Mirrors inhibit growth and stimulate lateralized response to prey in larvae of the Mexican axolotl Ambystoma mexicanum

Abstract

Laterality of brain and behavior at the individual and/or group level is a characteristic of all vertebrate classes, including amphibians. It is well recognized that the right-eye/left-hemisphere system is more efficient in discrimination of edible and non-edible items. However, the ontogenesis of this or other lateralized responses has rarely been investigated. Here we present the first evidence of ontogenesis of right-eye/left-hemisphere response to live food in the Mexican axolotl, Ambystoma mexicanum. Furthermore, we test a possible influence of mirror images imitating the presence of conspecifics on laterality of the reaction to food, as well as on the growth and development of axolotls; we then compare the results to those known for anuran tadpoles. We placed the newly hatched axolotls into aquaria either with transparent or mirror walls individually (first experiment), or in a group of 25 animals (second experiment). The larvae were fed Artemia salina nauplii and regularly scored for the number and direction (either left or right) of their attacks towards the prey. Body length was measured twice a month. The statistically significant lateralized response to food was first detected in only three-week-old larvae and was right-sided, suggesting processing of visual information in the left brain hemisphere. At the same age, the left hemisphere seems to start its specialization in more efficient perception of prey. In both experiments we found a significant difference between the larvae growing in the transparent and mirror aquaria, with a tendency towards a stronger right-sided lateralization in the mirror aquaria. Also, the mirror walls inhibited larval growth, especially at the age of 60–75 days. A similar phenomenon is known in overcrowding of anuran tadpoles, which is discussed in terms of visual brain stimulation.