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September 17, 1999
September 17, 1999
Researchers at the University of Chicago have discovered a substance produced by male salamanders that acts on female salamanders as a chemical signal to speed up the courtship process and hasten mating.
This is the first time that researchers have clearly pinpointed a single protein that directly influences female receptivity.
The researchers report their findings in the September 17, 1999 issue of Science. "The protein we isolated from the male salamander mental gland is the active component," says Stephanie Rollmann, a graduate student in the lab of Martin Kreitman, associate professor in the department of ecology and evolution at the University of Chicago. "It alone can significantly shorten the time that a pair spends in courtship."
The protein, called plethodontid receptivity factor (PRF), is part of a complex mixture of chemicals that the male salamander uses to stimulate the female and encourage her to mate. The male pheromones are produced by the mental gland, located under the male salamander's chin.
The male delivers these pheromones directly to the female during a stage in courtship called "tail-straddling walk." In this phase, the female straddles the male's tail while they both walk forward together. The male periodically pauses to rub or slap his mental gland on the female to deliver his chemical message. If all goes well, the male will deposit a spermatophore (sperm mass resting on a gelatinous base) on the ground. Still straddling the male's tail, the female then walks forward and lowers herself onto the spermatophore and takes in sperm to fertilize her eggs.
Rollmann, who is interested in how olfactory signals influence communication, wanted to find out which part of the male's pheromone cocktail was responsible for influencing the female's decision to mate.
She and her co-researchers collected courtship pheromones from the mental glands of Plethodon jordani salamanders caught in the field. They zeroed in on PRF when previous behavioral work indicated that a sample of the pheromone mixture containing PRF had biological activity.
To test the activity of PRF, they removed the mental glands of male salamanders and placed the males in boxes with sexually receptive females. "The males acted as though their glands were still present and would still attempt to deliver courtship pheromones," explains Rollmann. But because the glands were removed, Rollmann could control which chemicals were delivered to the females.
For each pair, Rollmann delivered either purified PRF or saline to the females. In pairs where the female was given PRF, courtship time was reduced by approximately 15 percent compared to pairs where the female received saline. "This reduction is a conservative estimate of female response in that the females in the study had been pre-selected for a high propensity to court in laboratory conditions," says Rollmann.
Pheromones such as PRF that can encourage and speed up the mating process play an important role in survival. "By shortening the amount of time spent in tail-straddling walk, the chances of courtship being interrupted by predators, or other males, is reduced," Rollmann explains.
Rollmann and colleagues also pinpointed the gene that codes for PRF. "By studying the gene, we can get a better idea of how this courtship pheromone evolved and begin to study its mechanism of action," she says.