Extra-pair paternity and sperm length variation in the socially monogamous Fieldfare Turdus pilaris

Abstract

Basic knowledge about the genetic mating system is lacking for the great majority of the approximately 10,000 extant bird species. Filling this knowledge gap is not only critical for a comprehensive understanding of the reproductive ecology of each particular species, but also for increasing the power of comparative approaches to uncover and explain interspecific patterns of variation in avian reproductive traits. Using six polymorphic microsatellite markers, we here present the first parentage study in the socially monogamous Fieldfare Turdus pilaris. In parallel, we also examined variation in sperm morphology and relationships between sperm traits and paternity loss of social males. Across two study years, extra-pair paternity was detected in 46.4% (95% CI: 28.9%−64.9%) of 28 broods, and on average 27.6% (95% CI: 16.8%−41.9%) of nestlings per brood were extra-pair offspring in a population in central Norway. The observed extra-pair paternity rates fall within the range of reported estimates of extra-pair paternity for four congeneric Turdus species (between 36% and 65% of broods and 27% and 46% of nestlings). Sperm total length was 87.0 ± 2.9 (SD) μm (range 79.7-96.8 μm) and 59.3% (95% CI: 37.1%−73.3%) of the total phenotypic variation in sperm total length was explained by differences between sperm samples collected from 17 different males. The among-sample coefficient of variation in mean sperm total length was 2.70% (95% CI: 1.99%−3.17%). We found no evidence for effects of sperm total length or relative midpiece length on loss of paternity among broods of 13 males.

Effects of fixatives and storage duration on avian sperm morphology

“…Thavarajah et al 2012;Hykin et al 2015), most museums have developed special collections for genomic research based on cryopreservation of fresh or ethanol-preserved tissues. Avian sperm samples have commonly been fixed and preserved in formalin for the analysis of sperm morphology (Lüpold et al 2009;Helfenstein et al 2010;Schmoll and Kleven 2011;Immler et al 2012;Albrecht et al 2013;Cramer et al 2013;Rowe et al 2015;Kleven et al 2019). However, if such samples are also intended for DNA analyses, ethanol might be a preferred fixative and storage medium.…”

Measuring Pre- and Post-Copulatory Sexual Selection and Their Interaction in Socially Monogamous Species with Extra-Pair Paternity

“…Laboratory studies where copulations are experimentally controlled similarly show that longer and faster-swimming sperm have higher fertilization success [ 38 , 39 ]. In contrast, field studies on wild populations often find no evidence of selection on sperm traits [ 40 , 41 , 42 , 43 , 44 ]. Potentially, the lack of patterns in these field studies is because post-copulatory sexual selection is not sufficiently strong in the species studied to result in statistical power, and indeed, in some species with high levels of extra-pair paternity, selection on sperm has been detected [ 45 , 46 ].…”