Sperm characteristics associated with different male reproductive tactics in bluegills (Lepomis macrochirus)

We examined the availability and motility of sperm from parental and sneake r male bluegills (Lepomis macrochirus), a colonially nesting sunfish (Famil y Centrarchidae) with male parental care and a high incidence of cuckoldry by both sneaker and satellite males. We found no differences between sneake rs and parentals in length and swimming speed of sperm, or percent and dura tion of sperm activity. In sneaker milt, however, sperm was almost 50% more concentrated than in parental milt (16.5 x 10(6) vs 11.5 x 10(6) sperm/mul of milt, respectively). Despite this difference in sperm concentration, st ripped ejaculates from sneakers contained almost 400 million fewer sperm (o nly 32% as many sperm) than those from parentals due to their much smaller stripped ejaculate volumes (only about 19% that of parentals). Thus unless sneakers can compensate by releasing more sperm or gaining closer proximity to eggs at the time of spawning, they may be at a disadvantage with respec t to sperm competition. We discuss these results in relation to models for the evolution of alternative reproductive behaviours in this species and su ggest that the cuckolders may be making the best of a bad situation.