-
- Laboratory rats usually groom and
yawn in a stress situation (Dourish
& Cooper, 1990). It has been proposed
that they groom to reduce arousal (Isaacson
& Gispen, 1990) and yawn when the situation
inducing the stress is over. Some findings seem
to support this observation. For example, rats
placed in a novel box groom excessively during
the first minutes of exposure. Rats gradually
diminish their activity until movement ceases;
frequently they yawn and fall asleep. Gulls
exposed to a slight, brief disturbance
sequentially display orienting, preening, and
yawning. The gradual return to normality is
marked hy drowsiness and sleepiness (Delius,
1988). Similarly, cats respond to a
disturbing stimulus by sniffing, grooming,
yawning, lying down, curling up, dozing, and
sleeping (Parmeggiani, 1962). Even humans may
respond to stress, particularly to situational
anxiety, by scratching their head and yawning.
Thus, it seems that these behaviors show a
gradation of change from arousal to quiescence,
which may reflect adjustment to the mild stress
that produced them.
-
- Findings from other approaches also offer
evidence that grooming and yawning and other
comfort behaviors are closely related, either
motivationally or functionally. For example, the
electrical stimulation of forebrain and
brainstem structures of gulls produces a
sequence of comfort behaviors similar to that
caused by an environmental disturbance. The
intraventricular irnection of
adrenocorticotrophic hormone in cats produces
excessive grooming, followed bv stretching and
yawning that is accompanied by drowsiness
(Gessa, Pisano, Vargiu,
Crabai, & Ferrari, 1967).
-
- In spite of the extensive work on these
behaviors, there are still no studies that have
analyzed the temporal relationship between
grooming and yawning in situations of mild
stress. Such studies would be relevant for
understanding how grooming and yawnin contribute
to the adjustment to unfamiliar environments. If
grooming reduces arousal and yawning signals the
termination of the event inducing the stress,
then, in situations of moderate stress, rats
would prolong grooming and would put off
yawning. There are two practical difficulties,
however, when testing this prediction. First it
is difficult to find a stressor that is
stressful, but inoffensive, enou-h. A solution
to this problem is to use unpredictable and
predictable stimuli, which cause more and less
stress, respectively ). Furthermore, these
stimuli may be controlled for lasting effects
without freezing the animais. Second, most
strains of rats tend to yawn infrequently,
making it difficult to measure any significant
effect of nonpharmacological experiments.
-
- Our laboratory at Benernérita
Universidad Autônoma de Puebla (Puebla,
Mexico) has produced two strains of Sprague-Dawley
rats, which were selected for high-yawning (HY)
and low-yawning (LY) frequencies (Urbà-Holmgren
et al., 1990). The HY strain also grooms
more than the LY strain in a novel environment.
Therefore, HY rats offer unusual advantages to
study the relation between yawning and grooming
in response to stress situations.
-
- This study investigated the temporal
relationship between grooming and yawning in HY
rats exposed to mildly stressful situations. The
use of brief electric font shocks (which are
disturhing rather than painful) allowed us to
control the intensity of the stres, event. In
addition, we were able to vary the effect of the
foot shocks by applying, sequences at fixed
intervals (Fis) or randorn intervals (Ris). In
the first case, the rats could anticipate the
application of the shocks and so adjust
accordingly. In the second case, the shocks were
unpredictable, and hence, the rats experienced a
situation in which it was more difficult to
adjust. [...]
-
- Discussion :
- Although grooming and yawning have been
extensively studied, this is the first work that
documents their relationship during mild stress.
The findings suggest that stress delays the
onset of yawning and prolongs grooming. The
number of foot shocks does not seem to be a
determining factor in such effects, as the rats
exposed to FI received twice as many shocks as
those subjected to RI, yet yawning diminished
more with the latter than with the former. In
addition, foot shock number was not a
significant predictor of yawning. However, this
result should be treated with caution because
the data analysis was based on three values of
the independent variable (i.e., foot shock
number) and because shock number could be
confounded with the type of shock regime. The
results, though, support the explanation that
the degree of predictability of the appearance
of the foot shocks determined the way the rats
responded. This interpretation accords with the
suggestion that the predictability of a stressor
or pleasurable stimuli modulates its
physiological and behavioral consequences.
-
- Because the NS group was in an undisturbed
environment (the experimental chamber), it is
reasonahle to assume that the rats habituated to
it and, hence, that the distribution of
groorning and yawning in such a condition
reflected that adjustment. Similarly, the
decrease in grooming and the increase in yawning
in the second session is likely to be a
consequence of the adjustment to the chamber.
Other studies (Hannigan & Isaacson, 1981;
Jolles, Rompa-Barendregt, & Gispen, 1979;
Van Erp, Kruk, Melis, & Willekens-Bramer,
1994), however, have not detected changes in
groomin- across repeated presentations to
stressors (e.g. novelty handling, restraint).
This disagreement with the results presented in
this study may arise from differences in
experimental conditions or from strain-specific
responses to novel environments.
-
- In any case, if grooming is a direct
response to stressful events (Van Erp et al.,
1994), then it should decrease after repeated
exposures of rats to the same stressor, unless
it is a severe stressor. Similarly, if grooming
occurs after the end of the stressful event,
then an increase should appear later in trials.
Our findings, however, show that in every group
the highest number of groomings occurred within
the first half hour of presentation to the
stress situation. This suggests that, at least
for the levels of arousal produced in the
present experiment, grooming is the initial
response to the stressful situation. The
findings also suggest that whether foot shocks
increase or decrease, total grooming is not, in
tact, the issue; what matters is how its
distribution changes in response to the
stressors. In other words, there seems to be no
direct relation between the magnitude of the
stressor and the amount of grooming, but rather
a dynamic shift in behavior as a function of
context.
