Video-Sound Recording Devices Reveal Multiple Drivers of Nocturnal Vocalizations in Tibetan Macaques

Nocturnal vocalizations not only reflect communications at night, but also reveal adaptive strategies of animals. Previous research focused on the vocalizations of nocturnal species, with little attention to nocturnal vocalizations in diurnal animals. Furthermore, sound studies traditionally employ an active acoustic monitoring approach which is limited in terms of the range of sound collection and is not combined with video data. To fill in these gaps, we studied the nocturnal vocalization of diurnal primates, Tibetan macaques (Macaca thibetana) in Huangshan, China by 4G infrared cameras and passive acoustic monitoring devices. Results showed that Tibetan macaques had a possible bimodal distribution of nocturnal vocalization count, with peaks occurring at 18:00-19:00 and 21:00-23:00. In terms of socio-biological factors, males or individuals with lower social centrality exhibited more vocalization count, and kinship also promoted the emission of vocalization, whereas vocalization was negatively correlated with age. As for ecological factors, wind direction significantly affected the vocal activity level. Vocal activity level was correlated with environmental variables, showing lower values under high wind speed, and higher values under low temperature and high humidity, with additional interactions among climatic factors such as temperature and precipitation. An analysis of the behavioral context within 10 s before and after vocalizations found that, diverse social behaviors were frequent around 18:00, but affiliative behaviors were most frequent between 21:00 and 22:00. Based on video-sound recording devices, we first revealed the pattern of nocturnal vocalization in Macaca thibetana and its factors. Nocturnal vocalizations of diurnal primates likely serve to maintain group cohesion, manage conflicts, adapt to environmental pressures, and facilitate group integration, reflecting the multifaceted strategies by primates for survival and social organization.