Sperm as a speciation phenotype in promiscuous songbirds

The paper compares 20 population pairs of Passerida songbirds and shows that their rate of divergence in sperm length was positively associated with the level of female promiscuity (expressed by the proportion of offspring sired by extrapair males). The effect was largely caused by a reduced population variance in sperm length in more promiscuous species. The paper suggests that rapid sperm length divergence can promote postcopulatory prezygotic isolation and hence rapid speciation in promiscuous songbirds.

Methods
Population pairs had estimates of total sperm length from minimum six males in each population, a published estimate of their divergence time using a molecular clock model, and an estimate of the proportion of extrapair young (EPY) for the actual populations or a population in the species complex. The two populations were either of the same taxonomic subspecies, different subspecies of the same species, or congeneric species with a known hybrid zone. Maximum divergence time was 2.41 million years since last common ancestor. For three population pairs, EPY was estimated from relative testes mass (no paternity studies known). 

Sperm length data were extracted from raw data in published studies or from the database of the Avian Sperm Collection at the Natural History Museum, University of Oslo (Lifjeld 2019). In some cases, data from published studies were extended with additional unpublished data from the same study populations available in the museum’s database. As a standard, ten or more sperm cells were measured by light microscopy from formalin-fixed ejaculates to calculate the mean sperm length for each male (Grønstøl et al. 2023). Each mean value was log-transformed. From these log-means we calculated mean sperm length and the standard deviation for each population. Sperm length divergence was expressed in Hedges' g (Hedges 1981). We also calculated an overlap index for the sperm length distributions.

References:

Grønstøl, G., Danielsen, M., Cramer, E. R. A., Johannessen, L. E., Johnsen, A., Whittington, E., & Lifjeld, J. T. (2023). Effects of fixatives and storage duration on avian sperm morphology. Journal of Ornithology, 164(1), 171-181. doi:10.1007/s10336-022-02015-x

Hedges, L. V. (1981). Distribution theory for Glass's estimator of effect size and related estimators. Journal of Educational Statistics, 6(2), 107-128. doi:10.3102/10769986006002107

Jetz, W., Thomas, G. H., Joy, J. B., Hartmann, K., & Mooers, A. O. (2012). The global diversity of birds in space and time. Nature, 491(7424), 444-448. doi:10.1038/nature11631

Lifjeld, J. T. (2019). The avian sperm collection in the Natural History Museum, University of Oslo. Alauda, 87(3), 93–101.