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Many primate species currently subsist in fragmented and anthropogenically disturbed habitats. Different threats arise depending on the species' life history strategy, dietary requirements and habitat preference. Additionally, anthropogenic disturbance is far from uniform and may affect individual forest fragments in a single landscape in differing ways. We studied the effects of fragmentation on three species of diurnal primate, Cebus albifrons, Alouatta seniculus and Ateles hybridus, in Magdalena Valley, Colombia. We tested the assumption that generalist species are more resilient than specialist species to habitat degradation by examining the fragments' vegetation and spatial structure and how these affected primate presence and abundance patterns. We found C. albifrons, a generalist, to be the most abundant species in 9 of 10 forest fragments, regardless of the level of habitat disturbance. A. hybridus, a large-bodied primate with a specialist diet, was either absent or low in abundance in fragments that had experienced recent disturbances and was found only in higher-quality fragments, regardless of the fragment size. A. seniculus, a species considered to have a highly flexible diet and the ability to survive in degraded habitat, was found in intermediate abundances between those of Cebus spp. and Ateles spp., and was more frequently found in high-quality fragments.

Forest structure, defined as the three-dimensional vertical and horizontal distribution of canopy vegetation, has great influence on the distribution patterns and abundance of forest primates. The complexity of this structural canopy produces a diverse range of microhabitats and distinct ecological niches, allowing ecologically similar species to co-exist. Degradation of forests through anthropogenic factors significantly alters forest structure, and arboreal species such as gibbons are particularly vulnerable to these changes due to their reliance on canopy for survival. We investigated how forest structural variables influenced the density of two sympatric gibbon species (siamangs Symphalangus syndactylus and lar gibbons Hylobates lar) in Sikundur, a historically disturbed tropical lowland forest in north Sumatra. We used auditory sampling to establish group density in 10 locations and assessed structural characteristics of forest within 4-6 vegetation plots in each location. Lar gibbon group densities were 0.53-3.10 groups/km2 and siamang group densities were lower, with 0.0-1.0 groups/km2. The densities of both species were positively influenced by median height of first bole and the percentage of canopy connectivity. Lar gibbon group density was positively related to large (DBH 30-100 cm), tall (20-25 m) trees with a large crown area (100-300m2), while siamang group density showed no significant relationships with these variables. These findings show canopy connectivity and height to first bole are significant structural variables for the continued presence of

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. Electronic supplementary material The online version of this article (10.1007/s10329-020-00849-8) contains supplementary material, which is available to authorized users.