Abstract
Sperm competition, in which the ejaculates of multiple males compete to fertilize a female's ova, results in strong selection on sperm traits. Although sperm size and swimming velocity are known to independently affect fertilization success in certain species, exploring the relationship between sperm length, swimming velocity and fertilization success still remains a challenge. Here, we use the zebra finch (Taeniopygia guttata), where sperm size influences sperm swimming velocity, to determine the effect of sperm total length on fertilization success. Sperm competition experiments, in which pairs of males whose sperm differed only in length and swimming speed, revealed that males producing long sperm were more successful in terms of (i) the number of sperm reaching the ova and (ii) fertilizing those ova. Our results reveal that although sperm length is the main factor determining the outcome of sperm competition, complex interactions between male and female reproductive traits may also be important. The mechanisms underlying these interactions are poorly understood, but we suggest that differences in sperm storage and utilization by females may contribute to the outcome of sperm competition.
Highlights
Sperm competition is almost ubiquitous across the animal kingdom [1] and imposes strong selection on males to produce high-quality sperm
We examined the half of the outer perivitelline layer (OPVL) that contained the germinal disc (GD) because the majority of sperm are observed around the GD [57]
Our results show that the proportion of sperm on the OPVL from a given male determines his likelihood of successful fertilization (GLMM; estimate 1⁄4 7.86 + 1.42; z 1⁄4 5.52; p, 0.0001; figure 2)
Summary
Sperm competition is almost ubiquitous across the animal kingdom [1] and imposes strong selection on males to produce high-quality sperm. In previous studies of the zebra finch, we have shown that: (i) sperm length is extremely consistent both within and between the ejaculates of individual males [41], (ii) length and swimming speed are heritable and positively genetically correlated [39,42], and (iii) longer sperm swim at greater velocities than shorter sperm [42] It is still not known whether, in a competitive scenario, males producing long sperm enjoy greater fertilization success than males producing relatively short sperm. Data on copulation rate and SG mass were opportunistically collected from long and short sperm males that were not used in the experiment (refer to the electronic supplementary material, tables S3 and S4). Model simplification was carried out using log-likelihood tests and Akaike information criterion (AIC) values to obtain the minimal adequate models
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