| Abstract: | Recently Sillén-Tullberg & Leimar (1988) modelled a general explanation for the evolution of gregariousness in prey organisms that live exposed, have no means of escape when discovered by a predator, and are small in relation to a potential predator (who thus can sample many prey individuals in one encounter). The model predicts that gregarious prey organisms of that type ought to be distasteful, and that the evolution of gregariousness will be favoured by aposematic coloration facilitating avoidance learning in a predator. Obviously, any protective power of grouping depends on group size. According to the Sillén-Tullberg & Leimar model, (1) “members of small groups may have a higher rate of death from predation than solitary individuals, but above a certain minimum group size, group members do better than solitary individuals; … as group size increases above the minimum value, group members suffer fewer and fewer deaths from predation”. They benefit from the “decreased risk of predator attack on any particular individual”, called dilution effect. (2) “The more prey specimens that the predator needs to sample during avoidance learning, the larger an aggregation needs to be in order for gregariousness to be advantageous”. It is further explained that (3) selection resulting from predation favours increase in group size until it “acts like a predator-satiation mechanism”. |
