Nemo through the looking-glass: a commentary on Desjardins & Fernald.

Comment Nemo through the looking-glass: a commentary on Desjardins & Fernald Mirror-elicited aggression has been widely used in animal behaviour research. Recently, Desjardins & Fernald [1] reported that males of the cichlid fish Asta-totilapia burtoni fighting a mirror had higher expression of immediate early genes in brain areas homologous to the amygdala and the hippocampus than males fighting a real opponent, despite the fact that the behavioural and androgen responses did not differ between the two conditions. Surprisingly, Desjardins & Fernald [1] omitted two recent papers (Oliveira et al. [2] in another cichlid fish, Oreochromis mossambicus, and Hirschenhauser et al. [3] in quail, Coturnix japonica), which also found a dissociation between the behavio-ural and the physiological responses towards mirror images. In these two studies, the mirror image elicited an aggressive response but failed to trigger the andro-gen response present when fighting a real opponent, suggesting that the physiological response to the fight depends on the cognitive appraisal that the individuals make of the situation [4]. This omission is even more surprising as the two divergent results support contrasting adaptive roles for socially driven changes in androgen levels (aka 'challenge hypothesis', [5]): the lack of hormonal response to a mirror fight suggests the need for asymmetric feedback to adjust androgen production, which is therefore only functional for subsequent interactions; the presence of an androgen response triggered by a mirror fight suggests a role for androgens in the modulation of aggressive behaviour in the ongoing fight. Both the O. mossambicus and quail studies support the first hypothesis, while the A. burtoni study is the first evidence for the latter. Three different kinds of factors may contribute to the divergent androgen responses. — Phylogenetic bias. Phylogeny is an unlikely source of variation in this case since two of the species showing divergent responses are closely related cichlid fishes, one of which shares the same pattern of response with the phylogenetically relatively distant quail. — Differences in the behavioural tasks. In the real opponent treatment, Desjardins & Fernald [1] staged fights between males previously placed in separate compartments. Therefore, the fish interacting across the transparent partition were mimicking a territorial border fight between two territorial males, with no initial asymmetry in resource-holding potential and no clear outcome. In contrast, in the other two studies (Hirschenhauser et al. [6], was used as a real opponent control for the mirror fights in Oliveira et al. [2]) …