Cooperative breeding favours maternal investment in size over number of eggs in spiders

The transition to cooperative breeding may alter maternal investment strategies depending on density of breeders, extent of reproductive skew and allo-maternal care. Change in optimal investment from solitary to cooperative breeding can be investigated by comparing social species with non-social congeners. We tested two hypotheses in a mainly semelparous system: that social, cooperative breeders, compared to subsocial, solitarily breeding congeners, 1) lay fewer and larger eggs because larger offspring compete better for limited resources and become reproducers; 2) induce egg size variation within clutches as a bet-hedging strategy to ensure that some offspring become reproducers. Within two spider genera, Anelosimus and Stegodyphus, we compared species from similar habitats and augmented the results with a mini-meta-analysis of egg numbers depicted in phylogenies. We found that social species indeed laid fewer, larger eggs than subsocials, while egg size variation was low overall, givi...

向合作繁殖（cooperative breeding）的转变，会根据繁殖者密度、生殖偏倚（reproductive skew）程度与异亲抚育（allo-maternal care）的规模，改变母本投资策略。若要探究从独居繁殖转向合作繁殖后的最优投资变化，可通过比较社会性物种与非社会性近缘类群来开展研究。我们在以单次生殖（semelparous）类群为主的系统中验证了两项假说：相较于亚社会性（subsocial）、独居繁殖的近缘类群，社会性合作繁殖类群将呈现如下特征：1）产下更少但体积更大的卵，因为体型更大的后代在有限资源的竞争中更具优势，更易成长为繁殖个体；2）在卵簇（clutches）内诱导卵大小变异，以此作为风险分摊策略（bet-hedging），保障部分后代能够成为繁殖个体。我们选取了阿内蛛属（Anelosimus）与Stegodyphus属两个蜘蛛类群中栖息环境相似的物种进行比较，并通过对系统发育树（phylogenies）中记载的卵数量开展小型元分析以补充研究结果。研究发现，社会性物种确实比亚社会性物种产出更少且更大的卵，而整体的卵大小变异程度较低，