Age-related breeding success in little penguins: A result of selection and ontogenetic changes in foraging and phenology

Reproductive performance typically improves with age, reaching a plateau at middle age and subsequently declining in older age classes (senescing individuals). Three potential non-exclusive mechanisms can explain the improvement in reproductive performance with age: (1) selection (poor quality individuals are removed from the population with increasing age), (2) constraint (individual efficiency increases through experience) and (3) restraint (reproductive investment increases with age as the residual reproductive value decreases). While all three mechanisms received strong empirical support, few studies have aimed at teasing apart those hypotheses and understanding their underlying functioning. In little penguins (Eudyptula minor), we used a 19-year longitudinal dataset on breeding and foraging of more than 450 individuals to investigate the effect of age on breeding success. We separated within- from among-individual age-effects using state-of-the-art statistical methods (within-subject centering and population change decomposition). We then assessed whether within-individual changes in breeding resulted from ontogenetic changes in foraging performances, breeding phenology or access to mates and nest sites. Fidelity and assortative pairing explained the high correlation in male and female ages within a pair. Breeding performances followed a typical bell-shaped curve with performance increasing up to 8 years-old, before reaching a plateau and subsequently declining after age 16. Both selection and within-individual processes occurred, although within-individual changes dominated differences in age-dependent breeding success. The selective appearance had almost no effect (apart from ages 2 to 3), and selective disappearance mostly affected changes at old ages (above 16), although they were also responsible for the slight increase in reproductive performances from ages 5 to 8. Focusing on within-individual changes, birds exhibited higher performances at middle ages, with birds foraging better, laying earlier and changing partner and nest less often. Their reproductive investment did not vary with age for females and slightly decreased for males. This supports the constraint hypothesis but not the restraint one. Finally, the increase in breeding performances at young ages was explained by the age-related increase in foraging performances during chick-rearing and advancement of laying. In contrast, reproductive senescence was defined by a general decrease in bird performances.

繁殖表现通常随年龄增长而提升，在中年阶段达到平台期，随后在高龄组（衰老个体）中逐渐下降。有三种非互斥的潜在机制可解释繁殖表现随年龄的提升：(1) 选择效应（劣质个体随年龄增长被种群淘汰）；(2) 约束效应（个体效率通过经验积累得到提升）；(3) 限制效应（随着剩余繁殖价值降低，繁殖投入随年龄增加）。尽管三种机制均获得了强有力的实证支持，但鲜有研究尝试厘清这些假说并解析其内在运作机制。
针对小蓝企鹅（Eudyptula minor），我们利用涵盖450余只个体、长达19年的繁殖与觅食纵向数据集，探究年龄对繁殖成功率的影响。我们运用前沿统计方法（个体内中心化与种群变化分解），区分了个体内年龄效应与个体间年龄效应。随后我们评估了个体内繁殖变化是否源于觅食表现、繁殖物候或配偶与巢址获取机会的个体发育变化。配偶忠诚性与选型配对解释了繁殖对中雌雄个体年龄的高度相关性。
繁殖表现呈现典型的钟形曲线：8岁前表现持续提升，随后进入平台期，16岁后逐渐下降。选择效应与个体内过程均存在，但个体内变化是年龄依赖的繁殖成功率差异的主导因素。选择性出现几乎无影响（2~3岁龄除外），而选择性消失主要影响高龄（16岁以上）的繁殖变化，不过其也促成了5~8岁龄繁殖表现的小幅提升。
聚焦个体内变化时，繁殖个体在中年阶段表现更优：觅食效率更高、产卵更早，且更少更换配偶与巢址。雌性的繁殖投入未随年龄发生变化，而雄性的繁殖投入则略有下降。这一结果支持约束假说，而非限制假说。
最后，幼年阶段繁殖表现的提升，可由育雏期觅食表现随年龄的提升以及产卵时间提前得到解释。与之相反，繁殖衰老则表现为个体繁殖表现的普遍下降。