Same-sex sexual behaviour (ssb) in free-ranging toque macaques (macaca sinica aurifrons) in the lower Hanthana region, Sri Lanka

Abstract

Same-sex sexual behaviour (SSB) is common in animals, including primates and is often viewed as an “evolutionary paradox” as unlike different-sex sexual behaviour (DSB), it persists without any obvious contribution to reproductive success. While SSB has been documented in many primate species, SBS in Macaca sinica has not been reported in previous studies. We conducted a behavioural study in free-ranging M. sinica as a part of a long-term study to document the occurrences of sexual encounters and to provide a qualitative description of SSB. Data were collected from August 2020 to August 2021 in lower Hanthana (7⁰15′24″ N 80⁰36′08″ E) from two groups of macaques A (n=37-53) and B (n=20-24). The adult male/female ratio in A and B ranged from 0.6-0.8 and 0.5-0.6, respectively. Scan sampling was carried out for 3-5 days/month from 0700 to 1700 h resulting in a total of 1,123 observational hours. Sexual behaviours were observed for the bout, kiss, masturbation, ventral hug, clutching reaction, mount position, partial mount, sideway mount, mount refusal, genital manipulation and mount rhythm. In troop A, 162 DSB and 163 SSB encounters were documented, while 64 DSB and 51 SSB occurrences were recorded for troop B. Mounting was the most frequently recorded SSB and DSB activity for both groups. Similar to M. fuscata, the fundamental factor of homosexual mounting is challenging to examine with no physiological information available. Adult male–adult female DSB encounters were higher in troop B (81%) compared to A (54%). Female-female SSB in troop A was recorded as 1%, whereas in troop B, it was 10%. Adult male–adult male SSB was higher in troop B (37%) compared to A (18%). There was no significant association between sexual behaviour and the troop size (χ2=1.146, df =1, p=0.284), depicting that when the number of individuals in a troop is high, the sexual encounters were also high and individuals can involve in both SSB and DSB. The unusual SSB among free-ranging adult and sub-adult males of M. sinica could be serving as a practice for DSB, avoiding/managing stress or simply pleasure-seeking.