Brood parasites that care: alternative nesting tactics in a subsocial wasp

Hosts and brood parasites are a classic example of conflict. Parasites typically provide no offspring care after laying eggs, imposing costs on hosts. Female subsocial wasps, Ammophila pubescens, alternated between initiating their own nests and an 'intruder' tactic of replacing eggs in nests of unrelated conspecifics. Hosts could respond by substituting new eggs of their own, with up to eight reciprocal replacements. Remarkably, intruders usually provisioned offspring in host nests, often alongside hosts. We used field data to investigate why intruders provision and to understand the basis of interactions. We found that intruders could not increase their fitness payoffs by using the typical brood parasite tactic of not provisioning offspring. Intruders using the typical tactic would benefit when hosts provisioned in their stead, but their offspring would starve when hosts failed to provision. Although some hosts obtained positive payoffs when intruders mistakenly provisioned their offspring, on average utilizing a conspecific nest represents parasitism: hosts pay costs while intruders benefit. Both females used the same tactic of egg replacement, but intruders more often laid the final egg. Selection should favour better discrimination of offspring, which could lead to repeated cycles of costly egg replacement.寄主与育雏寄生物是冲突关系的经典范例。寄生物通常在产卵后不会对后代提供抚育，进而给寄主施加生存成本。亚社会性雌性长背泥蜂（Ammophila pubescens）会在两种行为范式间交替：一是自主筑巢繁育，二是采取‘入侵者’策略，即替换非亲缘同种个体巢穴内的卵。寄主可通过替换自身新卵进行应对，最多可发生8次交替替换。值得注意的是，入侵者通常会在寄主巢穴中为后代提供育幼投喂，且常与寄主共同抚育后代。本研究利用野外实地数据，探究入侵者为何会开展育幼抚育，并解析此类交互行为的内在机制。研究发现，若入侵者采用典型育雏寄生物的不抚育后代策略，将无法提升自身的适合度收益。采取典型策略的入侵者，仅在寄主代为抚育后代时可获得收益，但倘若寄主未提供抚育，其后代便会饿死。尽管部分寄主会在入侵者误为其后代抚育时获得正向收益，但平均而言，利用同种个体巢穴本质上属于寄生行为：寄主承担成本，而入侵者从中获利。两类雌性个体均采用卵替换策略，但入侵者更常产下最后一枚卵。自然选择应会青睐更精准的后代识别能力，这可能会引发代价高昂的卵替换行为反复循环。