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

Lizards are cold-blooded animals and regulate their body temperature by basking in the sun. A lizard can bask at will to maintain a desired body temperature.

When Robert and her colleague Michael Thompson gave female E. tympanum lizards the opportunity to bask in the laboratory unimpeded by predators or weather conditions, the female lizards all chose to maintain a warm body temperature of 89.6 degrees Fahrenheit (32 degrees Celsius).

The warm temperature speeds up the development of lizard embryos, which reduces the length and burden of pregnancy, said Robert. At the temperatures the female lizards maintained in the laboratory, they produced exclusively male offspring.

In the wild, female and male lizards are born in roughly equal numbers. The researchers don't fully understand what mechanism enables E. tympanum females to control the body temperatures needed to produce equal ratios of male and female offspring in the wild. One possibility, said Robert, is that they are restricted from maintaining the higher temperatures by predators or weather conditions.

"Alternatively, they may not be thermally restricted and are actively choosing to maintain lower temperatures in the field to produce balanced litters in response to adult sex ratios," she said.

The laboratory experiments were conducted exclusively with female lizards. The researchers said it's also possible that the female lizards noticed the absence of males in the laboratory and actively maintained higher body temperatures to restore sexual balance to the population.

"The suggestion is that this animal behaviorally thermoregulates," said Crews. "There may be some selection by mom for the sex of the offspring."

Robert is currently conducting experiments in the field to determine whether the lizards are thermally restricted by factors such as predators and weather, or if they actively choose to maintain lower temperatures in response to sex ratios.

Scientists do not know the extent to which temperature-dependent sex determination occurs in live-bearing reptiles, many of which are restricted to small island populations. If the trait is common to these species, they would be extremely vulnerable to climate change for similar reasons as E. tympanum, said Robert.

Crews urges caution when it comes to such musing about climate change. "It is characteristic of evolutionary biologists to make these kinds of sweeping statements—statements that can never be proven," he said. "There is great debate as to whether global warming is occurring."

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