Faithfulness accuracy (proportion of trials in which the faithful model was correctly chosen in a forced choice task) in Experiment 1 and Experiment 2.

<p>The x-axis represents chance level (0.5) and S.E bars are shown.</p

Sperm Competition in Humans: Mate Guarding Behavior Negatively Correlates with Ejaculate Quality

<div><p>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.</p></div

Relationship between mate guarding frequency and PC1 (after accounting for the influence of lifestyle and collection variables), which describes ejaculates with a high percentage of motile sperm and high swimming speed.

<p>Relationship between mate guarding frequency and PC1 (after accounting for the influence of lifestyle and collection variables), which describes ejaculates with a high percentage of motile sperm and high swimming speed.</p

Relationship between mate guarding frequency and the unstandardized residuals of PC3 (after accounting for the influence of lifestyle and collection variables), which describes ejaculates with a high concentration of sperm and sperm that do not move erratically.

<p>Relationship between mate guarding frequency and the unstandardized residuals of PC3 (after accounting for the influence of lifestyle and collection variables), which describes ejaculates with a high concentration of sperm and sperm that do not move erratically.</p

Principal components analysis on all sperm quality parameters to produce principal components that account for sperm quality.

<p>VAP =  average path velocity, VSL =  straight line velocity, VCL =  velocity along the sperm cells point-to-point track, ALH =  lateral amplitude of sperm head movement, BCF =  frequency with which the sperm head crosses the average sperm path, STR =  straightness of the sperm's path, LIN =  linearity of the sperm's path, % motile =  percentage of motile sperm in the ejaculate, concentration =  concentration of sperm in the ejaculate (million sperm/ml).</p><p>Principal components analysis on all sperm quality parameters to produce principal components that account for sperm quality.</p