A central goal in behavioral ecology is to understand factors that explain variations in mating systems. The role that ecological variation plays in structuring mating systems is poorly understood. In Mr. Timothy Sparkes' doctoral-dissertation research, he will use a combination of physiological techniques and laboratory experiments to examine the role that ecological factors play in affecting mate-choice decisions. The focal organism is a freshwater crustacean, Lirceus fontinalis. Completed work has documented that variation in ecological factors (in particular, predation risk) among populations results in population-specific optima for life-history traits. Population differences in life histories, in turn, influence the optima for other traits, e.g., mate choice. The research will test hypotheses about the role that ecological variation plays in determining mate-choice decisions. Overall, this research program should provide an usually complete synthesis on the effects of ecological factors on a suite of interrelated prey adaptations. There are 3 components: (A.) Behavioral experiments to determine mate-choice criteria for individuals of each sex. (B.) A combination of physiological and behavioral experiments to examine variation in mating behavior as a consequence of physiological state. (C.) A behavioral experiment to determine the role that predation plays in explaining variations in mate-choice decisions.