Data from: Alien eggs in duck nests: brood parasitism or a help from Grandma?

Intraspecific brood parasitism (IBP) is a remarkable phenomenon by which parasitic females can increase their reproductive output by laying eggs in conspecific females' nests in addition to incubating eggs in their own nest. Kin selection could explain the tolerance, or even the selective advantage, of IBP, but different models of IBP based on game theory yield contradicting predictions. Our analyses of 7 polymorphic autosomal microsatellites in two eider duck colonies indicate that relatedness between host and parasitizing females is significantly higher than the background relatedness within the colony. This result is unlikely to be a by-product of relatives nesting in close vicinity, as nest distance and genetic identity are not correlated. For eider females which had been ring-marked during the decades prior to our study, our analyses indicate that (i) the average age of parasitized females is higher than the age of non-parasitized females, (ii) the percentage of nests with alien eggs increases with age of nesting females, (iii) the level of IBP increases with the host females' age, and (iv) the number of own eggs in the nest of parasitized females significantly decreases with age. IBP may allow those older females unable to produce as many eggs as they can incubate to gain indirect fitness without impairing their direct fitness: genetically-related females specialize in their energy allocation, with young females producing more eggs than they can incubate and entrusting these to their older relatives. Intraspecific brood parasitism in ducks may constitute cooperation among generations of closely-related females.

种内巢寄生（Intraspecific brood parasitism, IBP）是一类特殊的繁殖现象：寄生雌性个体除在自身巢穴中孵育卵粒外，还可将卵产于同种雌性的巢穴内，以此提升自身繁殖产出。亲缘选择理论可解释宿主对寄生者的容忍乃至选择优势，但基于博弈论构建的不同种内巢寄生模型却得出了相互矛盾的预测。我们对两个绒鸭（eider duck）繁殖群的7个多态性常染色体微卫星位点的分析结果显示：宿主雌性与寄生雌性间的亲缘关系显著高于种群内的背景亲缘水平。该结果不太可能是亲缘个体就近筑巢带来的副产物，因为巢间距与遗传同一性并无相关性。针对本研究开展前数十年间已被环志标记的绒鸭雌性个体，我们的分析表明：（i）被寄生雌性的平均年龄高于未被寄生的雌性；（ii）带有外来卵的巢穴占比随筑巢雌性的年龄增长而升高；（iii）种内巢寄生的发生水平随宿主雌性的年龄增加而提升；（iv）被寄生巢穴内自身卵的数量随雌性年龄增长显著减少。种内巢寄生或可使那些无法产出足够卵粒以满足自身孵卵需求的年长雌性，在不损害自身直接适合度的前提下获得间接适合度：具有亲缘关系的雌性个体在能量分配上形成特化，年轻雌性产出超出自身孵卵能力的卵，并将这些卵托付给年长亲属。鸭类的种内巢寄生或许代表了近缘雌性不同世代间的合作行为。