Peter Kappeler’s seminal work on animal social systems has built a strong conceptual foundation for generations of researchers (Kappeler (2019) Behav Ecol Sociobiol 73:13). The most notable and important contribution of his work has been a clear framework for animal social systems, clarifying the muddling of definitions. Unfortunately, many have used flawed definitions in their research on social systems, or even worse, no definitions at all. In contrast, Kappeler`s framework emphasizes that there are four inter-related components of the social system: (i) social organization, (ii) social structure, (iii) mating system, and (iv) care system. The framework is powerful for numerous reasons. Most importantly, these clearly defined components promote empirical and comparative research that does not confuse or confound behaviours coming under different forms of selection. Moreover, the framework permits the study of how these components are interconnected, instead of taking mixed definitions as an a priori. It allows for the identification of > 1000 types of social systems. Even so, the framework has not solved all challenges, most notably how to quantify social structure, which is arguably, the most complex social system component. Despite this limitation, the framework has enormous potential to shape our understanding of social diversity across a broad range of taxa. This Topical Collection advocates for the greater use of the framework in behavioural ecology and sociobiology while allowing for space to critically address its limitations.
