mise à jour du
15 février 2009
Animal Behaviour
Yawning and thermoregulation in
budgerigars, Melopsittacus undulatus
Gallup AC, Miller ML, Clark AB
Departmentof Biology, Binghamton University


 Read objections and review and letter 73
Andrew C. Gallup. Yawning and the thermoregulatory hypothesis
Yawning is a widespread behavioural response expressed ina ll classes of vertebrates. There is, however, little agreement on its biological significance. One current hypothesis states that yawning serves
a thermoregulatory mechanism that occurs in response to increases in brain and/or bodytemperature. The brain-cooling hypothesis further stipulates that, as ambient temperature increases and approaches (but does not exceed) body temperature, yawning should increase as a consequence. We tested this hypothesis in a sample of 20 budgerigars, Melopsittacus undulatus, through the manipulation of room temperature.
Birds we reexposed to three separate conditions (control temperature (22C), increasing temperature (22&endash;34C), and hightemperature (34&endash;38C)) in a repeated measures design, with each condition lasting 21min. The incidence of yawning differed significantly across conditions (4.20 +-2.39) yawns per bird in the increasing temperature condition, compared to 2.05 +- 1.90 and 1.25 +- 0.72 yawns per bird, in the high temperature and control conditions, respectively). Thesef indings are consistent with the hypothesis that yawning serves a thermoregulatory function.
Yawning is characterized by a large gaping of the mouth, accompanied by a deep inhalation of air, and a shorter expiration. Although typically studied in humans, yawning is a widely expressed, stereotyped phenomenon occurring in all classes of vertebrates (Baenninger 1987). but little is known about the function of yawning in any species. Research has shown that yawning coincides with a variety of neurochemical interactions in the brain (Argiolas & Melis 1998). While the neurological mechanisms underlying yawning are not entirely clear, research on yawning under laboratory conditions has proven valuable in understanding the physiopathology of certain diseases, as well as the action of new drugs (Daquin et al. 2001). However, numerous attempts at identifying the adaptive or biological significance of the yawn (reviewed by Smith 1999) have led to little consensus (Provine 2005).
Yawning is under involuntary control, and it cannot be inhibited or elicited by individual commahd (Provine 2005). Yawning is also contagious in humans and some nonhuman primates (Anderson et al. 2004; Paukner & Anderson 2006). In humans, attempts [o shield a yawn do not prevent its contagion (Provine 2005). The spontaneous and uncontrollable nature of yawning across species lends support for it having adaptive significance. In humans, yawning occurs before birth as early as 20 weeks after conception (Sherer et al. 1991), testifying to its importance postnatally, as many important postnatal behaviours begin to appear prenatally (e.g. breathing movements, swallowing and eye movements) before they develop any functional significance (Nijhuis 2003).
Throughout the lives of healthy adult humans, yawning occurs in a consistent pattern (Gallup & Gallup 2008), occurring most often during the first hour after wakening and the last hour before sleeping (Provine et al. 1987a; Baenninger et al. 1996; Zilli et al. 2007). Similarly, variation in yawning among rats appears to have a circadian pattern (Anias et al. 1984). In addition, stretching has been shown to accompany yawning almost 50% of the time in humans (Provine et al. 1987a). Researchers have attributed such findings to an association between yawning and increases in arousal and activity that accompany transitional states (Provine et al. 1987a; Greco & Baenninger 1991; Greco et al. 1993; Baenfinger et al. 1996). Aside from observational reports, comparative studies investigating yawning in nonhumans are few and the ethology of yawning in nonhuman species remains mysterious. Baenninger (1987) proposed that yawning may actually serve different functions in different species. Nevertheless, the tendency for yawning to correspond with state changes in humans (Provine et al. 1987a; Greco et al. 1993; Baenninger et al. 1996) suggests possible adaptive contexts for this behaviour across species. New evidence suggests that yawning may be involved in thermoregulation (Gallup & Gallup 2007, 2008) and may act as a braincooling mechanism. This hypothesis has been developed for humans but suggests one general utility across endotherms. Based on this theory, the yawn serves as a cooling mechanism that keeps the brain and/or body in thermal homeostasis, thus maintening mental efficiency. Increases in facial blood flow resulting from a yawn may operate like a radiator, removing hyperthermic blood from specific areas, while introducing cooler blood from the lungs and extremities. Increases in facial blood flow may alter cerebral blood flow as well (Heusner 1946; Barbizet 1958; Zajonc 1985). Consistent with the radiator hypothesis of human brain evolution (see Falk 1990), the respiratory and arterial actions that follow the yawn match those required to cool the brain effectively. An increase in cranial blood flow due to yawning may aid in the dissipation of heat via the emissary veins. In humans, increased arousal, as measured by skin conductance, occurs during yawning (Greco & Baenninger 1991), and vasodialation has been hypothesized to promote further cooling. Gaping of the mouth and deep inhalation of air taken into the lungs during a yawn can also alter the temperature of the blood travelling from the lungs to the brain through convection (Gallup & Gallup 2007). This hypothesis proposes that it is the temperature of the air that gives the yawn its utility, not the air's composition. In fact, variation in 02 and/or CO2 concentrations has no effect on yawn frequency (Provine et al. 1987b).
