mise à jour du
21 décembre 2009
Am. J. Primato
Variation and Context of Yawns
in Captive Chimpanzees (Pan troglodytes)
Sarah-Jane Vic, Annika Paukner
Department of Psychology, University of Stirling, Stirling, Scotland
Laboratory of Comparative Ethology, Poolesville, Maryland USA


Primate yawns are usually categorized according to context (e.g. as a threat, anxious, or rest yawn), but there has been little consideration of whether these yawns are best regarded as a unitary behavior that only differs with respect to the context in which it is observed.
This study examined the context and precise morphology of yawns in a group of 11 captive chimpanzees. Focal video sampling was used to describe the morphology and intensity of 124 yawns using ChimpFACS, a system for coding facial movements. Two distinct forms of yawn were identified, a full yawn and a yawn which is modified by additional actions that reduce the mouth aperture. These modified yawns may indicate some degree of voluntary control over facial movement in chimpanzees and, consequently, multiple functions of yawning according to context.
To assess context effects, mean activity levels (resting, locomotion, and grooming) and scratching rates were compared one minute before and after each yawn. Locomotion was significantly increased following both types of yawn, whereas scratching rates significantly increased following modified yawns but decreased following full yawns. In terms of individual differences, males did not yawn more than females, although male yawns were of higher intensity, both in the degree of mouth opening and in the amount of associated head movement.
These data indicate that yawning is associated with a change in activity levels in chimpanzees, but only modified yawns may be related to increased arousal. Different types of yawn can therefore be differentiated at the morphological level as well as context level.
Microanalyses indicate that yawns can be differentiated in terms of intensity (amount of head movement, degree of mouth opening) and accompanying facial movements, i.e. chimpanzees may modify their yawns with the addition of facial movements which reduce the mouth aperture.
Interestingly, these modified yawns were accompanied by more head movement than full yawns, perhaps indicating the yawns were being directed away from others, i.e. non-directed yawns [Hall & Devore, 1965]. The presence of these additional movements could indicate that chimpanzees may have some voluntary control over facial actions and that these actions may be used to reduce the salience of the yawn by reducing the mouth aperture.
Unfortunately, given the focal video sampling method (needed for microanalyses) and the enclosure layout, it was not possible to accurately record the presence and orientation of other group members; we cannot ascertain whether yawns were directed toward or away from conspecifics, whether yawning was more likely in the presence of dominant others, or whether the behavior of non-focal individuals differed in response to the different types of yawns.
Factors, such as social relationships and specific yawn types, should be more fully examined using both experimental and observational methods, if we are to better understand yawning in nonhuman primates. Another factor which may impact upon yawning rates may be the presence of human observers; we know that the presence and behavior of visitors can impact upon zoo-housed primates [e.g. Hosey, 2000], and higher visitor density or noise, or even the sustained proximity of the researchers (particularly in the inside areas where proximity was higher), may have elevated stress and increased the yawning rates observed [Baker & Aureli, 1997].
On the other hand, these primates were well habituated to visitors and the study was conducted in winter, that is, outside the peak season for visitor numbers, so we would expect any visitor effects on behavior to be less pronounced. As in humans [Provine & Hamernik, 1986; Schino & Aureli, 1989], we did not find any sex difference in yawn frequency, but male yawns were longer in duration and of higher intensity. There was no sex difference in the tendency to modify yawns with additional facial actions.
In humans, women are reported to cover their yawns more than men [Schino & Aureli, 1989], but we cannot compare this finding with any yawn modification seen in primates without studying human yawn morphology in more detail. It would be interesting to look in more detail at yawning morphology in species which use yawns as part of a threatening display, in terms of both intensity [e.g. Palagi et al., 2009; Setchell & Wickings, 2005] and also whether additional modifying actions can be identified.
The results also indicate that similar to humans [Baenninger et al., 1996], yawning in chimpanzees is related to a change in general activity levels with increased locomotion during the one minute interval following a yawn. This effect seems to be independent of yawn morphology and indicates that yawning may relate to synchronization of group activity and indicate changes in activity.
Although chimpanzees' yawns are not generally considered as a display [e.g.van Hooff, 1967], if yawns reliably indicate a change in activity state, they may be a source of information for other group members and help with synchronizing group behaviors. Given that there were increased levels of self-scratching following a modified yawn while full yawns led to reduced self-scratching, the modified yawns seem to be associated with arousal [e.g. Baker & Aureli, 1997; Pomerantz & Terkel, 2009], whereas full yawns may be considered true or rest yawns.
The overall pattern of results indicates that chimpanzee yawns are not used in display but rather reflect physiological factors. However, the phenomenon of yawn contagion in chimpanzees [Anderson et al., 2004; Campbell et al., 2009] indicates that yawns may also be signals as they might impact upon receiver behavior. It remains to be seen whether conspecifics perceive any difference in yawn types. In humans, attempts to suppress a yawn do not prevent it inducing yawns in human observers [Provine, 1997] but it would, nonetheless, be interesting to test these different yawn types within a contagious yawning paradigm similar to Anderson et al. [2004]. It is apparent that a combination of experimental methods and detailed and systematic observations of spontaneous behavior are both necessary to untangle the complexities of yawning in primates.