The role pheromones play in the mating game may have more to do with repelling than attracting, a new study suggests.

Researchers experimenting with fruit flies (Drosophila melanogaster) found that without certain pheromones, a sexual free-for-all ensued, with male flies suddenly attracted to other males and females attracting suitors from other species.

The study, published in the Oct. 15 issue of the journal Nature, suggests a link between sex, species recognition and a specific chemical mechanism.

"These pheromones provide recognition cues that facilitate reproductive behavior," said Joel Levine, a Biology professor at the University of Toronto who led the study. "Lacking these chemical signals (pheromones) eliminated barriers to mating."

To understand how fruit flies recognize the differences between the sexes and other species, the researchers genetically eliminated a certain class of these chemicals, called cuticular hydrocarbon pheromones. The resulting pheromone-free male flies attracted other male fruit flies, while the females without the pheromone became so irresistible that even other fly species attempted to mate with them.

But when the researchers treated females bred without the pheromones with an aphrodisiac, the barrier preventing sex between species was restored.

"That means the same chemical signals and genes are underlying not only social behavior in groups, like courtship and mating, but also behavior between species, Levine said.

There is evidence that men and women can differentiate between scents coming from the same sex or other sexes. How an individual discriminates those scents may reflect their gender preference, researchers suspect. But it's not yet clear how pheromones work in humans. One thing is certain: the cues for attraction are far more complex in our species, Levine said.

"We may rely more on the visual system," Levine said. "We may have a more complex way of assessing other individuals and classifying them and determining how we're going to relate to them than a fly does."

The study was funded by the Swiss National Science Foundation, the Canadian Institutes of Health Research and other grants.