Female-male mounting in Japanese macaques: the proximate role of sexual reward.

Female Japanese macaques (Macaca fuscata), in certain populations, routinely engage in female-male mounting. In this study, we documented patterns of vulvar, perineal and anal (VPA) stimulation during female-male mounting in Japanese macaques. During approximately 45% of the female-male mounts analyzed, two thirds of female mounters engaged in VPA stimulation. Given the VPA region's primary role in mediating sexual response, the results of this research provide direct evidence bearing on the sexual nature of a substantial proportion of female-male mounts in Japanese macaques.

Sexual reward via vulvar, perineal, and anal stimulation: a proximate mechanism for female homosexual mounting in Japanese macaques.

Female Japanese macaques (Macaca fuscata), in certain captive and free-ranging populations, frequently engage in same-sex mounting. Traditionally, same-sex mounting interactions in animals have been characterized as "sociosexual," that is, sexual in terms of their superficial form, but enacted to facilitate adaptive social goals. Sexual motivation is rarely ascribed to sociosexual interactions because their adaptive functions are often seen as their primary purpose, thus diminishing, or even negating, any sexual component that such activity might have. A substantial number of studies indicate that female-female mounting in Japanese macaques is not a sociosexual behavior. In contrast, several lines of circumstantial evidence suggest that these interactions are, indeed, sexual. In this study, we documented patterns of vulvar, perineal, and anal (VPA) stimulation during same-sex mounting in female Japanese macaques. During the majority of female-female mounts analyzed, female mounters engaged in repetitive VPA stimulation. Two forms of VPA manipulation by female mounters were observed. First, while sitting in a jockey-style position on the mountee, a female mounter would rub her VPA region against the mountee's back. Males never executed this type of mount posture or pelvic movement. Second, female mounters rubbed their VPA regions with their tails during same-sex mounts. Females mounters moved their tails in a voluntary, sex-specific manner and were never observed to do so in non-sexual contexts. Given the primary role of the VPA regions in mediating sexual response in primates, the results of this research provide direct evidence bearing on the sexual nature of female-female mounting in Japanese macaques and give further support for the conclusion that the term "homosexual behavior" is an appropriate label for these interactions.

Male-female and female-female mounting in Japanese macaques: a comparative study of posture and movement.

Mounting is generally considered to be a male-typical behavior. Female Japanese macaques, in certain populations, are unusual, in that they routinely mount other females. In this study, we examined to what extent female Japanese macaques mount same-sex partners in a male-typical manner. We compared the mount postures males and females adopt and their rate of pelvic thrusting. In addition, we employed a modified form of Laban Movement Analysis (LMA) to compare patterns of pelvic movement during mounts. LMA is a universal language for movement that describes quantitative features of movement, such as changes in the relation of the body segments, as well as qualitative features, such as the style of movements. Our results indicate that female Japanese macaques do not mount in a male-typical manner. Females exhibited a much greater variety of mount postures than did males. Some of the most common types of mount postures employed by females were never exhibited by males. Females performed fewer pelvic thrusts per mount than males, but they executed more pelvic movements per mount, as well as, greater variety and complexity of movement. In addition, the qualitative style of pelvic mounting that females employed differed, in general, from that of males. We argue that these sex differences in mounting can be explained by the fact that both sexes sought sexual reward via genital stimulation during mounting, but they did so in different ways owing to the constraints imposed by their genital architecture. This study raises the larger question as to what constitutes a male-typical or female-typical behavior.