Yawning is a stereotyped behaviour akin
to a reflex. It is mysterious as to its
function, and medical people have tried ever
since antiquity to assign it a physiological
role. All theories about breathing and
circulation proposed from the days of
Hippocrates until the middle of the XXth
century have been debunked by
contemporary physiological explorations.
Neurophysiology is now starting to explain
its cerebral mechanisms. Psychopharmacology
has integrated it to preclinical tests on Man
and animals in order to better classify
mechanisms of action in the various ancient
and developing psychotropic families.
Yawning circumstances
Among mammals, there are three types of
morphologically identical yawns occurring in
three distinct situations: situations
relative to circadian rest-activity rhythms,
situations relative to feeding, situations
relative to sexuality or social
interactions.
Situations relative to circadian
rest-activity rhythms. In human, the
most pronounced occurrences at at awakening
in the morning, in association with the
stretching of muscles (pandiculation), and as
sleep is about to occur, without stretching,
as well as in any condition of lessened
vigilance. Repetitive and monotonous
activities trigger repeated yawns as have
shown studies of individuals at their work
stations. Activities involving a weekly
change of shift, called 3 times 8, help
trigger episodes of somnolence preceded by
salvos of yawning, lack of sleep and
perturbations of circadian rhythms
(cortisol). When driving a vehicle,
particularly on long stretches of highways,
repeated yawns are an alarm signal warning
the driver about the risk of falling asleep.
R.
Provine proposed to students that they
watch both stimulating and monotonous videos.
Not surprisingly, he found a significant
correlation between the frequency of yawns
and the viewing of monotonous videos.
Environmental conditions such as being
confined to a small room or subjected to
excessive heat proved to be factors that
augment the frequency of yawns caused by
boredom. M.
Greco et R. Baenninger have established
that the frequency of yawns is high under
four daily life situations: reading,
travelling in public transport, driving and
waiting. In children, there is also a
relationship between yawning and schooling
rhythms and lifestyles. When children move
from kindergarten, which is not really
stressful, to the first year of grade school,
where they learn to read and count, the
proportion of yawners increases
markedly.
G.
Schino et F. Aureli have shown, through
the study of subway users in Rome at rush
hour, that there is no difference in the
frequency of yawns between men and women
(38). Yawning evolves over the course of
human ontogeny: from high frequency during
the first year of life to a gradual reduction
as age advances, in parallel with the
decrease in the the total time spend asleep
during the nycthemeron. Just as there are
light and deep sleepers, there are light and
deep yawners. Among non-human primates,
unspecific yawning is common to all species
regardless of age and sex, and constant as to
its parameters in a given male before canines
push out and after they have reached full
size. Yawning happens before sleep but mainly
after sleep (or various phases of daytime
rest). This type of yawning which occurs
during changes of life rhythms and activities
is common throughout the animal kingdom. For
instance, the
ostrich yawns and stretches as soon as it
has hatched, as well as throughout its life
following a prolonged period of immobility,
when it wakes up, etc.
Situations relative to
feeding. While observing lions at the
Philadelphia zoo, R.
Baenninger noted an increased frequency
of yawning before feeding times, as well as
in function of the outside temperature
(maximum of 20 to 24°C). In the wild,
carnivores yawn when they wake up and before
they go hunting. Carrion-eating animals, such
as hyenas, yawn repeatedly while circling a
dead animal before tearing it to pieces.
Similar observations of anticipatory yawning
at meal time have been noted among mandrill
monkeys in captivity.
Among birds, yawning immediately after
eclosion can be interpreted as a feed-me
signal addressed to the parents. The approach
of any other member of the species elicits a
fear recoil, which shows that the yawn is
specifically related to the food-providing
adults.
B.
Holmgren has experimented on the effect
the feeding rhythm of Sprague-Dawley rats has
on the schedule of yawns, but under constant
lighting conditions to eliminate the
light-darkness rhythm. After three weeks of
getting them used to one meal a day always at
the same time, he found increases in the
number of yawns in the hour preceding feeding
time, as well as in the moving-about
activity. After three days of food
deprivation, this behaviour faded away, as
well as all spontaneous yawns or those
pharmacologically induced with apomorphine.
It would appear that, beyond a day-night
rhythm, there is a rhythm associated with
ingestion which is expressed by an increase
in the frequency of anticipatory yawns. This
condition is in correlation with a rise in
the concentration of circulating
corticosteroids.
Of note is the fact that hypocretine
or orexine is a neuromediator involved in
the stimulation of vigilance and appetite. It
so happens that the injection of hypocretine
in the paraventricular nucleus of the
hypothalamus also triggers yawns.
Though long claimed non-existent, yawning
is also found in herbivores, but with a
reduced frequency. The energy contribution of
grasses is low, as opposed to that of meat.
Herbivores spend a great amount of time
feeding, do not sleep as much as carnivores
and yawn infrequently. There is a correlation
between the number of yawns, the duration of
sleep, particularly of paradoxical sleep, and
the calorific value of food intake. It seems
logical to propose the following: the more an
animal is under pressure from predators, the
case of herbivores for instance, the less it
sleeps and the less it yawns. The more it
ingests a high amount of calories in a short
span of time, the case of predators for
instance, the more it can afford to sleep for
long stretches and the more it yawns. The
exception to this rule is found in non-human
primates, the frugivorous, whose paradoxical
sleep and number of yawns is comparable to
that of carnivores.
In human, fasting is also a trigger of
yawns (hypoglycemia). Culinary excesses with
overeating and alcohol consumption quickly
lead to somnolence accompanied by yawns.
