Non-human primates: a comparative developmental perspective on yawning.

There is a long history of yawning in Old World monkeys being viewed as a form of communication, in particular, as a kind of threat. Yawning in agonistic and tense situations is seen in adult males, in particular, and it varies with male hormonal levels and social status. Experiments are reviewed that demonstrate operant control of the rate of yawning in adult male macaques, using food rewards. This indicates a degree of flexibility in the production of yawning. However, although adult male Old World monkeys often engage in 'canine contests', there is little evidence for the contagious yawning seen in humans. Experiments are reviewed showing that chimpanzees tested under comparable conditions to human adults, namely exposed to video sequences showing yawns, may yawn contagiously to yawn stimuli. Chimpanzees also yawn to computer animations of yawns. There is controversy in the literature over whether other species, including dogs and some monkeys, may also show contagious yawning. Further research is required to address unresolved issues. A hypothesis is put forward that in modern industrial society adults' natural pattern of yawning is inhibited, and that being reminded to yawn by seeing another individual yawn (contagious yawning) can help us to catch up on missed yawns. This would explain the lack of contagious yawning reported in young children and chimpanzees in natural surroundings, as these populations do not have the same social constraints on yawning.