Intraspecific variation in sperm length in two passerine species, the Bluethroat Luscinia svecica and the Willow Warbler Phylloscopus trochilus

Abstract

Sperm cells are highly diversified in birds and considerable research effort has focused on variation in sperm morphology between species. However, surprisingly little is known about intraspecific variation in sperm morphology in birds. We analyzed between-and within-male variation in total sperm length in two passerine species, the Bluethroat (Luscinia svecica) and the Willow Warbler (Phylloscopus trochilus). In both species, the variance in sperm length was nearly twice as high between as within males, resulting in high repeatability of sperm length for individual males (Bluethroat: r = 0.73 and Willow Warbler: r = 0.79). These results suggest that sperm traits are more variable among than within males. With a resampling approach, we illustrate how the spread in estimated mean sperm length and coefficient of variation (CV) is affected by increasing the number of males measured. Further, we illustrate how the CV of sperm length for individual males change with the number of spermatozoa measured. For the two species in our analyses, it seems that measuring 10 males and 10 spermatozoa per male gives adequate estimates of both between-and within-male sperm length and CV.