Within-shoal phenotypic homogeneity affects shoaling preference in a killifish

Anti-predator benefits associated with living in groups are multiple and taxonomically widespread. In fish shoals, individuals can exploit the confusion effect, whereby predators struggle to target a single individual among several individuals. Theory predicts that the confusion effect could be aided by homogeneity in appearance; thus, individuals should group by phenotypic characteristics, contributing to generating high within-shoal phenotypic homogeneity. While assortments by body size have been extensively documented, almost nothing is known about whether within-shoal homogeneity in body pigmentation affects shoaling preference. To investigate this issue, we used the Mediterranean killifish, Aphanius fasciatus , a shoaling species characterized by conspicuous vertical bars on body sides. Individual females were given a choice between two novel shoals characterized by either a high or low degree of homogeneity in the number of bars. As predicted, individual females preferentially associated with the shoal showing the higher phenotypic homogeneity. Our data demonstrated that fish might associate with the shoal that maximizes phenotypic homogeneity in body pigmentation, irrespective of their own phenotype.

The confusion effect when attacking simulated three-dimensional starling flocks

The confusion effect describes the phenomenon of decreasing predator attack success with increasing prey group size. However, there is a paucity of research into the influence of this effect in coherent groups, such as flocks of European starlings ( Sturnus vulgaris ). Here, for the first time, we use a computer game style experiment to investigate the confusion effect in three dimensions. To date, computerized studies on the confusion effect have used two-dimensional simulations with simplistic prey movement and dynamics. Our experiment is the first investigation of the effects of flock size and density on the ability of a (human) predator to track and capture a target starling in a realistically simulated three-dimensional flock of starlings. In line with the predictions of the confusion effect, modelled starlings appear to be safer from predation in larger and denser flocks. This finding lends credence to previous suggestions that starling flocks have anti-predator benefits and, more generally, it suggests that active increases in density in animal groups in response to predation may increase the effectiveness of the confusion effect.