Proximate causes of cannibalistic polyphenism in larval tiger salamanders

Larval tiger salamanders, Ambystoma tigrinum, exist as small-headed "typical" morphs that feed mainly on aquatic invertebrates and as large-headed "cannibal" morphs that often prey on conspecifics. Previous experiments have demonstrated that cannibals are induced facultatively when crowded with conspecific larvae. We asked: (1)What sensory cues trigger expression of cannibals? (2) Does larval age influence expression of cannibals? (3) Are cannibals also induced by crowding with heterospecifics? (4) Do cannibals suppress others from developing into cannibals? We found that tactile cues from other salamander larvae were necessary to elicit cannibals. Cannibals were produced when tiger salamander larvae were crowded with congeneric larvae, and these heterospecific-induced cannibals were expressed at an earlier age than those induced by conspecifics. Furthermore, younger larvae were more likely to develop into cannibals. Finally, visual and chemical cues from cannibals did not suppress other larvae from developing into cannibals. We infer the adaptive significance of polyphenism in this and other species.