For several species representing a diversity of taxonomic groups, the benefits of cannibalism appear to exceed the costs as evidenced by the development of specialized cannibalistic morphologies in the population. These are termed cannibalistic polyphenisms, in which cannibals are phenotypically distinct from noncannibalistic individuals. A behavioral polyphenism occurs in some birds (gulls and skuas) in which some individuals act as cannibalistic specialists. Alternating morphologies have been documented in slime molds, ciliates, rotifers, and larval amphibians. Cannibalistic polyphenism has been observed in larvae of the tiger salamander complex (Ambystoma tigrinum), as well as in the tadpoles of at least three species of spadefoot toads (Spea bombifrons, S. intermontanus, and S. multiplicata). Recently, a cannibalistic morphology has been described in larvae of other salamanders as well (Ambystoma macro-dactylum columbianum and Hynobius retardatus). Thus, evidence for such polymorphism appears to be increasing more for larval amphibians than for any other taxonomic group, despite the fact that cannibalism in amphibians has received much less attention in recent years compared to some other vertebrate groups (see the section titled 'Taxonomic distribution').
Cannibal morphs in larval salamanders are distinguished by their disproportionately broader and longer heads and enlarged vomerine teeth, compared to 'typical' morphs. Proximate factors responsible for this trophic polymorphism are varied: emergence of cannibals in a population may be influenced by population density, food level, and the genetic relatedness of other larvae. Moreover, ingestion of large prey (anuran tadpoles and heterospecific larval salamanders) can induce the cannibalistic morphology, although actual acts of cannibalism are not necessary for this to occur. In at least one species (Hynobius retardatus), the presence of either (or both) visual and chemical cues from conspecifics is sufficient to induce the cannibalistic morphology.
The carnivorous (cannibalistic) morph of spadefoot tadpoles is characterized by an enlarged head, enlarged jaw musculature, shortened intestines, and increased ker-atinization of the mouth to form a beak. These features are not observed in the omnivorous (typical) morph.
Originally, exogenous thyroxin (acquired by feeding on coexisting fairy shrimp, order Anostraca) and feeding on conspecific tadpoles were thought to be the agents that triggered development of the carnivorous morph in S. multiplicata. However, a recent reassessment of earlier research has challenged this idea and calls into question the environmental model of polyphenism in spadefoot toads that has been in existence for over 20 years. Thus, the environmental cue(s) that induce developmental polyphenism in this group are not yet clear. As with larval salamanders, however, trophic polymorphism in spade-foot toads is thought to be adaptive (in terms of faster development, greater foraging efficiency, and higher postmetamorphic survivorship) under certain environmental conditions such as in unpredictable, temporary ponds with crowded conditions.
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