Abstract
Sleeping tree selection and related behaviours of a family group and a solitary female siamang (Symphalangus syndactylus) were investigated over a 5-month period in northern Sumatra, Indonesia. We performed all day follows, sleeping tree surveys and forest plot enumerations in the field. We tested whether: (1) physical characteristics of sleeping trees and the surrounding trees, together with siamang behaviours, supported selection based on predation risk and access requirements; (2) the preferences of a solitary siamang were similar to those of a family group; and (3) sleeping site locations within home ranges were indicative of home range defence, scramble competition with other groups or other species, or food requirements. Our data showed that (1) sleeping trees were tall, emergent trees with some, albeit low, connectivity to the neighbouring canopy, and that they were surrounded by other tall trees. Siamangs showed early entry into and departure from sleeping trees, and slept at the ends of branches. These results indicate that the siamangs’ choice of sleeping trees and related behaviours were strongly driven by predator avoidance. The observed regular reuse of sleeping sites, however, did not support anti-predation theory. (2) The solitary female displayed selection criteria for sleeping trees that were similar to those of the family group, but she slept more frequently in smaller trees than the latter. (3) Siamangs selected sleeping trees to avoid neighbouring groups, monopolise resources (competition), and to be near their last feeding tree. Our findings indicate selectivity in the siamangs’ use of sleeping trees, with only a few trees in the study site being used for this purpose. Any reduction in the availability of such trees might make otherwise suitable habitat unsuitable for these highly arboreal small apes.
Highlights
Studies of sleeping site selection across the Primate order have revealed a diverse range of behaviours and choice of sites
Sleeping location in the tree can be related to predation risk, as it can be argued that sleeping at the end of a branch could expose an individual to avian predators, or that it may enhance the early detection of arboreal predators through branch vibrations (Fan and Jiang 2008; Fei et al 2012)
We aimed to (1) determine the physical characteristics of siamang sleeping trees and the surrounding forest, and record siamang behaviours to determine whether predation risk or access requirements supported tree selection; (2) compare sleeping tree preferences of a solitary siamang with those of a family group; and (3) assess whether sleeping site locations were indicative of range defence, scramble competition or access to food resources (Table 1)
Summary
Studies of sleeping site selection across the Primate order have revealed a diverse range of behaviours and choice of sites. Behaviours exhibited around sleeping trees include selecting suitable (e.g. the tallest) trees to sleep in (Brividoro et al 2019); entering sleeping sites before nocturnal predators become active (Anderson 1998; Reichard 1998); irregularly using the same sleeping tree (Teichroeb et al 2012; Whitten 1982); using familiar sleeping sites with known escape routes (Di Bitetti et al 2000; Struhsaker 1967); moving rapidly into the sleeping tree and remaining quiet near or at the sleeping site (Qihai et al 2009); defecating away from the sleeping site, thereby reducing olfactory cues that alert predators to their presence (Fan and Jiang 2008); and moving away from the sleeping site immediately after use (Reichard 1998). The documented killing of a juvenile siamang (Symphalangus syndactylus) by a clouded leopard (Neofelis diardi) in southern Sumatra (Morino 2010) highlights the importance of anti-predation strategies in these apes
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