Abstract: In species with flexible grouping dynamics (i.e., fission-fusion), party (or subgroup) size is often shaped by available resources. Food resources are thought to limit party size in a range of mammalian species, reflecting a strategy of reducing feeding competition. In montane habitats, where food is highly seasonal, we may expect to see strong effects of ecological constraints on party size. In the montane forest of Nyungwe National Park, Rwanda, we quantified changes in chimpanzee (Pan troglodytes schweinfurthii) party size. We used path analysis to analyze the direct and indirect effects of (i) ecological variables and (ii) a reproductive variable (estrous females) on party size. Our path analysis adds precision and directionality to the hypothesis that food availability and estrous females influence party size. We found that the presence of estrous females had the strongest effect on party size. Interestingly, the availability and distribution of important and preferred fruits did not directly influence party size but did influence the presence of estrous females. These findings indicate that fruit distribution may be the ecological precondition that attracts estrous females, which is the main driver of larger parties. Party size was, however, positively correlated with patch size, indicating that specific fruit species may be of particular importance to the ecology of these chimpanzees. Significance statement: In montane habitats with lower fruit tree density and diversity than lowland sites, it may be expected that ecological factors play a more important role in fission-fusion grouping patterns than reproductive factors. This is because the cost of competition for food among group members may be higher in a more resource-limited (montane) environment. We investigated the ecological and reproductive factors that influence grouping patterns of chimpanzees in a montane forest that encompasses the upper limit of this species’ altitudinal range. Our results showed that the presence of estrous females had the strongest influence on party size. These results have implications for evaluating ecological and reproductive drivers of chimpanzee party size across differing habitats.