April 29, 2009 -- Females of an Australian species of lizard rely on testosterone for a most unusual method of keeping amorous males off their backs, researchers have found.
Evolutionary ecologist Devi Stuart-Fox of the University of Melbourne and colleagues report their findings online ahead of print publication in the Journal of Comparative Physiology A.
In most animals that use colorful displays for attraction, such as the peacock, it's usually the male that's flashy.
But the female Lake Eyre dragon lizard (Ctenophorus maculosus) is an exception. She displays a bright orange belly and throat during parts of her breeding season, which researchers think is driven by the hormone testosterone.
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Interestingly, the color features prominently when the female wants to put off a male from copulating with her.
Stuart-Fox and colleagues took a close look at a number of female lizards taken from Lake Eyre in South Australia and observed what happened when they were in the company of males.
When Lake Eyre lizards copulate, the male bites the female's neck, climbs on top of her, wraps his tail around hers and inserts one of his two penises.
This can be hazardous to the health of the female, because when the males bite them on the neck they can pierce the female's spine, killing her. Therefore, once the female's eggs have been fertilized, she will try to avoid mating. But males aren't easily put off.
"The males are really persistent," said Stuart-Fox. "They try and force copulation and they harass females all through the breeding season."
Unreceptive females scare off advancing males by taking on a threatening posture. If that doesn't work, they throw themselves on their backs and reveal their bright orange underside.
"Males can't actually force themselves onto a female when she's on her back," said Stuart-Fox.
The researchers also measured the levels of sex steroids in blood samples taken from the lizards over time.
Progesterone and testosterone usually decrease once female lizards are no longer receptive to mating, but not in the Lake Eyre dragon lizard.
"They maintain high testosterone levels all through the reproductive cycle including when they weren't receptive later in the cycle," said Stuart-Fox.
The researchers believe the testosterone is used to drive the female courtship rejection behaviors.
Sexual conflict between persistent males and reluctant females has led to an evolutionary tit for tat involving all manner of behaviors, said Stuart-Fox.
"You get this runaway process where males evolve elaborate ways of trying to gain mating and females evolve elaborate ways of trying to avoid mating," she added.
But she wondered about the evolutionary benefit to females of spending time on their back, which makes it difficult for them to run away from predators.
Especially given they live in a rather drab landscape, the bright orange could be a flag to predators that says "come and get me," said Stuart-Fox.
But she says more recent studies have suggested predators do not recognize the brightly-colored female lizards as prey.
So, the risk of being vulnerable to prey, for these lizards, is lower than the risk of having unwanted sex.
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