Department of Psychology,
University of Stirling, Stirling, Scotland
Laboratory of Comparative
Ethology, Poolesville, Maryland USA
Abstract
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.
Discussion
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.
