Few systematic attempts have been undertaken to test hypotheses concerning the functional significance of concealed ovulation or prolonged sexual receptivity in extant populations of nonhuman primates. Here, evidence is presented that female vervet monkeys (Cercopithecus aethiops) successfully conceal the time of ovulation from males. Three hypotheses are considered concerning the evolutionary advantages of concealed ovulation for vervet females. Since females mate with numerous males and since males do not provide any parental investment, concealed ovulation cannot be considered an adaptation to promote monogamy in this species. Limited evidence indicates that concealed ovulation in vervets may facilitate female choice of older males as mates; however, because females mate with several males and because it was not possible to pinpoint the exact day of ovulation in the field, this hypothesis could be neither confirmed nor rejected. Indirect evidence is consistent with the idea that concealed ovulation red...