Sperm competition in humans

Samantha Leivers

    Research output: ThesisDoctoral Thesis

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    In species where females mate with multiple males, sperm competition may occur whereby the sperm of two or more males compete on a post copulatory level to fertilise the ova of a single female. Selection via sperm competition has given rise to the evolution of adaptations that affect a male’s chances of paternity when a female has mated, or is likely to mate, with another male. These adaptations improve a male’s chances of paternity by i) avoiding or preventing sperm competition from occurring (defensive tactics) and ii) by engaging in sperm competition (offensive tactics). Adaptations to sperm competition can be morphological, behavioural or physiological. The aim of this thesis was to investigate the role that sperm competition has played in human evolution, particularly the evolution of defensive psychological adaptations and offensive physiological adaptations.
    In Chapter 1, I examine the current evidence for morphological, behavioural and physiological adaptations for sperm competition in non-human animals and consider what this tells us about sperm competition in humans. The available evidence suggests that humans have primarily evolved defensive adaptations in response to the risk of sperm competition but that considerable further research investigating offensive adaptations to sperm competition in humans must be conducted before firm conclusions can be drawn.
    In Chapter 2, I investigate the evolution of psychological adaptations in humans and provide the first known evidence that men can show accuracy in their judgements of faithfulness. This accuracy is dependent on the experimental task and stimuli used. Further investigation showed that priming men to an environment depicting sexual competition from rival males does not improve men’s accuracy in judgements of faithfulness.
    In Chapters 3 and 4, I examine the evolution of physiological adaptations to sperm competition in men by investigating the factors that influence ejaculate quality and how these relate to sperm competition theory. Sperm competition theory predicts that males will increase ejaculate investment when mating with attractive females, although increasing evidence suggests that the quality of the male himself can also influence ejaculate quality (phenotype-linked fertility hypothesis). In Chapter 3, I show that ejaculate quality increases with a composite measure of male mate value, but only when men view images of highly attractivewomen.
    In Chapter 4, I investigate the extent to which men covary their performance of defensive and offensive sperm competition tactics. If a male’s defensive tactics are highly successful at preventing or avoiding sperm competition, one would expect reduced investment in offensive adaptations for the engagement in sperm competition, and vice versa. In this study, I show that men in committed heterosexual relationships who perform more mate guarding behaviours produce ejaculates of poorer quality. These findings suggest that men covary their investment in defensive adaptations that function to avoid or prevent sperm competitions and offensive adaptations that function to engage in sperm competition.
    Together, these studies examine the role that sperm competition has played through human evolution. I contribute to the literature that suggests that men have evolved psychological adaptations that function to avoid sperm competition by showing that men display some accuracy in their judgements of female faithfulness. Furthermore, I add to a growing literature that variation in men’sejaculate quality can, in part, be attributed to sperm competition risk.
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2014

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