Mammal Research Institute,
University of Pretoria, South Africa
We do not know why we and other animals
yawn. Yet because this behaviour is so
commonplace, we are obliged to accept either
that yawning has no physiologic function or
that, because it involves gaping of the mouth,
it has some obscure respiratory function. It is
suggested that yawning is a powerful reflex to
evacuate the palatine tonsillar fossae. As this
strong reflex does not have any immediate
urgency, it he become allocated to times of
minimal inconvenience. This is reflected in the
circadian pattern of yawning in our own species,
and explains why we have come to associate
yawning with drowsiness and boredom, even though
this association is not a causal one. If yawning
does indeed have this function, then by our
social sanctions against we may be supressing
its effectiveness, leading to endemic
tonsillitis in our species.
L'auteur suggère que la fonction du
bâillement serait d'évacuer, par
compression, la sécrétion
amygdalienne. Comme ce puissnt réflexe
n'a pas d'urgence immédiate, il
surviendrait aux moments de moindre
interférences avec la vie quotidienne.
Les conventions sociales en refrénant les
opportunités seraient contraire aux
bénéfices attendus pour la
santé. De là à penser que
bâiller pourrait réduire les
indications à l'amygalectomie et les
prescriptions antibiotiques, voilà un pas
que l'auteur n'a pas peur de franchir.
Hypothèse, hypothèse...
The familiar reflex known as yawning has
been defined as "the involuntary act of opening
the mouth widely, accompanied by deep
inspiration, and frequently stretching of the
arms, shoulders and chest to assist in the
inspiratory act, followed by relaxation of the
muscles involved, usually performed when sleepy
or bored". This definition incorporates, without
formal statement, the conventional wisdom, as
yet unproven, that yawning has an inspiratory
function. It is a remarkable fact that to this
day the cause, physiological function and, most
intriguing of all, the contagiousness of this
everyday habit remain unexplained. This article
explores an alternative function for yawning
behaviour, by looking laterally, not straight
down the throat but slightly to the side.
Yawning, as in the act of opening the mouth
widely, has been recorded in all vertebrate
classes with the exception of the Agnatha. Among
mammals this stereotyped behaviour is most
obvious in the carnivores and primates. Several
authors have cautioned that resemblances between
taxa may be superficial only, and that the role
of yawning may not be the same in all groups of
animals. This discussion will be limited to
mammals, yet even within this group it is
recognized that yawning serves two discrete
purposes: the first is the as yet
unsatisfactorily defined
physiological/homeostatic function, and the
second is a communication function. Provine and
Hamernik have appropriately labelled the
signalling function in humans as "a
paralinguistic signal for boredom", emphasizing
the strong relationship between yawning and
fatigue or drowsiness.
The mystery surrounding the function of
yawning has intrigued us for centuries:
Hippocrates is recorded as having been
fascinated by its role. Numerous hypotheses have
been proposed to fill this rather worrisome gap
in our knowledge about ourselves - none of which
has adequately explained all the observed
features of this complex behaviour. The
hypothesis proposed by Russel, that yawning is
an "automatic impulse" caused by "bad air in the
lungs" intended to "awaken the respiratory
organs into activity" and to effect a
stimulation of the brain through increased
activity of the circulation, seems to form the
basis of present-day conventional wisdom on
yawning.
However, Provine et al. have demonstrated
that anoxia does not play a role in the
regulation of yawning. This can be confirmed by
the reader by holding his or her breath for as
long as possible - this does not result in a
yawn! We may therefore bury the old ideas about
yawning and take a look at some of the
alternative explanations: yawning is an
integrated discharge in a bulbo-reticular motor
structure occurring at a particular level of
activity of the reticular formation
corresponding to a decrease of wakefulness
preceding sleep; yawning has no physiological
role; yawning stretches the facial and neck
muscles; yawning prevents a loss of lung
compliance during normal breathing; yawning
synchronizes sleep in group-living animals;
yawning provides a pulse of thyroid hormones by
squeezing the thyroid gland (unknown source in
ref. 6); yawning is an attempt of the body to
delay onset of sleep and reinforce wakefulness
after sleep; yawning is a means of increasing
attention when sleep is pressing in the face of
a danger or social circumstance; yawning is an
arousal defence reflex.
The following conclusions can be drawn
from current knowledge on the subject.
