Garter snakes yield sexual chemistry.

Garter snakes yield sexual chemistry

Scientists have isolated, identified and synthesized two pheromones in the Canadian red-sided garter snake, marking a rare instance in which the potent compounds have been characterized in a vertebrate, they say. Pheromones are chemicals produced by an organism that cue specific behaviors when smelled by the creature's compatriots.

The researchers, led by biochemist Robert T. Mason of the National Heart, Lung, and Blood Institute in Bethesda, Md., analyzed the female sex-attractiveness pheromone and the male sex-recognition pheromone that guide the species' mating behavior. They report their findings in the July 21 SCIENCE.

For one month every spring, male garter snakes gather in groups of several thousand waiting for mates to show up. Females appear sporadically, each immediately surrounded by 10 to 100 males engaging in "courtship" behavior -- rapid tongue-flicking and chin-rubbing up and down the female's back. One male eventually mates with the female.

Mason and his co-workers collected sexually primed, unmated adult garter snakes, 18 females and 24 males. They killed the animals and extracted their skin lipids, which contain the sex pheromones. The lipids were then broken down into several solutions containing different chemical compounds.

The researchers poured solutions of the female lipid on paper towels and placed these in a den with courting male garter snakes. Only one preparation, containing a series of previously undescribed long-chain methyl ketones, elicited courtship behavior. Scientists had found long-chain methyl ketones in some insects and snakes but had not established their behavioral functions.

When Mason's group added extracts of male lipids to female extracts, male courtship stopped, suggesting males emit specific chemical cues that identify them as males, the researchers say. One chemical in the male lipid, squalene, caused a significant drop in courting and is important in the male sex-recognition pheromone, Mason adds.

The researchers also collected skin lipids from "she-males," a small group of male garter snakes that other males court as if they were females. Their skin chemistry contains no squalene and is more like that of females, Mason notes.

Preliminary studies of related groups of snakes, such as brown tree snakes, suggest females possess some of the same methyl ketones as well as variations of the compound, he says. Different sex pheromones may help to maintain snake species or promote their evolution, Mason contends.