Many biological and social systems show significant levels of collective action. Several cooperation mechanisms have been proposed, yet they have been mostly studied independently. Among these, direct reciprocity supports cooperation on the basis of repeated interactions among individuals. Signals and quorum dynamics may also drive cooperation. Here, we resort to an evolutionary game theoretical model to jointly analyse these two mechanisms and study the conditions in which evolution selects for direct reciprocity, signalling, or their combination. We show that signalling alone leads to higher levels of cooperation than when combined with reciprocity, while offering additional robustness against errors. Specifically, successful strategies in the realm of direct reciprocity are often not selected in the presence of signalling, and memory of past interactions is only exploited opportunistically in case of earlier coordination failure. Differently, signalling always evolves, even when costly. In the light of these results, it may be easier to understand why direct reciprocity has been observed only in a limited number of cases among non-humans, whereas signalling is widespread at all levels of complexity.

翻译：许多生物和社会体系表现出了相当程度的集体行动。一些合作机制已经提出,但大多是独立研究。其中,直接互惠支持在个人之间反复互动的基础上开展合作。信号和法定人数动态也可能推动合作。在这里,我们采用进化游戏理论模型,共同分析这两种机制,研究进化选择直接互惠、信号或组合的条件。我们表明,单凭信号就能导致更高层次的合作,而不是与互惠相结合,同时提供针对错误的额外强健性。具体地说,直接互惠领域的成功战略往往不是在信号出现时选择的,对过去互动的记忆只有在早期协调失败时才被随机利用。不同的是,信号总是演变,即使代价高昂。根据这些结果,也许更容易理解为什么直接对等只在非人类的有限情况下得到遵守,而信号在各种复杂程度都是普遍的。