The response of fish to novel prey: evidence that dietary conservatism is not restricted to birds

Foragers continually encounter new objects in their environment, some being potential food. Sampling new food resources carries risks of poisoning or injury and can reduce foraging efficiency. Foragers typically show a brief aversion to novel objects (neophobia). An additional persistent wariness of novel foods, termed dietary conservatism (DC), has been described only in birds and is exhibited by only a proportion of individuals in each of the populations studied so far. The rest of the individuals in each population are relatively adventurous consumers (AC), eating novel foods as soon as their neophobia has been overcome. The presence of DC foragers is important because it may alter the selection pressures on novel types of prey. We demonstrate both AC and DC in a previously untested predator taxon, fish (specifically, the three-spined stickleback, Gasterosteus aculeatus). The fish showed sufficient avoidance of novel prey colors to allow these prey to spread in 33% of prey populations, from being rare (5% of prey phenotypes) to fixation (100%). In the remaining prey populations, the novel morph became extinct. Numerical simulation models based on these empirical studies showed that the success of the novel color morph (independent of the color used) was best explained by DC rather than by drift or apostatic selection. This study provides the first evidence of AC and DC foraging strategies in fish, demonstrating that this phenomenon is not restricted to birds and suggesting that this mixture of strategies might be a more general feature of foragers than was previously appreciated.