Same-Sex Sexual Behavior in Male Toque Macaques (Macaca sinica aurifrons) of Lower Hanthana, Sri Lanka

To date, 51 nonhuman primates have been reported to exhibit same-sex sexual behaviors. This study adds to that growing list with the first description and preliminary analysis of SSB in Macaca sinica of Sri Lanka. Using the scan sampling method, two free-ranging troops of M. s. aurifrons, in lower Hanthana, were observed from August 2020 to August 2021. We defined sexual encounters as including courtship display, mounting, and genital contact or stimulation. In total, 443 sexual encounters were recorded; 51.2% (N = 227) were different-sex sexual behaviors and 47.1% (N = 209) were same-sex sexual behaviors. Male SSB interactions were more prevalent (97.1%, N = 204) than those of females. The high frequency of male SSB described in this study resembles that of other South Asian macaques, suggesting conserved phylogenetic traits. Closer attention to the roles of kinship, ovarian status, hormones, dominance rank, sexual partner preferences, intra- and inter-sexual competition, and courtship pair formation is expected to shed further light on the motivation and function of sexual behavior in M. sinica society.