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Only a handful of primate taxa use ultrasonic vocalisations (those ≥20 kHz) to communicate. The extent and uses of ultrasonic communication remain poorly understood, potentially ranging from echolocation, advertisement of reproductive status and resource availability, social cohesion, to predator avoidance. Here, using active acoustics whereby the study subjects were observed throughout their activity period, we describe the first purely ultrasonic call from a strepsirrhine primate (family Lorisidae), recorded in a completely wild setting, and hypothesise about its function. We identified one type of ultrasonic call, the doublet click, from 14 Javan slow lorises (Nycticebus javanicus) produced by males and females of juvenile, subadult and adult ages within their social groups (n = 791, mean = 46.0 kHz). We ran quadratic discriminant function analysis, finding dominant frequency and doublet click duration as the key parameters for identifying individuals' sex and age. Significantly more vocalisations were produced during affiliative social behaviour, suggesting that the call serves a social cohesion function. Considering the range of other cryptic behaviours, including slow and silent locomotion, and the high degree of territoriality associated with venomous attacks on conspecifics, the call may also serve as a safety strategy, allowing family members to regulate distance from other slow lorises and to communicate cryptically whilst avoiding predators.

The Use of Ultrasonic Communication to Maintain Social Cohesion in the Javan Slow Loris (Nycticebus javanicus)