Analyses from >100 species of birds & mammals revealed robust positive correlations between yawn duration and brain mass and overall & cortical/pallium neuron totals; the observation of yawning in conspecifics selectively enhances vigilance

The causes and consequences of yawning in animal groups. Andrew C.Gallup. Animal Behaviour, Volume 187, May 2022, Pages 209-219. https://doi.org/10.1016/j.anbehav.2022.03.011 

Highlights

• Yawning is a highly conserved neurophysiological adaptation among vertebrates.

• The detection of yawning appears to be biologically important in social species.

• The observation of yawning in conspecifics selectively enhances vigilance.

• Yawn contagion appears to function in promoting motor synchrony in groups.

Yawning is a stereotyped action pattern that is prevalent across vertebrates. While there is growing consensus on the physiological functions of spontaneous yawning in neurovascular circulation and brain cooling, far less is known about how the act of yawning alters the cognition and behaviour of observers. By bridging and synthesizing a wide range of literature, this review attempts to provide a unifying framework for understanding the evolution and elaboration of derived features of yawning in social vertebrates. Recent studies in animal behaviour, psychology and neuroscience now provide evidence that yawns serve as a cue that improves the vigilance of observers, and that contagious yawning functions to synchronize and/or coordinate group activity patterns. These social responses to yawning align with research on the physiological significance of this behaviour, as well as the ubiquitous temporal and contextual variation in yawn frequency across mammals and birds. In addition, these changes in mental processing and behaviour resulting from the detection of yawning in others are consistent with variability in the expression of yawn contagion based on affinity and social status in primates. Topics for further research in these areas are discussed.

Keywords: arousalcircadian rhythmscollective behaviourmotor synchronystate changestressthermoregulationvigilance

Summary and future directions

Yawning is a neurophysiological adaptation that is omnipresent across vertebrates (Massen et al., 2021), and the detection of this action pattern in others appears to be biologically important among social species (Tsurumi et al., 2019). Moreover, recent studies indicate that yawning serves as a cue that enhances individual vigilance and promotes motor synchrony through contagion (see Fig. 1 for a graphic illustration of these processes). However, additional research is needed to replicate and further examine the nature of these effects, as well as investigate potential comparative differences in these responses based by on ecological factors and evolutionary history. In particular, future studies could examine how exposure to yawns alters the detection of threatening stimuli across different species, as well as how experimentally induced yawn contagion influences different patterns of motor synchrony and group coordination among human and nonhuman animals in the laboratory. In addition, naturalistic studies could investigate how the detection of yawning alters scanning rates and vigilance monitoring in free-moving groups, as well as how yawning and other patterns of behavioural contagion influence collective movement across different species. For example, among many species, yawning and stretching tend to co-occur, and both behaviours have been found to be contagious in budgerigars (Gallup et al., 2017; Miller, Gallup, Vogel, Vicario, et al., 2012). Since yawn and stretch contagion could have similar functions among animal groups in initiating collective action, future research could assess whether behavioural contagion in general is a key feature in initiating synchrony.

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Figure 1

The current evidence suggests that yawning serves as a cue rather than as a signal, but future studies could further examine whether spontaneous yawns evolved specifically to communicate internal states and/or alter the behaviour of observers in some species. For example, studies could investigate whether yawning occurs more readily in the presence of others and in contexts in which synchrony and/or vigilance would be most advantageous to the group. In addition, researchers could examine patterns in the variability of yawn expression. A recent study on macaques (Macaca tonkeana and M. fuscata) suggests differences in the morphology and duration of yawning are predictive of the contexts in which this behaviour arises (Zannella et al., 2021), so follow-up studies could also assess how different types of yawns differentially impact the subsequent vigilance behaviour and synchronization of observers. Similarly, researchers could assess differences in yawn-induced changes in behaviour based on the presence or absence of auditory cues. Vocal components to yawning appear to be common among humans and nonprimates (e.g. Massen et al., 2015; Palagi et al., 2009), yet seem unnecessary for the physiological function(s) of this action pattern. Thus, studies could investigate the factors that contribute to variation in vocal yawning and how the social outcomes of yawning vary based on visual and/or auditory detection.

Further examination of yawning in animals could provide important insights into the social role of this behaviour and its function in altering group dynamics, which could in turn offer applications for improving performance in surveillance settings and organized group activities in our own species. Based on what is already known about the social nature of yawning, it appears time to systemically examine some of the more overt social features of this behaviour in humans. This includes the stigmatization of yawning in some cultures (Schiller, 2002), which leads to the active concealment (Schino & Aureli, 1989) and/or inhibition of yawning when in the presence of others (Gallup, Church, Miller, et al., 2016; Gallup et al., 2019). For example, further research is needed to fully understand and disentangle the potential physiologic and social causes and consequences of inhibiting spontaneous and contagious yawning in groups. In line with the comparative perspective highlighted throughout this review, the bridging of both human and nonhuman animal research will provide the most comprehensive understanding of this evolutionarily conserved behaviour.