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.

Inhibition of grooming by pilocarpine differs in high- and low-yawning sublines of Sprague-Dawley rats.

A comparative study of the effect of pilocarpine, a muscarinic receptor agonist, on grooming, scored during 45 min via a time-sampling procedure, was carried out on two sublines of male rats selectively bred for high-(HY) and low-yawning (LY) frequency. In one condition, we introduced rats in a novel cage and observed them immediately after receiving an I.P. injection of pilocarpine (0.5-3.75 mg/Kg) or an equivalent volume of saline. Besides grooming, the occurrence of yawns was continuously recorded. In the other condition, we immersed rats in water for 60 s, then they received an I.P. injection of pilocarpine (3.75 mg/Kg) or an equivalent volume of saline and we placed them in an open field, in which we recorded the number of crossed squares. Grooming scores were significantly higher in the condition after water immersion than in the novel situation; in both conditions HY had a grooming response higher than that of LY rats. Pilocarpine produced a dose-dependent inhibition of novelty-induced grooming in HY rats, whereas LY grooming was reduced only with the highest dose. In contrast, yawning increased in a dose-dependent manner with HY rats curve over that of LY animals, except for the highest dose. Pilocarpine inhibited water immersion-induced grooming in both sublines of rats, but it did not reduce grooming as much as it did in the novel condition. Pilocarpine affected distinctly each of the components of grooming, without inhibiting animals locomotor activity. The results indicate that HY rats also have a higher number of grooms than LY rats, and because grooming and yawning can appear after stressful circumstances, HY rats may be used to study the role that both behaviors could have in counteracting the effects of stress. Similarly, HY animals might be utilized to study the underlying neurochemical mechanisms of grooming. This study also indicates that the cholinergic systems exert an inhibitory influence on grooming which contrasts with the excitatory effect on yawning.