The brain-cooling hypothesis leads to several testable predictions. First, it predicts that there will be a fairly narrow range of external temperatures, a 'thermal window', over which yawning can be triggered (Gallup & Gallup 2007, 2008). As ambient temperature rises, it becomes increasingly difficult to maintain thermal homeostasis, but it also becomes less effective to lower body temperature by using environmental heat transfer. The model's central predictions are that (1) the frequency of yawning should rise as ambient temperature approaches body temperature and (2) yawning should not occur when ambient temperature reaches or exceeds body temperature, because its cooling component will no longer occur. Likewise, when temperatures fall below a certain point, yawning should cease to be adaptive and could become maladaptive by sending unusually cool blood to the brain. This hypothesis is intriguing because it applies generally across endotherms and suggests differences in the importance of yawning for different species, dependent on both morphology and environment.
To test the central hypothesis, we manipulated the ambient temperature experienced by budgerigars in a laboratory environment while recording yawning, stretching and gular fluttering, a thermoregulatory response that promotes evaporative cooling in birds experiencing heat stress (Bartholomew et al. 1968). Body temperature is a balance between heat production and heat dissipation, and raising the ambient temperature would be expected to trigger compensatory thermoregulatory mechanisms. We therefore hypothesized that the frequency of yawning would increase in response to rising ambient temperatures, as opposed to when temperature is held constant. We chose M. undulatus as our study species because of its large relative brain size (Iwaniuk & Nelson 2002) as well as the fact that its natural habitats include arid Australia where it would be subject to wide swings in temperature. In addition, a recent study found n evidence for contagiûlls yawning in this species (M. L Miller, S. M. Vicario & A. B. Clark, unpublished data). Thus, we were able to investigate the frequency of yawning within small groups with confidence that any individual's yawns would not influence yawning in others.
The frequency of yawning was significantly affected by ambient temperature. As ambient temperature increased, birds were over twice as likely to yawn, compared to when temperatures were held constant (both low and high). Yawning occurred less frequently at low temperatures (1.25 + 0.72 yawns per bird), slightly more when held at high temperatures (2.05 ± 1.90 yawns per bird), and most frequently with increasing temperatures (4.20 ± 2.39 yawns per bird). Likewise, the strong quadratic correlation between yawning frequency and temperature supports the relationship between yawning and ambient temperature change. These data are consistent with the hypothesis that yawning, like gular fluttering, is connected with thermoregulation. Stretching, although often seen with yawning at control temperatures, was not influenced by ambient temperature manipulation.
