Why Do Monkeys Smack Their Lips in Social Groups?

Monkeys engage in a variety of facial and vocal gestures to interact with one another, and lip-smacking is a common behavior in social settings. This rhythmic movement of the lips, often accompanied by soft sounds, plays a key role in group dynamics. While it may seem simple, researchers have found it carries deeper significance in primate communication.

Understanding why monkeys engage in lip-smacking provides insight into how nonverbal signals contribute to bonding, coordination, and possibly even the origins of human speech.

Communication Role in Social Groups

Lip-smacking serves as a social signal that facilitates interactions within monkey groups. Unlike aggressive displays or alarm calls, it is often associated with affiliative contexts, reinforcing bonds and reducing tension. Studies on macaques show individuals frequently engage in lip-smacking during grooming, strengthening relationships and maintaining group cohesion. Its rhythmic nature and non-threatening appearance make it an effective tool for fostering trust.

Beyond individual interactions, lip-smacking plays a role in group dynamics. Research on rhesus macaques (Macaca mulatta) shows dominant individuals use it to signal approachability, allowing lower-ranking members to engage without fear of aggression. Mothers also use it to soothe infants, reinforcing attachment and encouraging social learning. Its consistency across different contexts suggests it is deeply embedded in primate social structures.

The acoustic and visual components of lip-smacking enhance its communicative function. Unlike vocalizations that carry over long distances, it is used in close-range interactions where subtle facial cues are more perceptible. Observational studies on Tonkean macaques (Macaca tonkeana) reveal that lip-smacking often precedes friendly interactions like play or grooming, acting as a precursor to positive exchanges. The predictability of this signal allows individuals to anticipate and respond appropriately, reducing misunderstandings or conflicts.

Physical Mechanics and Sound Production

Lip-smacking involves a coordinated sequence of rapid, rhythmic lip movements, often producing soft, repetitive sounds. Unlike vocalizations that rely on the larynx and respiratory system, this gesture primarily engages orofacial musculature, particularly the orbicularis oris and surrounding facial muscles. High-speed video analysis of macaques reveals the lips move at a frequency of approximately 5 Hz, similar to slow speech-related movements in humans. This rhythmic pattern is achieved through precise neuromuscular control.

Electromyographic recordings show that the cyclic opening and closing of the lips in lip-smacking differs from chewing or spontaneous jaw movements. Unlike mastication, which follows a rigid mechanical sequence, lip-smacking is flexible and dynamic, suggesting voluntary modulation. Cineradiography shows the tongue and jaw remain relatively passive, reinforcing that this behavior is primarily controlled by facial motor pathways rather than the broader oral-motor system involved in feeding.

The acoustic component arises from the brief closure and release of the lips, generating small bursts of sound. These sounds lack the harmonic structure of vocalizations and are instead characterized by transient broadband energy. Acoustic analyses show remarkable consistency in the temporal regularity of these sounds across individuals, indicating a shared motor pattern within species. Playback experiments confirm that conspecifics recognize and respond to these sounds even without accompanying visual cues, highlighting their significance as auditory social signals.

Observations in Different Monkey Species

Lip-smacking is widely observed across monkey species, though its expression and function vary depending on social structure and evolutionary lineage. Among macaques, it is particularly well-documented. Long-tailed macaques (Macaca fascicularis) use it extensively in affiliative interactions, frequently during face-to-face grooming, reinforcing social bonds and reducing tension. In Japanese macaques (Macaca fuscata), it is more context-dependent, often appearing in maternal interactions. Mothers direct it toward infants, seemingly to soothe them and facilitate early social learning.

Outside macaques, other primates display variations in lip-smacking use. Among geladas (Theropithecus gelada), it integrates into their vocal repertoire, blending with complex vocalized sounds known as “wobbles.” Unlike macaques, where lip-smacking is primarily visual and occasionally auditory, geladas produce a more conspicuous version central to their social conversations. This suggests that in some species, lip-smacking functions as an intermediate step between facial gestures and vocal communication. Similarly, in colobus monkeys (Colobus guereza), a related behavior known as “tongue-smacking” involves rhythmic mouth movements with soft clicking sounds. Though structurally different, it serves a similar social function, reinforcing cohesion within small, tightly bonded groups.

