Yawning involves opening the mouth involuntarily while taking a long, deep breath of air. This is usually done as a result of drowsiness or weariness [1]. Yawning occurs throughout the vertebrates, including fish, amphibians, reptiles, birds, and mammals [2].
Around 20 functions of yawning have been proposed [3], but there is little agreement about the major function of the behaviour. However, few would doubt that one of the most remarkable aspects of the behaviour in humans is its contagiousness. Just seeing, hearing, reading or even thinking about yawning can induce yawning, at least in adults [4][5][6].
Contagious yawning (i.e. yawning triggered by perceiving others' yawning) is a well-documented phenomenon [4][5][7][8] (Platek et al. 2003), but surprisingly little is known about the mechanisms underlying it. Contagious yawning is reported in humans and in only a few other primates such as chimpanzee (Pan troglodytes; [9]) and possibly in stumptail macaques (Macaca arctoides; Paukner & Anderson 2006), even though spontaneous yawning is widespread among vertebrate species [2]. Some authors argue that contagious yawning is a response to an innate releasing mechanism [4][5], whereas others claim that it is based on the capacity for empathy ([7]; Preston & de Waal 2002; Platek et al. 2003).
Sources:
[1] Yawning - excessive, MedlinePlus Medical Encyclopedia
[2] Baenninger, R. (1987). Some comparative aspects of yawning in Betta splendens,
[3] Smith, E. O. (1999). Yawning: An evolutionary perspective. Human Evolution, 14, 91-198.
[4] Baenninger, R., & Greco, M. (1991). Some antecedents and consequences of yawning. Psychological Record, 41, 453-460.
[5] Provine, R. R. (1986). Yawning as a sterotyped action pattern and releasing stimulus. Ethology, 72, 109-122.
[6] Provine, R. R. (1989a). Contagious yawning and infant imitation. Bulletin of the Psychonomic Society, 27, 125-126.
[7] Lehmann H.E. Yawning: a homeostatic reflex and its psychological significance. Bull. Meninger Clin. 1979;43:123-136.
[8] Anderson J.R, Meno P. Psychological influences on yawning in children. Curr. Psychol. Lett. Behav. Brain Cogn. 2003;11 http://cpl.revues.org/document390.html
[9] Anderson J.R, Myowa-Yamakoshi M, Matsuzawa T. Contagious yawning in chimpanzees. Proc. R. Soc. B. 2004;271(Suppl. 6):S468-S470. doi:10.1098/rsbl.2004.0224
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