SummaryThe color patterns of many organisms change rapidly with social context but such dynamic signals have been little studied with current methods. In this study we applied objective spectrophotometry techniques to the color displays of unrestrained male threespine sticklebacks, to assess the in uence of social context on coloration. Analyzing our data with a color space model based on stickleback visual physiology, we found that unrestrained males enhanced saturation of both their blue eye and red jaw color in response to the presence of a mature male or female conspeci c. Divergence between the eye and the jaw lead to enhanced contrast, likely increasing conspicuousness. We found little relationship between measures of color saturation and condition, but the color of males in better condition varied more with social context. This study is the rst to evaluate contrast between stickleback color pattern elements quantitatively and the rst in which re ectance spectra were collected from freeswimming sh. The methods presented here could be used in future studies of sticklebacks and could potentially be adapted to other animals.2) Present address: Tucker-Davis Technologies,

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