Male mating signals convey important mate-quality information to females and are regulated by androgens. Endocrine disrupting compounds (EDCs) are chemicals that interfere with proper hormonal functioning in exposed animals, causing altered hormone levels and resulting in changed reproductive characteristics, including mating signals. Altered signals can have ecological implications by influencing population and community dynamics and evolutionary implications via trans-generational reduction in signal reliability leading to reduced preference and eventual loss of the signal trait. I examined the effects of exposure to environmentally relevant concentrations of atrazine, a widely used herbicide and EDC, on mating signals and behaviors in male guppies, a sexually dimorphic freshwater fish. Guppies were exposed either during adulthood or embryonic development. Prolonged atrazine exposure during adulthood reduced the size of the carotenoid-based ornament, the number of courtship displays performed, and aggression towards competing males. Embryonic exposure did not affect survival to adulthood and the time to develop male-specific morphologies. But there was a trend for smaller genitalia, and the ornament size was significantly increased. Possible increases in immunocompetence as a result of slight estrogenecity may have allowed for greater carotenoid allocation to the ornament. Embryonic exposure also resulted in reduced courtship behavior, forced copulatory attempts and aggression towards competitors; female guppies found these males less attractive. The low dose had the strongest effects with embryonic exposure, indicating the importance of low-dose exposures. These studies highlight the effects of low and environmentally relevant doses of atrazine on mating signals and behaviors in exposed wildlife. A mathematical model was used to understand the evolutionary effects of EDCs on the optimal allocation of carotenoids between ornament and immunocompetence. Animals obtain carotenoids through their diet, and allocate some of this to enhance immune function and the rest to ornaments for mate attraction. The model replicates the disruption of carotenoid-based ornaments as a result of EDC-exposure, and predicts that signal reliability will be reduced. The model simulates an evolutionary shift in the optimal allocation if exposure spanned multiple generations, but signal reliability is not restored. Including additional selective forces like predation further suppresses signal reliability
