Yawning: a cue and a signal

Yawning behaviour has been associated with a variety of physiological and social events and a number of corresponding functions have been attributed to it. Non-directed (self-directed behaviour) and directed yawning (display behaviour) might nonetheless encompass all expressions of yawning, although it is difficult to differentiate one type from the other in a social context. Here we analysed more fully the data from a study in which four combinations of sensory cues were presented to pairs of either cage mate or stranger rats. The aim of the re-analysis was to demonstrate that non-directed and directed yawning might be identified by their distinctive functions. All pairs of rats used olfactory cues to recognise each other as stranger or cage mate companions, but only stranger rats used auditory cues to detect and respond to each other's yawning. Increasing defecation rates (i.e. an index of emotional reactivity) inhibited yawning in cage mate rats such that yawning frequency reflected each rat's physiological state. These results suggest that non-directed yawning functions as a cue in cage mate rats and directed yawning as a signal in stranger rats. We hypothesize that cue yawning might be a regulatory act that animals perform to adjust muscle tone for a coordinated change of state. Signal yawning might indicate the physiological capacity of rats in male-male conflicts.