Sperm Numbers as a Paternity Guard in a Wild Bird.

Melissah Rowe,Jan T Lifjeld,Michael S Webster,Daniel T Baldassarre,Lyanne Brouwer,Joseph F Welklin,Annabel Van Oort

doi:10.3390/cells11020231

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.

Highlights

In 1970, Geoff Parker [1] fundamentally shifted our understanding of sexual selection
Understanding how selection imposed by sperm competition shapes sperm traits and the role of sperm in fertilization success has been a major focus of both theoretical and empirical studies
While birds have been the focus of sperm competition studies for decades [29], our understanding of how sperm traits influence male reproductive success in natural populations remains limited

Summary

Introduction

In 1970, Geoff Parker [1] fundamentally shifted our understanding of sexual selection. It has become clear that females frequently mate with more than one male during a single reproductive event, and selection extends beyond mating as post-copulatory sexual selection in the form of sperm competition (i.e., the competition between sperm/ejaculates from different males for fertilization of a given set of ova [1,2]) and cryptic female choice (i.e., female-regulated fertilization bias towards the sperm/ejaculates of specific males [3,4]). While comparative studies provide clear evidence of post-copulatory sexual selection acting on sperm traits, our understanding of how different sperm traits influence male fertilization success remains limited. Studies investigating the relationship between the three most commonly assessed sperm traits—sperm morphology, velocity, and viability—and fertilization success report both positive and negative associations, as well as no association between sperm quality and fertilization success (reviewed in [25])

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