To our knowledge, the present study is the first to systematically investigate spontaneous imitation in free interactions between zoo visitors and zoo-housed chimpanzees. Previous visitor studies have found, in general, little interaction between H. sapiens and P. troglodytes. Moreover, visitor-directed chimpanzee actions appeared to have been generally unrelated to visitors’ behaviours unless food was involved, which elicited begging behaviours (e.g. Cook and Hosey 1995; Wood 1998). As suggested by reviewers, factors that might account for such different results are exhibit design and animal husbandry practices. The presence of large glass walls might facilitate close proximity between the species and thus higher levels of interaction. With respect to husbandry practices, zoo animals that are engaged by keepers in social interactions subsequently respond more positively to unfamiliar visitors (see Hosey and Melfi 2015).

In this study, during 52 h of observation, we recorded 354 episodes of cross-species reciprocal interactions. Food-related gestures were exclusively performed by AF. Imitative actions by one species or by both occurred in 37% of the 354 reciprocal interactions, and had an effect of prolonging and increasing engagement in the interaction. Indeed, episodes that included imitation contained twice as many actions as those without imitation. For both species, all imitated actions were familiar ones. This supports the view that, for a scientific understanding of the roles of behaviour copying, studies of imitation should not only be limited to the area of social learning.

The imitative responses documented here did not exhibit the characteristics of reflexive responding. On the contrary, several of their features suggest intentional communication as both species produced ‘targeted’ imitations, i.e. while visually and bodily oriented towards the model and while monitoring the interaction. Directedness toward a specific receiver and sensitivity to attentional stances are the most commonly accepted behavioural criteria for intentional communication in apes (e.g. Hobaiter and Byrne 2011). It has been argued, however, that apes’ intentionality in communicative situations lacks a crucial ‘shared’ aspect, which in humans supports the motivation to share experiences with others by means of communication (e.g. Carpenter and Call 2009; Nielsen 2009). As a consequence, these authors have proposed that, in apes, imitative exchanges might lack the communicative component. For example, in imitation learning tasks, apes are found to engage in shorter bouts of communicative behaviours (joint attention, gazing at the model’s face) than children (e.g. Carpenter and Tomasello 1995). However, it has also been found that communicative imitation in children is favoured by non-learning contexts, and that communicative behaviours decrease or even vanish in instrumental imitation learning tasks (Cordonier and Deschenaux 2014). With respect to imitation recognition, previously tested apes have not been reported to show signs of enjoyment and playfulness, e.g. to engage in imitative games (Nielsen 2009). The evidence in our study suggests, however, that apes can show playfulness in imitative contexts and that imitation might serve a communicative purpose. We base this conclusion on the fact that, on several occasions, the chimpanzees engaged in prolonged imitative exchanges that satisfy the criteria to qualify as imitative games. Additionally, we observed that, at least on some occasions, AM exhibited play face expressions in the context of imitative exchanges. We therefore propose that the imitative episodes described here are reminiscent of toddlers’ communicative imitation. In such contexts familiar actions are overtly imitated for initiating and maintaining interaction, resulting in mutually rewarding and sustained engagement in the interaction, occasionally leading to imitation games (Eckerman et al. 1989). This is in agreement with the suggestion that apes exhibit signs of shared intentionality in the context of social games, especially considering that in at least three documented cases—two with bonobos (Pika and Zuberbühler 2008) and one with gorillas (Tanner and Byrne 2010)—the social game involved symmetrical object exchanges, and thus could have been potentially imitative in nature.

Further argumenting for the intentional and communicative character of the imitation documented in this study is evidence of flexibility and selectivity. Interestingly, the most flexible chimpanzee exhibited an imitative repertoire of a similar size to that of the most imitative visitor categories (adults, subadults and juveniles). On the other hand, only few actions were preferentially selected for seemingly imitative–communicative purposes by both species. This selectivity is likely the result of a long interactional history between visitors and chimpanzees that—among other things—allowed the chimpanzees to learn the effect that copying specific actions has on visitors. Such an interpretation is plausible, considering that apes show selectivity for behaviours experienced as communicatively efficient (Hobaiter and Byrne 2011). Even if repeated interaction had a reinforcing role in shaping chimpanzees’ imitative responses, it is worth emphasizing that the only plausible reinforcement was the rewarding nature of (continued) social interaction itself. While zoo staff uses food incentives in many interactions with the chimpanzees, in our experience these interactions do not involve imitation, and are thus not likely to reinforce such responses. They more likely result in begging gestures, which we do see directed at zoo visitors as well. The actions we observed in imitative games, however, were typically not ritualized begging gestures.

