Spend any time in a barnyard and you’ll notice that male goats produce a characteristic “goaty smell” that repels humans, but attracts does. Through a phenomenon called the “male effect,” just the presence of a male sends females into heat. Scientists had long suspected that bucks emit a pheromone to turn on does’ reproductive systems, and they’ve finally found the active ingredient. By using a specially made gas-absorbing cap, the researchers collected scent compounds from the heads of male goats and compared them with compounds from castrated ones. They isolated individual chemicals and then tested whether they stimulated a response when females took a whiff. The compound 4-ethyloctanal starts a hormonal chain reaction in the female’s brain, which triggers ovulation, researchers report online today in Current Biology. When exposed to air, 4-ethyloctanal converts to 4-ethyloctanoic acid, which makes that “goaty odor.” So males can attract females and turn them on using the same compound.