Although the rate of yawning peaked around 30°C, during the increasing temperature condition, it began to decrease in frequency as temperature further increased (i.e. 34-38 °C during the high temperature condition). This trend appeared to be influenced by the prevalence of gular fluttering; while fluttering was originally positively correlated with the incidence of yawning at around 25.6 °c, this trend was reversed by the time all birds were engaged in this behaviour (i.e. 35.4°C). As gular fluttering is widely associated with thermoregulation (Bartholomew et al. 1968), we argue that this respiratory mechanism may supplant yawning, especially when temperature exceeds some critical point around 35.4 oC. That is, yawning may be inhibited when continuous gular fluttering is required to prevent hyperthermia. Yawning appears to be an initial response associated with thermal homeostasis; as temperature increases and heat dissipation becomes more difficult, more effective regulatory mechanisms, such as the gular flutter, are triggered. This corroborates the view that yawning serves as a compensatory rather than primary cooling mechanism (Gallup & Gallup 2007). Furthermore, as ambient temperature approaches body temperature, one would expect yawning to diminish in frequency (Gallup & Gallup 2007). Although the ambient temperature in this study never exceeded budgerigar body temperature (39.5 °C), attenuation of yawn frequency at 35.4 oc is consistent with this prediction. At 35.4 °C, the cooling capacity of the yawn (i.e. difference between ambient and body temperature) was less than that at lower temperatures.
The incidence of stretching was not affected by ambient temperature. There was no difference in stretching among temperature conditions, and the incidence of stretching did pot vary across the range of temperatures within this experiment (P> 0.9), nor was there a correlation between the incidence of yawning and stretching. Within the increasing and high temperature conditions, there was also no observed relationship between stretching and gular fluttering. Therefore, we propose that unlike yawning, stretching appears to be independent of thermoregulation in this species. In humans, at room temperature, stretching is accompanied by yawning nearly half of the time (Provine et al. 1987a), with the incidence of yawning predicting stretching, but not vice versa. The yawn/stretch relationship in budgerigars should be studied at lower ambient temperatures before a similar relationship can be dismissed.
This comparative evidence provides novel insight into yawning as a thermoregulatory mechanism, revealing that rising ambient temperature promotes excessive yawning in parakeets. This effect
could be tested further among an array of species, including humans. Recent interdisciplinary research has strengthened this connection between yawning and thermoregulation (Gallup & Gallup 2008). A growing body of medical and physiological evidence implicates instances of abnormal thermoregulation and heat stress with symptoms of atypical yawning. For instance, there is a link between the negative symptoms of epilepsy, multiple sclerosis and migraine headaches and increases in the ambient temperature. More importantly, individuals suffering from these disorders also yawn excessively (Gallup & Gallup 2008). Therefore, applications of this research range from basic physiological understanding to improved health and treatment of patients with thermoregulatory dysfunction.
These findings have significant ramifications regarding the way in which we study yawning in humans and other species. Yawning is widely associated with states of fatigue, frequently occurring when an individual wakes or gets ready for sleep (Provine et al. 1987a; Baenninger et al. 1996). Evidence shows that sleep and thermoregulation appear to be interrelated, with prolonged sleep deprivation in rats producing an increase in deep brain temperature (EverSon et al. 1994). Likewise, it has been argued that core body temperature and sleep vary inversely (Gilbert et al. 2004). Following this rationale, subjective ratings of sleepiness are correlated with increases in body temperature (Krauchi et al. 2005). These results may explain the empirical correlates of yawning with transitional states of fatigue. Moreover, the metabolic activity and locomotor changes associated with awakening may disrupt thermal homeostasis, and this underlying change in thermal homeostasis may trigger the association between yawning and awakening.
The thermoregulatory model complements and may also help explain models highlighting the association between yawning and other transitional states, such as alertness and arousal (Greco & Baenninger 1991; Walusinski 2006). Gallup & Gallup (2007) proposed that the cooling component of yawning may actually facilitate these processes (i.e. mental efficiency and vigilance) by reinstating optimal brain temperature. Moreover, this model has implications for understanding contagious yawning in humans as well as in nonhuman primates (e.g. see Anderson et al. 2004; Paukner & Anderson 2006), as the infectiousness of the yawn may have evolved to facilitate group vigilance.
