Oil palm cultivation critically affects sociality in a threatened Malaysian primate

Abstract

Human-induced habitat alterations globally threaten animal populations, often evoking complex behavioural responses in wildlife. This may be particularly dramatic when negatively affecting social behaviour, which fundamentally determines individual fitness and offspring survival in group-living animals. Here, we provide first evidence for significant behavioural modifications in sociality of southern pig-tailed macaques visiting Malaysian oil palm plantations in search of food despite elevated predation risk. Specifically, we found critical reductions of key positive social interactions but higher rates of aggression in the plantation interior compared to the plantation edge (i.e. plantation areas bordering the forest) and the forest. At the plantation edge, affiliation even increased compared to the forest, while central positions in the macaques' social network structure shifted from high-ranking adult females and immatures to low-ranking individuals. Further, plantations also affected mother–infant relationships, with macaque mothers being more protective in the open plantation environment. We suggest that although primates can temporarily persist in human-altered habitats, their ability to permanently adapt requires the presence of close-by forest and comes with a trade-off in sociality, potentially hampering individual fitness and infant survival. Studies like ours remain critical for understanding species’ adaptability to anthropogenic landscapes, which may ultimately contribute to facilitating their coexistence with humans and preserving biodiversity.

Occupancy of wild southern pig-tailed macaques in intact and degraded forests in Peninsular Malaysia

“…Although M. nemestrina readily leaves its natural forest habitat to enter oil palm plantations in search of food ( Ruppert et al, 2018 ; Holzner et al, 2019 ), it is described as a shy and elusive macaque species that tends to avoid human-dominated areas ( Bernstein, 1967 ; Oi, 1990 ). Recent studies highlighted the macaques’ dependency on primary forest habitat as a safe retreat to sleep and socialize ( Ruppert, Mansor & Anuar, 2014 ; Holzner et al, 2021 ), confirming previous doubts on their ability to permanently persist in highly disturbed habitats ( Caldecott, 1986 ). Accordingly, we predicted that forest clearance negatively affects the macaques’ ability to occupy a specific habitat, and hence a general decline in site occupancy in PFR during the sampling period.…”

“…In degraded forests, wildlife may suffer from habitat fragmentation, reduced availability of natural food sources, and a high human hunting pressure facilitated by the increased accessibility to the remaining forest patches ( Johns, 1985 ; Tilker et al, 2019 ). Previous studies emphasized the primates’ dependency on preserved natural forests in and around these highly disturbed landscapes to successfully disperse and reproduce ( Ancrenaz et al, 2021 ), as well as to perform the full range of their natural behavioural repertoire ( Holzner et al, 2021 ). The latter includes the formation of strong social relationships, which are critical characteristics of group-living animals owing to their crucial link to individual fitness ( Cameron, Setsaas & Linklater, 2009 ; Schülke et al, 2010 ; Ellis et al, 2019 ) and offspring survival ( Silk, Alberts & Altmann, 2003 ).…”

The Predicament of Macaque Conservation in Malaysia

“…Further research on habitat needs for all macaque species is imperative in order to understand the disparity of population density between the species, despite the similarities of natural habitats, diet, and behavior. A good example is a study conducted by Holzner et al [66] citing the significant changes in sociality behavior of pig-tailed macaques that visit oil palm plantations in Malaysia, which may debilitate individual fitness and infant survival. This proves that despite the ability of pig-tailed macaques to temporarily adapt to human-altered habitats, the proximity of forest is vital for the survival of the species.…”