Why do we yawn? The function of this
behavior has remained elusive for centuries, but
we are now closer to discovering the answer. The
leading theory of yawning as a brain cooling
mechanism has been bolstered by recent evidence
showing a biphasic effect of ambient temperature
on yawning.
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 two 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).
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 follow-up 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 threats.
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;In
Press.
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.
4. Gallup AC, Miller ML, Clark AB. Yawning
and thermoregulation in budgerigars
(Melopsittacus undulatus). Anim Behav
2009;77:109-113.
5. Gallup AC, Miller RR, Clark AB. Changes
in ambient temperature trigger yawning but not
stretching in rats. Ethology
2011;117:145-153.
6. Gallup AC, Eldakar OT. Contagious yawning
and seasonal climate variation. Front Evol
Neurosci 2011;3:1-4.
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;130:145-148.