Another finding is that, in both species, imitation rate increased in the indoor environment, suggesting that physical proximity plays a role in facilitating imitation. In both species, physical proximity further coincided with increased partial imitation; conversely, exact imitation dominated almost exclusively in distal imitative exchanges. It is unclear, however, whether such results are due to higher requirements for precision in distal communication, or by the different repertoires of imitated actions indoors and outdoors. For example, Call (2001) found an orangutan subject to be more successful at matching actions performed with gross body parts than those involving smaller parts, e.g. a raise-finger action was mimicked with a raise-arm action. Thus the almost exclusive predominance of exact imitation in the outdoor locations could be related to the fact that larger gestures and gross motor patterns were more likely to be performed and imitated in that environment. In the indoor locations, on the other hand, imitated actions involved more detailed matching, such as, for example, knocking on the window with knuckles or palm. In any case, it has to be stressed that this increase in partial imitation was similar in both species. Physical proximity, especially in cases when space can be shared, such as a windowpane, might be a crucial variable and requires further attention in imitation research. It is also possible, however, that glass surfaces (rather than proximity), afforded actions that facilitated imitation.

Besides cross-species commonalities there were also notable differences. For example, humans showed overall higher imitative precision. The data further indicate differences concerning the size of the cumulative repertoire of imitated behaviours, which was larger among visitors than chimpanzees. This can be due to the large difference in the number of individuals that represented each species, but also to the fact that—unlike humans—the chimpanzees did not imitate bodily postures or behaviours with a physiological function. In this respect, we found a potential parallelism with imitation recognition, i.e. how the chimpanzees responded when their behaviours were imitated by the visitors. The chimpanzees responded when visitors imitated their actions, but they were not observed to react when visitors imitated their postures or physiological behaviours (e.g. yawn, scratch). That chimpanzees failed to show a response when visitors copied their postures could be related to the generally non-conscious nature of this type of mirroring behaviour, which, as opposed to overt imitation, does not elicit behaviours indicative of explicit imitation recognition. Although recent studies highlight the similar positive social consequences of both overt imitation and nonconscious mimicry and regard them as related phenomena (e.g. Carpenter et al. 2013; Duffy and Chartrand 2015), further research is needed to specify the relationship between these imitative behaviours.

Further research should moreover be directed towards establishing how representative imitative communication is of chimpanzees’ intra- and cross-specific interactions, and if the present results can be replicated. Our data show that four of the five chimpanzees that were observed produced imitative actions in cross-species interactions. These results contrast with previous experimental studies that found familiar action imitation to be restricted to certain, ‘gifted’ individuals (Tennie et al. 2012), or to specific non-voluntary behaviours such as yawning (Amici et al. 2014). Since human-raised chimpanzees show a higher orientation towards human behaviour and better imitative skills (e.g. Byrne and Tanner 2006; Tennie et al. 2012 for discussions), rearing history is a potential candidate to explain these differences. In the case of this group, however, it is not possible to draw any firm conclusions. The only mother-reared chimpanzee (SF1) in the group interacted only minimally with visitors, while the four imitating chimpanzees, which had mixed rearing histories, exhibited variations in imitation rates. Interestingly, AF, the only chimpanzee in the group that was temporarily (i.e. for 18 months) raised in a human home without simultaneous contact with conspecifics, was also the chimpanzee that imitated the least. One of the chimpanzees (AM) was by far the most interactive individual in the group, as well as the most flexible imitator, which might support the view that certain ‘gifted’ individuals are more prone to imitation (Tennie et al. 2012). Additionally, imitation recognition in chimpanzees appears to be related to sex, as males are more responsive than females when being imitated by a human experimenter (Pope et al. 2015). Yet AM’s overall imitation rate was relatively similar to those of SF2 and JF. It thus remains to be established whether other factors can further account for a chimpanzee’s propensity for copying behaviour. Finally, the results reported could be attributed to the nature of the familiar actions performed in these imitative interactions, as well as to the context in which they were deployed. In our study, we capture the imitation of familiar communicative or play behaviours in the context of relaxed and positive social interactions. Moreover, food acquisition was not involved, unlike in previous studies.

With respect to visitors’ behaviour, subadult males were the most imitating category outdoors, while toddlers almost never imitated in this environment. Indoors, however, toddlers showed an increase in both interactivity and imitation rate, becoming the most imitating category in this proximal setting.

An advantage of using an observational approach is that we were able to simultaneously document a broader range of variables than is possible in an experimental setting, while capturing spontaneous occurrences in a naturalistic setting. Inherent limitations to this method (e.g. the inability to study potentially moderating variables in controlled conditions) limit, however, the conclusions which can be drawn from this study. Overall, the present study highlights the imitation of familiar behaviours in apes as a relevant research topic and provides testable hypotheses concerning its role in promoting and maintaining play and interaction, as well as concerning the potential role of proximity in promoting imitation. Our data add to previous observations of spontaneous imitation by apes and indicate that, unlike previously suggested, the chimpanzees’ imitative actions might serve a socio-communicative function by maintaining social interaction. Moreover, imitation seemed to take on the characteristics of play, giving rise to so-called imitation games. We hope that the results presented here will incite further inquiries into this topic.