Edward M. Miller1 University of New Orleans A system by which males send, and females receive, evidence of the male's continued presence is hypothesized. Such a system would promote the fitness of both sexes. Females increase their fertility when they can anticipate a mate to help rear the child. The existence of such a system could explain amongst others the following observations: that the menstrual cycle is more often of a fertile type when there is regular intercourse, that exposure to males influences ovulation, the large amount of non-reproductive sexual activity, the presence of what may be a functional vomeronasal organ, and the apparent design of the axilla for emitting odors or pheromones. Key words: Pheromones, fertility, odors, pair bonding, copulation, axillary hair, androstenol. Both sexes could benefit from having mate detectors and mechanisms for adjusting behavior to the existence of a mate. These would be devices that detect the presence of a mate (opposite sex by definition) and adjust fertility and behavior to the optimal level. Such mechanisms would plausibly operate by the emission and detection of pheromones, chemical messages from one individual to another (see Kohl & Francoeur, 1995 for more on pheromones). For reproductive success, a woman might be expected to have some physiological mechanism to increase fertility when there is a man around providing her with both food and security and likely to remain around to assist her in rearing the child. Possibly, the mechanism would operate after impregnation, checking to see if a male was still around, and if he was, permitting the blastocyst to implant and to develop. If the coupling had been a one night stand, or the result of a rape, the mechanism would discover there was no male around, and the probability of pregnancy would be reduced. The mechanism would ideally reduce the probability of pregnancy in the absence of a mate, but would not eliminate it since sometimes women, unassisted by a male, can raise offspring. Having a higher probability of copulation resulting in pregnancy in a mate's presence than in his absence (but with there still being some chance of pregnancy resulting from one night stands) would appear to be the optimal design. There is evidence that female fertility is higher in the presence of a man, with possible mechanisms involving either repeated copulations, or pheromones, or both. Pheromones are chemical stimuli emitted by one animal (here the male) that affect another animal (here the human female) at a distance. These may be odors that can be smelled, or they may be substances that are not consciously perceived. Interest in possible human pheromones has been increasing with the discovery that a human vomeronasal organ similar to that which detects pheromones in other mammals indeed exists and could be functional in humans (Monti-Bloch et al. 1994; Jennings-White 1995, Berliner, et al. 1996), and with the formation of at least two new companies to exploit them. Erox raised $15,000,000 (Bishop 1992), and is now selling pheromone containing perfumes. This paper presents evidence that regular copulation promotes fertility, even when done at times when the woman is not fertile. It also presents evidence that suggests that male pheromones promote fertility in human females. These will be argued to be adaptations by which females adjust their fertility to the continued presence of a male (implying likely assistance in rearing offspring). Other, previously puzzling traits found in Homo sapiens, including frequent non-reproductive sexual activity, cuddling, sleeping together, odors arising from apocrine secretions, axillary hair, keeping a minimal distance between males and females who are not romantically involved, male sleepiness after copulation, nasal flushing during copulation, and sweating during copulation, will be argued to be adaptations by which males signal their presence, and by which females induce males to signal their presence. …