Flexibility of nuptial colouration in a unique ecotype of threespine stickleback (Gasterosteus aculeatus)

Nuptial colouration in animals may serve as a signal of competitor and (or) mate quality during breeding. In many temperate fishes, nuptial colouration develops during discrete breeding seasons and is a target of sexual selection. We examine nuptial colouration and behaviour of a unique ecotype of threespine stickleback (Gasterosteus aculeatus Linnaeus, 1758), wherein males turn from dull brown-grey to pearlescent white during the breeding season. The main goal of this work was to determine the relative role of white colouration in intersexual competition and mate choice. In a combination of field and laboratory work, we found that males are brightest white when engaging in courtship activities in the presence of a female; this indicates that white colouration may be primarily related to enhancing signalling during mate attraction. White colouration intensity increased as the breeding season progressed and may be related to an influx of conspecifics. Colour change from cryptic grey to bright white occurred rapidly (<90 s) and may be deployed to enhance behavioural signals. We conclude that bright white colouration in the white ecotype is a potential signal of mate quality and may have evolved from a previously existing capacity for colour plasticity in common threespine stickleback.