Separating differential allocation by females from direct effects of male condition in a beetle

Differential allocation is the adjustment of reproductive allocation, typically by a female, in response to the quality of her male partner. A recent theoretical model suggests that differential allocation may influence trade-offs between reproductive traits within a breeding attempt. Furthermore, it is often difficult to distinguish differential allocation from direct effects of male condition. We address these gaps using a novel cross-fostering design to exclude direct effects of male condition and to test whether differential allocation affects trade-offs between and within breeding attempts. This design detects differential allocation as effects of a female’s mating partner and direct effects of male condition as effects of the larvae’s sire. We used the burying beetle Nicrophorus vespilloides, a species which adjusts reproductive allocation by culling some larvae after hatching. We used food-deprivation to manipulate the nutritional condition of both the female’s mating partner and the larvae’s sire. We find clear evidence for differential allocation as females mating with food-deprived males had fewer larvae than females mating with control males. There was a trade-off between number and size of larvae when females mated with control males, but a positive relationship when females mated with food-deprived males. Thus, differential allocation influenced relationships between reproductive traits within a breeding attempt, but not necessarily through trade-offs. Instead, we suggest that there may be cryptic heterogeneity in quality among females or their mating partners that was only exposed when females mated with a male in poor condition.

差异繁殖分配（differential allocation）指的是雌性个体根据其配偶的质量调整自身繁殖分配的行为。近期的理论模型表明，差异繁殖分配可能会影响单次繁殖事件中繁殖性状间的权衡关系。此外，研究者往往难以区分差异繁殖分配与雄性自身状态的直接效应。本研究通过一种新颖的交叉抚育实验设计（cross-fostering design）填补了上述研究空白，该设计可排除雄性自身状态的直接效应，并用于检验差异繁殖分配是否会影响不同繁殖事件间以及单次繁殖事件内部的性状权衡。该实验设计将雌性配偶的效应作为差异繁殖分配的检测指标，将幼虫父本的效应作为雄性自身状态直接效应的检测指标。本研究选取葬甲（Nicrophorus vespilloides）作为实验对象，该物种会在幼虫孵化后通过剔除部分幼虫来调整繁殖分配。我们通过食物剥夺处理操控雌性配偶以及幼虫父本的营养状态。研究结果为差异繁殖分配提供了明确的证据：与经过食物剥夺处理的雄性交配的雌性，其产下的幼虫数量显著少于与对照雄性交配的雌性。当雌性与对照雄性交配时，幼虫数量与幼虫体型间呈现权衡关系；而当雌性与食物剥夺处理的雄性交配时，二者则呈现正相关关系。由此可见，差异繁殖分配会影响单次繁殖事件内繁殖性状间的关联，但未必是通过权衡关系实现的。我们推测，雌性个体或其配偶的质量可能存在未被观测到的异质性，这种异质性仅在雌性与状态较差的雄性交配时才会显现出来。