Sperm Morphology and Male Age in Black-Throated Blue Warblers, an Ecological Model System

SIMPLE SUMMARY: Sperm cell characteristics can impact the number of offspring a male sires, particularly when females copulate with more than one male in a reproductive cycle. Females copulate with multiple males in many songbird species, but for many of these species, little is known about sperm characteristics. In this paper, we examine the shape of sperm cells in black-throated blue warblers, a species where multiple copulations are common but where sperm morphology was previously unknown. Sperm shape did not correlate with plumage characteristics that may make males more successful at gaining copulation partners. However, older males tended to have longer sperm cells. Previous work shows that older males sire more offspring, and work in other species indicates that longer sperm cells are more effective at fertilizing eggs. Thus, sperm shape may help explain the higher success of older males in this species. ABSTRACT: Extra-pair paternity may drive selection on spermatozoa and ejaculate characteristics through sperm competition and cryptic female choice. Here, we examine sperm morphology in the black-throated blue warbler (Setophaga caerulescens), an ecological model species where extra-pair paternity is frequent and is linked with male age. We test whether sperm morphology relates to several aspects of male phenotype known or suspected to affect extra-pair paternity success. Sperm morphology did not correlate with the size of the white wing spot, a social status signal, nor with the volume of the cloacal protuberance. However, older males tended to have longer sperm cells. Although the sample size was limited, this pattern is intriguing, as longer cells may be advantageous in post-copulatory sexual selection and older males have larger testes and higher extra-pair paternity success in this species. Changes in sperm morphology with age are not observed in other birds, though they have been observed in insects and fishes. More research on sperm morphology is needed to clarify its role in extra-pair fertilizations in this well-studied species.