Sperm quantity and quality effects on fertilization success in a highly promiscuous passerine, the tree swallow Tachycineta bicolor

Sperm competition is widespread among animal taxa and considered a major force in sperm evolution. Recent comparative studies have indicated that sperm competition selects for high sperm production capacity and long and fast-swimming spermatozoa across species. Here, we examine the role of sperm quantity and quality for fertilization success of individual males in a Canadian population of tree swallows Tachycineta bicolor, a socially monogamous, but highly promiscuous passerine. Male fertilization success (the sum of withinpair and extrapair young) was significantly associated with the size of the cloacal protuberance (a proxy for sperm quantity), but not with sperm size or in vitro sperm swimming speed. In a multivariate analysis, both cloacal protuberance volume and relative sperm midpiece size (i.e. high mitochondrial loading) had significant effects on male fertilization success. However, relative sperm midpiece size was not associated with fertilization success in a simple regression. Further, both cloacal protuberance volume and relative midpiece size had significant effects on sperm velocity, both in simple regressions and in a multivariate analysis. The finding that males with large relative midpiece size had both higher fertilization success and faster swimming sperm, suggests an indirect link between sperm morphology and male fertility mediated through sperm velocity. In conclusion, both quantitative and qualitative sperm traits seem to affect male fertilization success in tree swallows.     

Is female promiscuity constrained by the presence of her social mate? An experiment with bluethroats <Emphasis Type="Italic">Luscinia svecica</Emphasis>

Extra-pair paternity is common in birds and much research has focussed on the selective advantage of extra-pair matings for both sexes. In contrast, little attention has been given to the fact that in most species the majority of offspring are sired by the social male. We investigated whether extra-pair matings of female bluethroats (Luscinia svecica) are constrained by the presence of the pair male, by detaining males in cages on their territories for one morning during the peak of female fertility. The proportion of offspring sired by extra-pair males was higher in broods where males had been detained (35%) than in control broods (16%), while the proportion of broods that had at least one extra-pair offspring did not differ significantly between experimental (65%) and control broods (44%). Within the experimental group, levels of extra-pair paternity were not related to the day of experiment in relation to start of egg laying, but males caught early in the morning lost more paternity than males caught later on. Our results show that pair males exert constraints on the frequency of extra-pair paternity by being present during the period of peak fertility, which could be a direct effect of their mate guarding effort and/or due to an advantage in sperm competition for pair males.