1. With only a few exceptions, the potential contribution of heterospecific prey to the evolution of cannibalism and accompanying morphologies in amphibians and other taxa is unknown. In this study we tested the role of invertebrate and vertebrate prey in controlling variation in frequencies of a non-cannibal and an environmentally cued cannibal morph among populations of the Arizona tiger salamander, Ambystoma tigrinum nebulosum. 2. In parts of its range, A. t. nebulosum is syntopic with larvae of the chorus frog, Pseudacris triseriata. Our field measurements revealed a significant positive correlation between the frequency of cannibal morphs in larval tiger salamanders and density of chorus frog tadpoles. Cannibal frequency did not correlate significantly with larval salamander density, although frequency of cannibals varies positively with conspecific density in the laboratory. Analysis of stomach contents of salamanders revealed that cannibalism was low in the populations studied and P. triseriata larvae comprised about 24% of the volume of prey in the stomachs of cannibals. 3. We raised larval salamanders with various prey to test the effect of invertebrates, tadpoles and salamander larvae on expression of cannibalism. 4. Our experiments demonstrated that a diet of only P. triseriata tadpoles did not induce the cannibal morphology in salamander larvae. Likewise, salamanders fed only invertebrates failed to express the cannibal morphology, but those fed invertebrates while also having access to conspecifics as potential prey developed as cannibals. 5. These results support the conclusion that availability of conspecifics as prey is a key proximate factor controlling expression of the cannibal morphology in A. tigrinum and suggest this trait evolved as an adaptation to predation on conspecifics and not as a general response to selection for feeding on heterospecific vertebrates or invertebrates.
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