Sperm length variation as a predictor of extrapair paternity in passerine birds

• Published in: PloS one 2010-10, Vol.5 (10), p.e13456-e13456
• Main Authors: Lifjeld, Jan T, Laskemoen, Terje, Kleven, Oddmund, Albrecht, Tomas, Robertson, Raleigh J
• Format: Article
• Language: English
• Subjects: Agelaius phoeniceus; Analysis; Animal behavior; Animal breeding; Animal reproduction; Animals; Biology; Birds; Breeding; Coefficient of variation; Competition; Cost analysis; Ecology/Behavioral Ecology; Ecology/Evolutionary Ecology; Ecology/Population Ecology; Evolution; Evolutionary Biology/Animal Behavior; Evolutionary Biology/Evolutionary Ecology; Fish; Infertility; Luscinia svecica; Male; Males; Mammalia; Morphology; Museums; Natural history; Passeriformes; Paternity; Phylloscopus trochilus; Phylogenetics; Phylogeny; Population studies; Sexual Behavior, Animal; Soricidae; Species; Sperm; Sperm competition; Spermatozoa; Success; Tachycineta bicolor; Variables; Variation; Zoology
• ISSN: 1932-6203; 1932-6203
• DOI: 10.1371/journal.pone.0013456

The 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. We 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 (CV(bm)) and within-male variation (CV(wm)) 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 CV(bm) 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. We propose the CV(bm) 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.