Sperm investment and alternative mating tactics in bluegill sunfish (Lepomis macrochirus)

Although alternative mating tactics are found in males of many species, little is known about tactic-specific adaptations to sperm competition and the mechanism by which fertilization success is obtained. We now report on the sperm investment patterns of males that use alternative mating tactics in bluegill sunfish (Lepomis macrochirus). Bluegill males are characterized by three alternative mating tactics: parentals, sneakers, and satellites. Parentals delay maturation and adopt a courting and guarding tactic, whereas sneakers and satellites mature precociously and, respectively, use sneaking and female mimicry to steal fertilizations from parentals. We found differences among the tactics in testes weight, sperm longevity, and ejaculate sperm density and competitiveness. Parentals had the largest testes, but the smallest relative to their body weight. Satellites had intermediate-sized testes, and sneakers had the smallest testes, but the largest relative to their body weight. Within each tactic, there was a positive relationship between testes weight and body weight, but the exact relationship differed among the tactics and could not be attributed solely to allometry. Parentals had longer-lived sperm compared with that of both sneakers and satellites. Ejaculate sperm density was greatest in sneakers, satellites were intermediate, and parentals had the lowest. Competition experiments involving equal volumes of ejaculate showed that fertilization success increased linearly with sperm number, consistent with a rafflelike process. However, independent of sperm number, parental sperm were superior at fertilizing eggs. We interpret these sperm investment patterns in relation to differences in sperm competition risk, number of mating opportunities, and alternative investment options.