A comparative approach to the evolution of kissing

Kissing can be observed across the animal kingdom. This presents an evolutionary puzzle, since the fitness benefits of kissing are unclear. We use a non-anthropocentric approach to define kissing as a non-agonistic interaction involving directed, intraspecific, oral-oral contact with some movement of the lips/mouthparts and no food transfer. Using this definition we collate basic observational data across the Afro-Eurasian primates and employ Bayesian phylogenetic methods to reconstruct the evolutionary history of kissing. We find that kissing occurs in most extant large apes, and likely also occurred in Neanderthals (Homo neanderthalensis), first evolving in the ancestor to this group ~21.5-16.9 mya. Additionally, we highlight various life history variables that correlate reasonably, but not perfectly, with kissing across the apes (multi-male mating systems, non-folivorous diets, and premastication). With a major caveat about the quantity of available data at present, we hope that our results provide a useful starting point for further research into the adaptive function of kissing that highlights hypothesis generation and testing within a phylogenetic framework.