Variation in the antipredator behavior of Ambystoma salamanders.

Abstract

A thorough investigation into the evolution of behavioral traits requires the study of both intraspecific variation (assessing stereotypy in behavior and proximal sources of variation) and interspecific variation (making inferences about historical transitions and evaluating hypotheses of the functions and origins of behavioral traits). In this study, I used observation, experimentation, and comparison to examine the evolution of antipredator behavior of the salamander genus Ambystoma (Caudata: Ambystomatidae). I tested the responses of juvenile and adult Ambystoma to contact with live and artificial predators. I assessed inter-individual variation to uniform stimuli, as well as variation due to differences in the predatory stimuli and environmental context of an encounter. Even with uniform stimuli, salamanders showed considerable among-individual variation in response. When tested under a variety of predatory situations, they demonstrated variability in response related to differences in type of predator contact and environmental context. Juveniles and adults behaved differently in many species, with juveniles running more and posturing less than adults. To investigate evolutionary changes in behavior, I incorporated phylogenetic information into the comparative analysis of antipredator behaviors. By mapping behavioral components on an independently-derived cladogram for Ambystoma, and by examining the behavior of outgroup taxa, I made inferences about the transitions (independent derivations and losses) in behavior that occurred during the evolutionary history of the genus. Although the phylogenetic distributions of some behavioral components indicated frequent transitions, many components showed variation that reflected the genealogical relationships among the species. Several major behavioral transition points were noted. By comparing transitions in behavior with historical transitions in certain morphological and ecological traits, I evaluated specific hypotheses of the adaptive functions of particular behavioral components. In addition, I studied the repertoire of these salamanders and identified origins of some antipredator components from among movements used by salamanders in non-defensive contexts. The combination of methods used in this study proved useful in uncovering the current variation in behavior and in identifying both proximate and evolutionary sources of this variation. The incorporation of information on phylogenetic relationships permitted evaluation of hypotheses concerning evolutionary events.