AbstractLaboratory research on the great apes has revealed similarities in the proximate regulation of sexual behavior by hormonal, social, and spatial variables but differences among the species in various sexual and reproductive characteristics. Laboratory research indicated that all three species of great ape exhibited heightened female sexual motivation during the midcycle, periovulatory phase of the menstrual cycle, consistent with an interpretation of estrus. All three species were also similar in terms of mating at other times in the cycle as a result, primarily, of male sexual initiative/agression. Field research on the sexual behavior and social organization of the great apes under natural conditions suggested that the species differences were related, in part, to the species' mating systems—that is, to differences in intermale competition and female choice at estrus. Consideration of both proximate and ultimate influences on behavioral regulation facilitates the separate objectives of research directed primarily toward one or the other type of influence. © 1995 Wiley‐Liss,