Data from: Multiple benefits of alloparental care in a fluctuating environment

Although cooperatively breeding vertebrates occur disproportionately in unpredictable environments, the underlying mechanism shaping this biogeographic pattern remains unclear. Cooperative breeding may buffer against harsh conditions (hard life hypothesis), or additionally allow for sustained breeding under benign conditions (temporal variability hypothesis). To distinguish between the hard life and temporal variability hypotheses, we investigated whether the number of alloparents at a nest increased reproductive success or load-lightening in superb starlings (Lamprotornis superbus), and whether these two types of benefits varied in harsh and benign years. We found that mothers experienced both types of benefits consistent with the temporal variability hypothesis, as larger contingents of alloparents increased the number of young fledged while simultaneously allowing mothers to reduce their provisioning rates under both harsh and benign rainfall conditions. In contrast, fathers experienced load-lightening only under benign rainfall conditions, suggesting that cooperative breeding may serve to take advantage of unpredictable benign breeding seasons when they do occur. Cooperative breeding in unpredictable environments may thus promote flexibility in offspring care behaviour, which could mitigate variability in the cost of raising young. Our results highlight the importance of considering how offspring care decisions vary among breeding roles and across fluctuating environmental conditions.

尽管合作繁殖脊椎动物在不可预测环境中的分布比例偏高，但塑造这一生物地理格局的潜在机制仍未明晰。合作繁殖既可缓冲恶劣环境带来的生存压力（艰苦生活假说（hard life hypothesis）），也能使种群在适宜环境中维持持续繁殖（时间变异性假说（temporal variability hypothesis））。为区分这两种假说，我们以辉椋鸟（Lamprotornis superbus）为研究对象，探究巢内异亲个体（alloparents）的数量是否能提升繁殖成功率或产生减负效应（load-lightening），以及这两类收益在恶劣与适宜年份中是否存在差异。我们发现，雌鸟同时获得了两类收益，这与时间变异性假说相符：无论在降雨匮乏的恶劣年份还是降雨充足的适宜年份，更多的异亲个体均能增加离巢幼鸟的数量，同时让雌鸟降低自身的育雏投喂率。与之相反，雄鸟仅在降雨充足的适宜年份中才会获得减负效应，这表明合作繁殖或许可帮助种群利用偶然出现的适宜繁殖季。由此可见，不可预测环境中的合作繁殖或许能提升育雏行为的灵活性，进而缓解育幼成本的波动。我们的研究结果强调，需考量不同繁殖角色的育雏决策如何随波动环境条件发生变化。