Giant cannibals drive selection for inducible defence in heterospecific prey

Abstract

Predation can strongly influence the evolution of prey species by selecting for defensive phenotypes that reduce the risk of predation. Although cannibalism is frequent in predators and is known to strongly influence predator–prey dynamics, it is largely neglected when studying the evolution of phenotypic defences in prey. Changes in cannibalism can alter the abundance, size-structure, and even phenotypes within predator populations, and thereby change the numerical and per-capita effects driving the selection pressure on defensive phenotypes in prey. In the present study, we experimentally manipulated the presence and absence of cannibalism in predatory salamander larvae Hynobius retardatus (Dunn) and examined how this affects the expression of and selection pressure on inducible defensive phenotype (bulgy phenotype) in its prey, Rana pirica (Matsui) tadpoles. In the absence of cannibalism, salamanders were generally not sufficiently large to consume tadpoles, tadpoles did not express the defensive phenotype, and all phenotypes had equally high survival. By contrast, cannibalism of salamanders accelerated growth rates, leading to the development of ‘giant’ cannibalistic morphs that were able to consume tadpoles. Importantly, only in the presence of these giant cannibals did tadpoles express a defensive ‘bulgy’ phenotype that had significantly higher survival rates than phenotypes without this defence, indicating that cannibalism altered the selective regime for defensive traits in prey. The results of the present study demonstrate that cannibalism can drive trait evolution in predator–prey systems and suggest that we cannot predict how changes in numerical effects in predator populations affect selection regimes on prey traits without accounting for concurrent changes in predator per-capita effects.