Intraspecific Variation in Sperm Length Is Negatively Related to Sperm Competition in Passerine Birds

Abstract

Spermatozoa are among the most diversified cells in the animal kingdom, but the underlying evolutionary forces affecting intraspecific variation in sperm morphology are poorly understood. It has been hypothesized that sperm competition is a potent selection pressure on sperm variation within species. Here, we examine intraspecific variation in total sperm length of 22 wild passerine bird species (21 genera, 11 families) in relation to the risk of sperm competition, as expressed by the frequency of extrapair paternity and relative testis size. We demonstrate, by using phylogenetic comparative methods, that between-male variation in sperm length within species is closely and negatively linked to the risk of sperm competition. This relationship was even stronger when only considering species in which data on sperm length and extrapair paternity originated from the same populations. Intramale variation in sperm length within species was also negatively, although nonsignificantly, related to sperm competition risk. Our findings suggest that postcopulatory sexual selection is a powerful evolutionary force reducing the intraspecific phenotypic variation in sperm-size traits, potentially driving the diversification of sperm morphology across populations and species.

Sperm competition in tropical versus temperate zone birds

“…Theoretical models also predict lower variation in sperm dimensions within males under strong sperm competition levels [29,30]. In line with these predictions, comparative studies have shown a strong inverse relationship between sperm competition indices and coefficients of both between-male (CV bm ) and within-male (CV wm ) variation in sperm length in insects [31] and birds [32,33]. In contrast to sperm CV wm , the relationship between sperm CV bm and sperm competition risk does not show any phylogenetic bias (cf.…”

“…Similarly, another study [18] used a rather heterogeneous sample of passerine and non-passerine species in which most tropical passerines were represented by island populations, characterized by low sperm competition levels [54]. Our analysis is based on a relatively high number of tropical species and a proxy for sperm competition risk that is closely associated with EPP frequencies in passerine species [25,27,33]. Our results support the view that tropical avian species exhibit variable levels of sperm competition, similar to that observed among temperate species.…”

The evolutionary ecology of testicular function: size isn't everything

“…To elucidate the origin of these patterns comparative analyses in different bird groups have suggested that betweenindividual variation in sperm size is most pronounced in species subject to low levels of sperm competition, probably due to relaxed selection on an optimal sperm morphometry and so too presumably on other aspects of spermatogenesis (Birkhead et al, 2005;Calhim, Immler & Birkhead, 2007;Kleven et al, 2008;Lüpold, Linz & Birkhead, 2009a). Moreover, there may also be a role for post-mating sexual selection in explaining patterns of within-individual variation in sperm morphometry, since bird species subject to high levels of sperm competition are also those that show the lowest levels of standardised within-individual variation in sperm size (Immler, Calhim & Birkhead, 2008), which has led to the suggestion that sperm competition may select for enhanced quality control during spermatogenesis (see also Lüpold et al, 2011).…”

Intra‐ and intermale variability of mature sperm traits analysed in two brackish water populations of the pipefish Syngnathus abaster (Syngnathidae)

“…Recently, comparative studies of both passerine birds and some teleost species have showed that the variations are also present in normal spermatozoa within a species within each male of the same species. These types of variations are negatively associated with the risk of sperm competition (Calhim et al 2007;Th€ unken et al 2007;Immler et al 2008;Kleven et al 2008;Lifjeld et al 2010). Sperm design is commonly assumed to be under male and hence diploid genetic control (Beatty 1970;Parker and Begon 1993;Birkhead et al 2005).…”