Pattern of Lateralized Behaviors in a Caudate Amphibian, <i>Ambystoma mexicanum</i>

Abstract

Laterality of brain and behavior at the individual and/or group level is a characteristic of all vertebrate classes. The general pattern of left-right asymmetry in vertebrates is so that left-eye/right-hemisphere system is used to detect socially important images, like conspecifics, and also recognize danger, while the right-eye/left-hemisphere system is used to discriminate prey and food items from inedible objects. However, the pattern of motor and visual lateralization in caudate amphibians is poorly investigated to date. Here we present the first evidence of individual and group level motor and visual lateralization in sexually mature and immature larvae of the Mexican axolotl, Ambystoma mexicanum , thus presenting a comprehensive characteristic of the lateralized behavioral pattern in a salamander species. First, we performed four successive tests to assess asymmetry in rapid C-start and measure its characteristics in responses to startle stimuli in sexually immature larvae. We experimentally showed that axolotls exhibited a leftward population bias in each of four C-start tests, of which the bias was significant in the second test and in the pooled data for all four tests. Second, in mature axolotls we have found right-sided individual level asymmetry and, at the group level, a tendency to react more actively to food items (live chironomid larvae) appeared in their right visual hemifield. Third, individual axolotl larvae been placed in the square aquarium with two opposite mat walls and two opposite mirror walls preferred to stay near to mirror walls, rather than near to mat walls. At the group level, this preference increased over three successive tests and reached significance in the third test and in the pooled data. At both individual and group levels axolotls showed statistically significant lateralization, preferring to inspect the mirror walls with their left eyes in all three tests. Overall, the expressed pattern of lateralization in the Mexican axolotl corresponds to the general pattern of lateralization established for other vertebrates including anuran amphibians.