Introduction

The ability to coordinate behaviour with conspecifics can confer substantial fitness benefits to group-living animals, for example by strengthening social cohesion, facilitating collective vigilance, improving the effectiveness of anti-predator strategies and increasing foraging efficiency through cooperative hunting (Conradt and Roper 2000; Duranton and Gaunet 2016; Gallup and Gallup 2007; Lakin et al. 2003). One mechanism that may support such coordination is behavioural contagion, which occurs when observing the behaviour of one individual increases the probability of performing the same behaviour (Zentall 2003; Palagi et al. 2020). By promoting temporal alignment of activities within dyads and groups, behavioural contagion may contribute to behavioural synchrony without requiring explicit signalling or complex cognitive skills, and it is therefore considered a powerful tool to facilitate coordination and collective behaviour (Duranton and Gaunet 2016; Massen and Gallup 2017).

Behavioural contagion is thought to arise from simple perception–action mechanisms such as non-conscious mimicry, whereby observing a behaviour automatically activates the corresponding motor representation in the observer, without requiring explicit representation of others’ internal states (Chartrand and Bargh 1999; Lakin et al. 2003; Yoon and Tennie 2010). Empirical research on behavioural contagion has focused primarily on yawning, and to a lesser extent on other behaviours such as scratching, laughter or play (Massen and Gallup 2017). The primary focus on yawning likely stems from its being relatively common, highly stereotyped and easily identifiable (Provine 1986; Yoon and Tennie 2010), which makes it particularly suitable also for observational settings. Spontaneous yawning is widespread in vertebrates and has been associated to a variety of physiological functions, including arousal regulation, thermoregulation and brain oxygenation (Baenninger 1997; Gallup 2010; Massen et al. 2014; Smith 1999). In contrast, contagious yawning is triggered by social stimuli and appears to occur in fewer taxa than spontaneous yawning (Massen and Gallup 2017). Supporting this notion, experimental studies have failed to show contagious yawning in some species that are solitary or facultatively social, such as red-footed tortoises (Wilkinson et al. 2011). This absence suggests that contagious yawning is unlikely to arise solely from a simple perceptual-motor reflex and may instead depend on the underlying social or cognitive conditions.

Evidence for contagious yawning has been reported primarily in non-human primates, including most great apes (e.g., Amici et al. 2013; Anderson et al. 2004; Campbell et al. 2009; Demuru and Palagi 2012; Palagi et al. 2014), several catarrhines (e.g., Gallo et al. 2021; Palagi et al. 2009; Paukner and Anderson 2006) and, more recently, both platyrrhines (Valdivieso-Cortadella et al. 2023) and strepsirrhines (Lemes et al. 2024; Valente et al. 2023). Beyond primates, evidence for contagious yawning has also been reported in other taxa, including wolves (Canis lupus; Romero et al. 2014), pigs (Sus scrofa; Norscia et al. 2021), sheep (Ovis aries, Yonezawa et al. 2017), African elephants (Loxodonta africana; Rossman et al. 2020) and parrots (Melopsittacus undulatus; Gallup et al. 2015; Miller et al. 2012). In contrast, evidence in some species remains equivocal. In domestic dogs (Canis lupus familiaris), for example, findings are mixed: while several studies report yawning contagion in response to human yawns (Joly-Mascheroni et al. 2008; Madsen and Persson 2013; Neilands et al. 2020; Romero et al. 2013; Silva et al. 2012), others have found no such effect (Buttner and Strasser 2014; Harr et al. 2009; O’Hara and Reeve 2011). Moreover, there is currently no evidence supporting contagious yawning in dog–dog dyads (Harr et al. 2009; O’Hara and Reeve 2011), suggesting that behavioural contagion in dogs might have been shaped over the course of domestication for interspecific interactions with humans rather than conspecific partners (see Call et al. 2003, and Palagi and Cordoni 2020).

Some authors have also linked behavioural contagion to empathy, arguing that perception–action mechanisms may allow observers to match the emotional states of others, giving rise to emotional contagion, which is often considered a basic form of empathy (Palagi et al. 2009, 2020; Platek et al. 2003; Preston and de Waal 2002). From this perspective, behavioural contagion is expected to vary across individuals and social contexts, reflecting differences in empathic sensitivity. For example, individuals sharing stronger social bonds or higher familiarity are predicted to show higher levels of behavioural contagion, as empathy is assumed to be stronger in these dyads (Palagi et al. 2009; Preston and de Waal 2002). Similarly, some researchers have suggested that in some species females may show higher levels of empathy and, consequently, more pronounced behavioural contagion than males, reflecting sex differences in caregiving and parental investment (Norscia et al. 2016; see Gallup and Massen 2016). However, whether such patterns provide direct evidence for emotional contagion remains debated, as they may instead reflect attentional biases toward more familiar or socially salient group members (e.g., Gallup 2021; Massen et al. 2012; Massen and Gallup 2017). From this perspective, contagion may be more likely to occur when observing individuals with prominent social roles, such as high-ranking or socially well-integrated group members. Supporting this view, in Tibetan macaques (Macaca thibetana), the individuals that are more central in the social network elicit significantly higher levels of contagion than more peripheral group members (Zhang et al. 2022).

