In its most basic form, communication occurs when one individual (the sender) emits a signal that conveys information, which is detected by another individual (the receiver). We have discussed several aspects of primate sociality in this chapter, all of which require the communication of information between individuals. But exactly how does a female chimpanzee communicate her sexual availability? How does a vervet monkey communicate the approach of a leopard or that a python is nearby? How do solitary, nocturnal primates, like the slow loris, communicate information about themselves to conspecifics? Primate communication comes in four forms: vocal, visual, olfactory, and tactile. Species vary in their reliance on each.
Primates use sound to communicate danger or threats, to claim and maintain a territory, or make contact with other group members. Alarm calls are given in response to predators. In some cases, alarm calls are used to alert members of the group to the presence of a predator so they can take evasive action. In other cases, they are directed at the predator itself, signaling that it has been detected. You can learn more about alarm calls as forms of vocal communication in the highlight box in this chapter entitled “Dig Deeper: Alarm Calls: Signals to Friends or Foes?.”
Loud calls are designed to travel great distances and are used in territorial defense by many primate species including indris (Indri indri), orangutans, gibbons, and howler monkeys (Alouatta). In dense forest, where visual communication can be difficult, loud calls can be useful in signaling to conspecifics that a group or individual occupies a specific area. Howler monkeys are named for their loud calls, or “roars,” which can be heard one kilometer or more away (Schön Ybarra 1986). Howler monkey roars may act to maintain distance between neighboring groups or keep extragroup males from entering the home range (Schön Ybarra 1986).
Other vocalizations are intended to communicate with individuals in one’s own group. These include vocalizations given as part of threat displays or dominance interactions, as well as contact calls that provide information about one’s location to other group members. Chacma baboons (Papio ursinus) have a rich repertoire of vocalizations for communicating with other group members (Fischer et al. 2008). Adult males give specific vocalizations during threat displays and physical confrontations. Subordinates “screech” when retreating from a dominant individual, signaling submission. Since baboons rely on membership in their group for finding food and detecting predators, a baboon separated from his group will vocalize in an attempt to regain contact. Young baboons emit their own contact calls when separated from their mothers.
Visual communication, which involves signals that can be seen, is an important component of nonhuman primate behavior, alone or in combination with other forms of communication. Piloerection, or raising one’s hair or fur, is used in aggressive interactions to make an individual appear larger than it actually is. Female macaques (Macaca), baboons (Papio), and chimpanzees, signal sexual receptivity through changes in the size, shape, and, often, color of their hindquarters, called a sexual swelling (Figure 6.19a). The sexual swelling reaches its maximum size at ovulation. When females are not receptive, either because they are pregnant or are nursing, they do not display a sexual swelling (Figure 6.19b). Thus, the presence or absence of a sexual swelling signals a female’s reproductive state.
Monkeys and apes use diverse facial expressions in visual communication. Showing your teeth in a “smile” sends a signal of friendship in humans. Displaying teeth in this way is a sign of anxiety or fear in primates. That male mandrill you see “yawning” at your local zoo is actually displaying his teeth to signal tension or to threaten a rival (Figure 6.20a). In addition to showing their canines, male gelada baboons use “lip flips,” in which the gums and teeth are exposed by flipping the upper lip up over the nostrils (Figure 6.20b), and “raised eyelids,” in which the pale eyelids are exposed by pulling the scalp back as threatening gestures (Aich, Moos-Heilen, and Zimmerman 1990). Submissive males respond by fleeing or presenting their hindquarters.
Primates also communicate through color. In female and male mandrills, facial coloration provides information about an individual’s health, competitive ability, and reproductive state to conspecifics (Figure 6.21; Setchell et al. 2008; Setchell, Wickings, and Knapp 2006). Variation in facial coloration among monkeys of Central and South America ranges from very simple (Figure 6.22a) to complex (Figure 6.22b). Species living with larger numbers of other primate species have evolved more complex facial coloration patterns, suggesting that this trait evolved as a form of species recognition, or the ability to differentiate conspecifics from members of other species (Santana, Lynch Alfaro, and Alfaro 2012).
All primates use scent to communicate. Females secrete chemicals from their anogenital region (the area of the anus and genitals) that provide males with information about their reproductive state. In some species, like macaques and chimpanzees, this olfactory signal is enhanced by a sexual swelling, as discussed above. Olfactory communication, or communicating through scent, is particularly important for monkeys of Central and South America, lemurs, and lorises. Male and female common squirrel monkeys (Saimiri sciureus) (Figure 6.23a) engage in “urine washing,” in which an individual urinates on its hands and feet and then uses them to spread urine all over its body. Urine washing may be used to mark trails for others to follow, to control body temperature, as part of dominance displays, or to communicate reproductive state (Boinski 1992). During aggressive interactions with other males, male ring-tailed lemurs rub their tails with scent from glands on their wrists and chests. They use their “perfumed” tails in aggressive interactions with other males, who may respond by waiving their own scented tail, with physical aggression, or by fleeing (Jolly 1966). Males also waive their tails, saturated in scent, to attract females (Shirasu et al. 2020). Males use scent glands in their wrists to mark territorial boundaries (Figure 6.23b; Mertl-Millhollen 1988).
Tactile communication, or communicating through touch, is very important in all primate species. Physical contact is used to comfort and reassure, is part of courtship and mating, and is used to establish dominance and alliances. Grooming is an important and clearly enjoyable form of tactile communication for all primates (Figure 6.24). Not only does grooming serve to clean the skin and fur, removing parasites and debris, but it is an important affiliative behavior that helps reinforce social bonds, repair relationships, and cement alliances.
Alarm calls are common among group-living primates. They often serve to notify conspecifics of potential danger, as is the case with vervet monkeys. Research has shown that: (1) vervets classify predators based on hunting style; (2) alarm calls convey information to other vervets about that hunting style; and (3) other vervets respond in ways appropriate for evading that type of predator (Seyfarth, Cheney, and Marler 1980a). When a vervet gives a “leopard” alarm call (directed at mammalian carnivores like leopards, Figure 6.25a), monkeys on the ground climb the nearest tree, while monkeys already in trees stay there or climb higher. Since most mammalian carnivores hunt on the ground, getting into, and staying in, a tree is the best option for escape. When the “snake” alarm call is given, vervets stand on their hind legs and look down at the ground (Figure 6.25b). Since snakes are not pursuit predators, locating them quickly so as to avoid them is the best strategy. Lastly, when an “eagle” alarm call is given, vervets look up or run into bushes, both of which are useful responses for avoiding hawks and eagles, which attack from above (Figure 6.25c). Vervets clearly understand the meaning of each type of alarm call, as they respond appropriately even when they do not see the actual predator (Seyfarth, Cheney, and Marler 1980b). Such semantic communication, which involves the systematic use of signals to refer to objects in the environment, was once believed to be unique to humans. It may be a precursor to the symbolic capacities of human language.
Research on other African monkeys indicates that some species use alarm calls to signal to the predator that it has been detected. Diana monkeys (Cercopithecus diana) give alarm calls to leopards (Panthera pardus) but not chimpanzees (Zuberbühler, Noë, and Seyfarth 1997). Because leopards are stealth predators, they rely on the element of surprise to sneak up on their prey (Figure 6.26a). Alarm calling at leopards appears to tell the leopard that it has been seen and therefore its chance of success will be low. Research shows leopards are more likely to stop hunting after an alarm call has been emitted. Unlike leopards, chimpanzees are pursuit predators and may even use alarm calls to locate potential prey (Figure 6.26b). With such a predator, prey are better off remaining as silent as possible so as not to alert the predator to their location (Zuberbühler et al. 1999).