For red-sided garter snakes, beauty is literally skin deep. Sex pheromones wafting from a female's skin drive males crazy, but how they work has long been a mystery. In a new study, researchers implanted estrogen capsules near the testes of male snakes to mimic the location of the hormone's production in females. Then the team allowed unaltered males to choose between scent trails of altered males, large and small females, unaltered males, and she-males (unaltered males that naturally emit small amounts of female pheromones). The scientists found that altered males and large females were equally attractive to unaltered males and that their pheromone blends were identical, the team reports in The Journal of Experimental Biology. The authors suspected estrogen played a key role, and by implanting males with hormone capsules, researchers avoided confounding influences from female physiology and confirmed their suspicions. The results are worrisome, the team notes, because estrogen-like pollutants could disrupt normal mating activities if they induced males to smell like females in the wild.