To date, several studies have investigated whether behavioural contagion is modulated by the relationship quality, which has been operationalized in terms of familiarity, kinship or strength of the dyadic bond (Silk et al. 2009). In chimpanzees, for instance, contagious yawning is more frequent in ingroup or familiar individuals, as compared to outgroup or unfamiliar ones (Campbell and de Waal 2011, 2014). Similarly, contagious yawning is modulated by relationship quality in bonobos (Demuru and Palagi 2012; Palagi et al. 2014), geladas (Theropithecus gelada; Palagi et al. 2009) and wolves (Romero et al. 2014). However, other studies have found no effect of relationship quality on contagious yawning (chimpanzees: Massen et al. 2012; Madsen et al. 2013; spider monkeys: Valdivieso-Cortadella et al. 2023; parrots: Gallup et al. 2015), or even reported opposite patterns (in rats, Rattus norvegicus: Moyaho et al. 2015). In domestic dogs, some studies found that relationship quality positively affected the likelihood of yawning contagion in response to human stimuli (Joly-Mascheroni et al. 2008; Romero et al. 2013; Silva et al. 2012), whereas others reported no effect (Neilands et al. 2020; O’Hara and Reeve 2011; Madsen and Persson 2013).

With respect to sex differences, there is currently no consistent support for the prediction that females are generally more likely than males to show contagious yawning across mammals (Massen and Gallup 2017). Although one study on wolves showed that females had shorter reaction times than males to yawns produced by close social partners (Romero et al. 2014), several other studies reported no sex biases in contagious yawning (e.g., Campbell et al. 2009; Valente et al. 2023). Given that these patterns are inconsistent across species, some researchers have suggested that they may align more closely with an attention-bias account than with the hypothesis that behavioural contagion reflects emotional contagion or empathy (Massen and Gallup 2017). In chimpanzees, for example, yawns produced by males are more contagious than those produced by females (Massen et al. 2012), whereas the opposite pattern has been reported in bonobos (Demuru and Palagi 2012), suggesting attentional biases toward the dominant sex (Massen and Gallup 2017). Similarly, female geladas show higher levels of contagious yawning than males, but only when the observed yawner is female (Palagi et al. 2009). In dogs there is no evidence for sex-based modulation of contagious yawning (Neilands et al. 2020; Romero et al. 2013).

In this study, we investigated contagious yawning in wolves and wolves admixed with dogs (hereafter, admixed wolves), the latter comprising introgressed individuals of second- or later generation backcrosses to wolves (see below). From a theoretical perspective, comparing admixed and non-admixed wolves can shed light on the selective pressures shaping behavioural contagion and, more broadly, behavioural coordination. While evidence for contagious yawning is relatively well established in primates (e.g., Massen and Gallup 2017), patterns in canids are less clear. In wolves, only a single study has documented contagious yawning (Romero et al. 2014). In this study, wolves were more likely to yawn after observing a conspecific yawning, which the authors interpreted as evidence that contagious yawning may help highly social species such as wolves synchronize behavioural and physiological states within the group, thereby promoting coordination and social cohesion within the pack. In domestic dogs, however, there is no clear evidence for dog–dog contagion (Harr et al. 2009; O’Hara and Reeve 2011), and the role of familiarity and relationship quality remains inconsistent. This raises the possibility that domestication may have shaped behavioural contagion (see Palagi and Cordoni 2020), favouring the emergence of traits that enhance attention to humans (e.g., Miklósi et al. 2003) and sensitivity to human social cues (e.g., Hare and Tomasello 2005). If these traits are inherited from dogs through introgressive hybridization, admixed wolves may show contagion patterns that are intermediate between wolves and domestic dogs. Determining whether introgressed dog alleles influence behavioural contagion, therefore, may help clarify whether and how hybridization affects group coordination and social cohesion in admixed wolves, with important implications for their ecology and social behaviour.

Here, we made the following hypotheses and predictions. Given that wolves form cohesive packs characterised by strong affiliative bonds and high levels of cooperation (Packard 2003, 2019), and that contagious yawning has been documented in wolves (Romero et al. 2014) but not in dog–dog dyads (Harr et al. 2009; O’Hara and Reeve 2011), we expected that both admixed and non-admixed wolves would exhibit behavioural contagion (Prediction 1a), but that this effect would be stronger in wolves (Prediction 1b). Furthermore, if the distribution of behavioural contagion within groups is shaped by attention- and/or empathy-based biases, we predicted that contagious yawning would be more likely between individuals having stronger social bonds (Prediction 2a); when the initial yawner was socially more prominent, by having a higher rank (Prediction 2b); or when the initial yawner occupied a more central position in the social network (Prediction 2c). However, such biases may be more pronounced in cohesive groups, where individuals form stronger affiliative relationships, dominance hierarchies are more stable, and individuals may differentially allocate attention to socially salient partners (see Amici et al. 2024, for a preliminary comparison of social cohesion in admixed and non-admixed wolves). Therefore, we also predicted that the modulating effects of social bond strength, rank and centrality would be stronger in non-admixed than in admixed wolves (Prediction 2d).