Character displacement of male nuptial colour in threespine sticklebacks (Gasterosteus aculeatus)

Character displacement in signalling traits occurs when differences between species are greater in sympatry than where either species occurs alone. Finding character displacement in a signalling trait suggests that the trait has diverged as a result of interspecific interactions such as competition, aggression, predation, or reproductive interference. We tested for character displacement of male nuptial coloration between sympatric species of threespine sticklebacks (Gasterosteus aculeatus spp.). The sympatric pairs consist of a large 'benthic' species, which feeds on benthic invertebrates, and a smaller planktivorous 'limnetic' species. Breeding males of both species develop red throats and blue bodies, although limnetic males appear brighter. To test for character displacement, we compared the nuptial colour of benthics and limnetics from two species-pair lakes (sympatric) with that of males from three similar allopatric lakes (only one species present). We measured the intensity of blue and red coloration using reflectance spectra taken from live fish. We found that allopatric males were intermediate between limnetic and benthic males in the intensity of red colour, indicating character displacement in that trait in sympatry. By contrast, we found no evidence for character displacement in blue intensity, although it differed sharply between the species pairs in one lake (Priest). (c) 2007 The Linnean Society of London.