In our study, it was unclear whether the change in yawn frequency resulted from the increase in ambient temperature or the change in temperature (irrespective of direction). If yawning serves to maintain optimal thermal homeostasis, yawning frequency should increase with temperature change. Thus, a decreasing temperature condition may impose similar effects on yawn frequency. Our results remain consistent with the view that yawning is associated with behavioural state change (Provine et al. 1987a; Greco et al. 1993; Baenninger et al. 1996). In addition, we propose that the difference in yawning frequency among trial procedures may be due in part to the control condition in the first trial inadvertently lengthening the initial acclimation period. As a result, the second trial procedure (increase-high-control) may have coupled an already mildly stressful situation of environmental change with the manipulation of ambient temperature, increasing the likelihood of hyperthermia in the first two thermal conditions (Cabanac & Guillemente 2001).
Because of the potential multifunctionality of yawning across species (Baenninger 1987), we suggest that further comparative research is necessary to more completely understand the relationship between yawning, ambient temperature and other factors. For instance, the thermoregulatory model suggests that there should be differences in the potential adaptive significance of yawning between endotherms and ectotherms, as well as between endothermic species selected to different degrees for cooling abilities in challenging thermal environments.
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Andrew C. Gallup. Yawning and the thermoregulatory hypothesis
mise à jour du
26 décembre 2010
Animal Behaviour
Yawning and thermoregulation in budgerigars:
lack of support from results
Leandro de Castro Siqueira
Labo rató no de Comportamento Animal, Universidade de Brasilia, Brazil


Many attempts have been made to find physiological stimuli responsible for evoking spontaneous yawning. Smith (1999) enumerates many functions that have been suggested for yawning, ranging from increasing alertness and increasing oxygen levels in the blood to evacuation of infectious substances from tonsils. But Smith (1999) also points out that those propositions are deficient in presenting empirical verification to validate them. Therefore, yawning remains remarkable for being a behaviour so widespread among vertebrates whose physiological functions are still unclear (Baenninger 1987). The contagiousness of the yawn in certain social animals suggests it has not only physiological functions, but maybe also a social role (Deputte 1994). It can signal tiredness and boredom, and is observable in animals in stressful psychosocial situations (Bell 1980; Maestripieri et al. 1992; Baenninger 1997).
The emergence of different theories to explain the yawn is very positive, and should be encouraged, for it contributes new elements and points of view about this issue. Features of the yawn unidentified up to the present moment can be extremely valuable in better understanding this behaviour, as it is not just a simple reflex of short duration, but has a complex spatio-temporal organization with facial, respiratory and other components (Argiolas & Melis 1998).
Therefore, the hypothesis of yawning as a compensatory brain-cooling mechanism (Gallup & Gallup 2007, 2008) has its
merits as a new approach to the study of yawn. But this hypothesis still deserves more evaluation, considering that anatomical and physiological variables associated with selective brain cooling were not considered. In humans, for instance, Mekjavic et al. (2002) demonstrated that inhalation of cold air is not capable of influencing the temperature of the brain. Based on this finding, airflow at ambient temperature that occurs during a human yawn is unlikely to be enough to cool the brain. But Gallup et al.'s (2009) experiment used birds, in which moist surfaces of nasal and buccal cavities and of the eyes are known as evaporative heatdissipating organs (Caputa et al. 1998; Jessen 2001). In this case, yawning as a brain-cooling mechanism is a hypothesis worth investigating.
Gallup et al. (2009) interpreted the results of their study with budgerigars as consistent with the hypothesis that yawning is connected with thermoregulation'. It is worth noting, however, that Gallup et al. (2009) presented no evidence that yawning maintained or altered the birds' body temperature. Their results revealed, in effect, that yawns occurred more frequently when the ambient temperature was increased. The allegation that yawning is an initial response associated with thermal homeostasis also lacks support from their data.
