Data from: Within-female plasticity in sex allocation is associated with a behavioral polyphenism in house wrens

Sex-allocation theory assumes individual plasticity in maternal strategies, but few studies have investigated within-individual changes across environments. In house wrens, differences between nests in the degree of hatching synchrony of eggs represent a behavioral polyphenism in females, and its expression varies with seasonal changes in the environment. Between-nest differences in hatching asynchrony also create different environments for offspring, and sons are more strongly affected than daughters by sibling competition when hatching occurs asynchronously over several days. Here, we examined variation in hatching asynchrony and sex allocation, and its consequences for offspring fitness. The number and condition of fledglings declined seasonally, and the frequency of asynchronous hatching increased. In broods hatched asynchronously, sons, which are over-represented in earlier-laid eggs, were in better condition than daughters, which are over-represented in later-laid eggs. Nonetheless, asynchronous broods were more productive later within seasons. The proportion of sons in asynchronous broods increased seasonally, whereas there was a seasonal increase in the production of daughters by mothers hatching their eggs synchronously, which was characterized by within-female changes in offspring sex and not by sex-biased mortality. As adults, sons from asynchronous broods were in better condition and produced more broods of their own than males from synchronous broods, and both males and females from asynchronous broods had higher lifetime reproductive success than those from synchronous broods. In conclusion, hatching patterns are under maternal control, representing distinct strategies for allocating offspring within broods, and are associated with offspring sex ratios and differences in offspring reproductive success.

性分配理论（Sex-allocation theory）假定母方策略具备个体可塑性，但目前鲜有研究探讨个体内性状随环境的变化规律。针对家鹪鹩（house wrens）而言，不同繁殖窝之间的卵孵化同步性差异，体现了雌鸟的行为多态性（behavioral polyphenism），且该性状的表达随环境的季节变化而改变。繁殖窝间的孵化异步性差异同样会为子代创设差异化的生长环境；当孵化过程持续数日呈异步状态时，同胞竞争（sibling competition）对雄性子代的影响显著强于雌性。本研究针对孵化异步性与性分配的变异特征，及其对子代适合度（offspring fitness）的影响展开了系统探究。研究结果显示，离巢幼鸟（fledglings）的数量与身体状况随季节推移逐渐下降，而孵化异步性的发生频率则随季节升高。在异步孵化的窝卵中，前期产出的卵中雄性子代占比偏高，这类雄性幼鸟的身体状况优于后期产出卵中的雌性幼鸟（后者在后期卵中占比更高）。尽管如此，季节后期的异步孵化窝繁殖成效更佳。异步孵化窝中的雄性子代比例随季节逐步升高，而采用同步孵化策略的雌鸟，其所产雌性后代的数量随季节增加；该现象由雌鸟自身子代性比的个体内变化所驱动，而非性偏倚的死亡率所致。作为成体时，异步孵化窝产出的雄性个体身体状况更佳，且比同步孵化窝产出的雄性拥有更多的自主繁殖窝数；同时，异步孵化窝产出的雌雄个体的终身繁殖成功率（lifetime reproductive success）均高于同步孵化窝的对应个体。综上，孵化模式受母方调控，代表了窝内子代分配的独特策略，且与子代性比（sex ratios）以及子代繁殖成功率的差异密切相关。