Abstract
In species where females mate with multiple males, the sperm from these males must compete to fertilise available ova. Sexual selection from sperm competition is expected to favor opposing adaptations in males that function either in the avoidance of sperm competition (by guarding females from rival males) or in the engagement in sperm competition (by increased expenditure on the ejaculate). The extent to which males may adjust the relative use of these opposing tactics has been relatively neglected. Where males can successfully avoid sperm competition from rivals, one might expect a decrease in their expenditure on tactics for the engagement in sperm competition and vice versa. In this study, we examine the relationship between mate guarding and ejaculate quality using humans as an empirical model. We found that men who performed fewer mate guarding behaviors produced higher quality ejaculates, having a greater concentration of sperm, a higher percentage of motile sperm and sperm that swam faster and less erratically. These effects were found independent of lifestyle factors or factors related to male quality. Our findings suggest that male expenditure on mate guarding and on the ejaculate may represent alternative routes to paternity assurance in humans.
Highlights
In species where females mate with multiple males, the sperm of two or more males must compete to fertilise available ova [1]
Little is known about how individual males might balance their investments into tactics for the engagement in and avoidance of sperm competition when paired with a given female
PC1 described ejaculates with high motility and high swimming speeds, being most heavily loaded by the percentage of motile sperm in the ejaculate, VAP, VSL and linearity of the sperm’s path (LIN), which have all been linked to fertilization success in IVF treatments [60,64]
Summary
In species where females mate with multiple males, the sperm of two or more males must compete to fertilise available ova [1]. Adaptations for the avoidance of sperm competition can include the use of mate guarding or antiaphrodisiac odors and copulatory plugs, while adaptations for the engagement in sperm competition include copulation frequency and duration or strategic adjustments in ejaculate quality [4,5,6]. There is evidence to show that males will adjust their investment in mate guarding dependent on the perceived risk of sperm competition from rival males [2,11,12]. Physiological investments into ejaculate production are costly for males, and there is widespread evidence that males will adjust their investment into the ejaculate dependent on their perceptions of sperm competition risk [for a review, see 4]. Were males unable to effectively guard their mates, we might expect them to increase their expenditure on the ejaculate
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