Omar Tonsi Eldakar, Melissa
Dauzonne, Yana Prilutzkaya, Daniel Garcia,
Carolyn Thadal, Andrew C. Gallup
Abstract
The thermoregulatory theory posits that
yawns mnction to cool the brain in part due to
counter-current heat exchange with the deep
inhalation of ambient air. In support of this
theory, previous cross-cultural research on
humans has shown that self-reported conta-gious
yawning frequency varies between seasons with
distinct ambient température ranges.
However, it remains possible that
différences in yawning across seasons are
a resuit of physiological circadian changes
across the year rather than variation in ambient
température. In an attempt to address
this question, hère we discuss the
results of a study investigating the variation
in the frequency of self-reported contagious
yawning within a restricted range of a single
season in one géographie location. A
total of 142 pedestrians were recraited outdoors
during an 18-day period over the summer in an
equatorial monsoon climate in southem Florida,
USA. Consistent with the thermoregulatory theory
of yawning, results showed that self-reported
contagious yawning frequency varied predictably
across température gradients. This was
true after statistically controlling for
relative humidity, time of day, time spent
outside, testing day, âge of participant,
and amount of sleep the night before.
Thèse fîndings provide further
évidence suggesting a brain cooling
function to yawning.
Yawning is characterized by a large gaping
of the mouth with a deep inspiration, a brief
acme period with peak muscle contraction, and
passive closure of the jaw with a shorter
expiration of air (Barbizet 1958). This behavior
appears highly conserved, with report-ed
observations of yawns having been documented
across vertebrate classes (Craemer 1924;
Luttenberger 1975; Baenninger 1987; Sauer and
Sauer 1967). Overtime numer-ous fonctions of
yawning hâve been suggested (Smith 1999),
ranging from regulating middle ear pressure
(Laskiewicz 1953) to preventing lung atelectasis
(Cahill 1978). While many hypothèses
hâve been proposed for why we yawn, few
hâve been empirically supported (Gallup
2011). For example, it is commonly believed that
we yawn to increase oxygen in the blood, yet
this respiratory hypothesis was tested and
falsified over 25 years ago (Provine et al.
1987). In récent years, the
thermoregulatory theory has emerged as the most
empirically supported physiological explanation
for why we yawn (see Gallup and Eldakar 2013 for
a discussion of évidence and critiques).
This theory states that the motor action pattern
of yawmng functions as a brain cooling mechanism
(Gallup and Gallup 2007, 2008). Consistent with
this theory, research on both rats (Rattus
norvégiens) and humans has demonstrated
that yawns are surrounded by predicted
fluctuations in brain and skull
température, Le., yawning is preceded by
rises in température and followed by
decreases in température thereafter
(humans, Gallup and Gallup 2010; rats,
Shoup-Knox et al. 2010). Furthermore, research
on birds {Melopsittacus undulatus) has shown
that following stress-induced hyperthermia the
latency to yawn is negatively correlated with
the peak body température (Miller et al.
2010). Literature from diverse fields of
medicine, physiology and pharmacology also
support the connection between yawning and brain
thermorégulation (Gallup and Gallup
2008).
According to the thermoregulatory theory,
yawning acts to cool brain température
through thermoregulatory mechanisms of
countercurrent heat exchange, evaporative
cooling and enhanced cérébral
blood flow (for a discussion, see Gallup and
Hack 2011). As a resuit, the ambient air
température is critical for yawmng to
fonction as a brain cooling mechanism, and
therefore yawns should be constrained to a
relatively narrow range of température,
Le., a thermal window (Gallup and Gallup 2007).
The prédictions of this hypothesis are
that yawns should (1) increase in frequency with
initial rises in ambient température from
a thermal neutral zone, as this stimulâtes
thermoregulatory cooling mechanisms to control
températures within a normal range, (2)
decrease as ambient températures approach
or exceed body température, since yawns
would be counterproductive by introducing deep
inhalations of elevated tem-pérature into
the body, and likewise (3) diminish when
températures fall below a certain point,
because thermoregulatory cooling responses are
no longer necessary and coun-tercurrent heat
exchange may cause déviations in internai
température which fall below homeostasis
(Massen et al. 2014). Aside from having
différent triggers, the motor action
patterns of spontaneous and contagious yawns are
indistinguishable and there-fore thèse
prédictions should apply to both forms.
