Courtship and genetic quality: asymmetric males show their best side

Fluctuating asymmetry (FA), the small random deviations from perfect morphological symmetry that result during development, is ubiquitous throughout the animal kingdom. In many species, FA seems to play a role in mate choice, perhaps because it signals an individual's genetic quality and health. However, the relationship between an individual's FA and behaviour is generally unknown: what do more asymmetric individuals do about their own asymmetry? We now show for the first time that individuals respond behaviourally to their own morphological FA in what appears to be an adaptive manner. During courtship, male guppies exhibiting high FA in ornamental colour, bias their displays towards their more colourful body side, thus potentially increasing their attractiveness by exaggerating the quantity of their orange signal. This appears to be a strictly behavioural male response to cues provided by females, as it does not occur when males court a non-reactive model female. Whether inferior males realize any mating advantage remains uncertain, but our study clearly demonstrates a behavioural response to random morphological asymmetries that appears to be adaptive. We propose that the tendency to show or otherwise use a 'best side' is common in nature, with implications for sexual signalling and the evolution of more pronounced asymmetries.