Scrounging marmosets eat more when the finder's share is low without changing their searching effort

Abstract

Animals that forage in groups may either search actively for food sources (producers, P) or monitor other members of the group in an attempt to consume resources that producers have encountered (scroungers, S). One factor that may influence the choice of foraging strategy is the finder's share, defined as the proportion of the resource consumed by the producer before the arrival of scroungers. Several models predict that when the finder's share is relatively small, the frequency of scrounging will be high because producing gives only a small benefit compared to playing scrounger. A model based on a linear operator learning rule confirmed this prediction and further predicted that individuals would change their foraging strategy gradually in response to environmental changes. We tested these predictions in an experiment involving three groups (N = 18 individuals) of free-ranging black-tufted marmosets, Callithrix penicillata. We manipulated the finder's share at artificial feeding stations by controlling the distribution of food rewards (slices of banana) among baited platforms. We divided our field experiments into two conditions – low finder's share (few food-rich patches) and high finder's share (many food-poor patches). Most (15/18) marmosets consumed more banana slices as scroungers in the low finder's share condition than in the high finder's share condition, as expected by the theory. However, individuals did not modify their frequency of use of the strategies in both conditions and did not show gradual learning as expected by the linear operator learning rule. We suggest that in social groups with high levels of tolerance and cooperation, such as those of marmosets, scrounging individuals can have higher foraging success without increasing their searching effort by sharing productive food patches with producers and other scroungers.