Sperm Length Variation as a Predictor of Extrapair Paternity in Passerine Birds

Abstract

BackgroundThe rate of extrapair paternity is a commonly used index for the risk of sperm competition in birds, but paternity data exist for only a few percent of the approximately 10400 extant species. As paternity analyses require extensive field sampling and costly lab work, species coverage in this field will probably not improve much in the foreseeable future. Recent findings from passerine birds, which constitute the largest avian order (∼5 900 species), suggest that sperm phenotypes carry a signature of sperm competition. Here we examine how well standardized measures of sperm length variation can predict the rate of extrapair paternity in passerine birds.Methodology/Principal FindingsWe collected sperm samples from 55 passerine species in Canada and Europe for which extrapair paternity rates were already available from either the same (n = 24) or a different (n = 31) study population. We measured the total length of individual spermatozoa and found that both the coefficient of between-male variation (CVbm) and within-male variation (CVwm) in sperm length were strong predictors of the rate of extrapair paternity, explaining as much as 65% and 58%, respectively, of the variation in extrapair paternity among species. However, only the CVbm predictor was independent of phylogeny, which implies that it can readily be converted into a currency of extrapair paternity without the need for phylogenetic correction.Conclusion/SignificanceWe propose the CVbm index as an alternative measure to extrapair paternity for passerine birds. Given the ease of sperm extraction from male birds in breeding condition, and a modest number of sampled males required for a robust estimate, this new index holds a great potential for mapping the risk of sperm competition across a wide range of passerine birds.

Inbreeding depression of sperm traits in the zebra finch Taeniopygia guttata

“…2002; Lifjeld et al. 2010). If our results can be generalized to other bird species, it would suggest that inbreeding has the potential to affect ejaculate quality, and hence within‐pair paternity loss and extra‐pair paternity gain.…”

Evolution of sperm structure and energetics in passerine birds

“…Data on sperm length variation were calculated from sperm as the between-male coefficient of variation (CV bm ¼ s.d./mean  100), adjusted for small sample size according to the formula adjusted CV bm ¼ (1 þ 1/4n)  CV bm [40]. Adjusted CV bm (hereafter referred to as simply spermCV) provides a proxy measure of extra-pair paternity, and thus levels of sperm competition, in passerine birds [41].…”

Patterns of sperm damage in Chernobyl passerine birds suggest a trade-off between sperm length and integrity

“…Passerine birds have spermatozoa adapted to internal fertilization and sperm storage, with a sevenfold variation in length, i.e. approximately 40-290 mm [2,4,5]. Post-copulatory sexual selection seems to promote the evolution of longer sperm [2,4,6] although the adaptive function of longer sperm in sperm competition is not well understood.…”