanum, is surprising. Reptiles keep their body temperature relatively constant.

"It is the first demonstration in a live-bearing reptile," said David Crews, a biologist at the University of Texas in Austin. "It goes against one of the basic assumptions about [temperature-dependent sex determination]—that it occurs in egg-laying reptiles."

Restoring Balance

Continued on Next Page >>


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