Breaking bonds: maternal and offspring state shape constraint-based brood adoption in a seaduck

The adaptiveness of adoption for donors, recipients, and offspring remains
unsettled despite long-standing interest. Using decade-long data on
individually marked female and duckling common eiders (Somateria
mollissima), displaying frequent alloparenting, we examined factors
influencing the likelihood of females donating young and ducklings being
adopted. We explored how donor traits, including maternal body condition,
relative head size (a proxy of relative brain size), and relative hatching
date, and offspring characteristics such as body condition relative to
siblings, affect these processes. At least one offspring was permanently
adopted in 34.7% of brood observations. Females in better body condition
and larger relative head size were less likely to donate offspring, while
the likelihood of offspring transfer was higher in larger natal broods.
The odds of offspring donation peaked just before the population’s
hatching peak, suggesting that recipient availability influences adoption.
Ducklings in poorer body condition than their natal broodmates and those
whose mothers were in lower body condition were, respectively,
significantly and marginally significantly more likely to be adopted.
Offspring transfers may thus result from physiological and cognitive
constraints, rather than reflecting a fitness-maximizing strategy.
Donating young and becoming adopted ultimately align with a salvage
strategy for poor-condition donors and poor-condition offspring, but are
likely driven by constraints rather than active tactics. Multiple tenders
in broods prevented unique recipient identification, yet prior research
suggests that recipients may accrue fitness benefits. Future research
quantifying the fitness consequences for all parties in different
environmental contexts is required for a more comprehensive understanding
of alloparental behavior. Keywords: alternative reproductive tactics, body
condition, cognitive constraints, common eider, conspecific brood
parasitism, salvage strategy

尽管学界长期关注，但捐赠者、收养者与幼崽的收养行为适应性仍未明确。本研究依托长达十年的个体标记普通绒鸭（Somateria mollissima）雌性成体与雏鸭观测数据——该物种频繁表现出异亲抚育（alloparenting）行为——探究了影响雌性成体捐赠幼崽、雏鸭被收养概率的各类因素。我们分析了捐赠者特征（包括母体身体状况、相对头围（作为相对脑容量的替代指标）、相对孵化日期）以及幼崽特征（如相较于同窝同伴的身体状况）如何作用于上述过程。在34.7%的窝巢观测中，至少有1只幼崽被永久收养。身体状况更佳、相对头围更大的雌性成体捐赠幼崽的概率更低；而自身原生窝规模越大，幼崽被转移（收养）的概率越高。幼崽捐赠的发生概率在种群孵化高峰来临前达到峰值，这表明收养者的可获得性会影响收养行为的发生。相较于同窝同伴身体状况更差的雏鸭，以及母体身体状况较差的雏鸭，被收养的概率分别显著升高和边缘显著升高。由此可见，幼崽转移行为或许源于生理与认知限制，而非体现出适合度最大化策略。捐赠幼崽与被收养行为，最终与身体状况不佳的捐赠者和幼崽的补救策略相契合，但这类行为更可能由限制因素驱动，而非主动的生存策略。窝巢中存在多个抚育者，导致无法唯一确定收养者身份；但已有研究表明，收养者可获得适合度收益。未来需开展研究，量化不同环境背景下各方的适合度收益，以更全面地理解异亲抚育行为。关键词：替代繁殖策略、身体状况、认知限制、普通绒鸭、种内巢寄生、补救策略