- Andrew
C. Gallup. Yawning and the thermoregulatory
hypothesis
-
-
- The thermoregulatory theory of yawning
posits that yawns function to cool the brain in
part due to counter-current heat exchange with
the deep inhalation of ambient air.
Consequently, yawning should be constrained to
an optimal thermal zone or range of temperature,
i.e., a thermal window, in which we should
expect a lower frequency at extreme
temperatures. Previous research shows that yawn
frequency diminishes as ambient temperatures
rise and approach body temperature, but a lower
bound to the thermal window has not been
demonstrated. To test this, a total of 120
pedestrians were sampled for susceptibly to
self-reported yawn contagion during distinct
temperature ranges and seasons (winter: 1.4oC, n
= 60; summer: 19.4oC, n = 60). As predicted, the
proportion of pedestrians reporting yawning was
significantly lower during winter than in summer
(18.3% vs. 41.7%), with temperature being the
only significant predictor of these differences
across seasons. The underlying mechanism for
yawning in humans, both spontaneous and
contagious, appears to be involved in brain
thermoregulation.
-
-
- 1. Introduction
-
- Yawning occurs with an average duration of 4
to 7 sec, and consists of three distinct phases:
an active gaping of the jaw with inspiration, a
brief period of acme corresponding with apnea
and peak muscle contraction, and a passive
closure of the jaw with shorter expiration
[1]. In humans [2], as well as a
handful of other social vertebrates
[3-7], yawning can be categorized into
two basic forms: spontaneous and contagious.
Both forms include similar motor action
patterns, but spontaneous yawns seem to be
triggered by physiological mechanisms of
homeostasis and arousal since they reliably
occur during distinct behavioral contexts
[8, 9] and follow a consistent circadian
pattern [10]. In contrast, contagious
yawns are elicited simply by sensing or even
thinking about the action in others
[11]. Unlike its spontaneous form, which
appears evolutionarily older by its observed
presence in all classes of vertebrates
[12] and early onset in uterine
development [13], contagious yawning
appears to be a more recently derived behavior
as evidenced by its presence in relatively few
highly social species [2-7] and delayed
ontogeny [14-18]. Research investigating
contagious yawning has emphasized the influence
of interpersonal and emotional-cognitive
variables on its expression [4, 5,
19-28], but there have been few attempts to
combine theoretical frameworks when explaining
both contagious and spontaneous effects. Due to
the potential multifunctionality of yawning
across species [12,29], however, recent
reports on social primates have highlighted
potentially important differences in yawn
morphology or intensity [5, 30, 31]
Although it is commonly believed that yawns
serve a respiratory function, experimental
procedures have shown yawn frequency is
independent of brain/blood levels of O2 and CO2
[32]. A more recent theory, which posits
that the motor action of yawning functions as a
brain cooling mechanism [33, 34], has
received growing empirical support [reviewed
by 35]. For example, research on both rats
and humans shows that yawning is preceded by
intermittent rises in brain temperature and
localized mild hyperthermia and then followed by
equivalent decreases in temperature immediately
thereafter [36, 37]. While various
critiques have been proposed regarding the
thermoregulatory theory [38-42], no
study has found evidence contrary to its main
predictions and all current arguments remain
untenable [35, 43].
-
- According to the thermoregulatory theory,
the cooling effects of yawns occur through
thermoregulatory mechanisms of counter-current
heat exchange, evaporative cooling and enhanced
cerebral blood flow [44]. Consequently,
the effectiveness of yawning is dependent on the
ambient air temperature, and the expression of
this behavior should be constrained to an
optimal thermal zone or range of temperature,
i.e., a thermal window. In particular, this
theory posits that yawns should (1) increase in
frequency with initial rises in ambient
temperature, as this stimulates thermoregulatory
mechanisms to control temperatures within a
normal range, (2) decrease as ambient
temperatures draw near or exceed body
temperature, since taking a deep inhalation of
air above one's body temperature would be
counter productive, and likewise (3) should
diminish when temperatures fall below a certain
point, because thermoregulatory cooling
responses are no longer necessary and
countercurrent heat exchange could result in
deviations below optimal thermal homeostasis.
Since both spontaneous and contagious yawns are
indistinguishable, aside from different
triggers, the predictions of the thermal window
hypothesis should apply to both forms.
Experimental and observational research reports
of spontaneous yawning in non-human primates
[10, 5], birds [46, 47], and
rats [48] have confirmed the first two
predictions of this model. Additionally, it was
recently discovered that self-reported
contagious yawning frequency in humans varies
with seasonal climate variation
[49].
-
- In particular, two independent groups of
pedestrians were sampled in an arid desert
climate (Tucson, AZ, USA): the first in summer
(37°C) and the other during 'winter'
(22°C). Contagious yawning frequency was
significantly lower during the hot summer
climate (24% vs 45%), with temperature being the
only significant factor contributing to this
response after controlling for other variables,
such as humidity, sleep and time spent outside.
Here we tested the lower bound of the thermal
window hypothesis by investigating the frequency
of self-reported contagious yawning in a climate
with a colder winter season (Vienna, Austria).
In this case the summer condition provided
temperatures equivalent to those in winter
months of Tucson, while the winter condition
included temperatures at and slightly below
freezing.
-
-
- 4. Discussion
-
- Overall, these results show that
significantly fewer pedestrians reported
contagious yawning during the cold winter (-4 to
7oC), and that, similar to effects observed in
an arid desert climate [49], temperature
was the only significant predictor of this
response when controlling for other variables.
As predicted by the thermal window hypothesis,
reports of yawning were constrained to an
optimal thermal zone or range of ambient
temperature (figure 1). Importantly, changes in
daylight across the seasons cannot account for
these results. First, a particular time frame
was chosen for both studies (between 1 and 3 pm)
whereby contagious yawning frequencies remain
unchanged [53]. Second, the proportion
of people that reported yawning in the summer in
Vienna, Austria (current study) was comparable
to that of the winter in Tucson, Arizona USA
[49], whereas there is a large
difference in daylight hours between these
samples (Summer in Vienna: ± 16h vs. Winter
in Tucson: ± 10h). Lastly, an inverse
seasonal pattern emerged between the two study
locations; i.e., whereas in Tucson there was a
high frequency of reported yawning in winter,
and a low frequency of reported yawning in
summer, in Vienna there was a high frequency of
reported yawning in summer, and a low frequency
of reported yawning in winter. Thus, it cannot
be generalized that people yawn more or less in
winter vs. summer, nor with greater or fewer
hours of daylight. Instead, the ambient air
temperature accompanying the season appears to
determine reported yawn frequency. This report
adds to accumulating research suggesting that
the underlying mechanism for yawning, both
spontaneous and contagious, is involved in brain
thermoregulation. The thermoregulatory theory
provides clear predictions for both the
primitive and derived features of this behavior.
That is, the thermoregulatory benefits resulting
from yawning provide the mechanism by which
arousal or state change can be achieved [8,
10], while the spreading of this behavior,
i.e., yawn contagion, would therefore coordinate
arousal in a group and enhance overall group
vigilance [33]. In addition to enhancing
the basic understanding of why we yawn,
applications from this research include improved
treatment and diagnosis of patients with
thermoregulatory problems [34, 37, 54,
55].
-
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