Homosexual mounting in free-ranging female Hanuman langurs (Presbytis entellus).

Female-female mounting was studied for 3233 hr in a bisexual one-male troop of free-ranging Hanuman langurs in northwestern India over a period of 6 years. The population breeds throughout the year and female-female mounts (n = 524) occurred during all months. All 15 adult females mounted and were mounted, although only 20% of the individuals exhibited more than half of all mounter activity and 33% more than half of all mountee activity. Various similarities with heterosexual mounting suggest that sexual arousal is a stimulus of the activity. Cycling, pregnant, and lactating females acted as mounters as well as mountees. However, ovulating individuals were greatly overrepresented concerning their likelihood of mounting and being mounted. Females of any given rank in the dominance hierarchy showed mounter as well as mountee activity, although mounters were higher ranking than mountees in 84% of cases. Young mounters were overrepresented and middle- to old-aged mounters underrepresented, whereas no such differences existed for mountees. Mounting relations among four young adult paternal half-sisters who occupied top positions in the dominance hierarchy were significantly overrepresented. Although the ultimate function of homosexual female mounting is not fully understood, it is suspected to be connected with intrasexual competition: If "pseudocopulations" induce mountees to reduce the number of solicitations addressed to the male, the probability of insemination decreases and mounters thus reduce the number of future competitors.