Full compensation of parental care after partner loss in an annelid

In species with biparental care, each parent would benefit by exploiting its partner and reduce its investment over offspring care. The conflict is especially apparent when one parent deserts or reduces its investment, forcing the remaining parent to either abandon the brood or compensate for the loss of partner-care effort; however, compensation is expected to be partial for the evolutionary stability of biparental care. We investigated the level of parental care compensation in the simultaneously hermaphroditic annelid worm, Ophryotrocha diadema, where both parents care for egg clutches. We removed one parent and found that widowed parents fully compensated for the loss of their partner; the number of scan observations widows spent in clutch attendance was statistically equivalent to that of paired parents. Consistently, individually, widowed parents attended their clutch significantly more often than focal worms among paired parents. We discuss how full compensation can be compatible with evolutionary stability of biparental care in a rare species with sparse population and whose adaptations suggest that mate encounter might be rare. In this condition, nest attendance might be valuable as it promotes clutch protection from predators as well as mate encounters thus mitigating the conflict of interests over parental care.

在双亲育幼（biparental care）的物种中，每个亲代均可通过利用伴侣、降低自身对子代抚育的投入而获得演化收益。当一方亲代弃巢或减少抚育投入时，亲代间的冲突尤为凸显——这会迫使剩余亲代要么放弃整个卵群，要么弥补伴侣缺失的抚育工作量；不过从双亲育幼的进化稳定性角度来看，这种弥补通常是不完全的。我们针对同步雌雄同体（simultaneously hermaphroditic）的环节动物Ophryotrocha diadema的双亲育幼行为，探究了其亲代抚育补偿水平。该物种的两个个体均可承担卵簇抚育工作。我们移除其中一方亲代后发现，丧偶亲代完全弥补了伴侣缺失的抚育工作量：丧偶亲代用于看护卵簇的扫描观察（scan observation）次数，在统计学上与成对亲代的看护次数无显著差异。与此一致的是，从个体层面来看，丧偶亲代的卵簇看护频率显著高于成对亲组中的目标亲代个体。我们讨论了完全补偿如何与双亲育幼的进化稳定性兼容：该物种种群稀疏且罕见，其适应性特征表明配偶相遇概率极低。在此情境下，亲代留守巢穴可同时实现双重收益——既保护卵簇免受天敌侵扰，又能提升配偶相遇的可能，从而缓解亲代抚育中的利益冲突。