Cooperative work in social birds: how is a tragedy of the commons averted?

合作行为的进化是进化生物学中一个基本而持久的问题。自然选择预计会选择自私的行为，因为个体应该寻求最大限度地提高自己的繁殖成功率，以便在下一代留下更多的后代。因此，当个人在共同任务中合作时，不可避免地会在每个人应该多努力工作的问题上产生冲突：每个人都希望别人工作得更努力，而自己做得更少。因此，在社会群体中，合作行为很容易受到自私的个人或吃白食者的剥削。公共产品游戏抓住了悖论的本质：由合作个体组成的群体应该胜过由自私个体组成的群体，但在群体中，自私个体比合作个体表现得更好。这种社会困境被称为“公地悲剧”，它几乎存在于所有生物系统中，包括人类社会，其中过度开发鱼类等公共资源或未能遏制碳排放是明显的例子。大量的理论文献表明，在大多数情况下，自私取代了合作，但在某些情况下，合作行为可能保持不变。这些包括：1；当不合作的个人受到惩罚或通过执法系统被迫合作时。2. 当个体必须通过合作获得良好的声誉时，才能最大限度地提高自身的适应性。3. 当合作对亲属有利时，合作者通过亲缘选择获得适合度利益。本研究的目的是利用南部非洲社会编织者独特的社区繁殖系统来测试这些替代假设，以解决公地悲剧。群居编织者建造大量的公共巢穴，其中一个巢穴群为多达数百只鸟提供栖息和繁殖场所。公共鸟巢有两个不同但整合的组成部分：一个茅草屋顶，具有支持和温度调节功能；以及嵌在茅草下面的单独的巢室，在那里成对或小群的个体栖息和繁殖。一个关键特征是穹顶不是一个自组织结构，因为它不是投资于单个巢室的紧急属性。因此，群体成员必须投入时间和精力来建造和维护公共圆顶以及他们自己的巢室。社会编织者如何解决个人之间在合作投资上的冲突，并避免导致社会崩溃的公地悲剧？这项研究将首先确定群体成员对公共产品的个人贡献，并确定这种努力是否昂贵。公共产品的功能也将通过将茅草的温度调节特性与茅草的大小和巢室的位置相关联来量化。最后，也是最重要的是，本研究将使用实地观察和实验来测试是否通过与亲属合作来避免公地悲剧，通过其他群体成员强制合作工作，或者因为合作工作是向其他群体成员发出的一个信号，表明个体的品质与个体自身的繁殖成功相关。先前对社会编织者的研究表明，研究计划是容易处理的，项目目标是可以实现的。这项研究将与开普敦大学的珀西·菲茨帕特里克非洲鸟类学研究所合作，在南非金伯利的一个已建立的社会编织者研究种群中进行。由于生物系统中公地悲剧的普遍存在，本研究的结果将在广泛的学科范围内具有广泛的相关性。

The evolution of cooperative behaviour is a fundamental and persistent problem in evolutionary biology. Natural selection is expected to select for selfish behaviour because individuals should seek to maximise their own reproductive success to leave more offspring in the next generation. Therefore, when individuals cooperate in joint tasks, there is inevitably conflict over how hard each should work: every individual would like others to work harder and to do less themselves. Hence, in social groups, cooperative behaviour will be vulnerable to exploitation by selfish individuals, or freeloaders. Public goods games capture the essence of the paradox: groups composed of cooperating individuals should out-compete groups of selfish individuals, but selfish individuals do better than cooperative ones within groups. This social dilemma is known as the tragedy of the commons and it is found in virtually all biological systems, including human societies, where over-exploitation of common resources such as fish stocks or failure to curb carbon emissions are obvious examples. An extensive theoretical literature shows that selfishness replaces cooperation under most conditions, but that cooperative behaviour may be maintained under certain circumstances. These include: 1. When non-cooperating individuals are punished or coerced into cooperation through a system of enforcement. 2. When individuals must achieve a good reputation through cooperation in order to maximise their own fitness. 3. When cooperation benefits relatives so that cooperators gain fitness benefits through kin selection. The aim of this study is to use the unique communal breeding system of the sociable weaver of southern Africa to test these alternative hypotheses for resolution of a tragedy of the commons. The sociable weaver builds massive communal nests in which a single nest mass provides roosting and breeding sites for up to several hundred birds. The communal nest has two distinct but integrated components: a thatched dome that has a supportive and thermoregulatory function; and individual nest chambers embedded beneath the thatch in which pairs or small groups of individuals roost and breed. A key feature is that the dome is not a self-organising structure because it is not an emergent property of investment in individual nest chambers. Thus, colony members must invest time and effort in building and maintaining the communal dome as well as their own nest chamber. How do sociable weavers resolve the conflict among individuals over cooperative investment and avoid a tragedy of the commons that would result in social collapse? This study will first determine the individual contributions made by colony members to public goods, and determine whether this effort is costly. The function of the public goods will also be quantified by relating the thermoregulatory properties of the thatch to its size and the position of nest chambers. Finally, and most importantly, this study will use field observations and experiments to test whether a tragedy of the commons is averted by cooperating with kin, through enforcement of cooperative work by other colony members, or because cooperative work is a signal to other colony members of an individual's quality with associated benefits to an individual's own reproductive success. Previous studies of sociable weavers show that the programme of research is tractable and the project's objectives achievable. The study will be conducted on an established study population of sociable weavers at Kimberley, South Africa, in collaboration with the Percy FitzPatrick Institute of African Ornithology at the University of Cape Town. Because of the ubiquity of the tragedy of the commons in biological systems, the findings of this study will have broad relevance across a wide range of disciplines.