We describe here a novel communication system: food-requesting behaviour in wild, untrained monkeys, interacting with unfamiliar human beings. This behaviour appears to have emerged in at least two natural populations of bonnet macaques, an Old World cercopithecine primate species, in both of which it has already been established as a distinct behavioural tradition60,61. This signalling system involves, on occasion, signals of different modalities; specifically, a combination of vocal and two visual signals and remarkably, it was always exclusively directed towards humans. Although the macaques used communicative signals in different modalities within the same food-requesting event, we did not examine the temporal relationship between any two consecutive signals across modalities and hence, could not establish whether this communication system is multimodal in nature.

Communication signals in non-ape primates have rarely been established to be intentional and referential, especially in the wild35 (but see25,26,29,30 for studies in captivity). We tested for the compatibility of the behavioural components of food-requesting behaviour, as displayed by the study macaques, with the key criteria laid down to qualify a certain communication signal as being intentional in nature. First, our natural observations indicated that two key behavioural components—the coo-call and hand-extension gesture—were almost invariably used in contexts wherein the macaques interacted with humans. While the hand-extension gesture was never displayed in any other situation, coo-calls were produced in one other social context, when juvenile bonnet macaques were separated from their natal troops and unable to detect them. In the field experiments conducted during this study, the subject macaques performed all the behavioural components at significantly higher levels only when human subjects carried food during the trials (experimental trials), as compared to without food trials (control trials). This suggests that these communicative behaviours were being directed only towards a particular recipient, a human holding a food item. These signals thus qualify the first criterion of intentional communication, which proposes the presence of an appropriate recipient towards whom a signal needs to be directed3,12,20,44,71.

The hand-extension behaviour was displayed by the macaques only when there was a mutual visual-attention state established with the human recipient, as measured in terms of the two subjects exhibiting complete body orientation towards one another, as recorded during the natural observations and experimental trials. This establishes the credentials of this behaviour being intentional, as the second criterion of intentionality demands that the signaller be sensitive to the attentional state of the recipient. Visual gestures in primates have been defined to be movements of the limbs or head and body that are directed towards a recipient and that are goal-directed or intentional, and mechanically ineffective actions72. In accordance with this definition, we here report previously unreported “visual gesture” of hand-extension used in a specific communicative context for this population of bonnet macaques which was absent from gestural repertoire of this species35.

Coo-calls have traditionally been observed in different macaque species and preliminary used as a contact call that serves as a signal to maintain group cohesion68,69,70. During our focal animal sampling, we too recorded usage of coo-calls on eight occasions when juvenile macaques were separated from their natal troops. To our knowledge, this is the first record of wild bonnet macaques using the coo-calls, in a novel context of requesting food from humans. Our observations of the involvement of this call in the context of food acquisition by bonnet macaques may, however, not be entirely unprecedented. Rhesus macaques73,74 and Japanese macaques75, for example, could be trained to emit coo-calls while requesting food from humans but only under captive conditions. It is significant to note, in this connection, that two Japanese macaque individuals spontaneously produced coo-calls when taught to use a tool to retrieve food from an otherwise inaccessible source75. None of the studies, however, examined the actual function of the coo-call in those particular contexts.

We hypothesised that a potential function of the bonnet macaque coo-call in the context of food requesting would be to attract the visual attention of the human subject during the interaction. If true, we would predict that the macaques would produce this call at relatively higher rates in the absence of mutual visual attention—a condition that we tested for, both in our natural observations as well as in the field experiments. Our analysis of the coo-calls produced by the study macaques, however, revealed no significant differences in the rates of their production in relation to the attentional state of the human recipients, either during natural observations or in the field experiments. This would, therefore, tentatively argue against a possible role of the coo-calls to serve as a vocalisation to attract the visual attentional state of the humans, from whom the macaques were requesting food.

Alternatively, the call could be used to express the caller’s communicative intent to acquire food from humans; it could also serve the purpose of maintaining persistent contact with the human recipients. It has, in fact, been suggested that a signal could simultaneously function to express the internal motivational states of the signaller as well as respond to the external events experienced by the signaller5,76,77. We must, however, reiterate that it may be empirically difficult to distinguish between the latter possible functions of the coo-call. Additionally, it is entirely possible that certain other factors, such as partial occlusion of the food item, a specific position of the food item or some other unaccounted variable could explain the usage of coo-calls during food requesting. These possibilities, however, remain unexplored in our present study.

The study macaques displayed orientation behaviour both during natural observations and the field experiments when there was no mutual visual attention state with the humans involved. The macaques performed this behaviour consistently across food-requesting events and persistently within each event in a way that suggested that they were attempting to orient themselves to the visual field of the human recipients. This indicates that the bonnet macaque’s orientation behaviour functions to attract the attentional state of human interactants in this particular context by the subject macaque moving into the line of vision of the humans, illustrating the third criterion for intentional communication, which suggests that a signaller would orient itself with the line of view of a recipient.