In animal behaviour studies, as Milinski (1997) points out, the apparent correlation between variables can lead the researcher to draw unjustified conclusions. The results obtained by Gallup et al.(2009) unmistakably revealed that yawning occurred more frequently with the increase in ambient temperature or with changes in temperature (irrespective of direction). But, unlike gular fluttering, whose function as an evaporative-cooling mechanism has already been established (Lasiewski & Bartholomew 1966; Bartholomew et al. 1968), their results did not provide evidence that yawning is used to alter the temperature of any part of the body, because no measure of bodily temperature was taken. And, even the correlation between variation in ambient temperature and the occurrence of yawning may not be direct. It is important to observe that other variables not considered in their experiment may also have influenced elicitation of yawning. To investigate these variables, the experiment should be designed so as to adopt a more suitable experimental design.
Some studies suggest that yawns might have a derived function of social communication (Smith 1999), acting as a signal in social species (Rasa 1971). In some fish (Microspathodon chrysurus) and monkeys (Macaca nigra), yawning occurs in contexts of excitement and tension for the performers (Rasa 1971; Hadidian 1980). These findings are in agreement with Sauer & Sauer (1967), who found that yawning induced relaxation of tension in groups of excited South African ostriches. The rapid temperature changes in the experiment of Gallup et al. (2009) can be considered a context of high tension for budgerigars. It is not implausible, then, that the budgerigars' yawns may have been produced as a form of social signal among birds in the same cage. Yawning is not contagious in budgerigars, so when an individual yawns, it may not trigger yawns in other individuals within the group, but the yawning of that individual may have occurred because the others were present.
According to Gallup (personal communication), their budgerigars were tested as a group to reduce stress of being removed from the group. In fact, Soma & Hasegawa (2004) demonstrated the budgerigars show decreased aversion to new places when in groups. However sound this precautionary measure may have been, it introduced other variables that would have to be dealt with. These variables may have no influence at all in yawning, but that should be experimentally established. To eliminate this possibility, budgerigars should have been tested individually.
Personality differences also could have influenced the occurrence of yawning. Social animals respond differently to a situation when they are alone or with other individuals (Harcourt et al. 2009). Testing the birds individually would also have eliminated the possible influence of this variable.
Yawning could also be credited to stress, both from the new environment created by the wooden box cover used in Gallup et al's (2009) study and from the sudden increase in the temperature of the surroundings. Gallup & Gallup (2008) acknowledge and discuss the connection between yawning and stress. The experiment with budgerigars would have benefited immensely from taking this variable into account, because of its strong influence on the animal's physiology. To reduce the stress of the budgerigars in this type of environment, researchers should use stable temperature conditions, and for longer periods than in Gallup et al. (2009), and they should avoid recording yawns during rapid temperature transitions. Considering a range of stable environmental conditions, from below to above the animal's body temperature (or using as many conditions as the researcher deems necessary), would allow researchers to demonstrate that yawns are responses to environmental temperature and not responses to variation in environmental temperature.
In the experiment with budgerigars, yawning may have been a consequence of social influence or a consequence of stress from changes in environmental temperature. It may have also been a physiological mechanism to prevent brain tissue from overheating. Because of the possible influences of all these variables acting together, it seems that the best course of action would be to eliminate or at least to reduce the effect of the variables that are not under investigation.
After eliminating other intervening variables, a strong indication that yawning is related to brain cooling would be the satisfaction of the model's predictions: (1) the frequency of yawns rising as ambient temperature approaches body temperature and (2) yawning not occurring when ambient temperature reaches or exceeds body temperature. But even then, the conclusion that yawning is a compensatory cooling mechanism would need measures of bodily temperatures. Differences in temperature between the brain and the rest of the body are not uncommon in birds (Burgoon et al. 1987). Therefore, separate measures of brain and body temperature should be taken before and after yawns in each experimental temperature condition, taking into account circadian brain and body temperature variations. The maintenance or the cooling of brain temperature due to yawns would be a direct evidence of the temperature regulatory function of yawning, allowing the researchers to come to an unquestionable conclusion.
As an effort to elucidate the function of yawning, the thermoregulatory hypothesis has a strong appeal, and provides an invaluable contribution to research. But the limitations of experiments should lead to more cautious conclusions regarding the proximate causes of yawning, if we want to identify its adaptive or biological significance.