Foraging benefits of shoaling with familiars may be exploited by outsiders

Abstract

Previous studies have shown that social animals representing a range of taxa preferentially associate with individuals with whom they are familiar, gaining significant benefits from doing so. There is little information on whether outsiders might join such groups in order to participate in these benefits, however. We tested whether individual three-spined sticklebacks, Gasterosteus aculeatus, were able to distinguish between unfamiliar conspecific groups on the basis of intragroup familiarity. Focal fish were given a choice of two stimulus shoals of fish with which they were unfamiliar. In one shoal, fish were familiar with one another and in the other, fish were unfamiliar with one another. Focal fish showed significant association preferences for the shoal composed of familiar individuals. We then tested a potential mechanism underlying this preference behaviour by measuring the swimming behaviour of the stimulus fish; no differences were found between the stimulus shoals. Finally, we examined possible functional explanations for the discrimination behaviour in a foraging context. We found that familiar shoals located the food patch more quickly and consumed more prey items than unfamiliar shoals. Furthermore, focal fish that were unfamiliar with the other individuals in the foraging shoal were able to exploit these benefits, arriving at the food patch more quickly and consuming more prey items when in a shoal of familiar individuals than when in a shoal of unfamiliar individuals. There is therefore a functional benefit to a fish joining a familiar shoal and this may explain why exclusively familiar shoals of sticklebacks may be relatively rare in the wild.