Predator preferences for attacking particular prey group sizes: consequences for predator hunting success and prey predation risk

Abstract

Models of group living assume that the hunting success of predators and per capita predation risk of prey decrease with increasing prey group size. In contrast to numerous studies documenting anti-predator advantages of group living, little is known about the choice of predators for particular prey groups. Two shoals of guppies,Poecilia reticulata, which differed in size (2 versus 5 fish, 5 versus 10, 2 versus 10, 10 versus 16), were simultaneously presented to individual blue acara cichlid fish,Aequidens pulcher, and the shoal that was attacked first, the number of subsequent attacks and the total time cichlids spent near each shoal during a 3-min period were recorded. For all three criteria, the cichlids showed a significant preference for the larger of the two shoals, resulting in a higher per capita risk in terms of first attack and attack duration for individuals in larger shoals. Further trials, in which guppy activity was manipulated by using different water temperatures, showed that the cichlids preferred a smaller over a larger shoal if the larger shoal was kept in cold water (i.e. exhibited less movement). This result demonstrated that the visual conspicuousness of the shoal, and not its size per se, was the key factor for the cichlids' choice of a prey shoal. In a final experiment, wherein cichlids were given an opportunity to attack free-swimming shoals (comprised 1-8 individuals), predator hunting success decreased with increasing shoal size. We conclude that, despite the over-proportionately high attack risk for individuals in large shoals, per capita predation risk is lower than in small shoals, and joining larger shoals should be advantageous for individual guppies. Cichlids, however, would do better in terms of hunting success by attacking small rather than large shoals. Potential implications of such predator preference for larger shoals are discussed.