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The cao vit gibbon (Nomascus nasutus) is one of the rarest primates on Earth and now only survives in a single forest patch of less than 5000 ha on the Vietnam-China border. Accurate monitoring of the last remaining population is critical to inform ongoing conservation interventions and track conservation success over time. However, traditional methods for monitoring gibbons, involving triangulation of groups from their songs, are inherently subjective and likely subject to considerable measurement errors. To overcome this, we aimed to use 'vocal fingerprinting' to distinguish the different singing males in the population. During the 2021 population survey, we complemented the traditional observations made by survey teams with a concurrent passive acoustic monitoring array. Counts of gibbon group sizes were also assisted with a UAV-mounted thermal camera. After identifying eight family groups in the acoustic data and incorporating long-term data, we estimate that the population was comprised of 74 individuals in 11 family groups, which is 38% smaller than previously thought. We have no evidence that the population has declined-indeed it appears to be growing, with new groups having formed in recent years-and the difference is instead due to double-counting of groups in previous surveys employing the triangulation method. Indeed, using spatially explicit capture-recapture modelling, we uncovered substantial measurement error in the bearings and distances from field teams. We also applied semi- and fully-automatic approaches to clustering the male calls into groups, finding no evidence that we had missed any males with the manual approach. Given the very small size of the population, conservation actions are now even more urgent, in particular habitat restoration to allow the population to expand. Our new population estimate now serves as a more robust basis for informing management actions and tracking conservation success over time., (© 2024. The Author(s).)

Dance is a universal art form practiced by all human societies and has many functions including sexual attraction, social cohesion, and the therapeutic release of energy. Dance also has been reported in a small number of non-human primate species, in particular apes. However, its function has not been systematically evaluated. We observed 357 intentional, rhythmic, and nonverbal dance displays performed by four adult female cao vit gibbons (Nomascus nasutus) residing in four polygynous groups during 3000 h of observation in Bangliang, Guangxi, China. Females used dance to solicit copulations, as well as to promote a social bond with the group's lone adult male. In addition, this display appears to represent a form of non-aggressive competition among adult females living in the same group. We found that a female who had a weaker social relationship with the breeding male increased her social and sexual access to the male by an increase in dancing frequency. Given that gibbons dance in various behavioral contexts, and appears to serve several important social and sexual functions, a greater understanding of this form of gestural communication offers an instructive model for examining the origin and evolution of dance in humans and other apes.