In humans yawning can be triggered both
physiologically as well as psychologically
(i.e., spontaneous and contagious yawns), and it
is the distinct manifestation of these 2 yawn
types that has led researchers to traditionally
study them independently. However, evolution
produces adaptations that are constrained by and
build upon existing architecture, and since
contagious yawning is a derived feature of the
more primitive spontaneous form, it stands to
reason that at least some fundamental
mechanistic pathways should be shared between
them.
Recent research by Eldakar et al. has taken
this perspective by assessing how non-social
variables, i.e., those related to
thermoregulation, alter the expression of yawn
contagion. The thermoregulatory theory of
yawning posits that the motor action pattern of
this behavior functions as a brain cooling
mechanism, serving to counteract rising brain
temperature by altering the rate and temperature
of arterial blood flow to the skull.
Accordingly, the powerful stretching of the jaw
and deep inhalation of air forces hyperthermic
blood away from the skull while introducing
cooler blood from the lungs and
extremities.
The deep inspiration also allows for
countercurrent heat exchange with the ambient
air and subsequent cooling of venous return
surrounding arterial blood supply. Since the
inception of this theory in 2007, the primary
brain/ skull cooling predictions derived from
this theory have been confirmed and replicated
in different species (including humans).3 The
thermal window hypothesis is a derivative of the
thermoregulatory theory, which makes additional
predictions regarding the relationship between
yawning and ambient temperature.
First, yawns should increase in frequency
with initial rises in ambient temperature, as
this would trigger cooling mechanisms that
function to maintain thermal homeostasis.
Second, as air temperature continues to rise
toward body temperature, yawning should then
decrease since deep inhalations of warm air
would heat rather than cool internal tissues.
Third, yawning frequency would be expected to
diminish as ambient temperature falls below a
thermal neutral zone, as cooling mechanisms
would no longer be needed. In sum, yawning is
predicted to occur within a relatively narrow
range of ambient temperatures (i.e., a thermal
window).
Initial support for this hypothesis has come
from studies of birds and rodents, which
demonstrated that spontaneous yawning could be
modulated in predicted ways through the
experimental manipulation of ambient
temperature. More recently, a pair of
naturalistic experiments has provided further
support for these predictions by assessing the
influence of ambient temperature on the
expression of contagious yawning in humans.
By sampling pedestrians across divergent
seasons and climatic conditions, these studies
first demonstrated that contagious yawning (much
like the spontaneous form) became less common at
temperature extremes. In a followup study,1
which controlled for potential physiological
circadian changes that may produce distinct
patterns of yawning across different seasons,
142 pedestrians were shown a contagious yawning
stimulus while being outside during an 18-day
period over the summer in southern Florida, USA.
Consistent with the thermal window hypothesis,
self-reported contagious yawning was most
frequent (60.5%) at moderately warm
temperatures, but dropped precipitously (23.8%)
as air temperatures approached human body
temperature.
Ambient temperature was the only significant
predictor of contagion in this study, and this
was true even when controlling for a host of
other variables known to influence yawning
frequency. Before we can fully grasp why it is
that we yawn in response to the yawns of others,
we first need to understand why we yawn when we
are alone. These latest ambient temperature
findings provide further evidence indicating
that the underlying mechanisms controlling the
expression of yawning, both spontaneous and
contagious, are embedded in thermoregulatory
physiology.
Of the numerous hypotheses for why we yawn,
only the thermoregulatory theory predicts these
combined effects. In sum, the thermoregulatory
theory can explain the adaptive value of both
isolated and social forms of yawning. Since
yawning cools the brain, which in turn should
enhance mental processing efficiency, the
spreading of this behavior across the group
under natural conditions would be expected to
heighten collective vigilance and promote the
detection and avoidance of hreats.
References
[1] Eldakar OT, Dauzonne M,
Prilutzkaya Y, Garcia D, Thadal C, Gallup AC.
Temperature-dependent variation in self-reported
contagious yawning. Adap Hum Behav Physiol 2015;
1:460-6;
http://dx.doi.org/10.1007/s40750-015-0024-6
[2] Gallup AC, Gallup Jr. GG.
Yawning as a brain cooling mechanism: nasal
breathing and forehead cooling diminish the
incidence of contagious yawning. Evol Psychol
2007; 5:92-101.
[3] Gallup AC, Eldakar OT. The
thermoregulatory theory of yawning: what we know
from 5 years of research. Front Neurosci 2013;
6:1-13;
http://dx.doi.org/10.3389/fnins.2012.00188
[4] Gallup AC, Miller ML, Clark AB.
Yawning and thermoregulation in budgerigars
(Melopsittacus undulatus). Anim Behav 2009;
77:109-13;
http://dx.doi.org/10.1016/j.anbehav.2008.09.014
[5] Gallup AC, Miller RR, Clark AB.
Changes in ambient temperature trigger yawning
but not stretching in rats. Ethology 2011;
117:145-53;
http://dx.doi.org/10.1111/j.1439-0310.2010.01854.x
[6] Gallup AC, Eldakar OT.
Contagious yawning and seasonal climate
variation. Front Evol Neurosci 2011; 3:1-4;
PMID:21720531;
http://dx.doi.org/10.3389/fnevo.2011.00003
[7] Massen JJM, Dusch K, Eldakar OT,
Gallup AC. A thermal window for yawning in
humans: yawning as a brain cooling mechanism.
Physiol Behav 2014; 130:145-8; PMID:24721675;
http://dx.doi.org/10.1016/j.physbeh.2014.03.032
