MORPHOLOGICAL VARIATION AND CANNIBALISM IN A LARVAL SALAMANDER (AMBYSTOMA-MACRODACTYLUM-COLUMBIANUM)

We document morphological variation and its association with larval ca nnibalism in a population of long-toed salamanders, Ambystoma macrodac rylum columbianum. Larvae observed in a natural pond engaged in cannib alism. Additionally, these larvae possessed significantly longer and w ider heads, as well as larger vomerine teeth, than did conspecifics of the same size reared in the laboratory for 1 month after capture. Thi s variation in trophic structures is consistent with the characteristi cs of ''cannibal'' morphs that have been documented for larvae of anot her salamander species. We extend current knowledge of the development of this morphology by using multivariate statistical analysis to exam ine, fur-ther, a possible basis for this variation in head shape. Prin cipal component analysis indicated that after the removal of the effec ts of body size, variation in the distance between the eyes (interocul ar width) accounted for most of the remaining total overall variation in head shape. Multivariate ontogenetic trajectories of head shape, co nstructed from linear regressions of principal components (measures of size and shape), were equivalent for the two larval samples. These st atistical analyses indicate that this trophic polymorphism is not due to differences in either the rate of change in head shape relative to overall size or the size at which the divergence in head shape begins. Rather, morphological variation in larval A. m. columbianum may be du e to fluctuations in the presence of key, influential environmental fa ctors, as has been demonstrated for other larval amphibians that exhib it trophic polymorphism.