Dominance style is a key predictor of vocal use and evolution across nonhuman primates.

Animal communication has long been thought to be subject to pressures and constraints associated with social relationships. However, our understanding of how the nature and quality of social relationships relates to the use and evolution of communication is limited by a lack of directly comparable methods across multiple levels of analysis. Here, we analysed observational data from 111 wild groups belonging to 26 non-human primate species, to test how vocal communication relates to dominance style (the strictness with which a dominance hierarchy is enforced, ranging from 'despotic' to 'tolerant'). At the individual-level, we found that dominant individuals who were more tolerant vocalized at a higher rate than their despotic counterparts. This indicates that tolerance within a relationship may place pressure on the dominant partner to communicate more during social interactions. At the species-level, however, despotic species exhibited a larger repertoire of hierarchy-related vocalizations than their tolerant counterparts. Findings suggest primate signals are used and evolve in tandem with the nature of interactions that characterize individuals' social relationships.

Gestural communication of wild bonnet macaques in the Bandipur National Park, Southern India.

Nonhuman primate gestures are believed to be crucial evolutionary precursors of human language. Comparative studies on primate gestures in an evolutionary framework have, however, remained largely restricted to the great apes and the potential flexibility and richness of gestural communication in monkeys, especially in the wild, continue to be virtually unknown. In this paper, we followed several criteria, adapted from ape gesture studies, to identify gestures and evaluate their contexts of usage in the repertoire of wild bonnet macaques Macaca radiata in the Bandipur National Park of southern India. This report is the first of its kind to systematically identify gestures in any wild, non-ape species, thus providing a platform for comparative studies across primate taxa, particularly in our efforts to trace out the phylogenetic origins of language-like markers in the primate lineage, earlier than in the great apes.

Intentional communication between wild bonnet macaques and humans.

Comparative studies of nonhuman communication systems could provide insights into the origins and evolution of a distinct dimension of human language: intentionality. Recent studies have provided evidence for intentional communication in different species but generally in captive settings. We report here a novel behaviour of food requesting from humans displayed by wild bonnet macaques Macaca radiata, an Old World cercopithecine primate, in the Bandipur National Park of southern India. Using both natural observations and field experiments, we examined four different behavioural components-coo-calls, hand-extension gesture, orientation, and monitoring behaviour-of food requesting for their conformity with the established criteria of intentional communication. Our results suggest that food requesting by bonnet macaques is potentially an intentionally produced behavioural strategy as all the food requesting behaviours except coo-calls qualify the criteria for intentionality. We comment on plausible hypotheses for the origin and spread of this novel behavioural strategy in the study macaque population and speculate that the cognitive precursors for language production may be manifest in the usage of combination of signals of different modalities in communication, which could have emerged in simians earlier than in the anthropoid apes.

Not here, there! Possible referential gesturing during allogrooming by wild bonnet macaques, Macaca radiata.

Intentional referential gestures, a fundamental building block of symbolic human language, have been reported from a range of species, including non-human primates. While apes are known to spontaneously use intentional gestures, only captive macaques, amongst non-ape primates, appear to intentionally display learnt gestures. On the other hand, referential gestures have so far been reported only in chimpanzees, amongst non-human primates. We document here, for the first time, potentially referential gesturing, used intentionally as well, in a monkey species, the bonnet macaque Macaca radiata, in the wild. Bonnet macaques use four distinct actions during allogrooming, possibly to indicate a particular body part intended to be groomed. These acts were successful in drawing the recipients' attention to the indicated part, which they began to groom subsequently. This study enriches our understanding of non-ape primate gestural communication and adds to the growing evidence for early human language-like capacities in non-human species.

Watering holes: The use of arboreal sources of drinking water by Old World monkeys and apes.

Water is one of the most important components of an animal's diet, as it is essential for life. Primates, as do most animals, procure water directly from standing or free-flowing sources such as pools, ponds and rivers, or indirectly by the ingestion of certain plant parts. The latter is frequently described as the main source of water for predominantly arboreal species. However, in addition to these, many species are known to drink water accumulated in tree-holes. This has been commonly observed in several arboreal New World primate species, but rarely reported systematically from Old World primates. Here, we report observations of this behaviour from eight great ape and Old World monkey species, namely chimpanzee, orangutan, siamang, western hoolock gibbon, northern pig-tailed macaque, bonnet macaque, rhesus macaque and the central Himalayan langur. We hypothesise three possible reasons why these primates drink water from tree-holes: (1) coping with seasonal or habitat-specific water shortages, (2) predator/human conflict avoidance, and (3) potential medicinal benefits. We also suggest some alternative hypotheses that should be tested in future studies. This behaviour is likely to be more prevalent than currently thought, and may have significant, previously unknown, influences on primate survival and health, warranting further detailed studies.