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 

 

 

 

 

 

 

 

 

 

 

Mis à jour le
28 février 2026
iScience
2025;29(1):114408
 Social engagement modulates wild monkeys' vocal expressions
and the behavioral response to that of others
 
Galotti A, Pedruzzi L, Francesconi M, Quartesan A, Gamessa SA,
Serra V, Petroni G, Bogale BA, Lemasson A, Palagi E.

Chat-logomini
 
 Tous les articles sur la contagion du bâillement
All articles about contagious yawning
 
Animal vocal communication relies on the dynamic interaction between emitter and receiver, with signals shaped within a social and embodied context. To fully understand how such interactive processes operate, the authors used yawn vocalizations of geladas (Theropithecus gelada), a species showing exceptional yawning variability. They first examined yawn calls produced in three contexts: high-intensity social, low-intensity social, and non-social context and found clear acoustic differences among them, revealing context-dependent modulation in a typically stereotyped behavior. They conducted field playback experiments exposing wild geladas to unfamiliar male yawn vocalizations emitted in the three contexts. During playbacks, monkeys gazed more at the loudspeaker when yawns originated from a social rather than a non-social context, indicating that animals perceive the stimuli's differing nature. Although yawn responses did not vary across contexts, contagion was higher when geladas were grooming during test, suggesting that positive social engagement enhances, rather than reduces, susceptibility to contagion.
 
La communication vocale animale repose sur l'interaction dynamique entre l'émetteur et le récepteur, les signaux étant façonnés dans un contexte social et incarné. Afin de comprendre pleinement le fonctionnement de ces processus interactifs, les auteurs ont utilisé les vocalisations de bâillements des geladas (Theropithecus gelada), une espèce présentant une variabilité exceptionnelle en matière de bâillements. Ils ont d'abord examiné les cris de bâillement produits dans trois contextes : social à haute intensité, social à faible intensité et non social. Ils ont constaté des différences acoustiques nettes entre eux, révélant une modulation dépendante du contexte dans un comportement typiquement stéréotypé. Ils ont mené des expériences de lecture sur le terrain en exposant des geladas sauvages à des vocalisations de bâillement mâles inconnues émises dans les trois contextes. Pendant les écoutes, les singes regardaient davantage le haut-parleur lorsque les bâillements provenaient d'un contexte social plutôt que non social, ce qui indique que les animaux perçoivent la nature différente des stimuli. Bien que les réponses aux bâillements ne variaient pas selon les contextes, la contagion était plus forte lorsque les geladas se toilettaient pendant le test, ce qui suggère que l'engagement social positif renforce, plutôt que réduit, la susceptibilité à la contagion.
 
 
 
Introduction
Vocal signals often evoke responses that are influenced not only by their acoustically encoded information but also by the context in which they are emitted and the internal state of the receiver. This context-dependent modulation of vocalizations allows individuals to convey different meanings or enhance the efficacy of signals in various social situations. As such, the capacity to increase the variability of information transmission in animal communication should be further explored, particularly as interpreting behavioral response is often not straightforward. While referential call types and other monocontextual vocalizations are relatively rare, many call types are typically produced across a broad range of contexts. Many studies have highlighted context-specificity of vocalizations and their acoustic properties across various taxa, including birds, cetaceans, dogs,' bats, and nonhuman primates. These findings highlight that changes in the acoustic structure of calls are linked to various behaviors, including food-related activities, agonistic interactions, and longdistance communication.
 
1° Additionally, vocalizations encode emotional states, with specific call types linked to positive or negative valence (e.g., ultrasonic vocalizations in rats'), as well as acoustic features coding for low vs. high arousal.
In addition to the fact that vocal parameters reflect the emitter emotional and social contexts, the same call may also elicit different responses depending on the receiver's internal state, thus leading to the same call being perceived slightly differently. In meerkats (Suricata suricatta), subordinate females adjust their responses to dominant females' vocalizations based on whether they had recently experienced a conflict or a neutral interaction, suggesting that responses are context dependent.
 