Situations relative to sexuality or
social interactions. Yawning also
occurs outside the periods of sleep or rest,
after various social interactions linked
notably to sexuality or conflicts. Data is
not available for all animal species.
Examples used here refer to a fish and to
macaque monkeys.
Microspathodon
chrysurus, a fish of the Pomacentridae
family, exhibits natural aggressivity.
Yawning in fish, as observed in aquariums, is
associated with an interruption of swimming
motions, the stiffening of the body as a
whole and of the fins, along with a wide
opening of the mouth. This interruption of
swimming motions translates into a tendency
to sink vertically. In examining the
behaviour of this fish, it is possible to
identify yawns related to internal stimuli.
As mentioned earlier, yawns develop before
feeding time and during reduced motor
activity; turbulent water forces regular
swimming motion, which inhibits yawning.
However, Microspathodon chrysurus also yawns
in response to external stimuli. Two types of
experiments involving calm waters have been
conducted. Dangling a lure mimicking a
conspecific or dangling a grey moving ball
triggers an increase in the degree of
excitation. Chronologically, there are 2 to 4
yawns in the 30 seconds following the
dangling, then the body takes on a dark-green
hue, swimming picks up speed with frequent
direction changes at 90°, signalling a
state of excitation that will last about 10
minutes. An immobile ball does not trigger
the same reaction as the image of a
conspecific. This fish lure may be the target
of agressive attacks and will sustain this
state of excitation as long as it is dangled,
whereas the grey ball in motion will
gradually lose its triggering effect. These
observations confirm that yawning is the
first manifestation that the level of
excitation in the fish has increased, either
because the water is being agitated or
because a conspecific has been seen. Adding a
surrenal- stimulating hormone (ACTH) to the
aquarium water, which triggers the release of
the stress hormone cortisol, starts the same
chronology of events: yawning, darkening of
the skin, agitated swimming.
Males yawn more frequently than females,
be it among rodents (rats, cobayes) or
non-human primates. This behavioural
characteristic is widely used in
neurophysiological experiments.
Among non-human primates, the frequency
grows as secondary sexual traits develop
(testicular drop, growth of canines), and
reaches its maximum in adult males. Yawning
is partially governed by androgens; the
castration of adult macaques causes a marqued
reduction of yawns, while injections of
exogenous dihydrotestosterone re-establishes
their frequency. The injection of a
non-steroid anti-androgen (hydroxyflutamide)
blocks the effects of testosterone
injections, thus reproducing the effects of
castration.
Such yawns, which are basically
restricted to adult males and dominant males
in the group can be qualified as "emotivity
yawns" (BL
Deputte), which indicates that they are
triggered by a "psychic tension". What
communication value can yawning have in a
social group? In such a context, anything
that can be witnessed by a partner can have
meaning. Over the course of evolution, actual
communication signals have arisen to act as
codes shared by all members of a given
species and have led to the modification of
the fellow creature's behaviour to whom the
signal is adressed or who has noticed it. The
yawn can be perceived by a fellow creature
without being specifically adressed to same.
Such signal always appears at the end of an
interaction during which a more or less
significant quantity of signals, stricto
sensu, have been exchanged. It has not been
possible to detect any modification of the
fellow creature's behaviour at the sight of a
yawn. Therefore, it can only be seen as
having a secondary communication value. Since
the emotiviy yawn is associated with a given
individual, it can serve as a reinforcement
of the particular rank this individual has in
the group. Such a function cannot be assigned
to an unspecific yawn which, though
morphologically identical, is not associated
with any individual in particular.
The testosterone-dependent yawn uncovers
and exposes the male's long canines.
Non-human
primates being essentially vegetarians, a
utilitarian perspective would explain this
behaviour as a ritualised exhibition of
"weapons of dissuasion". But, this hypothesis
is not supported by observation, certainly
not among the white-cheeked mangabeys and the
long-tailed macaques. In the case of the
emotivity yawn, more particular to males
therefore more susceptible to expose powerful
canines, it occurs at the end of interactions
involving exchanges of numerous communicative
signals allowing for the specific pattern of
this interaction. To the contrary of other
signals, the yawn, and of course the canines,
have very little chance of being noticed. An
electromyographic analysis has shown that
during the yawn the exposure of the canines
is purely passive and dependent on the
maximal opening of the mouth. This study has
underlined an essential difference between a
threat, a specific signal to stay away, and a
yawn. Three characteristics support this
difference:
a- Whereas the duration of a threat and
its intensity are totally dependent of the
partner's behaviour, the sequence of events
during a yawn is unchanging and entirely
independent of any behavioural trait of the
partner.
b- Among all primates, a threat comprises
at least an intense visual fixation aimed by
the emitting party at the threatened one.
While this visual fixation is accompanied
solely by a retractation of the scalp among
white-cheeked mangabeys, it involves the
opening of the mouth among long-tailed
macaques. This visual fixation is held as
long as the partner has not backed off or
ended the interaction. Yawning does not
necessarily imply, at its inception, that the
yawner is looking at a fellow being; once
triggered, it involves the lifting of the
head, usually accompanied by the closing of
the eyes.
c- Finally, when a threat is a specific
signal devoid of ambiguity, the canines
remain hidden, whereas they are largely
exposed during a yawn that is not in any way
directed at a given partner. Under no
circumstances can yawning be seen as a type
of threat.
U.
Halder and R. Schenkel have described the
repeated yawns of bovids which, after having
calved, lick the amniotic liquid and the
embryonic sac. Is this an olfactive component
of yawning resting of the mother's specific
recognition of her calf later? (or a yet to
be investigated Ocytocine effect?).
