Nocturnal Mating Behavior in Tibetan Macaques: The Low-Ranking Male's Path to Mating Success

This study constitutes the first comprehensive study on nocturnal mating behavior in primates conducted to date. Our objective is to explore the patterns and the influencing factors of nocturnal mating in Tibetan macaques (Macaca thibetana), with a particular focus on the mating strategies employed by low-ranking males. To investigate nocturnal mating behavior in a typically diurnal primate, we conducted a 4-year study (2021-2025) on one group of free-ranging Tibetan macaques at Mount Huangshan, China, employing 4 G solar-powered night-vision cameras for systematic documentation and analysis. Our results indicated that the nocturnal mating behavior of Tibetan macaques follows a bimodal distribution. Successful mating at night did not differ significantly between high- and low-ranking males, indicating that subordinates gained increased mating opportunities under the cover of darkness. At the group level, nocturnal mating frequently occurred in the absence of high-ranking individuals, and the total duration, thrusting duration, and number of thrusts were all significantly greater than during daytime events. The number of nocturnal non-force mating events was significantly higher in low-ranking males than in their high-ranking ones. Still from the group perspective, a significant positive correlation was found between lunar illumination and mating frequency when considered independently of other factors. We proposed that nocturnal mating serves as an alternative reproductive tactic for Tibetan macaques, thereby enhancing low-ranking males' reproductive opportunities. These findings expanded our understanding of the behavioral flexibility inherent in the mating systems of diurnal primates, also offering novel insights into the study of human mating behavior.