Yawning is a reflex act whichmay be modified
or induced by higher cerebral activity. It
involves paroxysmal contraction of facial and
neck musculature which, once initiated, cannot
be fully supressed by concious effort (going) to
completion with the inevitability of a sneeze
(ref. 2, p. 119). The strength of the reflex is
such that it occasionally results in dislocation
of the jaw and may even fracture the styloid
process at the base of the skull. Yawning is
often, but not invariably, associated with
stretching. It is regulated by a centre in the
brain, possibly in the medulla. (This may
account for the yawn's strong, involuntary
nature, as the medulla is the site of regulation
of several involuntary physiological
reflexes.)
Yawning is mediated through an as yet
undefined neurochemical sequence involving ACTH,
dopamine D2 receptors and muscarinic cholinergic
neurons (reviewed in ref. 26). Oxytocin has also
been implicated in the cascade of events leading
to yawning, while adrenergic activity
antagonizes the yawning reflex. Yawning is
associated with inactivity and proximity of
sleep in animals, and with inactivity, boredom,
fatigue and proximity of sleep in humans,
although the causality of this relationship has
not yet been defined.
Yawning is contagious within groups of
social animals and in humans; in the case of
humans this contagiousness is extended to
include auto-suggestion evoked by thinking or
reading about yawning. It is the contagiousness
of yawning which eludes explanation by the
various hypotheses on its physiological role.
Because of its close association with drowsiness
and boredom, yawning has become an unmistakeable
signal of these states, possibly leading to the
present-day suppression of yawning as a socially
unacceptable human behaviour and to its
occasional use purposefully to indicate
boredom.
The tonsillar evacuation
hypothesis
Based on three disparate yet equally
incongruous observations, I formulated an
hypothesis to explain yawning behaviour. The
observations were:
1) The conventional respiratory explanation
of yawning does not appear to be valid although
it remains as established conventional
wisdom.
2) A yoga exercise (called Simhasana, or the
Lion Posture) is claimed to help prevent or even
heal a sore throat. (This exercise consists of
elevating the head and protruding the
tongue.)
3) How do the tonsils of humans, other
primates, and carnivores avoid beConurig a
permanent site of infection? The dark, moist,
sheltered tonsillar fossae, with their continual
supply of masticated food and associated
bacteria, seem an ideal site for the
proliferation of microbes and eventual
ulceration and abscessation.
These three unrelated observations gave rise
to the following hypothesis:
The yawn is a complex, centrally mediated,
reflex muscular contraction which ensures
intermittent evacuation of the palatine
tonsillar fossae, thereby ensuring that the
tonsils are exposed to new antigens and
preventing excessive accumulation of foreign
material, micro-organisas and inflammatory
products.
The function of the palatine tonsils is to
identify antigens passing via the pharynx, and
to initiate antibody production against
potentially harmful micro-organisms which have
entered the body via the mouth. Occlusion of the
tonsillar fossae with foreign material and
inflammatory products would eliminate this
function. The tonsillar fossa is a blind
diverticulum, open to the external environment,
which is lined with lymphatic tissue and mucous
membranes, and which is devoid of any apparent
evacuating reflex to clear itself of
obstruction. Such involuntary reflexes exist for
the eyes (blink, lachrymation), the nose
(sneeze), the mouth (swallow, gag) and the lungs
(cough). As this diverticulum is located in the
pharynx, where it is continually exposed to food
particles and bacteria, it is reasonable to
expect that some mechanism would exist to
evacuate foreign material.
A question immediately raised by this
alternative explanation of yawning is: Why then
is this reflex associated with drowsiness,
boredom or proximity to sleep? This can be
explained in terms of the tonsillar evacuation
hypothesis. A mechanism to ensure that the
tonsillar fossae are frequently evacuated would
have to be of an involuntary nature; if it
remained a voluntary function, it would be
improperly or insufficiently performed. If there
was such a reflex, it could be one of two types.
Centrally mediated reflexes controlled by
centres in the medulla oblongata maintain
essential homeostasis. Some of these reflexes
may serve a protective function - for example,
related to the vomition centre, the satiety
centre, etc. Other, more directly released
protective reflexes mediate a reaction to
potentially harmful circumstances - for example,
blinking, swallowing, gagging, sneezing and
coughing. While reflex control of yawning is
indeed required, it need not be of the immediate
type as is encountered in the direct reflexes
such as sneezing and coughing: even total
occlusion of the tonsillar fossae is not a
life-threatening event.
