Our study revealed for the first time that pet dogs exhibit a social preference for people who behave in synchrony with them. Additionally, this preference is modulated by breed: molossoid dogs preferred the person who was synchronized with them, whereas shepherd dogs chose randomly. Our working hypotheses were thus confirmed.

However, after choosing between the synchronized and unsynchronized experimenters, dogs spent the same amount of time close to each experimenter, demonstrating that they were not motivated by fear or stress toward any of the experimenters, and supporting the hypothesis that behavioral synchronization alone influenced their choice. One could also argue that this result reflects a low preference between the synchronized and unsynchronized experimenters. To disentangle this point, a longer testing time could be carried out, to determine if prolonged exposure to synchronization would lead to a greater, longer lasting preference in dogs.

Interestingly, we found a strong breed effect that did not appear to be due to any difference in the dogs’ vigilance: (1) both breeds exhibited minimal numbers of stress-associated behaviors; and (2) we visually controlled for activity level/velocity: it did not differ between the breeds, as dogs only stayed still or walked (breed affects dogs’ velocity for faster activities such as when trotting or running, see Duranton et al. 2018; Voss et al. 2010). Molossoid dogs clearly preferred the experimenter with the synchronized behavior, whereas shepherd dogs did not. This is in line with previous findings showing that when the owner acts neutral, and does not provide any cues that could influence the dogs’ behavior, pet dogs behave according to the temperament for which their breed was selected during the course of domestication and selective breeding (Duranton et al. 2016, Merkham and; Wynne 2014). Shepherd dogs were selected for tending and herding livestock, and for paying attention to their owner for direction during shared activities. Molossoid dogs, by contrast, were originally selected for guarding and attending to strangers. Various studies of dogs’ personality have found that guarding breeds, including molossoids, are the boldest breed group (Turcsán et al. 2011; Starling et al. 2013), and are notably bolder than shepherd dogs (Svartberg 2006; Duffy et al. 2008). Molossoid dogs were selected to respond independently to novel and unusual situations (Starling et al. 2013), whereas shepherd dogs were expected to be more focused on their owners and less interested in unfamiliar people than other breeds (Vas et al. 2005; Passalacqua et al. 2011; Duranton et al. 2016). We, therefore, concluded that the molossoid dogs we observed were more interested in and thus more sensitive to the behavior of the unfamiliar person than shepherd dogs were, which would explain why they alone exhibited increased affiliation with the unfamiliar person who mimicked them. Our results are in line with recent findings (Kis et al. 2014; Nagasawa et al. 2015) evidencing that domestication, and thus genetic selection, affect dogs’ human-directed social behaviors.

Regarding the mechanisms, although the present paradigm allows us to state that activity (moving or staying still), temporal (changing from move to still or still to move) and local (staying close to) synchronies contribute to increasing dogs’ social attunement towards humans, it is prudent to note that the effect of each type of synchrony cannot be disambiguated using this paradigm. By testing the dogs in their natural living environment, we indeed aimed at creating a scenario as ecologically meaningful for the tested dogs as possible; and it is known that in natural settings, the three different nonconscious behavioral synchronizations are often not possible to isolate (see Duranton and Gaunet 2016 for more details). One could argue that the important feature explaining dogs’ preference is not behavioral synchronization in the broad sense, but only local synchrony, due to a simple mechanism: the mere exposure effect stating that the repeated, unreinforced exposure is sufficient to enhance attitude toward a stimulus (Zajonc 1968), by increasing positive affects towards it (Harmon-Jones and Allen 2001). Such an effect is known to be at play in preference tests (Bornstein and D’Agostino 1992). We would like to emphasize that we controlled as much as possible to avoid this effect. First, dogs were exposed only once to each experimenter, i.e., without any repetition, and the dogs received the same amount of exposure to the two experimenters as they stayed in a close space around the dogs for the same amount of time in both conditions. Second, tests started only after the dogs first explored the experimenters, to ensure that the dogs became attentive to the experimenters, and allowing them the time to recognize/categorize them as humans (Autier-Dérian et al. 2013). Mere exposure effects produced by stimuli that are clearly recognized are weaker than mere exposure effects produced by exposure to stimuli that are not recognized (Bornstein and D’Agostino 1992). Finally, our setting is evaluating the short-term effect of behavioral synchronization. Dogs were tested in Phase P, the preference test, immediately following completion of the exposure phases, Phase S and Phase R. It has been demonstrated in non-human mammals that mere exposure effect positively affects long-term preference for social companions but decreases the preference for the exposed stimulus in short-term situations (Hill 1978). We, however, did not find such an effect in our setting, allowing us to say that mere exposure effect was likely not at play in our setting.

