We investigate how social experience can alter traits, and thereby influence sexual selection, using threespine sticklebacks (Gasterosteus aculeatus spp.). Specifically, we examine how female and male responses to social conditions affect their reproductive investments and mating success.We first focus on how mate availability impacts female reproductive investment as measured by the timing and amount of clutches and eggs. Using treatments that mimic high and low availability of reproductive males, we found that Paxton Lake females, although previously shown to adjust their mate choices, did not alter their reproductive investment strategies. Because females must make relatively fixed reproductive investments prior to courtship, but have plastic mate choice, the strength of sexual selection acting on males during mating is subject to change. This work therefore highlights the potential consequences when reproductive investment and female choice operate out of sync. We focus second on how males allocate resources into several traits, and when and how those traits are assessed in courtship. With Cranby Lake sticklebacks, we show that the sexual signal preferred by females in many populations (red throats) is neither condition-dependent, nor preferred in mating. Males indeed allocate resources to their morphological displays, courtship behavior, and nests, but only a subset of traits matter to females at each stage of the courtship progression. Specifically, females only prefer high male condition early in courtship, and they increasingly prefer more vigorous, display-oriented courtship behavior as courtship progresses. This work suggests that the relative importance of traits upon which females choose mates is not only a function of whether those traits indicate male quality, but also depends on what stage in courtship traits are assessed. Third, because population demography is likely to vary spatially and temporally for many organisms, we address how male traits might change in response to the intensity of mate competition. We experimentally manipulated male density (‘perceived mate competition’), and examined the effects on male nesting probability, mating traits, and mating success. We found males to display different coloration and build nests differently in response to a two-week exposure to either low or high competition. Given that these changes resulted from such a short-term exposure, this work clarifies how rapid changes in demography affect the courtship and nesting strategies adopted by males. If females are assessing male traits throughout courtship, understanding whether and how male traits change with competitive conditions suggests how mate competition and female choice may interact to shape traits, and how acclimation to social conditions may influence adaptation. Finally, we inquire how experience and success in mating affect male traits. Because previous experience with other males altered male traits, we were inspired to discover how male traits and mating success respond to prior experience and success in courtship with females. This is particularly important as changes to population demography often alter mating opportunities, competition, and the likelihood of success. Here we show that males alter their body coloration and court more quickly with experience, but males do not alter their nests when they experience mating success. The trait responses to experience suggest a cycle: the male traits used in mating are responsive to prior experiences and success, and these modified traits are used in future mating attempts. Thus, trait variation can be maintained in part by the responses to experiences individuals have across their lifetime.
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