2º Similarly, in baboons (P. hamadryas ursinus), females react more strongly to threat vocalizations from kin in stressful situations compared to non-kin or neutral situations.? However, all the mentioned studies investigated contextual and/or affective changes to animal calls focusing on vocalizations that have primarily evolved for communicative reasons. Here, to avoid any referential confusing factor, we studied a state-based affected sound by focusing on the effect of the context on the expressive and perceptual components using as a model derived vocalizations shown by a non-human primate while yawning. It is a widespread behavior whose complexity and communicative functions are still highly debated. Despite its stereotypical nature, in humans yawning serves as a multimodal cue, in which the visual component is often integrated by vocalizations.
 
The social significance of yawning is particularly evident in its contagiousness, triggering similar responses in others. The phenomenon of yawn contagion extends beyond our species, being observed in various non-human social species. Recent research further highlights the role of yawn contagion in synchronizing collective behavior in non-human social animals. In lions (Panthera leo), contagious yawning among group members facilitates behavioral alignment, ensuring coordinated activities. Such synchronization is essential for fostering social cohesion and supporting cooperative interactions within social species. Moreover, additional adaptive benefits of yawn contagion should be considered. In particular, some studies have explored its role in promoting group vigilance. From an evolutionary perspective, maintaining alertness to potential threats would provide an immediate survival advantage.
 
Geladas (Theropithecus gelada), a highly social monkey species endemic to the Ethiopian highlands, have a rich vocal repertoire and, along with humans, are the only primates known to produce specific derived vocalizations during yawning. Recent experimental and observational studies showed that the sound of a yawn is alone sufficient to elicit contagious yawning in the species, hinting at the communicative role of yawn vocalizations in the coordination of social interactions. However, data on the contextual and emotional triggers of yawn vocalizations, as well as the variability in their production and perception, are still lacking. Considering its role in group synchronization, the evolutionary advantage of auditory yawn contagion may lie in enabling individuals to maintain acoustic contact when visual information is prevented. This function could be particularly significant in societies characterized by modularity, flexibility, and high social complexity, where individuals rely on multimodal communicative strategies to manage interactions effectively. The high morphological variability of vocalized and non-vocalized yawns in geladas, ranging from yawns with covered teeth to those with uncovered teeth and gums, together with the wide range of contexts in which geladas yawn, suggest that yawn vocalizations may also exhibit a greater variability than expected, potentially serving different communicative functions. In geladas, the acoustic component of yawns is mainly produced by males.
 
Here, we conducted field playback experiments on wild geladas, exposing the animals to unfamiliar male yawn vocalizations emitted during three different contexts: (i) high-social (e.g., post-mounting), (i) low-social (when involved in grooming), and (iii) non-social (solitary activities).
 
We predicted that the acoustic structure of yawn vocalizations (expressive component sensu Tsutsuit) would vary depending on the context in which they are emitted (Prediction 1) and that geladas are able to distinguish the contextual origin of different yawn vocalizations (perception component sensu Mateo 50), differently responding to yawns recorded during a social context compared to yawns recorded in a non-social context (i.e., increased number of gazes directed to the loudspeaker) (Prediction 2). Furthermore, we hypothesized an increase of yawn contagion when the context of the stimulus broadcasted has a social compared to non-social origin (Prediction 3a). If the socio-emotional context experienced by the tested subjects during the stimulus administration has a role in modulating the yawn contagion response, two alternative scenarios are possible. If an individual is engaged in grooming, its focus on the ongoing interaction might reduce the motivation to respond to external stimuli (Prediction 3b); if the social engagement produced by grooming fosters the receiver's positive emotional state, we expect a higher motivation to respond to yawn vocalizations during grooming than during a non-social situation (Prediction 3c).
 
Discussion
Our results provide valuable insights into the relationship between the sender's social contexts at the time of vocalization emission and call acoustic structure, as well as the relationship between the social context experienced by the receiver and their behavioral response to others' vocalizations. Our study investigated a derived vocalization, the yawn call, which uniquely emerged in humans and gelada monkeys and is produced across a range of contexts. First, the DFA revealed systematic variations in the acoustic parameters of male yawn vocalizations produced across different social contexts with classification accuracy significantly above chance, confirming a non-random acoustic variability (Prediction 1 supported).
 
