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
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mystery of yawning 

 

 

 

 

mise à jour du
25 octobre 2016
Biol Lett
2016;12(10)
Yawn duration predicts brain weight
and cortical neuron number in mammals
 
Gallup AC, Church AM, Pelegrino AJ.

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Abstract
 
Research indicates that the motor action pattern of yawning functions to promote cortical arousal and state change through enhanced intracranial circulation and brain cooling. Because the magnitude of this response likely corresponds to the degree of neurophysiological change, we hypothesized that interspecies variation in yawn duration would correlate with underlying neurological differences. Using openly accessible data, we show that both the mean and variance in yawn duration are robust predictors of mammalian brain weight and cortical neuron number (_-values > 0.9). Consistent with these effects, primates tend to have longer and more variable yawn durations compared with other mammals. Although yawning has long been considered a stereotyped action pattern, these findings reveal substantial variation in this response and highlight the importance of measuring yawn duration in future research.
 
1. Background
 
Yawning is characterized by a powerful gaping of the jaw with inspiration, a brief period of peak muscle contraction and a passive closure of the jaw with shorter expiration [1]. Yawn-like mandibular gaping patterns have been identified across vertebrate classes [2], though it remains unknown whether the jaw stretching observed in fish, amphibians and reptiles functions in the same way as yawns in birds and mammals [3]. Nonetheless, the relative ubiquity of this response in vertebrates suggests it is an evolutionarily conserved behaviour that holds basic and important adaptive value. Although the function to yawning remains debated [3&endash;6], previous research supports a role in promoting cortical arousal and state change [7&endash;11] through enhanced intracranial circulation and brain cooling [12&endash;13].
 
Yawning is controlled by several neurotransmitters and neuropeptides [14] and has been linked with numerous neurological diseases and clinical conditions [15&endash;16]. Despite the potential applications of studying differences in yawning as a marker of neural processing, variation in this response has yet to be fully explored. Naturalistic reports on the frequency of yawning across taxa are quite limited [17,18], and yawns have long been considered a stereotyped action pattern with limited variation in expression within or between species [8]. However, recent studies on non-human primates have identified previously overlooked variation in the mouth aperture associated with yawning, and it has been shown that different forms of yawning tend to correspond to distinct contexts and situations [18,19].
 
Another measure of variability that could be easily catalogued across species is the duration of yawning. Although early research examined average yawn duration in humans (approx. 6 s) [8], this variable has not been used in studies of non-human animals. Given that the primary circulatory effects associated with yawning are localized within the skull, differences in the duration or magnitude of this response likely correspond to the degree of neurophysiological change. Therefore, we hypothesize that differences in yawn duration will correlate with neurological variation between species.
 
Here, we examine this possibility by linking openly accessible videos of yawning from the Internet (primarily from www.youtube.com) to previously published brain parameters in a representative sample of mammalian taxa [20]. Based on previous research supporting a neurophysiological function to yawning, we hypothesized that mammals with larger brains would yawn longer, even when controlling for body size, and that yawn duration would correlate with number of cortical neurons.
 
4. Discussion
 
Although yawning has been considered a stereotyped action pattern [8], we document substantial variation in the duration of this response in mammals. In particular, we found that both the mean and variance in yawn duration are robust predictors of brain weight and cortical neuron number (_-values > 0.9). Consistent with these results, we also found that primates tend to have longer and more variable yawn durations compared with other mammals. These results apply irrespective of variation in the mouth aperture of yawns in some non-human primates [18,19].
 
These combined effects represent a striking scaling relationship between brain and behaviour. Importantly, neither the size of the body nor the anatomical structures specific to yawning (cranium and mandible) are driving these effects, because gorillas, camels, horses, lions, walruses and African elephants all have shorter average yawns than humans. Furthermore, having a larger skull does not necessitate more variable motor pattern duration. Instead, differences in yawn duration appear to be specifically linked to interspecies variation in brain size and complexity, with cortical neuron number being the most significant factor. Future research should investigate whether mean and variation in yawn duration also predict similar brain parameters recently documented across avian taxa [21].
 
These findings are consistent with the view that yawning holds a basic neurophysiological function. In particular, previous research suggests that yawning is an adaptation to enhance intracranial circulation and brain cooling [11&endash;13], which in turn could promote cortical arousal and state change [7&endash;9]. While the neural structures necessary for yawning appear to be located within the brainstem [22], based on these results, we hypothesize that the neurophysiological consequences of yawning affect the brain more globally, whereby longer yawns may be necessary to more effectively modulate cortical arousal for animals with larger and more complex brains. Furthermore, we predict that the greater within-taxon variance in yawn duration observed for large-brained mammals may be related to increased cognitive capacities and more variable behavioural repertories. We note that while a primarily social/communication (i.e. signalling) function to yawning has been posited [4], it is unclear how such an explanation would account for the current effects.
 
Provine [8, p. 120] stated, 'yawning may have the dubious distinction of being the least understood, common, human behavior'. Unfortunately, 30 years later, we still know relatively little about the biological significance of this evolutionarily conserved response. The difficulty in uncovering the ultimate function(s) of yawning may, in part, be owing to the fact that it can be elicited by numerous stimuli and researchers have by and large overlooked subtle distinctions in the expression of this behaviour. Based on the current findings, we believe yawn duration deserves further attention. We close by offering suggestions for future research in this area