Morphological variation and cannibalism in a larval salamander (Ambystoma macrodactylum columbianum)

Abstract

We document morphological variation and its association with larval cannibalism in a population of long-toed salamanders, Ambystoma macrodactylum columbianum. Larvae observed in a natural pond engaged in cannibalism. Additionally, these larvae possessed significantly longer and wider heads, as well as larger vomerine teeth, than did conspecifics of the same size reared in the laboratory for 1 month after capture. This variation in trophic structures is consistent with the characteristics of "cannibal" morphs that have been documented for larvae of another salamander species. We extend current knowledge of the development of this morphology by using multivariate statistical analysis to examine, further, a possible basis for this variation in head shape. Principal component analysis indicated that after the removal of the effects of body size, variation in the distance between the eyes (interocular width) accounted for most of the remaining total overall variation in head shape. Multivariate ontogenetic trajectories of head shape, constructed from linear regressions of principal components (measures of size and shape), were equivalent for the two larval samples. These statistical 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 due to fluctuations in the presence of key, influential environmental factors, as has been demonstrated for other larval amphibians that exhibit trophic polymorphism.