Are cannibalistic morphs of the tiger salamander obligatory cannibals?

Tiger salamanders exhibit alternative trophic morphologies, with cannibals developing a larger head and longer teeth than typical larvae. Resource partitioning is known between morphs, with cannibal morphs usually foraging on conspecifics and rarely on small organisms. Our aim was to determine whether the cannibal and typical morphs shift their diets across time and particularly whether conspecifics are necessarily the main prey of cannibals and plankton the primary prey of typicals. We found that only the cannibal morph foraged on conspecifics, but not all the time. Cannibalism typically occurred only early after the ontogenetic divergence between morphs. Cannibals shifted their diet later in the summer to plankton, and this ontogenetic shift led to dietary overlap with the typical morph. In contrast to other studies, our findings suggest that the cannibal morphology actually allows the consumption of a larger variety of prey, rather than specialization on specific resources (i.e., conspecifics). The outcomes of the cannibalistic ontogenetic pathway include a higher biomass intake from food and a larger size than typicals. From a foraging perspective, the cannibalism pathway is clearly advantageous over the typical morphology. However, the increased diet breadth of cannibal morphs found in this study suggests that the maintenance of the polyphenism is more complex than has previously been suggested.