Data from: No evidence of inbreeding depression in sperm performance traits in wild song sparrows

Inbreeding is widely hypothesized to shape mating systems and population persistence, but such effects will depend on which traits show inbreeding depression. Population and evolutionary consequences could be substantial if inbreeding decreases sperm performance and hence decreases male fertilisation success and female fertility. However, the magnitude of inbreeding depression in sperm performance traits has rarely been estimated in wild populations experiencing natural variation in inbreeding. Further, the hypothesis that inbreeding could increase within-ejaculate variation in sperm traits, and thereby further affect male fertilisation success has not been explicitly tested. We used a wild pedigreed song sparrow (Melospiza melodia) population, where frequent extra-pair copulations likely create strong post-copulatory competition for fertilisation success, to quantify effects of male coefficient of inbreeding (f) on key sperm performance traits. We found no evidence of inbreeding depression in sperm motility, longevity or velocity, and the within-ejaculate variance in sperm velocity did not increase with male f. Contrary to inferences from highly inbred captive and experimental populations, our results imply that moderate inbreeding will not necessarily constrain sperm performance in wild populations. Consequently, the widely observed individual-level and population-level inbreeding depression in male and female fitness may not stem from reduced sperm performance in inbred males.近交（inbreeding）被学界广泛假设会影响交配系统与种群存续，但这类效应的强弱取决于哪些性状会出现近交衰退（inbreeding depression）。若近交降低精子性能，进而削弱雄性受精成功率与雌性生育力，那么其种群与进化后果可能极为显著。然而，针对自然近交水平存在变异的野生种群，学界极少估算精子性能相关性状的近交衰退程度。此外，“近交可能提升射精内精子性状的变异水平，进而进一步影响雄性受精成功率”这一假说尚未得到明确检验。我们依托一个具有完整谱系记录的野生歌带鹀（Melospiza melodia）种群——该种群频繁发生额外配对交配，大概率会引发针对受精成功的强烈交配后竞争——来量化雄性近交系数（f）对关键精子性能性状的影响。研究未发现精子活力、寿命或运动速度存在近交衰退的证据，且精子速度的射精内变异并未随雄性近交系数升高而增加。与高度近交的圈养及实验种群的研究推论相悖，本研究结果表明，适度近交未必会限制野生种群的精子性能。因此，学界广泛观测到的雄性与雌性适合度层面的个体及种群水平近交衰退，未必源于近交雄性的精子性能下降。