- University of Oslo
- Queen‘s University
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.
We sampled sperm from 17 passerine bird species in the vicinity of Queen's University Biological Station, Ontario, Canada, and in southern Norway during the breeding season 2006. Sperm were collected by gently massaging the cloacal protuberance of live birds using a similar technique as described by Wolfson (1952). The sperm samples were fixed in a 5% formalin solution and a droplet of this solution was placed on a microslide and air‐dried. Sperm were examined using a digital light microscope (Leica DM6000 B) at ×320 magnification and photographed with a digital camera (Leica DC500). Total sperm length (± 0.1 μm) was measured using Leica IM1000 (Leica Microsystems, Heerbruug, Switzerland) analysis software. A subset of spermatozoa from a single male was measured twice, revealing that the measurements of sperm length were highly repeatable (r = 0.98, F 19,20= 78.9, P < 0.001) (Lessells and Boag 1987). All measurements were made by one person to reduce potential interobserver variability. We measured 10 spermatozoa per male in 4–10 males of each of the 17 species. Measuring 10 spermatozoa per male gives a good species‐specific estimate of average within‐male variation in sperm length, as revealed by a repeatability analysis of individual coefficients of variation (CV) in sperm length for the 17 species (r = 0.59, F 16,129= 13.2, P < 0.001). We have also shown for two other passerine species that measuring 10 spermatozoa gives a representative estimate of the mean and variance of a male's sperm length (Laskemoen et al. 2007). In birds, sperm length within males is also highly consistent across ejaculates (Birkhead and Fletcher 1995; T. Laskemoen, O. Kleven, F. Fossøy, R. J. Robertson, G. Rudolfsen and J. T. Lifjeld, unpub. data). Data on intraspecific variation in sperm length were obtained for an additional five wild passerine species, in which figures on extrapair paternity were also available (see online Supplementary Table S1). For two of the five latter species (sedge warbler Acrocephalus schoenobaenus and splendid fairy‐wren Malurus splendens ), information on within‐male variation in total sperm length was not available. We did not include data on sperm length variation in the zebra finch in our analyses, because the available data were collected from domesticated birds (Birkhead and Fletcher 1995) that potentially have been subject to selection pressures deviating from those of wild birds. As a measure of total sperm length variability, we calculated the coefficient of variation adjusted for differences in sample size (Sokal and Rohlf 1995). Within‐male variation in total sperm length was normalized through log10‐transformation prior to analysis.
SPERM COMPETITION INDEX
Direct measures of sperm competition risk among free‐living birds are difficult to obtain, implying that sperm competition needs to be estimated indirectly. Information on patterns of paternity (obtained through molecular methods) are available for a wide range of bird species (e.g., Griffith et al. 2002) and such data are likely to represent appropriate estimates of the true level of sperm competition risk (cf. Briskie et al. 1997). We therefore chose to use extrapair paternity (defined as the proportion of fertilizations resulting from copulations outside the social pair bond) as an index of sperm competition. Hence, our sperm sampling strategy was targeted toward species with available data on extrapair paternity, and particularly those populations in which the paternity studies had been conducted (see Fig. 1 and see online Supplementary Table S2). Proportions of extrapair paternity were arcsine square‐root transformed prior to analysis. We also present the result of the main statistical analysis using relative testis mass (i.e., testis mass corrected for body mass) as an index of sperm competition. The rationale for including the latter analysis is that relative testis mass more commonly has been used as a proxy for the risk of sperm competition (e.g., Hosken 1997; Dunn et al. 2001; Gage and Freckleton 2003). Data on testis and body mass were obtained from the literature (Dunn et al. 2001) and log10‐transformed prior to analysis.