Experimental investigations of the evolutionary significance of sexually dimorphic nuptial colouration in Gasterosteus aculeatus (L.): temporal changes in the structure of the male mosaic signal

Abstract

Although classically described as "red," the three-spined stickleback male's nuptial signal is a complex mosaic of at least three colours: blue eyes, black dorsal–lateral body, and red ventral–lateral body. Seven variables (intensity, hue, and distribution of red body colour; intensity and hue of blue eye colour; intensity and distribution of black body colour) were scored for 19 males across an entire breeding cycle. These variables interacted to produce four distinct male colour mosaic signals corresponding with the stage a male had reached in the breeding cycle. No single variable was sufficient for the delineation of all breeding stages: however, the distribution of red body colour reliably distinguished the courting male from nesting and parental males. Variability in signal intensity between courting males was greatest for the red component of the nuptial signal and both intra- and inter-male variability in the overall intensity of red was greatest during courtship. Based upon these results, future investigations of the role of intersexual selection in shaping the male nuptial signal in this species should offer females a choice between males differing in the intensity of red rather than the traditional "red" versus "nonred" dichotomy.