The thermal window hypothesis is already well
supported by existing research in comparative
psychology and behavioral biology. Observational
research on non-human primates has documented
that spontaneous yawmng frequency becomes
elevated in warm ambient températures
(Campos and Fedigan 2009; Deputte 1994).
Furthermore, expérimental studies on
birds and rats in the laboratory are consistent
with the first two prédictions of this
model (Gallup et al. 2009, 2010, 2011). For
example, in budgerigars spontaneous yawns
increase when températures are elevated
from 22 to 34C, but then decrease in frequency
when températures are held between 34 and
38C. Similar effects have been documented in
Sprague Dawley rats using slightly
différent température ranges
(Gallup et al. 2011).
Two récent naturalistic studies on
humans have also investigated the relationship
between ambient température and yawning
frequency by surveying pedestrians while
outdoors in the same geographical location
during distinct seasonal climate conditions. The
ffrst study measured self-reported contagious
yawning among pedestrians during the summer and
winter months in the arid désert climate
of Arizona, USA following exposure to a
contagious yawning stimulus (Gallup and Eldakar
2011). The second study used the same
methodology but measured the response of summer
and winter yawning among pedestrians in the
temperate climate of Vienna, Austria (Massen et
al. 2014). Overall, the results from
thèse studies demonstrated that yawning
is signifîcantly less common at
température extrêmes. In
particular, self-reported contagious yawning was
highest in the winter in Arizona and in the
summer in Vienna, when ambient
températures were near room
température (19.4C; 21.9C), but yawning
rates declined signifîcantly in the summer
months of Arizona, when températures
averaged 37C, and in the winter months of
Vienna, when températures averaged 1.4C.
Furthermore, both studies demonstrated that
température was a significant predictor
of yawning even when controlling for other
variables associated with yawning frequency
The aforementioned literature, including
naturalistic observations and controlled
laboratory experiments, suggests that underlying
mechanisms controlling yawning are sensitive to
ambient température. Despite accounting
for changes in daylight cycles associated with
winter and summer months, and statistically
controlling for hours of sleep prior to testing,
it remains possible that physiological circadian
changes across the year contributed to the
pattern of self-reported contagious yawning in
humans during distinct seasons. Therefore, the
current study was designed to investigate
whether significant variation in yawning exists
within a short timeframe in a single season.
Using the same approach as Gallup and Eldakar
(2011) and Massen et al. (2014), we recruited
pedestrians outside during an 18-day period over
the summer in the equatorial monsoon climate of
southern Florida, USA (Kottek et al. 2006).
Ambient températures during this period
ranged from 27.8 to 36.1C. Based on the first
two prédictions of the thermal window
hypothesis, we expected yawns to be (1) most
fréquent during the lower bound of this
range, since thèse températures
are moderately warm and deviate from a thermal
neutral zone, and (2) least fréquent at
the highest températures as thèse
are drawing near human body
température.
Discussion
The themioregulatory theory states that the
physiological conséquences of yawning
fonction in brain cooling. Accordingly, the
thermal window hypothesis, an extension of this
theory, states that the expression of yawning
should be sensitive to ambient
température variation. Unlike previous
studies to investigate the relationship between
ambient température and yawning frequency
in humans, which hâve sampled pedes-trians
outside during distinct seasonal conditions and
daylight cycles, the current study controlled
for potential physiological changes that could
émerge from changes in daylight cycles
throughout the year by restricting data
acquisition across a short timeframe. Consistent
with the thermal window hypothesis of the
themioregulatory theory, results showed that
self-reported contagious yawning frequency
varied predict-ably across ambient
température gradients. That is, there was
an increase in self-reported contagious yawning
in moderately warm conditions deviating from
room température, but as ambient
température approached human body
température pedes-trians exposed to a
contagious yawning stimulus were significantly
less likely to report yawning. In addition, the
same pattern emerged for the self-reported urge
to yawn among pedestrians that did not yawn
while being tested.
This study demonstrates within season
variation in self-reported contagious yawning is
largely explained by déviations in
ambient température. Thèse
findings are présent even after
statistically controlling for relative humidity,
time of day, time spent outside prior to being
surveyed, testing day across the two and half
week period, âge of the participant, and
hours of sleep the night before. Overall,
thèse results support the
interprétations of previous naturalistic
reports on human yawning, and provide further
évidence suggesting that the underlying
mechamsms controlling the expression of yawning
are involved in thermoregulatory
physiology.
