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 http://www.baillement.com

mystery of yawning 

 

 

mise à jour du
12 novembre 2026
Behav Processes
2026 Oct 10:105448
Yawning in lungfishes: occurrence
and potential functional differences
between respiratory modes
Enokizu A, Hirakawa M, Wakabayashi I,
Morisaka T, Sakai M.

Chat-logomini
 
Abstract
Yawning is a stereotyped behavioural pattern observed across vertebrates and is considered multifunctional, but how its function changed during major physiological transitions remains poorly understood. Lungfishes provide an ideal model to address this question because species vary in their relative reliance on branchial and pulmonary respiration. The authors investigated whether yawning occurs in one individual each of the branchial-dominant Australian lungfish and three pulmonary-dominant species (marbled, spotted, and West African lungfishes) and, when present, characterized yawning frequency, yawning investment (frequency _ duration), and activity levels before and after yawning. Yawning was observed exclusively underwater in all species. The branchial-dominant Australian lungfish showed a significantly higher yawning frequency than the three individual pulmonary-dominant lungfishes (negative binomial model: p = 0.007), whereas yawning investment was broadly comparable among the four individuals. In the branchial-dominant Australian lungfish, activity levels significantly increased following yawning (Wilcoxon signed-rank test: p = 0.041), whereas no significant post-yawning increase was observed in the two pulmonary-dominant individuals. The magnitude of activity-level change associated with yawning differed significantly from that observed during control periods in the branchial-dominant Australian lungfish (Wilcoxon rank-sum test: p = 0.006), but not when the pulmonary-dominant individuals were pooled. These observations indicate that lungfish yawning occurs underwater regardless of respiratory mode and is associated with differences in yawning characteristics and post-yawning activity among the individuals. Although these findings cannot establish species-level differences, they suggest that yawning function may vary with respiratory mode and warrant further comparative study.
 
Résumé
Le bâillement est un comportement stéréotypé observé chez tous les vertébrés et considéré comme multifonctionnel, mais la manière dont sa fonction a évolué au cours des grandes transitions physiologiques reste mal comprise. Les dipneustes constituent un modèle idéal pour répondre à cette question, car les espèces varient dans leur recours relatif à la respiration branchiale et pulmonaire. Les auteurs ont cherché à déterminer si le bâillement se produisait chez un individu de chaque espèce parmi le dipneuste australien, à respiration branchiale dominante, et trois espèces à respiration pulmonaire dominante (dipneustes marbré, tacheté et d'Afrique de l'Ouest) et, le cas échéant, ils ont caractérisé la fréquence des bâillements, l'investissement en bâillements (fréquence _ durée) et les niveaux d'activité avant et après le bâillement. Le bâillement a été observé exclusivement sous l'eau chez toutes les espèces. Le dipneuste australien à respiration branchiale dominante a présenté une fréquence de bâillement significativement plus élevée que les trois dipneustes à respiration pulmonaire dominante (modèle binomial négatif : p = 0,007), tandis que l'occurrence des bâillements était globalement comparable chez les quatre individus. Chez le dipneuste australien à respiration branchiale dominante, les niveaux d'activité ont augmenté de manière significative après le bâillement (test de Wilcoxon des rangs signés : p = 0,041), alors qu'aucune augmentation significative post-bâillement n'a été observée chez les deux individus à respiration pulmonaire dominante. L'ampleur du changement du niveau d'activité associé au bâillement différait significativement de celle observée pendant les périodes de contrôle chez le dipneuste australien à respiration branchiale dominante (test de somme des rangs de Wilcoxon : p = 0,006), mais pas lorsque les individus à respiration pulmonaire dominante étaient regroupés. Ces observations indiquent que le bâillement chez les dipneustes se produit sous l'eau quel que soit le mode respiratoire et qu'il est associé à des différences dans les caractéristiques du bâillement et dans l'activité post-bâillement entre les individus. Bien que ces résultats ne permettent pas d'établir des différences au niveau de l'espèce, ils suggèrent que la fonction du bâillement peut varier en fonction du mode respiratoire et justifient la poursuite d'études comparatives.
1. Introduction
 
Yawning is a fixed, involuntary action pattern that starts with slow mouth opening, followed by a maximum gaping phase, and ends with rapid mouth closure (Enokizu et al., 2021). Yawning is widely observed across vertebrates, including fishes, amphibians, reptiles, birds, and mammals (e.g. Yamada and Wada, 2023; Bakkegard, 2017; Kotake et al., 2024; Sauer and Sauer, 1967; Baenninger, 1987). Yawning is considered multifunctional, with proposed roles in respiration, thermoregulation, neural processes, and social communication (Guggisberg et al., 2010; Gallup, 2022), but its evolutionary history remains poorly understood. Vertebrate evolution involved major physiological transitions, including the evolution of pulmonary respiration (Cupello et al., 2022) and endothermy (Legendre and Davesne, 2020), which may have influenced its occurrence and functions. We therefore focused on how yawning may have changed during the transition from branchial to pulmonary respiration.
 
