Despite the fact that all African great apes have overlapping diets, they differ substantially in both biogeographical distribution and social organisation: Gorilla lives in relatively small, cohesive groups within a small biogeographical area while Pan is much more widely distributed and lives in large, fluid groups in which the members are rarely all together. In this study we use a modelling approach to identify possible causes and consequences of these differences. We use a time budget model which is based on the relationship between time available for various activities, group size, body mass and climate. We demonstrate the importance of body mass as a critical determinant for maximum ecologically tolerable group size as well as ape distribution patterns. In addition, we show that predation pressure may play a strong role in limiting the distribution of smaller-bodied apes (Pan). Predation pressure appears to be especially important if the apes opt for a fission–fusion strategy because it obliges them to maintain larger (sub-) groups. In effect, the apes appear to face a tradeoff between solving the predation problem by increasing body size (at the expense of reduced ecological flexibility) and going for ecological flexibility (but at some cost in terms of how they handle predation).