Datafiles and code for Covas et al: The oxidative cost of helping and its minimisation in a cooperative breeder

Cooperative actions are beneficial to the group, but presumably costly to the individual co-operators. In cooperatively breeding species, helping to raise young is thought to involve important energetic costs, which could lead to elevated exposure to reactive oxygen species, resulting in oxidative stress. However, identifying such costs can be difficult if individuals adjust their investment in helping in relation to environmental conditions or their own physiological condition. Experimental approaches are therefore required to quantify the costs of helping but, to date, these have been infrequent. Here, we combined correlational and experimental data to investigate the oxidative cost of helping-at-the-nest and how this affects helping decisions in wild sociable weavers Philetairus socius, a colonial cooperatively breeding bird. At the correlational level, we found that the probability of helping was influenced by the interaction of an individual’s oxidative state and age: compared to younger birds, older individuals were more likely to help when they had higher oxidative damage, and the opposite trend was found for younger individuals. After experimentally increasing the energetic cost of flight, manipulated helpers in breeding colonies decreased nestling feeding rates and incurred an increase in oxidative damage, which was not present in manipulated helpers in non-breeding colonies. This indicates that individuals decreased their helping behaviour to minimise the associated costs. These results suggest that oxidative stress can influence helping decisions and underlie a trade-off between cooperation and self-maintenance, which is central to understanding when helping might take place in this and other species.

合作行为对群体有益，但对个体合作者而言通常需付出相应代价。在合作繁殖（cooperative breeding）物种中，育幼帮扶行为被认为会产生显著的能量成本，该成本可能导致个体暴露于活性氧物种（reactive oxygen species）的水平升高，进而引发氧化应激。但若个体可根据环境条件或自身生理状态调整帮扶投入，则此类成本的量化识别将颇具难度。因此，量化帮扶成本需采用实验方法，但迄今为止此类研究仍较为匮乏。本研究结合关联数据与实验数据，针对野生群居合作繁殖鸟类群居织巢鸟（Philetairus socius），探究了其巢边帮扶行为的氧化应激成本，以及该成本如何影响个体的帮扶决策。关联分析结果显示，个体的帮扶概率受其氧化状态与年龄的交互作用影响：相较于年轻个体，年长个体在氧化损伤程度更高时更倾向于实施帮扶，而年轻个体则呈现出相反的趋势。在通过实验提升飞行能量消耗后，繁殖群体中的实验处理帮扶者降低了雏鸟饲喂频率，且氧化损伤水平显著升高；而非繁殖群体中的实验处理帮扶者则未出现此类变化。这表明个体会通过降低帮扶行为的投入，以最小化相关的生理成本。上述结果表明，氧化应激可影响帮扶决策，且构成了合作行为与自我维持之间的权衡——这一权衡是理解该物种乃至其他物种何时会发生帮扶行为的核心所在。