Data from: Male-biased sex allocation in ageing parents; a longitudinal study in a long-lived seabird

Optimal sex allocation is frequency-dependent, but senescence may cause behaviour at old age to be suboptimal. We investigated whether sex allocation changes with parental age, using 16 years of data, comprising >2,500 molecularly sexed offspring of >600 known-age parents in common terns (Sterna hirundo), a slightly sexually size-dimorphic seabird. We decomposed parental age effects into within-individual change and sex allocation-associated selective (dis)appearance. Individual parents did not differ consistently in sex allocation, but offspring sex ratios at fledging changed from female- to male-biased as parents aged. Sex ratios at hatching were not affected by parental age, suggesting sons to outperform daughters after hatching in broods of old parents. Our results call for the integration of sex allocation theory with theory on ageing and demography, since a change in sex allocation with age per se will cause the age structure of a population to affect the frequency-dependent benefits and the age-specific strength of selection on sex allocation.

最优性别分配（optimal sex allocation）具有频率依赖性（frequency-dependent），但衰老（senescence）可能导致老年个体的行为呈现次优（suboptimal）状态。本研究依托16年的观测数据，以存在轻微两性体型二态性的海鸟——普通燕鸥（Sterna hirundo）——为研究对象，探究性别分配是否会随亲本年龄发生改变。该数据集包含600余只已知年龄的亲本所产生的2500余只经分子生物学鉴定性别的（molecularly sexed）后代。我们将亲本年龄的效应拆解为个体内的年龄相关变化，以及与性别分配相关的选择性出现与消失（selective (dis)appearance）效应。单个亲本的性别分配并不存在稳定的个体差异，但随着亲本年龄增长，雏鸟出飞（fledging）时的后代性别比例从雌性偏倚转向雄性偏倚。雏鸟孵化（hatching）时的性别比例并未受亲本年龄的影响，这表明老年亲本所产育雏群中的雄性后代在孵化后的表现优于雌性后代。本研究结果表明，亟需将性别分配理论与衰老及种群统计学（demography）理论相结合，因为性别分配随年龄本身发生的变化，会使得种群年龄结构影响频率依赖的收益，以及性别分配所受的年龄特异性选择强度。