Why do some younger individuals breed themselves, whereas other counterparts help? This question is addressed using 15 years of data from a Tibetan bird. The finding—kin neighbourhoods facilitate younger males to acquire a territory and breed independently—explains how the kin-based, facultative cooperative societies form in vertebrates. (Illustration: Y. Chen; Photo: D. Ke).

Abstract

In cooperatively breeding birds, why do some individuals breed independently but others have to help at home? This question has been rarely addressed despite its fundamental importance for understanding the evolution of social cooperation. We address it using 15 years of data from Tibetan ground tits Pseudopodoces humilis where helpers consist of younger males. Since whether younger males successfully breed depends critically on their chances to occupy territories nearby home, our analytic strategy is to identify the determinants of individual differences in gaining territory ownership among these ready-to-breed males. Across widowed, last-year helper and yearling males, an age advantage was evident in inheriting resident territories, occupying adjacent vacancies and budding off part of adjacent territories, which left some last-year helpers and most yearling males to take the latter two routes. These males were more likely to acquire a territory if they were genetically related to the previous or current territory owners; otherwise they remained on natal territories as helpers. The relatedness effect can arise from the prior residence advantage established in the preceding winter when younger males followed their parents to perform kin-directed off-territory forays. Our research highlights the key role of local kinship in determining younger males' territory acquisition and thus their fate in terms of independent reproduction versus help. This finding provides insight into the formation of kin-based, facultative cooperative societies prevailing among vertebrates.

摘要

在合作繁殖鸟类中，为什么一些性成熟的个体独立繁殖，而其它被迫留家帮助?尽管这一问题对于理解社会进化至关重要，但迄今没有得到充分回答。 我们使用来自青藏高原特有物种地山雀15年的野外数据，企图回答这一问题。研究种群中，帮助者都是年轻的雄性。这些个体的繁殖成功，取决于其是否能在家族领域附近获得自己的领域。因此，我们的分析策略就是识别决定繁殖雄性个体获得领域的关键因素。 在继承领域、占据相邻空缺领域和瓜分相邻领域方面，丧偶雄性、上年帮助者和一龄雄性的优势依次降低。这使得一些上年帮助者和大多数一龄雄性不得不采取后两种获得领域的途径。我们发现，如果这些年轻雄性与相邻空缺的前任、或被挤占领域的现任拥有者为亲属，他们就更可能获得领域而独立繁殖；否则便沦为留家的帮助者。这种亲属效应来自冬季年轻雄性跟随父亲频繁访问相邻亲属领域而预先建立的对潜在未来领域的熟悉性。 我们的研究凸显了邻里亲缘关系在决定年轻雄性能否获得领域、进而独立繁殖或成为帮助者的关键作用。该发现增进了人们对普遍存在于脊椎动物中以亲缘关系为基础的兼性合作社会形成机制的理解。