Sperm dimensions differ between two Coal Tit Periparus ater populations

The evolutionary forces that shape and maintain variation in sperm traits across species in birds have recently attracted increased attention. Much less is known about the patterns, the causes and the consequences of variation in sperm traits within and, in particular, between populations of the same species. We here analyse intraspecific variation in sperm dimensions within and between two populations of the Coal Tit Periparus ater, a socially monogamous passerine with a high frequency of extra-pair paternity. Spermatozoa from a Norwegian population had a greater mean total length compared to those from a German population, mainly as a result of longer sperm heads. Sperm head length in the Norwegian population also accounted for a larger percentage of sperm total length suggesting differences in sperm proportions between populations. Furthermore, spermatozoa from the Norwegian population showed lower mean within-male variation in sperm length and there was significant between-male variation in sperm total length within each of the two populations. We discuss these results in the light of recent comparative evidence suggesting relationships between sperm length, and variation in sperm length, and the frequency of extra-pair paternity across passerine birds.