Deconstructing sociality, social evolution and relevant nonapeptide functions

Although behavioral neuroendocrinologists often discuss "sociality" as a unitary variable, the term encompasses a wide diversity of behaviors that do not evolve in a linked fashion across species. Thus grouping, monogamy, paternal care, cooperative breeding/alloparental care, and various other forms of social contact are evolutionarily labile and evolve in an almost cafeteria-like fashion, indicating that relevant neural mechanisms are at least partially dissociable. This poses a challenge for the study of the nonapeptides (vasopressin, oxytocin, and homologous neuropeptides), because nonapeptides are known to modulate all of these aspects of sociality in one species or another. Hence, we may expect substantial diversity in the behavioral functions of nonapeptides across species, and indeed this is the case. Further compounding this complexity is the fact that the pleiotropic contributions of nonapeptides to social behavior are matched by pleiotropic contributions to physiology. Given these considerations, single "model systems" approaches to nonapeptide function will likely not have strong predictive validity for humans or other species. Rather, if we are to achieve predictive validity, we must sample a wide diversity of species in an attempt to derive general principles. In the present review, I discuss what is known about functional evolution of nonapeptide systems, and critically evaluate general assumptions about bonding and other functions that are based on the model systems approach. From this analysis I attempt to summarize what can and cannot be generalized across species, and highlight critical gaps in our knowledge about the functional evolution of nonapeptide systems as it relates to dimensions of sociality.