The present study is the first to demonstrate that pet dogs exhibit social preferences for people who are synchronized with them. These social preferences are similar to those observed in human infants, who are more attuned to individuals who have behaved in synchrony with them (Tunçgenç et al. 2015; Cirelli et al. 2016). Dogs are the third species to display this ability, as behavioral synchronization has already been found to increase affiliation in humans and capuchins (Paukner et al. 2009; Chartrand and Lakin 2013). It has been suggested that the link between behavioral synchronization and increased affiliation was key to human evolution, helping to maintain smooth relationships between individuals (Lakin et al. 2003). We believe that behavioral synchronization, a social glue that helps bind individuals together (Lakin et al. 2003), also exists between dogs and humans. Behavioral synchronization leads to greater affiliation between dogs and humans, thereby increasing dyad/group cohesion and enhancing dogs’ integration into human society. In humans, sensitivity to behavioral synchronization is considered a prerequisite for prosociality (Asendorpf et al. 1996; Kirschner and Tomasello 2010; Xavier et al. 2013). Prosociality toward familiar conspecifics has recently been highlighted in dogs (Quervel-Chaumette et al. 2016a), but studies have failed to observe prosociality toward familiar persons (e.g., Marshall-Pescini et al. 2016; Quervel-Chaumette et al. 2016b). It is significant that human infants are known to exhibit more prosocial behaviors towards people who behave in synchronization with them (Cirelli et al. 2014, 2016), because dogs are considered to have analogous social skills to those of human infants (Miklósi et al. 2007). As the present study evidenced that pet dogs, like infants, prefer people who synchronize with them, we recommend further studies investigating interspecific prosociality to consider dog–human partners with a high level of behavioral synchronization, as this may interfere with prosociality.

Finally, our results support recent suggestions that convergent selection pressures, such as a similar developmental environment, social networks or cooperative activities, explain the social skills of different species (Miklósi et al. 2007; Quervel-Chaumette 2016a), including sensitivity to others’ synchronized behaviors. The new data yielded by the present study support the hypothesis that human-like social skills have functionally developed in domestic dogs during the course of their evolution and selection to live together with humans (e.g., Hare et al. 2002; Gaunet and El Massioui 2014). Interestingly, we also showed that genetic selection of specific breeds and groups of dogs for varied working activities also modulates behavioral synchronization skills. Such findings add consistent data to the existing literature on dog–human behavioral synchronization (see e.g., Duranton et al. 2017b) but may even allow us to go further in the understanding of the phenomenon. We suggest that both the dogs’ ability to synchronize with humans, and the dogs’ sensitivity to human’s behavioral synchronization are skills which have been selected for at different times in the evolutionary history of dogs: the first one early in the domestication history of the species as it is adaptive and found in all dogs, and the second with later artificial selection to enhance specific breeds’ abilities. Genetics, therefore, appear to be at play in the evolution of sensitivity to the synchronized behaviors of others, suggesting its plausible implication in humans too, as found for other social skills (Ebstein et al. 2010). Still, the present study and available data (Paukner et al. 2009; Chartrand and Lakin 2013) do not disambiguate the origin of sensitivity to the behavioral synchronization of others. Did such a skill evolve due to genetic selection through the pressure of social life (through a common ancestor, or evolutive convergence), and/or is it due to ontogenic effect and learning experiences during individuals’ development in community? What are the respective roles of both genetic and ontogeny? We suggest five hypotheses: (1) the skill was present in a common ancestor to all three species (humans, capuchins, and dogs) and does not depend on life experiences, (2) the skill was present in a common ancestor to all three species, and is modulated by life experiences, (3) the skill was present in the common ancestor of primates, and appeared in dogs through evolutive convergence, (4) the skill was present in the common ancestor of primates, appeared in dogs through evolutive convergence, and is also modulated by life experiences, and (5) the skill is present in the three species only due to life experiences. Further studies are needed to identify the origins of behavioral synchronization and to better understand its appearance in humans’ evolutive history.

To conclude, the present study showed for the first time that dogs possess human-like sensitivity to behavioral synchronization, exhibiting social preference for people who mimic them. It also provided new evidence that human-like social skills have evolved in dogs and that behavioral synchronization acts as an interspecific social glue between dogs and humans.