The male and female members of each species must locate and identify each other. This is easy for gregarious species, but not for solitary ones, since there may be no member of the opposite gender within close proximity; so signals have to be emitted that are detectable over long distances. These may take the form of smell (e.g., with butterﬂies) or sound (e.g., with birds). Members of these species need to have extremely sensitive sense organs that are capable of detecting these signals. Since humans are gregarious they do not need to rely upon smell or sound to attract members of the opposite gender over long distances. Their main source of sexual attraction is the relatively short distance signal of vision.