Gibbon foraging decisions and the marginal value model

Abstract

We use data from an observational field study of frugivory in two sympatric gibbons, lar (Hylobates lar) and siamang (H. syndactylus), to test assumptions and predictions of the marginal value model (MVM). A key prediction of the MVM is that marginal gain rates at the time of leaving the patch are equal across patch types. We found that this is not the case for gibbons: rates of energy intake at the end of feeding sessions were significantly different for different types of fruit, and we could not attribute this to temporal variation in fruit availability. Initial and final caloric intake rates were highly correlated. This suggests that gibbons do not adjust the time spent in patches in order to maximize the average rate of energy intake. Similar results were obtained for all other currencies considered. Gibbon foraging appears to satisfy several, but not all, assumptions of the MVM. As required by the model, fruit patches occur as discrete units, patches are encountered sequentially, travel time between patches exceeds search time between items within a patch, search for and search within patches are incompatible activities, and intake rates decline over time spent in a patch. However, the declining rates we detected may be an effect of satiation instead of patch depletion, patches probably are not encountered at random, and group members may not forage independently. Thus, our results suggest that the MVM is not an adequate model of gibbon foraging behavior, but they do not invalidate the MVM per se.