Data from: The price of associating with breeders in the cooperatively breeding chestnut-crowned babbler: foraging constraints, survival and sociality

Understanding the costs of living with breeders might offer new insights into the factors that counter evolutionary transitions from selfish individuals to cooperative societies. While selection on early dispersal is well-understood, it is less clear whether costs are also associated with remaining with family members during subsequent breeding; a pre-requisite to the evolution of kin-based cooperation. We propose and test the hypothesis that living in groups containing breeders is costly and that such costs are exacerbated by increasing group size. For example, in group-living central-place foragers, group members might suffer from resource depletion when foraging in a restricted area during breeding and significant costs of repeatedly travelling between foraging patches and the site of offspring. Using the cooperatively breeding chestnut-crowned babbler (Pomatostomus ruficeps), for which grouping during breeding is obligatory, we show that reproduction is associated with substantially reduced foraging areas and evidence of resource depletion, particularly in larger groups. Such effects largely persisted from the onset of incubation through to offspring independence 4-5 months later. All group members, irrespective of their breeder or helper status, lost significant body mass over this period, and, in males, mass loss was associated with reduced inter-annual survival. Although babblers are constrained from living outside of breeding groups due to high risks of predation and the poor success of breeding without helpers, we suggest that the effects we describe may generally select against group-living during breeding attempts in species where constraints to independent breeding and costs of dispersal are less acute.

探究与繁殖者共居的代价，或可为理解对抗自私个体向合作社会演化转变的驱动因素提供全新视角。尽管学界已明晰早期扩散所受的选择压力，但对于后续繁殖阶段留居家族成员是否同样伴随生存代价，目前仍不甚明确——而这恰恰是基于亲缘关系的合作演化的先决条件。本研究提出并验证了一项假说：栖息于包含繁殖者的群体中需付出生存代价，且群体规模越大，此类代价愈为显著。例如，对于群居的中心地觅食者（central-place forager）而言，繁殖期在受限区域内觅食时，群体成员可能面临资源枯竭的问题，且反复往返于觅食斑块与育幼场所之间也会产生高额代价。本研究以繁殖期必须群居的合作繁殖鸟类——棕冠鹩鹛（Pomatostomus ruficeps）为研究对象，结果显示繁殖活动与觅食范围大幅缩减存在关联，且存在资源枯竭的迹象，这一现象在更大的群体中尤为明显。此类效应从孵卵开始，一直持续到幼鸟独立的4至5个月后。在此期间，所有群体成员——无论其为繁殖者还是协助者——均出现了显著的体重下降，且雄性个体的体重下降与年际存活率降低存在关联。尽管由于捕食风险较高且无协助者时繁殖成功率低下，棕冠鹩鹛无法脱离繁殖群体生存，但我们认为，本研究揭示的效应，通常会在独立繁殖限制与扩散代价相对较低的物种中，对繁殖期的群居行为产生选择抑制作用。