Primates other than humans communicate with each other in a number of ways. Many primates use touch to establish relationships, grooming each other to show friendship and to remove insects. Howler monkeys, living in the tropical forests of South America, make very loud calls. The male calls to mark his territory and can often be heard over 3km (almost 2 miles) away. Male gibbons, too, have loud calls, used for communicating with family members and to warn off other males. Some Old World monkeys have brightly coloured faces and bottoms, which the males use in courting displays and to frighten off enemies and rivals. Gorillas thump on the ground to warn off rival males, or beat their chests and roar to demonstrate their strength and power. Chimpanzees communicate with each other by using sounds and gestures.
Like other animals, primates communicate to satisfy their biological and social needs, such as avoiding predators, interacting with other group members, or maintaining cohesion during travel. To this end, they use a range of different signals, many of which have directly evolved as ritualised abbreviations of more basic behavioural or physiological processes. For example, chimpanzees sometimes react with pilo-erection (bristling of hair) during conflicts, which makes them appear bigger and more dangerous and conveys their willingness to escalate. Communication signals have thus evolved partly to be psychologically effective on receivers (Guilford & Dawkins 1991).
Most primates live in groups in which members know each other individually and maintain multifaceted social relations; factors which are thought to favour the evolution of advanced communication skills. However, other animals with complex social behaviour, such as dolphins, also show sophisticated communication skills, suggesting that complex communication is not limited to primates.
Monkeys sometimes produce terrestrial predator alarms when competing over food, even though no predator is around. As a result, other group members run to safety, which then gives the caller a foraging advantage. In general, however, primates rarely produce such dishonest signals, or ‘cry wolf’. Why is dishonest signalling not more common? One solution has been given by Zahavi’s ‘handicap principle’, which states that receivers will only attend to signals that are difficult to fake by low-quality or poorly motivated individuals. It has also been argued that, in primates, individuals know and need each other and thus gain little from deception. Moreover, primates can learn to ignore unreliable signallers, suggesting that ‘reputation’ acts as a further safeguard against dishonest signalling. Honest signalling prevails because of sceptical receivers.
Primate communication takes place in all major modalities. Olfaction is one of the least researched modalities, partly because it is difficult to measure and manipulate olfactory cues, especially in the wild. Nevertheless, probably all primates secrete scents that influence others. An interesting human example is women apparently influencing each other’s ovulation through odourless cues. Another remarkable example is the ‘stink fights’ of male ring-tailed lemurs. During conflicts, males rub their tails across their wrist and chest glands before waving them at each other. Generally, olfactory cues play important roles in stating claims over resources and displaying individual characteristics, such as reproductive state, social rank, immune-compatibility, and other genetic traits. One difficulty with research on olfactory communication is that it is often unclear whether scent-bearing substances are actively and strategically released into the environment, or whether they are mere by-products of general metabolic processes. Active scent marking and self-anointment (applying scent-bearing substances onto a substrate or body) are notable exceptions, but in many cases it is unclear whether olfactory cues qualify as proper communication signals.
In the visual domain, primates use a range of facial displays and body part movements as communication signals, sometimes combined with tactile components. Gorilla chest beating and Rhesus monkeys bared-teeth displays are examples of how different species express social rank with visual signals. Current research has focused much on gestures, which are interesting because of their partly flexible, partly species-specific use in a variety of social contexts. Gestures have been studied mainly in great apes, where considerable variation between individuals and groups has been found. Whether some of this variation is socially learned and thus potentially ‘cultural’ is still an unresolved question. A more established finding is that, during gesturing, apes take into account each other’s attention and deploy their signals accordingly. Interestingly, however, there is almost no evidence that primate gesture, or combinations thereof, carry symbolic meaning by referring to external entities. Instead, they appear to function primarily to facilitate ongoing social interactions, to bond with others, or to persuade others to behave in a desired way.
For many primates, vocalizations are the main channel of communication. Vocal repertoires tend to be species-specific, indicating that they develop under strong genetic control. Humans also possess a specific repertoire of such context-specific calls but, in addition, they also possess extensive control over vocal production, an ability that develops early and is crucial for the acquisition of speech. Such high degree of vocal control is not seen in other primates and one interesting hypothesis is that it is the product of relatively recent genetic changes during human evolution. If this is correct then our hominid ancestors must have relied on a primate-like communication system.
In sum, primates communicate using all major modalities. Olfactory communication is poorly researched but is probably widespread, mostly inflexible and contextually confined to basic biological functions. Within the visual modality, gestures are somewhat of an exception because of their flexibility and socially targeted use. Finally, vocal communication is based on species-specific repertoires, with some flexibility in use but little in structure.
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