High-social (produced after mounting, an affiliative behavior in geladas, or vocal interactions, affiliative derived calls with females 2), low-social (during grooming), and non-social yawns (solitary activities) were possibly associated with different degrees of emotional arousal. This suggests that variations in yawn vocalizations may unconsciously encode contextual information, indicating a wider within-call contextual variability allowing to increase the range of information conveyed, as it occurs for other primate vocalizations.
 
Our results are also in line with previous findings on other gelada vocalizations, grunts, which were found to vary according to the behavioral context in which they are produced. This unexpected finding calls into question the very characterization of yawning as a fixed action pattern, revealing a level of flexibility that contradicts its traditionally assumed rigidity, not only in its visual, but also in its acoustic component. Although the overall classification accuracy obtained through the DFA was moderate (48%), this outcome is not unexpected given that our analysis focused exclusively on the contextual dimension and did not include other potentially relevant sources of variation (e.g., individual identity, age, body size, or yawn morphology). Moreover, probably yawn calls are not signals selected to convey specific contextual meanings possibly explaining the partial overlap among categories.
 
The differences found raise interesting questions about the meaning and underlying mechanisms of such inter-contextual variability. They may indicate that yawning-associated vocalizations convey different information depending on context, for instance, during grooming (e.g., a relaxed state) versus after sexual interactions or intense cross-sex greetings (e.g., high-arousal or other cues involuntarily expressed toward neighboring males). Playback experiments showed that geladas gazed more frequently at the loudspeaker when exposed to vocalizations deriving from yawns emitted in both high- and low-social context compared to a non-social context. Furthermore, the absence of a significant difference in gazing at high- and low-social stimuli suggests that receivers are sensitive and/or motivated to react to yawn vocalizations whatever the level of positive social arousal the yawner is experiencing. This finding provides evidence for the perceptual component of the cue, 5° indicating that yawn vocalizations convey to some extent information about the social aspect of the context of production, thereby supporting Prediction 2. The ability to distinguish between socially relevant and less relevant yawns suggests an adaptive benefit in modulating attention to group-related cues, particularly in species with complex social dynamics.
 
While the interest toward the stimuli is influenced by their social or non-social origin, the same does not apply to yawn responses, as yawn contagion was consistently elicited by the broadcasted yawns regardless of the context (high-social, low-social, or non-social) in which they were recorded (Prediction 3a not supported). This finding aligns with existing knowledge on yawn contagion across species, including humans. In Homo sapiens, it is well established that merely hearing or reading the word "yawn," or even thinking about the behavior in the absence of any specific context, can automatically and unconsciously trigger the same behavioral pattern in many cases.2
 
The automaticity of yawn contagion is shown in geladas, where the responses do not seem to vary according to the modality of perception, the morphology of the yawn or the side of perception. The absence of a direct link between the stimulus contextual origin and yawn contagion further underscores the need to consider not only the external (e.g., emission context) factors but also the internal states of the receivers (e.g., social engagement at the time of exposure, internal affective state) in shaping behavioral responses. Our data support this last consideration. Specifically, geladas exhibited higher levels of yawn contagion in response to the stimuli while engaged in grooming compared to when involved in solitary activities, albeit their interest for the loudspeaker (i.e., gazing) was not affected by such activities. Yet, this finding is not sufficient per se to confirm that an ongoing grooming interaction directly facilitates yawn contagion.
 
To further explore this hypothesis, we needed to exclude the possibility that the increased yawn contagion during grooming was not simply due to an increase in spontaneous yawning but actually due to an increased propensity to respond to others' yawn vocalizations. To exclude this possibility, we administered to geladas engaging in grooming activities yawn and grunt vocalizations both recorded during the same social context. The sound of social yawns evoked a higher yawn response than social grunts. This suggests not only that being actively involved in a social interaction does not reduce contagion susceptibility to external vocal yawns, but that the positive emotional state deriving from grooming might foster such susceptibility. To aid interpretation of the playback results, it is essential to consider the naturally high spontaneous yawning rates in geladas. A previous study showed that gelada adult males yawn up to 0.25 times per minute, making the probability of at least one spontaneous yawn within 3 min exceed 20%-25%. This may explain the presence of yawn responses after grunts playbacks. Moreover, our last analyses included only grooming sessions, a context in which spontaneous yawning is very common in geladas.
 
