The immediate social environment dynamically shapes movement and foraging in wild Sumatran orangutans (Pongo abelii)

Social associations have well-established costs and benefits, and likely play a fundamental role in shaping animals' movement. Individuals are predicted to balance the costs of associating, such as feeding competition, against key benefits, including social learning and mating opportunities, resulting in context-dependent movement patterns. However, the effects of the immediate social environment on movement remain poorly understood, likely because most movement ecology research focusses on either strictly group-living or solitary species. We investigate how social associations shape fine-scale movement patterns and food resource exploitation in wild, semi-solitary Sumatran orangutans (Pongo abelii) who show fission-fusion social dynamics. We integrated 16 years of spatial data on 72 individuals with detailed behavioural observations. We predicted that associations shape movement in ways that reflect distinct costs and benefits across age-sex classes.

We quantified the effects of associations on daily path length (a proxy for energetic investment in movement) using a continuous-time speed and distance (CTSD) approach, path sinuosity (as a measure of movement efficiency), and the use of key food hotspots (as an indicator of feeding competition and spatial knowledge). Using linear mixed models, we tested how these metrics varied with time spent in association and age-sex class, controlling for ecological factors.

Across all age-sex classes, daily path length increased with time in association, driven by longer active periods. At the population level, there was a trend towards straighter paths when individuals were with associates, though this depended on the class of the focal and the associate. Foraging behaviour was socially sensitive, with all age-sex classes visiting fewer feeding hotspots when in associations, and spending less time feeding within them. Social influences on movement and foraging were most apparent in mothers, although flanged males also exhibited significant behavioural effects, including the greatest reduction in overall feeding time when in the presence of unflanged males compared to when alone.

Together, these results suggest that social associations entail costs and benefits for orangutans that are tangibly reflected in their movement and foraging decisions. Furthermore, different age-sex classes showed distinct adjustments in movement depending on who they associated with, highlighting the dynamic nature of social influences on behaviour.