In baboons (Papio spp.), lip-smacking appears in conciliatory contexts, particularly after conflicts. Subordinates may direct it toward dominants as a form of appeasement, signaling non-aggression and a willingness to reintegrate. This highlights the flexibility of lip-smacking in different social scenarios. Even in species where it is less pronounced, such as capuchins (Cebus spp.), similar rhythmic mouth movements occur in affiliative interactions between closely bonded individuals.

Comparison With Other Primate Vocal Gestures

Lip-smacking differs from traditional vocal gestures in primates due to its combination of visual and auditory components. While vocalizations like grunts, screams, and coos rely on the larynx and respiratory system, lip-smacking depends on rhythmic lip movements without vocal cord involvement. This suggests it may bridge facial expressions and more complex vocal communication. Some researchers propose its controlled rhythmicity shares similarities with human speech patterns, particularly in tempo and coordination, offering insights into vocal communication evolution.

A comparison can be drawn between lip-smacking and lip-flapping in great apes, particularly chimpanzees (Pan troglodytes). Unlike the affiliative nature of lip-smacking in monkeys, chimpanzee lip-flapping often appears in dominance displays or excitement, accompanied by vocal outbursts. This contrast highlights how similar orofacial gestures take on different meanings depending on species and social context. Gorillas (Gorilla gorilla) exhibit a related behavior known as “hoots with pouts,” where rhythmic lip movements pair with vocalized calls, reinforcing the idea that primates integrate facial and vocal elements in diverse ways to convey social information.

Potential Relation to Coordinated Activities

Lip-smacking may facilitate coordinated group behaviors beyond simple social bonding. Its rhythmic nature and predictable structure suggest it helps synchronize activities among individuals. In highly social species like macaques, where group cohesion is crucial for survival, these nonverbal cues regulate interactions and maintain order within dynamic environments. Lip-smacking often coincides with cooperative behaviors like grooming or play, functioning as a preparatory signal aligning individuals’ actions.

Observations in captive and wild populations indicate lip-smacking often occurs in sequences where individuals take turns reciprocating the gesture, resembling rudimentary turn-taking. This structured exchange mirrors coordination seen in more complex social behaviors, like synchronized movements during group travel or cooperative defense against predators. In geladas, lip-smacking frequently integrates into vocal interactions, reinforcing social bonds while maintaining group cohesion. Its rhythmic structure bears similarities to human conversational speech, raising questions about whether such behaviors represent an evolutionary precursor to structured communication. By establishing a predictable rhythm, lip-smacking may help individuals anticipate each other’s responses, reducing miscommunication in social settings.

Neurological Correlates

The rhythmic precision of lip-smacking suggests specialized neural mechanisms regulate orofacial movement. Research using neuroimaging and electrophysiological studies in nonhuman primates identifies brain regions associated with motor control and social cognition as involved in its production. The basal ganglia, a key structure in movement sequencing, appears to generate the rhythmic patterns characteristic of lip-smacking. This is notable given that similar neural circuits are implicated in human speech production, suggesting a possible evolutionary link.

Studies on macaques show cortical motor areas responsible for voluntary facial movements exhibit distinct activation patterns during lip-smacking compared to chewing or other orofacial behaviors. The orofacial motor cortex, which governs fine motor control of the lips and mouth, demonstrates activity aligning with the rhythmic cycles observed. This suggests lip-smacking is not merely reflexive but a controlled movement influenced by higher cognitive processing. Additionally, lesion studies indicate damage to these motor pathways can disrupt lip-smacking, reinforcing that it is governed by specific neural circuits rather than incidental facial movements.