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The release of pheromones is one of various forms of nonverbal communication many animals use to transmit messages to other members of the same species. The complex molecular structure of pheromones allows these chemical messages to contain a great deal of often very specific information. The pheromone released by sexually receptive silkworm moths, first isolated in the 1950s, is one of the best-studied examples. The pheromone bombykol, released by the female from a gland in her belly, is detectable by male silkworm moths up to several kilometers away. The male identifies the chemical in the environment with tiny receptors at the tip of his antennae and is then able to hone in on the female. Hornets, when disturbed, release an alarm pheromone that calls other hornets to their aid. Female mice pheromones may excite a male mouse to mate immediately. In addition to producing instinctive behavioral responses, pheromones can also produce changes in an animal's physiology, spurring the onset of puberty or bringing on estrus. Pheromones used by animals, such as cats and dogs, to mark territory can convey information about an animal's species, gender, age, social and reproductive status, size, and even when it was last in the area. But can humans communicate via chemicals, too? In the 1970s Martha McClintock showed that the menstrual cycles of women living closely together in dormitories tended to become synchronized, an effect thought by some to be mediated by pheromones. Despite such evidence, no pheromone receptors have yet been found in humans.