Sleeping trees and sleep-related behaviours of siamang (Symphalangus syndactylus) living in a degraded lowland forest, Sumatra, Indonesia.

Abstract

Tropical forests are hotspots for biodiversity and hold some of the world’s most unique flora and fauna, but anthropogenic pressures are causing large-scale tropical forest disruption and clearance. Southeast Asia is experiencing the highest rate of change, altering forest composition with intensive selective and mechanical logging practices. The loss of the tallest trees within primate habitat may negatively affect arboreal primates that spend the majority of their lives high in the canopy. Some primate species can spend up to 50% of their time at sleeping sites and must therefore select the most appropriate tree sites to sleep in. The behavioural ecology and conservation of primates are generally well documented, but small apes have gained far less attention compared to great ape species. In this study, sleeping tree selection of siamang (Symphalangus syndactylus) were investigated from April to August 2018 at the Sikundur Monitoring Post, a degraded lowland forest in Gunung Leuser National Park, Sumatra, Indonesia. Siamang were shown to sleep at the end of branches in tall, stable, emergent trees, high above the mean canopy height. Sleeping trees had an optimum percentage of canopy connectivity and number of large branches, as well as being surrounded by taller trees. Siamang entered sleeping trees before sunset and left before sunrise. All these factors suggest that antipredation is an important factor affecting sleeping behaviour. However, siamang in this study had regular sleeping trees, a quality so far undocumented amongst other hylobatids. This re-use of the same sleeping trees goes against the idea that predation is the main driver of sleeping site selection, as regularly used sleeping sites are more predictable to predators. Sleeping tree re-use may be explained by the degraded nature of the Sikundur forest or could be an adaptation of the siamang’s unique distribution, morphology and behaviour. Siamang did not position their sleeping trees close to fruiting trees but instead may select trees on a basis of comfort and stability. By understanding siamang sleeping sites in greater depth, conservation management plans will be able to mitigate the loss of an already endangered primate species.