Abstract
Sperm competition is expected to favour the evolution of traits that influence the performance of sperm when they compete to fertilize a female's eggs. While there is considerable evidence that selection favours increases in sperm numbers, much less is known about how sperm quality contributes towards competitive fertilization success. Here, we determine whether variation in sperm quality influences competitive fertilization success in the green swordtail Xiphophorus helleri, a highly promiscuous livebearing fish. We use artificial insemination as a method of controlled sperm delivery and show that sperm swimming velocity is the primary determinant of fertilization success when ejaculates from two males compete to fertilize a female's eggs. By contrast, we found no evidence that sperm length had any effect on siring success. We also found no evidence that pre- and postcopulatory sexual traits were phenotypically integrated in this species, suggesting that the previous observation that reproductive skew favours males with high mating rates is unlikely to be due to any direct association between sperm quality and male sexual ornamentation.
Highlights
Because females typically mate multiply during a single reproductive episode, a male’s reproductive fitness will depend on his ability to acquire mates, and the ability of his sperm to compete for fertilizations [1]
Such patterns may arise when pre- and postcopulatory traits reflect a male’s underlying condition [4], or where females mediate the outcome of sperm competition to favour attractive males [5], a mechanism of postcopulatory sexual selection coined ‘cryptic female choice’ [6,7]
In this study we focus on the role of sperm quality in influencing sperm competition and the relationship between male sexual ornamentation and sperm competitiveness in the green swordtail (Xiphophorus helleri), a model system in sexual selection [24,25]
Summary
Because females typically mate multiply during a single reproductive episode, a male’s reproductive fitness will depend on his ability to acquire mates, and the ability of his sperm to compete for fertilizations (sperm competition) [1]. We explored potential phenotypic relationships between the length of the male’s caudal ‘sword’ (an important target of precopulatory sexual selection) [24] and sperm traits, and determine whether variation in both male sexual attractiveness and ejaculate traits predict competitive fertilization success in this species.
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