Between-group competitive advantage offsets foraging costs for bigger groups in harsher seasons

Larger animal groups are widely understood to require more space and travel farther to mitigate the foraging costs of within-group competition. Yet, between-group interactions and shifting resource distributions can confound the relationship between group size and behavior, making responses to demographic change difficult to predict--- especially in highly seasonal environments. Leveraging 33 years of longitudinal data from 12 neighboring white-faced capuchin monkey (Cebus imitator) groups, combined with remotely-sensed environmental data, we show that within- and between-group competition jointly shape space-use patterns, with their relative importance shifting across environmental contexts. We found that: (1) larger groups compensated for reduced per-capita foraging efficiency by expanding into less-exploited areas over longer timescales rather than increasing daily travel; (2) range expansion by large groups was disproportionately directed toward smaller neighbors; (3) during the dry season, resource confinement to riparian zones reduced spatial overlap but increased intergroup encounters, especially in more productive areas; and (4) larger groups held higher-quality ranges in the dry season while maintaining consistent fruit intake across seasons, suggesting that benefits of between-group competitive ability offset within-group costs during resource-scarce conditions. Our findings show that environmental variation shifts the trade-offs of within- and between-group competition, shaping how group-living animals adjust to changing social and ecological conditions.