Human-induced habitat alterations globally threaten animal populations,
often evoking diverse and 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 critical behavioural modifications of Southern pig-tailed
macaques visiting Malaysian oil palm plantations in search of food.
Specifically, we found significant reductions of positive social
interactions, an increase of non-physical aggression and shifts in the
macaques' social network structure, with the central positions of
high-ranking adult females and immatures being passed to low-ranking
individuals likely resulting from socio-ecological risks posed by
plantations. Deviations from natural behaviour also affected the
smallest but crucial social units within groups, mother-infant pairs,
with increased maternal protectiveness at plantations. Our study
provides strong evidence that although primates, and more generally
group-living wildlife, can persist in human-altered habitats, their
ability to adapt may come with a trade-off for their natural sociality
potentially hampering infant development and individual survival.