Chemical communication plays a major role in the establishment of social or sexual interactions and in the control of reproduction in the animal kingdom. The substances acting as messengers are designated pheromones/or pherormones. They are released exteriorly and received by an animal within the same species. Indeed, pheromones may operate by stimulating or inhibiting different kinds of sensory receptors as they are perceived by olfactory or gustatory receptors. They may also function after being ingested. Evidence has been provided within the past 20 yr that olfactory-mediated stimuli in some subprimate mammalian species are more widely used, with respect to the control of reproductive processes, than any other sensory signals. We know little at present about the chemical nature of pheromones. Therefore we cannot adopt models such as those used by entomologists for the separation and characterization of pheromonal substances in insects to explain olfactory-mediated stimuli in mammals. A sensory stimulus whose action is prevented by either olfactory bulb removal or peripheral anosmia should be considered as pheromonal in nature until we obtain further information on the compounds involved. Therefore, in the absence of a more precise characterization of pheromones, we are led to use the terminology proposed by Wilson and Bossert, who classified these substances according to the effects they produced. These authors distinguished 'the releaser effects involving the classical type of stimulus mediated wholly by the central nervous system from the primer effects in which the endocrine and reproductive (and possibly other) systems are altered'. The term releaser or signaling pheromone is used when the pheromone elicits more or less immediate changes in behavioral or sexual activity from the recipient animal. If the pheromone triggers a chain of physiological events, i.e., neuroendocrine or endocrine reactions, the term primer or priming pheromone has to be used. Unquestionably the role of olfactory stimuli in creating a releaser effect in mammals was recognized before their primer function. Wilson and Bossert, in dealing with chemical communication in animals, alluded to location of a mate and to recognition of sexual states of activity in some species of mammals but mentioned neither Whitten's pioneer work on estrous synchronization in the mouse nor Van der Lee and Boot's observations on grouping effects in the same species. After these initial experiments real progress was made in this field because of findings derived from studies with the laboratory mouse and and their extension to other species. Moreover, a better understanding of the mechanisms whereby olfactory stimuli control reproduction in mammals has resulted from the use of either neuroendocrinological techniques or radioimmunological methods for measuring levels of both steroid and peptide hormones.
