Data from: Egg investment in response to the helper presence in cooperatively breeding Tibetan ground tits

Life-history theory predicts a trade-off between current and future reproduction to maximize lifetime fitness. In cooperatively breeding species, where offspring care is shared between breeders and helpers, helper presence may influence the female breeders' egg investment, and consequently, survival and future reproductive success. For example, female breeders may reduce egg investment in response to helper presence if this reduction is compensated by helpers during provisioning. Alternatively, female breeders may increase egg investment in response to helper presence if helpers allow the breeders to raise more or higher quality offspring successfully. In the facultatively cooperative-breeding Tibetan ground tit Pseudopodoces humilis, previous studies found that helpers improve total nestling provisioning rates and fledgling recruitment, but have no apparent effects on the number and body mass of fledglings produced, while breeders with helpers show reduced provisioning rates and higher survival. Here, we investigated whether some of these effects may be explained by female breeders reducing their investment in eggs in response to helper presence. In addition, we investigated whether egg investment is associated with the female breeder's future fitness. Our results showed that helper presence had no effect on the female breeders' egg investment, and that egg investment was not associated with breeder survival and reproductive success. Our findings suggest that the responses of breeders to helping should be investigated throughout the breeding cycle, because the conclusions regarding the breeders' adjustment of reproductive investment in response to being helped may depend on which stage of the breeding cycle is considered.

生活史理论（life-history theory）预测，生物需在当前繁殖与未来繁殖之间达成权衡，以实现终生适合度（lifetime fitness）的最大化。在合作繁殖物种中，育幼任务由繁殖者（breeders）与协助者（helpers）共同分担，协助者的存在可能会影响雌性繁殖者的卵投资（egg investment）策略，进而影响其存活与未来繁殖成功率。例如，若协助者在育雏投喂（provisioning）阶段能够弥补产卵投入的减少，雌性繁殖者可能会因协助者的存在而降低卵投资。反之，若协助者可帮助繁殖者成功抚育更多或质量更高的后代，雌性繁殖者则可能提高卵投资。针对兼性合作繁殖的地山雀（Pseudopodoces humilis），既往研究显示，协助者可提升总巢雏投喂率与离巢幼鸟种群招募率（fledgling recruitment），但对产出的离巢幼鸟的数量与体重无显著影响；而带有协助者的繁殖者，其育雏投喂率更低、存活率更高。本研究旨在探究，上述部分效应是否可由雌性繁殖者因协助者存在而降低卵投资这一机制所解释。此外，我们还探究了卵投资是否与雌性繁殖者的未来适合度存在关联。研究结果表明，协助者的存在对雌性繁殖者的卵投资无显著影响，且卵投资与繁殖者的存活率及繁殖成功率均无关联。本研究结果提示，应在整个繁殖周期内探究繁殖者对协助行为的响应，因为关于繁殖者因获得协助而调整繁殖投入的相关结论，可能取决于所考察的繁殖周期阶段。