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Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal
Le bâillement, du réflexe à la pathologie
Le bâillement : de l'éthologie à la médecine clinique
Le bâillement : phylogenèse, éthologie, nosogénie
 Le bâillement : un comportement universel
La parakinésie brachiale oscitante
Yawning: its cycle, its role
Warum gähnen wir ?
Fetal yawning assessed by 3D and 4D sonography
Le bâillement foetal

mise à jour du
21 mars 2010
American journal of physical anthropology

Yawning as a Behavioral Adaptation to Heat Stress
and Water Scarcity in White-Faced Capuchins
(Cebus capucinus)
Andrew C. Gallup
Department of Biological Sciences
Binghamton University, New York. USA


Andrew C. Gallup. Yawning and the thermoregulatory hypothesis
Recently, Campos and Fedigan (2009) described some behavioral adaptations to heat stress and water scarcity in three distinct groups of white-faced capuchins (Gebus capueinus) inhabiting the Santa Rosa National Park in Costa Rica. The authors conclude that at least one specific self-directed behavior ("tongue out") has a thermoregulatory function. Evidence for this comes from data showing that on average, capuchins exposed their tongues more frequently during hot and dry conditions. In particular, tongue out behavior occurred at mean temperatures exceeding the 99% bootstrap confidence interval (BCI) for temperature and below the 99% BCI for humidity. Although yawning met the same criteria (occurring during higher mean temperatures and lower mean humidity), the authors do not suggest a thermoregulatory function for this behavior. Campos and Fedigan (2009) state, instead, that the observed pattern is "likely a byproduct of the association between yawning and resting," and when considering resting and temperature are positively correlated, it was concluded that yawning does not participate in thermoregulation. I feel the authors are 1) overly vague when referring to yawning as a byproduct of resting and 2) too quick to discount a thermoregulatory value. Methodological constraints in this study do not allow for a more convincing examination of the potential role of yawning in regulating body temperature.
Recent research suggests the biological function of yawning among homeotherms is thermoregulation (for a review see Gallup, 2010). More specifically, it has been proposed that yawning serves to stimulate cortical arousal as a brain cooling mechanism (Gallup and Gallup, 2007). According to this model, yawning is triggered to maintain brain thermal homeostasis when other regulatory mechanisms fail to operate favorably. Brain temperature is affected by a number of variables, including the temperature of the blood supplying the brain, rate of blood flow through the brain, and rate of metabolic heat production. Yawning contributes to the first two of these variables. The deep inhalation of air taken into the lungs raises blood pressure (Askenasy and Askenasy, 1996) and causes acceleration in heart rate (Greco and Baenninger, 1991; Guggisberg et al., 2007). Likewise, the constriction and relaxation of facial muscles during a yawn increases facial blood flow and these changes are thought to increase cerebral blood flow (Zajonc, 1985; Askenasy, 1989). Together, these processes are believed to operate like a radiator by removing hyperthermic blood from specific areas while introducing cooler blood from the lungs and extremities, thereby cooling cortical surfaces through convection. The gaping of the mouth and deep inhalation of air during a yawn is thought to cool venous blood draining from the nasal and oral orifices into the cavernous sinus, which surrounds the internal carotid artery supplying blood to the rest of the brain. Recent research provides strong support for this model showing that 1) excessive yawning in humans is triggered during mild hyperthermia and produces significant decreases in body temperature (Gallup and Gallup, in press) and 2) spontaneous yawning in rats is preceded by rapid increases in brain temperature, which are followed by corresponding decreases in temperature after a yawn (Shoup ML et al., unpublished data).
One of the predictions of the brain cooling hypothesis, other than producing direct brain cooling effects, is that yawning frequency should be influenced by variation in ambient temperature. In particular, it is hypothesized that yawning should increase as ambient temperature approaches body temperature. It is assumed that a rise in ambient temperature activates thermoregulatory mechanisms that function to maintain brain and body temperature within a normal range. This prediction has been recently tested using budgerigars (Melopsittacus undulatus) as an avian model (Gallup et al., 2009). Results showed that yawning frequency in budgerigars increased during rising ambient temperature, as opposed to when temperatures were held constant. A more recent study manipulated ambient temperature in both directions and confirmed that yawning occurred more often during high-increasing temperatures compared with decreasing temperatures (Gallup et al., in press). In addition, yawning was positively correlated with ambient temperature and occurred more often in tandem with other thermoregulatory behaviors.
The results of Campos and Fedigan (2009) are in accord with these studies showing that yawning occurred at higher ambient temperatures and thus was more likely to have also occurred alongside tongue out behavior. By investigating the temporal relationship between yawning and tongue protrusion, it would be possible to provide a more convincing justification for whether to couple or disentangle these two behaviors. The reports of Campos and Fedigan (2009) and Gallup et al. (2009) show that yawning occurs at roughly the same average temperatures (30-32°C) across both species. One difference is that budgerigars do not rest more during higher ambient temperatures, whereas capuchins did. It should be noted, however, that the assumption of a relationship between yawning and resting in capuchins was not based on a formal analysis but rather on the general association between yawning and sleep. It is because of this yawn/rest association that yawning was overlooked as a thermoregulatory behavior in capuchins. Campos and Fedigan (2009) imply functionality to yawning when they describe it as a byproduct of resting but a specific role was not identified or discussed. It is well documented in humans that yawning occurs most often before sleep onset and after waking (Provine et al., 1987). However, it is important to realize that sleep and body temperature vary inversely, and yawning frequently occurs in the evening, when brain temperature is at its peak, and upon waking, when brain temperature begins increasing from its lowest point (Landolt et al., 1995). It would have been interesting to compare the rates of yawning among capuchins during resting periods of varying ambient temperatures.
In combination with existing evidence supporting a strong connection between yawning and changes in internal and external temperatures, it seems reasonable that both yawning and tongue protrusion are methods of behavioral thermoregulation in capuchins. Among homeotherms, ecological factors such as the relative need for water conservation could affect the way yawning is used to alleviate thermal stress, as has been suggested for budgerigars (Gallup et al., 2009). For instance, there could be differences in yawning between homeothermic species selected to different degrees for cooling abilities in challenging thermal environments. Thus, yawning may not be highly involved in behavioral thermoregulation across all primates. Similar to budgerigars, however, capuchins serve as an intriguing model for studying yawning and thermoregulation because they also live in hot, dry climates. It would be interesting to know whether yawning rates in capuchins vary as a function of increasing, decreasing, or constant ambient temperatures.
In summary, comparative research from birds, rats, and humans suggests that yawning reduces brain and body temperature, is influenced by the range and direction of ambient temperature change, and is inhibited by methods of behavioral cooling (Gallup, 2010). The data presented by Campos and Fedigan (2009) are consistent with the view that yawning may also be a thermoregulatory behavior in white-faced capuchins, yet more focused analyses are required to reach an informed conclusion. Although yawns are observed in all classes of vertebrates (Baenninger, 1987), researchers are only beginning to critically study this behavior in a comparative nature, and due to the potential multifunctionality of yawning, much more research is needed. Recently, Vick and Paunker (2009) have investigated the physiological role of yawning in primate behavior, providing thorough analysis of the variation and context of yawning in chimpanzees (Pan troglodytes). In addition, Palagi et al. (2009) have studied the social nature of yawning, providing support for a role of empathy in yawning contagion among gelada baboons (Theropithecus gelada). Hopefully, these efforts will pave the way for future comparative studies highlighting the importance of this evolutionarily conserved behavior.
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