Data from: Differential maternal testosterone allocation among siblings benefits both mother and offspring in the Zebra Finch Taeniopygia guttata

Parents are selected to preferentially invest in the offspring with highest reproductive value. One mechanism to achieve this is the modification of competitive asymmetries between siblings by maternal hormones. In many organisms, offspring value varies according to birth position in the brood, which determines survival chances and competitive advantage over access to resources. In birds, variation in yolk androgen allocation over the laying sequence is thought to modulate dominance of senior chicks over junior brood mates. We tested this hypothesis in zebra finches, which show a natural decreasing pattern of within-clutch testosterone allocation. We abolished these within-clutch differences by experimentally elevating yolk testosterone levels in eggs 2-6 up to the level of the egg 1, and assessed fitness measures for junior (eggs 2-6) and senior (egg 1) offspring, and their mothers. T-injected eggs hatched later than control eggs. Junior but not senior chicks in T broods attained poorer phenotypic quality compared with control broods, which was not compensated by positive effects on seniors. Mothers were generally unaffected by clutch treatment. Thus, naturally decreasing within-clutch yolk testosterone allocation appears to benefit all family members, and does not generally enhance brood reduction by favoring senior chicks, in contrast to widely held assumption.

亲代倾向于优先将投资投向繁殖价值（reproductive value）最高的后代。实现这一投资策略的机制之一，是通过母体激素（maternal hormones）调整同胞个体间的竞争不对称性（competitive asymmetries）。在众多生物类群中，后代的价值取决于其在一窝卵中的出生顺位，该顺位直接决定后代的存活概率以及获取资源时的竞争优势。在鸟类中，学界普遍认为，产蛋序列中卵黄雄激素分配（yolk androgen allocation）的差异，可调控年长雏鸟相较于年幼同胞的竞争主导地位。我们以斑胸草雀（zebra finches）为实验对象，该物种的窝内睾酮（testosterone）分配天然呈现递减模式。我们通过实验将第2至6枚卵的卵黄睾酮水平提升至第1枚卵的水平，以此消除窝内的睾酮分配差异，并对年幼（第2-6枚卵）、年长（第1枚卵）后代及其母本的适合度指标（fitness measures）进行了评估。实验结果显示，睾酮注射组的卵孵化时间晚于对照组卵。睾酮处理组中的年幼雏鸟（而非年长雏鸟），相较于对照组雏鸟表现出更差的表型质量，且该劣势未通过对年长雏鸟的正向效应得到弥补。母本整体未受窝卵处理的影响。综上，天然的窝内卵黄睾酮递减分配模式似乎对所有家族成员均有益处，且并未如学界广泛持有的假设那样，通过偏向年长雏鸟来促进窝雏淘汰（brood reduction）。