Data from: Extreme intraclutch egg-size dimorphism in Eudyptes penguins, an evolutionary response to clutch-size maladaptation

Eudyptes penguins (six species) are uniquely characterized by a two-egg clutch with extreme intraclutch egg-size dimorphism (ESD): the first-laid A-egg is 17.5%–56.9% smaller than the B-egg. Although A-eggs are viable, they almost never produce fledged chicks (genus average <1%). Using classical life-history theory and phylogenetic comparative methods, we demonstrate a marked slowdown in the life history of Eudyptes: age of first reproduction is 52% later and annual fecundity 48% lower compared with other two-egg clutch penguin species. All six Eudyptes species have retained a two-egg clutch, despite this pronounced life-history slowdown; this suggests evolutionary mismatch between clutch size and chicks fledged per clutch. Consistent with this, we show that Eudyptes fledge 43% fewer chicks per clutch than other two-egg clutch penguin species. Extreme intraclutch ESD in Eudyptes is associated primarily with a uniform (5%) increase in relative B-egg size, and B-egg size has evolved in accord with life history. We further show that intraclutch ESD is positively correlated with age of first reproduction in Eudyptes but not in other two-egg clutch penguin species. We argue that Eudyptes’ persistent failure to evolve a one-egg clutch constitutes a unique genus-wide evolutionary maladaptation and that extreme intraclutch ESD evolved as a correlated response to selection favoring a slower life history imposed by their extreme pelagic overwintering and migration ecology.

冠企鹅属（Eudyptes）企鹅共计6个物种，其最显著的特征为拥有双枚卵的窝卵数，且窝内卵大小二态性（intraclutch egg-size dimorphism，ESD）极强：首枚产出的A卵较B卵小17.5%~56.9%。尽管A卵具备可孵化性，但几乎从未成功育出离巢雏鸟（属内平均成功率不足1%）。本研究依托经典生活史理论与系统发育比较分析方法，证实冠企鹅属的生活史存在显著放缓现象：相较于其他双卵窝企鹅物种，其首次繁殖年龄晚52%，年繁殖力低48%。尽管生活史存在如此显著的放缓，6个冠企鹅物种仍保留了双卵窝的繁殖策略；这暗示窝卵数与每窝育雏数之间存在进化错配。与此结论一致，本研究发现冠企鹅每窝育雏数较其他双卵窝企鹅物种低43%。冠企鹅属极端的窝内卵大小二态性，主要源于各物种B卵的相对尺寸均匀提升5%，且B卵尺寸的进化符合其生活史特征。本研究进一步证实，冠企鹅属的窝内卵大小二态性与首次繁殖年龄呈正相关，但该关联并未在其他双卵窝企鹅物种中出现。我们认为，冠企鹅属始终未能进化出单卵窝繁殖策略，这是一种独特的全属性进化适应不良（evolutionary maladaptation）；而极端窝内卵大小二态性，乃是为响应其极端远洋越冬与迁徙生态所施加的、青睐较慢生活史的选择压力而演化出的关联进化结果。