Extra-pair paternity and sperm length variation in a far northern Great Tit (Parus major) population

Extra-pair paternity is common among socially monogamous bird species and considered an important driver of post-copulatory sexual selection on ejaculate traits including sperm traits. Patterns of extra-pair paternity and sperm size both show substantial variation among populations, yet we know little about the expression of these key reproductive traits at high latitudes. Here we report patterns of extra-pair paternity and describe variation in sperm dimensions in a Norwegian population of the socially monogamous Great Tit (Parus major) breeding beyond the polar circle at 69° northern latitude. Across six study years, we detected extra-pair paternity in 19.2% of 26 broods, and on average 4.7% of nestlings per brood were extra-pair offspring. As expected from results of previous intraspecific analyses of latitudinal variation in extra-pair paternity rates, the observed rate of extra-pair offspring was low in comparison to published estimates from more southern Great Tit populations (range: 2.9 − 20.4%). Our results therefore support a pattern of decreasing levels of extra-pair paternity with increasing latitude in this species also for extremely high latitudes. Overall mean sperm total length amounted to 97.5 ± 0.6 (SE) μm and 30.6% of the total phenotypic variation in sperm total length was explained by differences among sperm samples. The among-sample coefficient of variation in mean sperm total length per sample was 1.93%. Using previous comparative work as a yardstick, this value is substantially lower than expected for the observed frequency of 4.7% extra-pair offspring.