The fourth criterion of intentionality requires continuous monitoring of the recipient by a signaller. This was met by the monitoring behaviour shown by the study macaques during natural observations or in experimental trials whenever food was present, regardless of the attentional state of the human interactants.

The final key criterion of intentional communication refers to the persistence and elaboration of a signal. Although debated, persistence is generally displayed when a goal-directed behaviour is expressed repeatedly or a functionally similar signal is displayed until the desired goal is achieved6,13. The study macaques ceased to display the behavioural components of food requesting immediately after they received the food item from the human target, which, we argue, marked the achievement of their intended goal. We suggest that, in future studies, experimental manipulation of goals could possibly be used to quantify persistence in a more concrete, objective manner13.

Another aspect of persistent behaviour—that of elaboration—allows for the enhancement of a signal in order to achieve the same goal and appears to be a crucial factor in intentional communication19,78. Traditionally, elaboration has been observed and discussed in the light of unimodal communication and involves instances when the signaller may have switched from the principal signal to another one in a different modality79. In the present study, we propose that the bonnet macaques have already elaborated on their signal by developing a signalling strategy that involves at least two visual signals (hand-extension gesture and orientation behaviour) and coo-calls. We further argue that it may be a difficult exercise to determine the principal signal in this communication system. We, therefore, suggest that the study macaques have possibly achieved the function of signal enhancement through a use of multiple visual or vocal signals.

Food requesting by the bonnet macaques thus appears to be potentially an intentionally produced behavioural strategy, as all its behavioural components except coo-calls conform to all the key multiple criteria of intentionality, considered simultaneously12,80. The argument against this complex behaviour being a mere response to the stimuli of food presence is bolstered by our observations that the simple sight of food items, available naturally or held by conspecific individuals, neither elicited a coo-call nor a hand-extension gesture in any situation. Moreover, in five of the 86 food-requesting events that we recorded, the subject macaques changed their strategy of acquiring food from requesting to opportunistically snatching the food item, when it was left unattended. This too indicated that the macaques displayed the food-requesting behaviour with the intention of acquiring the desired food item.

The food-requesting behaviour displayed by our study bonnet macaques involves communication signals that are being used intentionally. What is novel, however, is that these signals are being employed by the macaques through frequent interactions with humans in the wild; the performance of these behaviours has never involved any active training by caretakers, unlike that reported in studies of intentional gesturing by captive monkeys25,26,29,30. It is important to speculate on the possible origins of this novel behavioural strategy in particular populations of bonnet macaques. In general, bonnet macaques rarely actively share food with conspecifics and are more likely to aggressively defend food resources from others6,59,81,82. Adult bonnet macaques in Bandipur rarely displayed the food-requesting behaviour while interacting with humans holding food items. They either opportunistically snatched the food items away or aggressively threatened their human targets, the latter often being an effective strategy to induce their targets to throw away the food (AD and AS pers. obs.).

Juvenile macaques, which have begun to forage independently, however, may have to depend on alternative foraging strategies such as food requesting, particularly given their small body size and general fear of humans. All the 18 juveniles observed to display food-requesting behaviour in Bandipur were between the ages of two to four years. The additional ecological opportunity to request food from heterospecific humans could have potentially given rise to the co-option of the plaintive coo-call, generally produced by individuals of this species when separated from their troops, and the development of a new gesture, that of hand extension, by juvenile macaques in a novel context. It is entirely possible that, initially, certain individuals could have acquired the hand-extension gesture from simply reaching out with their forelimbs for the food held by humans. This behavioural act could have then become ritualised through frequent interactions with humans and may have either been reinforced when positively rewarded and then spread by social learning mechanisms, such as social facilitation or emulation within the population. Many vertebrate species are, in fact, known to rely on social learning to develop and acquire novel and/or complex foraging techniques83,84,85,86,87,88,89. More focused studies are required in the future to test these hypotheses in this population of bonnet macaques.

In conclusion, our study is the first endeavour to explore intentionality underlying complex vocal and gestural communication between a wild, non-ape primate species and humans in an entirely natural context, thus providing support to similar studies conducted in captive settings25,27,28,29,30. It also highlights an inherent capacity of behavioural flexibility displayed by a non-human species in using communication signals of different modalities to achieve an intended goal. Our present study thus provides strong supporting evidence toward inherent capacities of intentional communication in macaques, thus encouraging us to think that the precursors of intentional behaviour are possibly evolutionarily older than previously thought, even preceding that in the anthropoid apes. This exploration also tempts us to speculate that the cognitive precursors for language production may possibly be manifest in the usage of a combination of signals of different modalities to communicate particular intent90. Our preliminary explorations of such potentially evolutionarily conserved signalling could also aid future studies in investigating the origins and cognitive roots of human language. More specifically, we establish once again the inherent capacity of bonnet macaques, an Old World cercopithecine species, to develop and employ novel behavioural strategies and communicate their intent to members of another species in an unusual context, reaffirming the remarkable behavioural flexibility and cognitive abilities of this species60,63,65,66,67.