OVERVIEW

The fission–fusion social organization that characterizes all chimpanzee communities is immediately recognizable when observers attempt to collect data on known individuals over extended periods of time. On a typical day, a researcher might begin by following a mother and her dependent offspring, accompanied by several adult males, as they make their way to a fruiting tree (Plate 4). At some point, after feeding in and then leaving this tree in search of another feeding location, one of the males might wander off, for no obvious reason or perhaps in the direction of vocalizations heard from another part of the forest, not to be seen again for a week. Alternatively, the five individuals might be joined by another small group like theirs and continue together for the rest of the day, or even several days. Or the initial small group might join a noisy group of twenty individuals feeding in a grove of trees laden with ripe fruits and remain with them, first feeding and then resting and socializing. Toward the end of the day, the mother and her offspring might leave the group to find their own spot to sleep for the night. And so on. The permutations are infinite and generally unpredictable.

A combination of ecological and social factors is implicated in this fluid, ever-shifting grouping pattern. Ecologically, the availability of ripe fruits sets the baseline conditions for ranging and association. Individual trees that are important sources of ripe fruit for chimpanzees can flower in different months in different years and may or may not bear fruit synchronously with other trees of the same species (Chapter 1, endnote 21). Edible fruits therefore occur in discrete and unpredictable “patches” of varying sizes scattered widely throughout community territories. A patch might consist of a single, isolated tree in fruit or a cluster of trees fruiting at the same time. Consequently, unlike temperate habitats where all members of a tree species may flower and fruit nearly simultaneously with reliable seasonality, in chimpanzee habitats the timing and location of fruit production is extremely variable. Chimpanzees are therefore forced to search almost constantly for new feeding areas, often monitoring known trees as they go. The size and distribution of food patches then influence the size and membership of foraging subgroups.