Variation in the expression of dietary conservatism within and between fish species

When animals encounter new food, they must decide whether to eat it and risk being poisoned, or avoid it but risk losing valuable food resources. Some individuals within a foraging population are 'adventurous consumers' and readily accept novel food items into their diets, while others display an active and long-term avoidance of novel food, called 'dietary conservatism' (DC). Previous studies have shown the phenomenon of DC in many bird species and in a temperate fish, the three-spined stickleback, Gasterosteus aculeatus. However, it is unknown to what extent DC exists in other fish species, and so in the current study, we investigated the occurrence of DC in four species of tropical fish belonging to the poeciliid family (guppy, Poecilia reticulata, mollie, Poecilia sphenops, platy, Xiphophorus maculatus, and green swordtail, Xiphophorus hellerii). We detected DC strong enough to drive a novel prey morph from initial rarity to fixation in a prey population in all four poeciliid species. Despite some underlying differences in pre-existing preferences for prey colour, there was no significant difference between poeciliid species, sexes or sizes in the likelihood of the novel morph reaching fixation; neither was there an effect of the relative conspicuousness or crypsis of the novel prey. Poeciliids were, however, less likely than sticklebacks to drive the novel prey morph to fixation. These results provide strong evidence for the widespread existence of DC in fish populations from a range of habitats, but not all fish species exhibit this trait at the same frequency.