Sleeping trees and sleep-related behaviours of the siamang (Symphalangus syndactylus) in a tropical lowland rainforest, Sumatra, Indonesia

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.

Keywords: Gunung Leuser National Park; Hylobatidae; Leuser Ecosystem; Predation; Primate.

Fig. 1

Frequency of sleeping tree use by the family group (FG; 60 observations) and the solitary female (SF; 35 observations). The dotted line represents the group mean level for FG if frequency was consistent (15 uses), and the dashed line represents the group mean level for SF if frequency was consistent (2.33 uses)

Fig. 2

Canopy connectivity (a) and number of branches over 20 cm in diameter (b). ST Sleeping trees, CT control emergent trees. Boxes represent quartiles, whiskers are set to the 95th percentile, dots represent outliers

Fig. 3

Tree height (a), bole height (b) and crown depth (c) of background trees. SP Sleeping plots, CP control plots. Boxes represent quartiles, whiskers are set to the 95th percentile, dots represent outliers

Fig. 4

Frequency distribution of siamang sleeping heights (measured from the ground to the sleeping place)

Fig. 5

Location of sleeping trees and control trees within the home ranges of both siamang units (SF and FG) calculated using minimum convex polygon (a), and the kernel density estimate (KDE; Kernel) method to subdivide the core (50% KDE) and periphery (95% KDE) areas of the home range, and the core (50% KDE) areas of feeding trees (b). For other abbreviations, see Fig. 1