Joint travel is a common social activity of many group-living animals, which requires some degree of coordination, sometimes through communication signals. Here, we studied the use of an acoustically distinct vocalisation in chimpanzees, the ‘travel hoo’, a signal given specifically in the travel context. We were interested in how this call type was produced to coordinate travel, whether it was aimed at specific individuals and how recipients responded. We found that ‘travel hoos’ were regularly given prior to impending departures and that silent travel initiations were less successful in recruiting than vocal initiations. Other behaviours associated with departure were unrelated to recruitment, suggesting that ‘travel hoos’ facilitated joint travel. Crucially, ‘travel hoos’ were more often produced in the presence of allies than other individuals, with high rates of recruitment success. We discuss these findings as evidence for how motivation to perform a specific social activity can lead to the production of a vocal signal that qualifies as ‘intentional’ according to most definitions, suggesting that a key psychological component of human language may have already been present in the common ancestor of chimpanzees and humans.

Funding: The study was funded by the Leverhulme Trust (UK) with additional support by the Wissenschaftskolleg zu Berlin, the European Research Council and the Fyssen Foundation during the writing of this article. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Introduction

In a recent comprehensive review of animal travel, Boinski and Garber concluded that group movement “…is as much a social behaviour as it is an ecological response to the distribution and availability of resources and risks” [1] (p. 680). We subscribe to this view with a study on the social dimension of group travel in chimpanzees. Wild chimpanzees often travel in small subgroups, and this requires individuals to engage in coordination and communication [2]. Travel therefore makes a promising context for investigating the social awareness available to individuals during this joint activity [3]. From previous work with chimpanzees it is already relatively well established that, to some extent, chimpanzees can take their audience into account during call production, particularly in contexts of aggression [4], sex [5], feeding [6], when encountering group members [7], and when discovering dangers [8]. Similar findings have also emerged from closely related bonobos (Pan paniscus). In one study, female bonobos engaging in sexual behaviours with high- (but not low-) ranking partners advertised this fact with ‘copulation’ calls [9]. These and other findings have led to the suggestion that great apes are able to adjust signal production to their surrounding audience in seemingly strategic ways. This is relevant because it suggests that the common ancestor of modern humans and the two Pan species might already have had some control over vocal production by taking into account the audience and the social implications of call production.

There is little doubt that chimpanzees, as well as many other primates and non-primate species, can engage in communal acts with potentially different roles, such as group hunting [10]. Another relevant example of a communal act in chimpanzees is food sharing, which mostly consists of field observations of individuals tolerating others’ scrounging on food that they control, known as ‘passive’ sharing. Actively handing a piece of food to another individual, or ‘active’ sharing, is much rarer [11]. Related experimental evidence comes from captive bonobos, who will unlock a door to let another individual into the same room in order to share food [12]. Both chimpanzees and bonobos produce food calls when discovering a new food source, sometimes also to newly arriving individuals who have not yet been feeding in the tree. This apparent vocal recruitment has been interpreted as an invitation for the recipient to feed jointly with the caller [2,6]. Whether this is to merely avoid aggression in a potentially competitive situation [13] or to actively inform them in an altruistic way is currently unclear and the topic of ongoing research. In sum, there are a considerable number of situations in which great apes engage in joint activities, which offer as many opportunities to study the psychological bases of such behaviour.

In this study, we focused on the travel behaviour of free-ranging chimpanzees (Pan troglodytes schweinfurthii) of the Sonso community in Budongo Forest, Uganda [14]. Travel represents one of the main daily activities of chimpanzees, notably to find food sources, but also to reach to nesting sites or to interact with neighbours. Travel typically occurs in parties of varying sizes, often without interruption for several kilometres as if pursuing a goal [15]. Travelling with others is likely to be adaptive due to the potential dangers of encountering predators or males of neighbouring groups, which can have fatal consequences especially for single individuals [16]. Although intergroup encounters have been observed at territory borders, Sonso males do not show much ‘patrolling behaviour’, as described for other communities. Rather, they appear to control their territory by adopting foraging patterns and choosing travel routes that include the peripheral areas of their range [15]. Joint travel, in other words, is particularly important in this community because of the dangers of being in the more peripheral area.

We have observed that, in the travel context, chimpanzees produce a brief and inconspicuous vocalisation, the so-called ‘travel hoo’, which is acoustically distinct from ‘hoos’ produced in other contexts (Figure 1) [2]. Pilot observations showed that travel initiations were usually accompanied by various non-vocal behaviours, particularly ‘waiting’ and ‘checking’ (see methods), suggesting that the initiator may be expecting others to follow. To investigate whether ‘travel hoos’ function in recruiting others for a joint activity, namely group travel, we analysed the production of ‘travel hoos’ across the various stages of a travel event. We were specifically interested in whether callers directed these signals at specific audiences and how their vocal behaviour was integrated in their wider recruitment efforts.

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larger image TIFF original image Download: Figure 1. ‘Hoo’ spectrograms obtained from an adult male (HW) and female (NB) of the Sonso community. Above: ‘hoos’ given during travel events (‘travel hoos’); below: ‘hoos’ given during resting events (‘resting hoos’). Compared to ‘resting hoos’, ‘travel hoos’ are significantly shorter (0.125s vs. 0.336s, t-test, N=20, t=4.455, p=0.001), have a lower maximum fundamental frequency F 0 (178.83Hz vs. 220.47Hz, t-test, N=20, t=3.139, p=0.006), are less modulated (difference D between F 0MAX and F 0MIN : 37.17Hz vs. 89.23Hz, t-test, N=20, t=3.796, p=0.001), and consist of more elements (mean 2.7 vs. 1.0, t-test, N=20, t=-3.042, p=0.014). Analyses were based on N=20 calls (N=5 travel hoos, N=5 resting hoos recorded from HW and NB, respectively). https://doi.org/10.1371/journal.pone.0076073.g001

Researchers interested in signals that function to influence others’ social behaviour and are putatively intentionally produced usually look for a range of accompanying behaviours, such as: (1) audience checking: the signaller monitors the state of attention of a recipient; (2) response waiting: the signaller pauses after producing the signal to wait for a behavioural response; (3) persistence: the signaller repeats the signal or produces a new one if the recipient’s response is unsatisfactory [17,18]. We predicted that, if ‘travel hoos’ functioned to recruit others, they should occur in situations when the caller was with others rather than alone and was actively attempting to recruit others to follow. We further predicted that, if ‘travel hoos’ functioned to initiate travel, they should also be consistently given before the locomotor behaviour occurred during the travel ‘initiation phase’, so as to alert others to forthcoming departure. We also predicted that if ‘travel hoos’ were socially directed to recruit specific group members, they should be produced preferably in the presence of a desirable audience. According to the current literature, the following classes of individuals should be particularly desirable travel partners: (i) individuals with whom the focal animal maintained strong bonds, i.e., ‘allies’ [8]; (ii) higher-ranking individuals, who might offer protection and other social benefits [9,19] and, in the case of males; (iii) oestrous females potentially available as mating partners [20]. In contrast, if ‘travel hoos’ were a reflection of a more general motivation to travel, and if individuals did not expect a specific answer from the audience, then they should be delivered randomly throughout the ‘initiation phase’. Similarly, ‘waiting’ and ‘checking’ behaviours should not necessarily follow call production.