Yawn - Scholarpedia

Stretching and yawning (known as pandiculation when they occur together) are under-researched features of behavior. Ethologists agree that almost all vertebrates yawn (Deputte, 1974). Yawning is morphologically similar in reptiles, birds, mammals and fish. These behaviors may be ancestral vestiges maintained throughout evolution with little variation (phylogenetic old origins). Systematic and coordinated pandiculations occur in a similar pattern and form across all animals, and consistently occur during behaviors associated with cyclic life rhythms: sleep-arousal, feeding and reproduction. Pandiculation appears as one undirected response to an inner stimulation, underlying the homeostasis of these three behaviors (Provine, 2005; Walusinski & Deputte, 2004).