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

Abstract

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.

Social relationships among captive male pygmy slow lorises (Nycticebus pygmaeus): Is forming male same‐sex pairs a feasible management strategy?

“…Studies have reported that vocalizations often accompany social behaviors (Daschbach et al, 1981; Zimmermann, 1985). A recent study in Javan slow loris reported that the lorises used ultrasonic vocalizations for communication (Geerah et al, 2019), and reported that lorises emitted vocalizations in the context of following or leading each other, and entering a sleep site. In this study, we sometimes observed that the pygmy slow lorises looked into a nest box where another individual was already inside, or kept in social proximity for a while in front of the nest box before entering the box together.…”

Social relationships among captive male pygmy slow lorises (Nycticebus pygmaeus): Is forming male same-sex pairs a feasible management strategy?

“…Studies have reported that vocalisations often accompany social behaviors (Daschbach, Schein, & Haines, 1981; Zimmermann, 1985). A recent study in Javan slow loris reported that the lorises used ultrasonic vocalisations for communication (Geerah, O’Hagan, Wirdateti, & Nekaris, 2019), and reported that lorises emitted vocalisations in the context of following or leading each other, and entering a sleep site. In this study, we sometimes observed that the pygmy slow lorises looked into a nest box where another individual was already inside, or kept in social proximity for a while in front of the nest box before entering the box together.…”

Venom in Furs: Facial Masks as Aposematic Signals in a Venomous Mammal

“…Yet, such potential predators still could perceive contrast. Slow lorises are highly cryptic, communicating in the ultrasonic range and moving slowly without leaping or disrupting vegetation [ 32 ]. If this cryptic strategy should fail and the loris is spotted by a predator, possession of a powerful aposematic signal could be an advantage.…”

“…Javan slow lorises largely live in uni-male, uni-female social groups with 1–3 offspring in heavily defended territories [ 21 ]. From our long-term field work, we know that pairs can maintain these territories for up to eight years, meaning that the establishment of a permanent territory is a vital component of slow loris development [ 32 ]. Distinct facial features may communicate a warning signal, indicating aggressiveness of young animals looking to settle in a permanent home range, such as also seen in sciurids [ 2 ] and blue-ringed octopus (Hapalochlaena lunulata) [ 38 ].…”