Postcopulatory consequences of female mate choice in a fish with alternative reproductive tactics

Mate choice plays a well-known role in the evolution of secondary sexual traits important in precopulatory competition. However, few studies have linked mate choice with the evolution of postcopulatory competitive traits. Here, we explore how variation in male mating behaviours and female mate choice influences male investment in reproductive traits that enhance sperm competition, a form of postcopulatory male-male competition. By combining ecological and physiological data from wild plainfin midshipman (Porichthys notatus), a marine fish species with two alternative reproductive tactics (guarding and sneaking males), we show that female mate choice is associated with uneven sperm competition risk between male reproductive tactics as well as among males using the same reproductive tactic. Larger guarding males attracted more females and experienced higher rates of attempted cuckoldry compared to smaller guarding males. In turn, larger guarding males appear adapted to this increased sperm competition risk, producing faster sperm than smaller guarding males. Sneaking males (the smallest males of all) had faster swimming sperm, with larger sperm midpieces and smaller sperm heads than did guarding males. These results suggest that female choice can amplify the selection gradient acting on males both between and within reproductive tactics.