Consistent with this, yawning responses during grooming sessions occurred across all stimuli conditions. Nevertheless, the probability of responding to high-social yawn calls remains substantially higher, supporting the interpretation that socially relevant (and emotionally aroused) yawning calls elicit a stronger response than other types of social or non-social vocalizations.
 
Our study was conducted in fully natural settings, where logistical constraints naturally limit sample size. However, the number of subjects in our study is consistent with standards in playback research across nonhuman primates and other mammals (chimpanzees, 58 bonobos, 59 geladas, 6° and cheetahs®1). Combined with repeated, controlled exposures per individual and careful spacing of sessions, this design ensures robust within-subject comparisons and meaningful insights into context-dependent communication and yawn contagion.
 
In conclusion, yawn vocalizations emitted both in high- and low-social contexts exhibit structural differences compared to those produced in non-social contexts, reflecting the expressive component of the vocal cue. This unexpected finding challenges the assumption that yawning is a purely stereotypic behavior, suggesting instead that it may carry nuanced social and communicative functions. Moreover, geladas seem to perceive them as distinct, showing a stronger interest (measured via their gazing behavior) to yawn calls emitted during social interactions, highlighting the perceptual component of the vocal cue. However, yawn contagion does not appear to depend on the context in which the cue originates but rather on the context currently experienced by the receiver. Specifically, positive contexts increase the likelihood of a contagious response. This "hic et nunc" effect, evidenced by our study, calls for a critical reconsideration of correlational data suggesting that yawn contagion covariates, both positively or negatively, with specific social variables such as relationship quality, kinship, or social rank.
 
If the immediate context modulates the yawn response, then correlational findings may not accurately reflect the actual dynamics of yawn contagion. For instance, if an individual is engaged in a grooming session with a closely bonded partner and perceives a yawn from another, less socially connected individual, the likelihood of a contagious response may be more influenced by the ongoing grooming activity than by the relationship with the yawning individual. A grooming session represents a kaleidoscope of underlying factors, including focused attention on conspecifics, ectoparasite removal, physical stillness, and a distinctive somatosensory experience, not to mention a potential reduction in alertness to external threats. Consequently, it is challenging to disentangle which of these elements may actually influence yawn contagion. What does seem evident, however, is that the activity being performed by the individual at the time of exposure substantially modulates their response.
 
Taken together, our results further highlight the remarkable socio-communicative complexity of geladas, which is reflected even in seemingly simple behaviors such as yawning and yawn-related vocalizations. Moreover, our findings raise intriguing questions about the potential communicative complexity of yawn calls in our own species. From this perspective, if we truly aim to unravel the proximate and ultimate drivers of yawn contagion, our attention must shift toward the immediate effects and underlying mechanisms that shape this phenomenon in real time. Only by embracing a dynamic, context-dependent approach can we move beyond mere correlations and capture the true nature of this intriguing social behavior. This shift in perspective not only refines our understanding of yawning and yawn contagion but also paves the way for deeper insights into the complex interplay between social context, perception, cognition, and behavioral synchronization.
 
Limitations of the study
Our study provides novel insights into the acoustic and social dynamics of yawn vocalizations in geladas; however, some limitations should be acknowledged. First, the sample size, although sufficient to detect systematic acoustic differences and contagion patterns, and comparable to those used in similar studies, was relatively small. A larger dataset would have allowed us to explore additional contexts, include more individuals, and increase the number of yawns per context. Second, although our observations were conducted under naturalistic conditions, we did not directly assess internal factors such as individual temperament or stress levels, which may influence yawning behavior. Third, we used stimuli collected from a different (captive) population to minimize potential biases related to familiarity; nonetheless, we cannot entirely rule out perceptual differences arising from inter-population variation. Finally, our analyses focused primarily on male vocalizations, as they are more frequent and conspicuous than those of females, leaving potential acoustic differences in female yawns for future investigation.