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.

凤头企鹅属（Eudyptes）共包含6个物种，其标志性特征为具有极端窝内卵大小二态性（intraclutch egg-size dimorphism, ESD）的双卵窝：第一枚产出的A卵较B卵小17.5%~56.9%。尽管A卵具备受精活力，但几乎无法成功育出出飞幼雏（该属平均成功率不足1%）。本研究依托经典生活史理论与系统发育比较方法，证实凤头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，其首次繁殖年龄延后52%，年繁殖力降低48%。尽管生活史显著放缓，该属全部6个物种均保留了双卵窝的繁殖策略，这暗示窝卵数与每窝出飞幼雏数之间存在进化错配。与此结论一致，本研究显示凤头企鹅属每窝出飞幼雏数较其他双卵窝企鹅物种低43%。凤头企鹅属的极端窝内卵大小二态性，主要与B卵相对尺寸均匀提升5%相关，且B卵尺寸的演化符合其自身生活史特征。本研究进一步证实，凤头企鹅属的窝内卵大小二态性与首次繁殖年龄呈正相关，但这一相关性在其他双卵窝企鹅物种中并未出现。本研究认为，凤头企鹅属始终未能演化出单卵窝繁殖策略，这是一种独特的属级进化适应不良；而极端的窝内卵大小二态性，乃是针对其极端远洋越冬与迁徙生态所施加的选择压力（青睐更缓慢的生活史）的关联演化结果。