Complete evacuation of the fossae is likely
to require a strong and co-ordinated contraction
of a complex array of phaxyngeal and associated
muscles. If this reflex was of the direct type
it could be elicited at any time by partial
occlusion of one of the fossae. If an animal or
person was sleeping, chewing, drinking,
vocalizing or mating at the time that a yawn was
initiated, this strong and prolonged (3-11
seconds) reflex would be highly inconvenient. If
the animal was swallowing food, fighting,
subduing prey or escaping predation, a yawn
could be a life-threatening event. The
allocation of a paroxysmal reflex such as
yawning to an appropriate time in order to avoid
potential disaster would be an essential event
in the evolution of animal behaviour. Yawning is
not unique in this regard. Spontaneous blinking
is another maintenance reflex which needs to
occur sufficiently often to perform its function
of keeping the eyes clean and moist, but which
is allocated to a more appropriate time if other
- potentially life-threatening - matters become
more urgent. Indeed, there is evidence that
blinking and yawning have a common
neuropharmacological basis.
Most earlier explanations of yawning
considered that its association with drowsiness
and the proximity of sleep has some causal
significance. The implication of the
tonsillar-evacuation hypothesis of yawning is
that its association with sleepiness is a
reflection of its disassociation from activity,
during which its strong, irresistible nature
could be inappropriate. This is supported by the
fact that yawning is generally of a low
frequency during times of high cerebral activity
and during times of physical activity?
Furthermore, it has been shown that adrenergic
activity, characteristic of fight-or-flight
situations, antagonizes drug-induced
yawning.
Suggestions that cerebral anoxia during
inactivity is the cause of yawning (list in ref.
14) do not address the logical extension of this
hypothesis - why do animals and humans not yawn
when they are asleep, when such anoxia could
potentially be greatest? The present hypothesis
implies that there is no association between
yawning and sleep. In fact, yawning could
disrupt sleep and, from a lack of any records of
yawning during sleep, it would appear that the
reflex is supressed during the hours of sleep
just as it is supressed during other times of
inappropriate activity.
The previously inexplicable contagiousness
of yawning conforms to the tonsillar evacuation
hypothesis. Social animals take many of their
behavioural cues from conspecifics within their
group - a phenomenon which has been dubbed
social facilitation ? The social milieu in which
an animal finds itself at any one time is thus a
cue to its own behaviour. As explained above,
yawning in an animal is a reflection of a state
of minimal cerebral, physical and adrenergic
activity: a yawn by an animal is thus a signal
of such an internal state. If another animal in
the group witnesses the yawn this signal (or
releaser) may invoke a yawn through social
facilitation, only if the observer is in an
appropriate physiological state; the induced
yawn is a reflection of the state of relaxation
in the observer, not of sottie magical
contagiousness of the reflex itself. In the
wild, times of relaxation for potential prey
animals (including our own ancestors, who were
heavily preyed upon by leopards and other
African predators) may be unpredictable,
irregular and scarce. If yawning is to occur at
least a few times per day in order to fulfil a
maintenance function, then it should, for
reasons of survival, happen at times of minimal
predation risk as signalled by relaxation in
group members: being caught in the middle of a
5-8-second yawn could mean the difference
between life and death. This may he the reason
for the strength of the contagiousness of
yawning in our and other species,
The tonsillar evacuation hypothesis may lead
to an explanation of speciesspecific differences
in yawning behaviour among mammals. It is well
known that carnivores are the most prodiguous
yawners, leading, through the association
between yawning and sleep, to the common
description of lions as lazy animals! As the
food of carnivores contains particularly high
levels of bacteria, and provides an ideal,
protein-rich culture medium for bacterial
multiplication, they may be obliged to yawn more
frequently than other mammals in order to keep
their tonsils clear.
Finally, it is suggested that the
tonsillar-evacuation hypothesis of yawning may
usefully explain the published observations that
uninhibited yawning leads to a feeling of
refreshment, and that yawning is frequently
followed by swallowing
Conclusion
The hypothesis presented here suggests that
yawning has important health implications for
Homo sapiens: if yawning does indeed function to
evacuate the tonsillar fossae, then by
supressing yawning in public through our social
sanctions against this behaviour we may have
encouraged the development of endemic
tonsillitis in our children, leading first to
the high incidence of tonsillectomy, and later
to the widespread use of antibiotics in its
place. This hypothesis suggests that accumulated
material is removed from the tonsillar fossae by
the act of yawning (if it is not supressed), and
the encouragement of this reflex could become an
adjunct to the routine treatment of tonsillitis
by clinicians. After all, few clinicians would
consider antibiotic therapy for an abscess
without assisting the healing process by
promoting drainage of the accumulated material.
The yoga exercise Simhasana - which stretches
the pharyngeal muscles to the extreme may well
be valuable physiotherapy for both the
prevention and treatment of tonsillitis. For
this reason, I hope this article will motivate
research and clinical trials to test the
hypothesis.
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