Within-group relatedness can lead to higher levels of exploitation: a model and empirical test

Abstract

When animals live in groups, individuals can invest in resources themselves or exploit the investments of other group members. Grouping with kin may reduce the frequency of exploitation because kin selection should favor individuals that imposed fewer costs on their kin. However, taking into account the gains of the exploited individual, allowing kin to exploit one's efforts may be less costly than allowing exploitation from nonkin. In this case, there may be higher frequencies of exploitative behaviors among related than unrelated individuals. In order to understand the net effect of genetic relatedness on intragroup exploitation, we developed a model that considers the inclusive fitness consequences of producing (searching for food) and (exploiting the food discoveries of others) when foraging with relatives, while simultaneously allowing individuals to show differential tolerance toward scrounging by kin versus nonkin. The model predicts that increased relatedness can lead to higher levels of exploitation when producers are kin-selected to be more tolerant of scrounging from relatives compared with unrelated scroungers, for example, by being more aggressive toward nonkin. We tested this prediction empirically in captive zebra finches (Taeniopygia guttata) foraging either in flocks with full siblings or in flocks of unrelated individuals. Flocks of related zebra finches had higher frequencies of scrounging and lower levels of aggressive interactions compared with flocks of unrelated zebra finches. The results suggest that producers may be kin-selected to allow relatives to scrounge.