Sex-specific effects of postnatal testosterone on lateralization in cichlid fish

Lateralization is a fundamental principle in the organization of brain and behaviour in humans and nonhuman animals. To what extent lateralization is, in addition to genetic factors, under the influence of testosterone, which would also explain sex differences in laterality, is the topic of a long-standing debate. This debate is partly hampered by confusion between organizational and activating effects of testosterone. Here we focused on activating effects, less often studied than organizational effects. Studies on humans have shown ambiguous results and few experimental studies on animals have been conducted. We studied Aequidens rivulatus, a cichlid species in which lateralization of visually guided behaviour has been demonstrated and related to aggressiveness. After treatment with testosterone, lateralization was tested in a rotational preference task where fish had to face a predator. Testosterone induced significant lateralization at the population level. Testosterone-treated fish watched a predator preferentially with the right eye, which is in line with the literature on population-level lateralization of predator viewing in fish. It has been suggested that species differences in laterality of predator escape are related to sociality and we speculate that, within species, testosterone may affect lateralization in relation to a change in sociality. Among the fish of known sex, only males reacted to the treatment; a greater responsiveness of males may explain sex differences in lateralization found in many animal species. This may be caused by sex-specific sensitivity to androgens, perhaps in the habenular area of the brain.