Nuptial Colour Loss and Signal Masking in Gasterosteus: an Analysis Using Video Imaging

Threespine stickleback vary greatly in male nuptial display, from full red expression between the throat and pelvis to total melanism without any traces of red. The dominant expression for any population is correlated with the underwater spectral irradiance and many populations with melanistic nuptial colour occur in bog habitats where the aquatic spectrum is shifted to long wavelengths. Field observations indicate that in these bog habitats, the typical red nuptial signal is 'spectrally masked' and this might reduce the effectiveness of display during male-male and male-female interactions. We used video imaging techniques to test whether the spectral composition of the visual background influences female preference for nuptial colouration in male threespine sticklebacks. Throat colour preference tests were carried out in which females chose between video images of red and black-throated males superimposed on either a blue or a red background. Despite belonging to a red-throated population, females were found to respond preferentially to the black-throated male image viewed against a red background, indicating that preference was not determined by throat patch hue. This suggests that preference for an epigamic optical signal is likely a function of its efficacy in generating visual contrast rather than its intrinsic spectral characteristics.