Male reproductive success is dependent on a correlated suite of traits related to a species ecology and mating system dynamics. Closely related species differing in their mating systems and ecology, such as the garter snakes (Thamnophis), are ideal for studying the correlated evolution of sexually selected traits. Here, we compare the degree of sexual size dimorphism (SSD), copulatory behavior, copulatory plug size, and traits associated with sperm competition between two sympatric and closely related Thamnophis species, T. sirtalis and T. radix with divergent mating aggregation size and density. Our findings indicate that T. sirtalis has greater female-biased SSD, shorter copulations, and larger, more strongly adhering copulatory plugs than T. radix. Our finding that T. sirtalis have longer sperm and higher numbers of sperm per ejaculate is further evidence of more intense sperm competition in this species than in T. radix. However, this reduced number of sperm in the ejaculate means that T. radix males are likely capable of more matings per season than T. sirtalis. This result may reflect differences in feeding during the breeding season (obligate aphagy in T. sirtalis) and the potential for sperm loss in T. radix during prolonged copulations that are prevented in T. sirtlais by their substantial copulatory plugs. Our findings demonstrate that ecological and mating system dynamics have the capacity to strongly influence correlated selection of pre- and postcopulatory traits.