Sperm Numbers as a Paternity Guard in a Wild Bird

Abstract

Sperm competition is thought to impose strong selection on males to produce competitive ejaculates to outcompete rival males under competitive mating conditions. Our understanding of how different sperm traits influence fertilization success, however, remains limited, especially in wild populations. Recent literature highlights the importance of incorporating multiple ejaculate traits and pre-copulatory sexually selected traits in analyses aimed at understanding how selection acts on sperm traits. However, variation in a male’s ability to gain fertilization success may also depend upon a range of social and ecological factors that determine the opportunity for mating events both within and outside of the social pair-bond. Here, we test for an effect of sperm quantity and sperm size on male reproductive success in the red-back fairy-wren (Malurus melanocephalus) while simultaneously accounting for pre-copulatory sexual selection and potential socio-ecological correlates of male mating success. We found that sperm number (i.e., cloacal protuberance volume), but not sperm morphology, was associated with reproductive success in male red-backed fairy-wrens. Most notably, males with large numbers of sperm available for copulation achieved greater within-pair paternity success. Our results suggest that males use large sperm numbers as a defensive strategy to guard within-pair paternity success in a system where there is a high risk of sperm competition and female control of copulation. Finally, our work highlights the importance of accounting for socio-ecological factors that may influence male mating opportunities when examining the role of sperm traits in determining male reproductive success.

Socio-sexual environment manipulates sperm allocation strategies in ladybird beetle, Cheilomenes sexmaculata (Fabricius)

“…Several studies have linked high sperm number to paternity success in many insect species, for example, water strider (Arnqvist & Dannielsson 1999), crickets (Gage & Morrow 2003) and scorpion y (Engqvist et al 2007). A recent study in wild red-backed fairy-wrens shows that males having large number of sperm available for copulation had greater within pair paternity success (Rowe et al 2022). Our results also demonstrated that males transferred consistently more sperm across successive mating (1:5 male biased condition).…”

Sperm competition favours intermediate sperm size in a hermaphrodite

“…Importantly, the vast majority of empirical studies assume a linear relationship between sperm size and reproductive (or post-mating) success and do not test for non-linear associations which limits our understanding on how selection operates on sperm size. To our knowledge, only five studies that have tested for such non-linear associations, but none of the studies found any significant non-linear associations between sperm length and estimates of fitness (Cramer et al, 2013; Lymbery et al, 2018; Kahrl et al, 2021; Rowe et al, 2022; Marie-Orleach et al, 2023).…”

Longitudinal covariation of testosterone and sperm quality across reproductive stages in the zebra finch

“…In both mammals and in birds, testosterone levels are related to the percentage of normal sperm found in ejaculates (birds Gumulka and Rozenboim, 2015;Penfold et al, 2000;mammals Walker, 2011) and experimentally increased testosterone levels can increase ejaculate size and sperm concentration (in a passerine; Kast et al, 1998). While the number of sperm (and proportion of normal sperm) are likely to affect fertilisation success (Rowe et al, 2022), to our knowledge, no study to date has examined the relationship between testosterone and sperm motility, which is also likely to be linked to fertilisation success (Birkhead et al, 1999;Denk et al, 2005;Gumulka and Rozenboim, 2015).…”