Certain Female Lizards Choose Sex of Offspring

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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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