Behavioural endocrinology: no hormonal response in tied fights

Fish androgens may start to surge only when there seems to be a good chance of winning a contest. Duel action: a cichlid attacks its reflection. Androgens are the principal sex steroids controlling reproduction and aggression in male fish1, but their production can also be affected by social interactions2, 3. Here we show that androgen concentrations are not significantly increased in cichlid fish (Oreochromis mossambicus) that are fighting their own image in a mirror, despite their aggressive behaviour towards the virtual intruder. Our results indicate that the hormonal response normally triggered in male contests is not induced under these circumstances by the act of fighting itself, and that it may therefore depend on some indicator of relative fighting ability that cannot be delivered by a mirror-image challenger