Institute of Ecology and Earth Sciences, University of Tartu, Tartu, 51014, Estonia.
Latvian Biomedical Research and Study Centre, Rīga, 1067, Latvia.
Sci Rep. 2020 Mar 4;10(1):4005. doi: 10.1038/s41598-020-60144-w.
Only dominant individuals have unrestricted access to contested resources in group-living animals. In birds, subordinates with restricted access to resources may respond to intragroup contests by acquiring extra body reserves to avoid periods of food shortage. In turn, higher body mass reduces agility and increases predation and mortality risk to subordinates. Birds often live in hierarchically organized mixed-species groups, in which heterospecific individuals are considered to substitute for conspecifics as protection against predators at a significantly reduced competition cost. Crested tits (Lophophanes cristatus) and willow tits (Poecile montanus) form mixed-species groups during the non-reproductive season that typically exhibit a nearly linear dominance hierarchy ('despotic' social structure) in which the highest ranking male willow tit is fourth in the overall hierarchy after the dominant male, female and subordinate juvenile crested tit, respectively. Much less frequently, 'egalitarian' dominance structures occur in which the adult willow tits rank second and the hierarchy is less steep, or linear. We present a rare long-term data set in which egalitarian flocks are common enough to assess the consequences of this simple change in hierarchy structure as well as a potential driver of the pattern. A comparison of individuals in the despotic mixed-species groups revealed a strong negative correlation between subcutaneous fat stores and dominance rank in the interspecific dominance hierarchy, whereas in egalitarian groups, subordinate willow tits had significantly lower fat reserves and they foraged in safer parts of the canopy than willow tits in despotic groups. Moreover, egalitarian groups exhibited markedly less within-group aggression, higher group cohesion and improved winter survival in both tit species. However, winter survival of birds in egalitarian groups was impaired relative to despotic groups in forests recently affected by industrial forestry. This suggests that the more egalitarian bird societies may best be adapted to less-disturbed environments.
只有优势个体才能不受限制地获得群居动物中具有争议性的资源。在鸟类中，资源获取受限的下属个体可能会通过获得额外的身体储备来应对群体内竞争，以避免食物短缺时期的到来。相反，更高的体重会降低敏捷性，并增加下属个体被捕食和死亡的风险。鸟类经常生活在等级组织的混合物种群体中，其中同种个体被认为可以替代同种群体，以显著降低竞争成本来抵御捕食者。在非繁殖季节，凤头䴙䴘（Lophophanes cristatus）和普通朱雀（Poecile montanus）会形成混合物种群体，这些群体通常表现出近乎线性的优势等级制度（“专制”社会结构），其中排名最高的雄性普通朱雀在整个等级制度中分别排在优势雄性、雌性和下属幼年凤头䴙䴘之后，位列第四。但也存在更为罕见的“平等主义”等级结构，其中成年普通朱雀排名第二，等级制度较为平缓或线性。我们提供了一个罕见的长期数据集，其中平等主义的鸟群数量足够多，足以评估这种简单的等级结构变化的后果以及这种模式的潜在驱动因素。对专制混合物种群体中的个体进行比较后发现，种间优势等级制度中，皮下脂肪储备与优势等级之间呈强烈负相关，而在平等主义群体中，下属普通朱雀的脂肪储备明显较低，它们在树冠的安全区域觅食的次数多于专制群体中的普通朱雀。此外，平等主义群体表现出明显较少的群体内攻击行为、更高的群体凝聚力以及两种䴙䴘在冬季的存活率都得到了提高。然而，在最近受到工业林业影响的森林中，平等主义群体中鸟类的冬季存活率相对于专制群体有所下降。这表明，更为平等主义的鸟类社会可能最适应未受干扰的环境。