In branchial-breathing fishes, yawning may serve both behavioural and respiratory functions. Previous studies suggest that yawning is associated with changes in behavioural state and arousal (Yamada and Wada, 2023; Yoshida et al., 2026). The state-change hypothesis proposes that yawning facilitates transitions between behavioural states, whereas the arousal hypothesis proposes that yawning contributes to the maintenance of vigilance and brain arousal (Guggisberg et al., 2010). In addition, increased yawning under hypoxic conditions has been reported in branchial-breathing fish, suggesting that the respiration-circulatory hypothesis may also apply to branchial-breathing fishes (Hasler et al., 2009). To understand how the occurrence and functions of yawning may have changed during the evolutionary transition from branchial to pulmonary respiration, lungfishes provide an important intermediate model because they possess both lungs and gills and differ in their reliance on these respiratory modes (Zhang et al., 2023).
 
The Australian lungfish (Neoceratodus forsteri) has well-developed gills and a single lung and relies strongly on branchial respiration, whereas African lungfishes, including West African lungfish (Protopterus annectens), marbled lungfish (P. aethiopicus) and spotted lungfish (P. dolloi), and the South American lungfish (Lepidosiren paradoxa) have reduced gills and well-developed lungs and rely predominantly on pulmonary respiration (Glass, 2011). Thus, lungfishes provide an opportunity to examine how yawning characteristics may vary with respiratory mode. However, yawning has not previously been documented in lungfishes.
 
In the present study, we investigated whether lungfishes yawn underwater and characterized yawning frequency, yawning investment, and activity-level changes associated with yawning in one Australian lungfish (branchial-dominant) and one individual from each of three African lungfish species (pulmonary-dominant: obligate air-breathers; Zhang et al., 2023). Because only a single individual was available for each species, results were interpreted at both the individual level and between respiratory modes while acknowledging potential individual effects.
 
4. Discussion
 
In the present study, we found that both the branchial-dominant Australian lungfish and the pulmonary-dominant marbled, spotted, and West African lungfishes yawn underwater, regardless of respiratory mode. This is the first report of yawning in lungfishes, suggesting yawning has been retained in both respiratory modes for approximately 140 million years since these lineages diverged (Brownstein et al., 2023), suggesting that underwater yawning may be important irrespective of respiratory mode.
 
Yawning characteristics, however, differed among individuals and between the two respiratory-mode groups. Yawning frequency was significantly higher in the branchial-dominant lungfish, whereas yawning investment did not differ markedly between the two groups. Activity levels significantly increased following yawning in the branchial-dominant lungfish, but not in the pulmonary-dominant lungfishes. The _ activity level associated with yawning was greater than during control periods in the branchial-dominant lungfish, but not in the pulmonary-dominant lungfishes; the marbled lungfish showed a significant difference when analysed separately. These findings suggest potential functional differences in yawning between branchial- and pulmonary-dominant lungfishes, although its function in pulmonarydominant lungfishes remains unclear.
 
4.1. Yawning characteristics and potential functions in lungfishes
 
In the branchial-dominant lungfish, yawning may contribute to respiratory function. Because branchial respiration involves water flow generated by mouth movements, the large gape during yawning may enhance oxygen uptake. A similar interpretation has been proposed for largemouth bass, in which yawning increases under hypoxic conditions (Hasler et al., 2009). Thus, the respiration-circulatory hypothesis may apply to the branchial-dominant lungfish. Activity levels also increased following yawning, and _ activity level was greater than during control periods, suggesting state-change and/or arousal functions. However, given the relatively high yawning frequency observed in the branchial-dominant lungfish, if yawning simultaneously enhances ventilation and precedes increases in activity, it is difficult to distinguish between oxygen-supply and state-change/arousal functions. Thus, it remains unclear which function is primary or whether multiple functions operate simultaneously.
 
In contrast, because pulmonary-dominant lungfishes have a much lower reliance on gills for oxygen uptake than the branchial-dominant Australian lungfish (Zhang et al., 2023), yawning would be expected to occur less frequently if the contribution of the respiration-circulatory function to yawning were limited; indeed, the pulmonary-dominant individuals observed in the present study showed lower yawning frequencies than the Australian lungfish. In addition, no significant increase in activity levels was observed following yawning, and the _ activity level associated with yawning did not differ significantly from the _ activity level during control periods, providing no clear support for state-change or arousal functions. Thus, no clear function of yawning could be identified in pulmonary-dominant lungfishes.
 
Taken together, our findings suggest that yawning may have been retained through the evolutionary transition from branchial- to pulmonary-dominant respiration, although the relative importance of its functions may have changed, particularly that of oxygen uptake.
 
4.2. Limitations and future directions
 
Only one individual per species was examined, and the findings should be regarded as preliminary observations because they may partly reflect individual variation rather than species-level characteristics (Nagasaka et al., 2025). Differences in housing environments, including visitor exposure at BRH, may have influenced activity, arousal, and yawning. Because these primarily nocturnal lungfishes were observed during the light period, subtle changes in activity may have gone undetected. In pulmonary-dominant lungfishes, activity and arousal changes associated with surfacing behaviour may also have obscured yawning-associated changes. Behavioural states were inferred solely from activity levels, and internal states such as arousal and motivation were not directly measured. Future studies involving multiple individuals per species, observations during the dark period, environmental manipulations, and physiological measurements are needed to clarify the functions of yawning in lungfishes.