The forms and behavioral correlates of yawning are described, and the phylogenetic and ontogenetic aspects of the act are examined with particular attention to its possible functions. Much evidence supports the view that yawning is an important mediator of behavioral arousal levels, a view that is further strengthened by a review of endocrine, neurotransmitter, and pharmacological mechanisms of yawning: A major function of yawning appears to involve maintenance or increase of arousal when environments provide relatively little stimulation.
Students of behavior have neglected yawning. It is rarely mentioned in animal behavior texts, and even books that would seem to be particularly relevant, such as Hinde's (1972) Non Verbal Communication, do not include yawning in the index. In physiology the standard texts such as Starlings Human Physiology (Davson & Eggleton, 1962) or the revised Johns Hopkins Atlas of Human Functional Anatomy (1977) devote only a few vague sentences to yawning, if it is mentioned at all. There is no precise knowledge of the eliciting stimuli, functions, development, or topography of yawning. Darwin (1873) described yawning in humans as an act of deep inspiration, followed by a lengthy, forceful expiration with simultaneous strong contraction of many skeletal muscle groups, and occasionally accompanied by lacrimation. Yawning is sometimes described as a reflex, and Moore (1942) reported that yawning is one of the first reflexes observed in newborn human infants.
Most rats maintained on a food-deprivation schedule for 2 wk. killed mice when hungry. A large proportion continued to kill after being food satiated for 2 or more wk. Rats that had never been food deprived were much less likely to kill. Rats first tested when food satiated, but with a history of food deprivation, were somewhat more likely to kill than were rats first tested without a history of food deprivation. These effects were obtained in male Long-Evans rats bred for killing, and in male randomly bred Long-Evans and Sprague-Dawley rats with low base rates of spontaneous killing. Hunger had a smaller effect on killing by Sprague-Dawley rats. Female Long-Evans rats, bred for killing, killed less frequently than did their male littermates, both while food satiated and while hungry. Water deprivation had no effect on the initiation of killing. Relationships of mouse killing to feeding and to less homeostatic instinctual responses were discussed.
Siamese fighting fish showed a waning tendency to view both their own reflections and a similar fish during trials lasting 32 hr. Although both stimuli elicited aggressive displays, all Ss spent more time viewing their ref1ections than viewing the live stimulus fis h. Recent experiments have shown that the opportunity to perform an aggressive display may serve as reinforcement in the acquisition of operant responses (Thompson, 1963; Thompson, 1964). The possibility exists that the demonstrated "extinction" of these re
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