Why do we yawn? Primitive versus derived features

Guggisberg et al. (2010) reviewed the evidence for the origin and function of yawning, and conclude that theories describing a physiological role lack support. Instead, they argue research supports the notion that yawning has a communicative function. Contrary to the authors' claim that the social/communication hypothesis has the "best experimental evidence", there is in fact no definitive experimental support for the predictions of this model. Furthermore, the authors claim to take an evolutionary perspective, but sufficient examples across the comparative (non-primate) literature are missing, and they fail to acknowledge phylogenic history. Due to the ubiquity of this behavior across vertebrates, and the regularity of its occurrence in a number of different physiological states and social contexts, it is likely that instead of serving one purpose, yawning is multifunctional across a number of species. The most parsimonious explanation for the origin of yawning suggests that any social value is a derived feature, while the primitive feature or function is physiological. The current paper addresses these concerns, and identifies a number of other weaknesses in the social/communication hypothesis as a global explanation for the origin and function of yawning.