Why do two-sex geckos triumph? (Lizards).

The smell of one invading species of gecko has a mysterious influence on the activity of the defending species, but the voodoo doesn't work on first exposure, reports a researcher in Hawaii.

The Hawaiian Islands and many other islands in the Pacific Ocean have long been home to Lepidodactylas lugubris, a species of unisexual lizards. These geckos, which are effective colonizers, probably reached Hawaii with the Polynesians. As more people moved around the Pacific during World War II, a bigger species (Hemidactylus frenatus) that has two sexes started spreading. When H. frenatus arrives on an island, the population of the unisexual species plummets. Susan Brown, a biologist at the University of Hawaii at Hilo, wonders why.

Scientists had speculated that because the geckos of H. frenatus are bigger, they poach the unisexuals' food. Brown doubts that explanation, since there seem to be plenty of bugs to go around, she says.

Instead, she proposes that H. frenatus inhibits the behavior of the smaller species. When the L. lugubris geckos are on their own, "these ladies are really aggressive" she says. But when she put the two species together, the L. lugubris were subdued. Moreover, the H. frenatus scent by itself could dampen aggression and reduce egg laying by the smaller geckos.

In new work, she raised the unisexual geckos in individual quarters and then exposed them to the dreaded scent. The first time a unisexual gecko encountered the other species, the smaller gecko attacked with her species' usual fervor. On her next exposure, however, she became subdued. Brown suspects that such a subtle interaction between the species is fueling the spread of H. frenatus.--S.M.