Search

Signal evolution is thought to depend on both a signal's detectability or conspicuousness (signal design) as well as any extractable information it may convey to a potential receiver (signal content). While theoretical and empirical work in sexual selection has largely focused on signal content, there has been a steady accrual of evidence that signal design is also important for trait evolution. Despite this, relatively little attention has been paid to spatial variation in the conspicuousness of a given signal, especially over small spatial scales (relative to an organism's dispersal distance). Here, we show that visual signals of male threespine stickleback vary in conspicuousness, depending on a male's nest depth within a given lake. Deeper nesting males were typically more chromatically conspicuous than shallow nesting males. This trend is partly because all male stickleback are more conspicuous in deep optical environments. However, deep males are even more conspicuous than environmentally driven null expectations, while shallow males tend to be disproportionally cryptic. Experimental manipulation of male nesting depth induced plastic changes in nuptial color that replicated the natural gradients in conspicuousness. We discuss a number of potential mechanisms that could produce depth gradients in conspicuousness in male stickleback, including concomitant depth gradients in diet, predation pressure, male/female density, female preference, and opportunity for sexual selection. [ABSTRACT FROM AUTHOR]

Multiple biological processes can generate sexual selection on male visual signals such as color. For example, females may prefer colorful males because those males are more readily detected (perceptual bias), or because male color conveys information about male quality and associated direct or indirect benefits to females. For example, male threespine stickleback often exhibit red throat coloration, which females prefer both because red is more visible in certain environments, and red color is correlated with male immune function and parasite load. However, not all light environments favor red nuptial coloration: more tannin-stained water tends to favor the evolution of a melanic male phenotype. Do such population differences in stickleback male color, driven by divergent light environments, lead to changes in the relationship between color and immunity? Here, we show that, within stickleback populations, multiple components of male color (brightness and hue of four body parts) are correlated with multiple immune variables (ROS production, phagocytosis rates, and lymphocyte:leukocyte ratios). Some of these color-immune associations persist across stickleback populations with very different male color patterns, whereas other color-immune associations are population-specific. Overall, lakes with red males exhibit stronger color-immune covariance while melanic male populations exhibit weak if any color-immune associations. Our finding that color-immunity relationships are labile implies that any evolution of male color traits (e.g., due to female perceptual bias in a given light environment), can alter the utility of color as an indicator of male quality. [ABSTRACT FROM AUTHOR]

Sexual selection is most often thought of as acting on organismal traits, such as size or color. However, individuals' habitat use may also affect mating success. Here, we show that, in threespine stickleback, nest depth can be a target of sexual selection. In postglacial lakes in British Columbia, male threespine stickleback nest in a narrow range of depths. Prior studies revealed heritable variation in males' preferred nest microhabitat. We surveyed four natural populations, finding that male stickleback with shallower nests were more successful at breeding. Indeed, nest depth was a much stronger predictor of male mating success than more commonly studied targets of sexual selection in stickleback (size, condition, shape, color, infection status). This selection on nest depth means that variance in fitness changed predictably across microhabitats, altering the opportunity for sexual selection to act on other traits. Accordingly, we show that sexual selection on other male traits is strongest where variance in nesting success is highest (at intermediate nest depths in some lakes). We conclude that males' choice of nesting microhabitat is an especially important target of sexual selection, resulting in fine-scale spatial variation in sexual selection on other traits. [ABSTRACT FROM AUTHOR]