-
- Although foot shocks delivered at Fls did
not affect total grooming, they probably delayed
its decline and prevented the quik increase of
yawning. It, therefore, seems that the foot
shocks caused arousal to increase, which later
diminished as the foot shocks became predictable
and the rats were able to adjust to them as
expected. An alternative is that the rats showed
an anticipatory fear of encountering an
environment that had prevously perturbed them.
In such case, fear would be expected to
increase with the second exposure and
consequently, yawning would be expected to
diminish and grooming to increase. The
results are not consistent with this possibility
rather, they suggest an adjustment to the
experimental condition. This is supported by the
similarity of the temporal course of the
behaviors between NS and FI groups, particularly
in the second session. This explanation accords
with the idea that the ability to cope with
stressors by predicting their appearance may
modulate the behavioral and physiological
consequences of those stressors. However, this
idea is based on studies that frequently used
conditioned stimuli to search for associations
with physiological or behavioral variables,
including grooming. Rarely, however, has the
likelihood of the occurrence of an unconditioned
stimulus been investigated as a predictive clue
to hehavioral responses. Our findings suggest
that rats may use such clues to initiate
behavioral actions in response to stress.
-
- A different scenario was that of RI in which
the HY rats could not anticipate the appearance
of the foot shocks, as they were randomly
presented. The uncertainty so produced might
have caused the rats to be anxious, as has been
suggested in other studies. Even biochemical
disturbances similar to those of depression may
appear if rats are subjected to chronic
treatment with unpredictable stressors (Katz,
Roth, & Carroll, 1981 ). The low rates in
grooming and yawning during the first session of
RI could be attributable to behavioral
inhibition because of the incapacity of the rats
to adjust to the situation, as revealed by the
lack of groorning habituation. Low levels of
grooming associated with decreased motor
activity in emotional rats have also been
reported as well as immobility with ultrasonic
vocalizations in repetitive encounters of rats
with resident conspecifics or potenlial
predators. Thus, the decrease in yawning and
grooming could be the result olf behavioral
inhibition.
-
- However, in the second session, yawning, and
grooming increased; yawning was expected to rise
but grooming was not. The fluctuations in
grooming and yawning that were sometinies upward
and sometimes downward suggest that there was a
trade-off between them, as though their courses
interfered with each other. A similar trend has
been proposed to account for the switching
between incompatible behaviors in a model for
the crayfish's behavioral commands during stress
(Edwards, 1991 ). Such oscillations increase
with the size of an inhibitory coefficient,
which is a quantitative driving factor of the
behavioral commands. leading to higher
excitation conditions. Therefore, the
fluctuations of yawning and grooming may be the
result of excitation caused by the
unpredictabilily of foot shocks.
-
- How do grooming and yawning help HY rats to
adjust to a stress situation? Our findings
indicate that the prevalence of grooming or
yawning reflects opposing arousal levels:
disturbance and quiet. In this study, the
relative dominance of one behavior over the
other represents shifts in either direction,
indicating that HY rats make constant
adjustrnents to cope with arousal changes. It
is thought that grooming serves for relaxation
and yawning for arousal (Baenninger,
1997). Consequently, the action of yawning
would be to prevent rats from being relaxed.
However, as they and other species eventually
fall asleep after exposure to stress situations,
it is possible that yawning modulates relaxation
so as to prepare for sleeping. The observation
that in the NS situation the HY rats yawned
frequently before sleeping, with the diminution
of yawning and the absence of sleepy rats in the
foot shock conditions, supports the role of
yawning in modulation of relaxation. In
situations of uncertainty, it may be more
difficult for the rats to adjust to the context
so that they prolong grooming and delay yawning.
But, if the disturbing condition continues, then
both behaviors tend to co-occur, leading to
conflict. The same is probably true in primates
(e.g., monkeys. Maccica fascicularis and M.
fuscata), including humans, because grooming and
yawning have been associated with situational
anxiety. If so, then in anxiety disorders, such
as obsessive-compulsive disorder, grooming and
yawning would tend to co-occur.
-
- In summary, this study is the fïrst
empirical evidence of' the interaction of
grooming and yawning in situations of mild
stress. The findings show that although grooming
is the forward reaction to diminish arousal,
yawning completes the process. Both are mutually
inhibitory but complementary behaviors that
serve to calm the animal down in arousing
conditions. These findings are potentially
relevant to an understanding ofhow animals cope
with stress-related disorders.
-
- Eguibar
JR et al Behavioral differences between
selectively bred rats: D1 versus D2 receptors in
yawning and grooming Pharmacology, Biochemistry
and Behavior 2003; 74; 827-832
- Eguibar JR
et Moyaho A Inhibition of grooming by
pilocarpine differs in high-and low yawning
sublines of Sprague-Dawley rats. Pharmacoology
Biochemistry and Bebavior 1997; 58: 2
317-322
- Moyaho A et
al Induced grooming transitions and open
field behaviour differ in high- and low-yawning
sublines of Sprague-Dawley rats Animal Behavior
1995; 50; 61-72
- Moyaho A,
Valencia J Grooming and yawning trace
adjustment to unfamiliar environments in
laboratory Sprague-Dawley rats J Comparative
Psychology 2002; 116; 3; 263-269
- Pandiculation:
the comparative phenomenon of systematic
stretching AF Fraser
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