Research indicates that the motor action
pattern of yawning functions to promote cortical
arousal and state change through enhanced
intracranial circulation and brain cooling.
Because the magnitude of this response likely
corresponds to the degree of neurophysiological
change, we hypothesized that interspecies
variation in yawn duration would correlate with
underlying neurological differences. Using
openly accessible data, we show that both the
mean and variance in yawn duration are robust
predictors of mammalian brain weight and
cortical neuron number (_-values > 0.9).
Consistent with these effects, primates tend to
have longer and more variable yawn durations
compared with other mammals. Although yawning
has long been considered a stereotyped action
pattern, these findings reveal substantial
variation in this response and highlight the
importance of measuring yawn duration in future
research.
1. Background
Yawning is characterized by a powerful
gaping of the jaw with inspiration, a brief
period of peak muscle contraction and a passive
closure of the jaw with shorter expiration
[1]. Yawn-like mandibular gaping
patterns have been identified across vertebrate
classes [2], though it remains unknown
whether the jaw stretching observed in fish,
amphibians and reptiles functions in the same
way as yawns in birds and mammals [3].
Nonetheless, the relative ubiquity of this
response in vertebrates suggests it is an
evolutionarily conserved behaviour that holds
basic and important adaptive value. Although the
function to yawning remains debated
[3&endash;6], previous research supports
a role in promoting cortical arousal and state
change [7&endash;11] through enhanced
intracranial circulation and brain cooling
[12&endash;13].
Yawning is controlled by several
neurotransmitters and neuropeptides [14]
and has been linked with numerous neurological
diseases and clinical conditions
[15&endash;16]. Despite the potential
applications of studying differences in yawning
as a marker of neural processing, variation in
this response has yet to be fully explored.
Naturalistic reports on the frequency of yawning
across taxa are quite limited [17,18],
and yawns have long been considered a
stereotyped action pattern with limited
variation in expression within or between
species [8]. However, recent studies on
non-human primates have identified previously
overlooked variation in the mouth aperture
associated with yawning, and it has been shown
that different forms of yawning tend to
correspond to distinct contexts and situations
[18,19].
Another measure of variability that could be
easily catalogued across species is the duration
of yawning. Although early research examined
average yawn duration in humans (approx. 6 s)
[8], this variable has not been used in
studies of non-human animals. Given that the
primary circulatory effects associated with
yawning are localized within the skull,
differences in the duration or magnitude of this
response likely correspond to the degree of
neurophysiological change. Therefore, we
hypothesize that differences in yawn duration
will correlate with neurological variation
between species.
Here, we examine this possibility by linking
openly accessible videos of yawning from the
Internet (primarily from www.youtube.com) to
previously published brain parameters in a
representative sample of mammalian taxa
[20]. Based on previous research
supporting a neurophysiological function to
yawning, we hypothesized that mammals with
larger brains would yawn longer, even when
controlling for body size, and that yawn
duration would correlate with number of cortical
neurons.
4. Discussion
Although yawning has been considered a
stereotyped action pattern [8], we
document substantial variation in the duration
of this response in mammals. In particular, we
found that both the mean and variance in yawn
duration are robust predictors of brain weight
and cortical neuron number (_-values > 0.9).
Consistent with these results, we also found
that primates tend to have longer and more
variable yawn durations compared with other
mammals. These results apply irrespective of
variation in the mouth aperture of yawns in some
non-human primates [18,19].
These combined effects represent a striking
scaling relationship between brain and
behaviour. Importantly, neither the size of the
body nor the anatomical structures specific to
yawning (cranium and mandible) are driving these
effects, because gorillas, camels, horses,
lions, walruses and African elephants all have
shorter average yawns than humans. Furthermore,
having a larger skull does not necessitate more
variable motor pattern duration. Instead,
differences in yawn duration appear to be
specifically linked to interspecies variation in
brain size and complexity, with cortical neuron
number being the most significant factor. Future
research should investigate whether mean and
variation in yawn duration also predict similar
brain parameters recently documented across
avian taxa [21].
These findings are consistent with the view
that yawning holds a basic neurophysiological
function. In particular, previous research
suggests that yawning is an adaptation to
enhance intracranial circulation and brain
cooling [11&endash;13], which in turn
could promote cortical arousal and state change
[7&endash;9]. While the neural
structures necessary for yawning appear to be
located within the brainstem [22], based
on these results, we hypothesize that the
neurophysiological consequences of yawning
affect the brain more globally, whereby longer
yawns may be necessary to more effectively
modulate cortical arousal for animals with
larger and more complex brains. Furthermore, we
predict that the greater within-taxon variance
in yawn duration observed for large-brained
mammals may be related to increased cognitive
capacities and more variable behavioural
repertories. We note that while a primarily
social/communication (i.e. signalling) function
to yawning has been posited [4], it is
unclear how such an explanation would account
for the current effects.
Provine [8, p. 120] stated, 'yawning
may have the dubious distinction of being the
least understood, common, human behavior'.
Unfortunately, 30 years later, we still know
relatively little about the biological
significance of this evolutionarily conserved
response. The difficulty in uncovering the
ultimate function(s) of yawning may, in part, be
owing to the fact that it can be elicited by
numerous stimuli and researchers have by and
large overlooked subtle distinctions in the
expression of this behaviour. Based on the
current findings, we believe yawn duration
deserves further attention. We close by offering
suggestions for future research in this
area
