Living in groups means that communication is key, and green monkeys have a wide vocal repertoire that helps them with this. Different types of calls, grunts, and barks together comprise a whole language, with specific sounds being used for specific purposes. 

Dominant adult males emit loud barks at dawn and sunset, from high up in the canopy, advertising their troop’s location, their ownership over their territory, and ensuring that spacing is maintained between neighboring groups. Intra- and inter-group conflicts or territory disputes can lead to noisy screams from both males and females. They also emit soft, friendly grunts during grooming, while handling infants, and to greet one another. 

Vocalizations allow troops to maintain coordination during travel. Adult females often make loud screaming sounds to alert the group that it is time to get on the move. Once the group is moving, green monkeys make a specific repetitive vocalization, termed ‘cohesion grunts’, that go ‘cro-cro-cro-cro’, allowing them to keep track of one another’s position as they travel and forage.

Communicating about food is crucial, and dominant adults may make aggressive grunts around food sources to threaten subordinates to wait their turn. Possessive gasps and clicks are used to assert ownership over a piece of food that a monkey is holding, indicating that it is not to be shared. Satiated gasps and clicks may follow once a monkey is resting after foraging. 

Subadults or low-ranking individuals may scream when they are experiencing threats or aggression from more dominant monkeys, or when they are startled or punished. They may also scream and make distressed calls to beg for food from high-ranking individuals, expressing submission and eliciting care instead of aggression. 

Baby green monkeys make cooing sounds when they want an adult’s attention, and may shriek loudly if their mother prevents them from breastfeeding. A sudden movement or harm can lead to surprised shrieks from infants, while being separated from their mother can make them squeal and scream loudly. 

A whole set of calls is dedicated to alerting each other about predators, with specific vocalisations for different threats. For example, if a leopard is spotted, males produce loud barks and females make longer, high-frequency ‘leopard-chrips’. Upon hearing these calls, other green monkeys flee high into trees to avoid predation from a leopard lurking on the ground. When the predator in question is a snake, males emit soft chutters, while females make shorter, lower-frequency ‘snake-chirps’. In response, while some monkeys flee, others stand up on their two hind legs to assess the threat, collectively mobbing the snake instead of running away. 

Natural threats from aerial predators are relatively lower in many green monkey populations, since large monkey-eating raptors like eagles are absent from large parts of their range. However, some populations do have a distinct, aerial-predator-specific call. A striking example of their ability to distinguish threats and communicate it vocally comes from an experimental study. When scientists exposed green monkeys in the wild to a novel aerial threat, a flying drone, they responded with distinct aerial calls, different from leopard and snake calls, and very similar to aerial calls made by other closely-related monkey species when faced with predation from eagles.

The ability to make and recognize distinct predator-specific calls is very useful, since escaping from different predators can require different responses. In Barbados, where introduced green monkey populations have been separated from their native predators for over 350 years, these monkeys have lost their predator-specific calls and instead make a generalised alarm call in response to threats. However, they retain the ability to recognise predator-specific calls and respond appropriately when such calls recorded from their West African cousins are played in the Barbados monkeys’ habitats. 

While vocalization is an important mode of communication for green monkeys, visual signals can play very crucial roles too. Adult males have a contrasting ‘Red-White-and-Blue’ display, wherein they present their red penis and blue scrotum against the surrounding white fur to assert their territorial and sexual dominance. During territorial disputes, they may also stand bipedally and display their bright white chest fur against their darker abdominal fur, along with jumping and shaking branches, as additional shows of strength. Scrotal color for males lightens as they grow older, and dominant males have brighter scrotums than submissive ones, so this visual signal plays a major role in sexual signalling of both age and status. 

Body postures are important to assert dominance too, and high-ranking individuals often walk with a more confident, upright stance. Low-ranking individuals may crouch or cower while walking to signal submissiveness and avoid any physical aggression from the dominant individuals. Facial expressions serve similar roles, with bared teeth signalling submission, while open-mouth expressions signal threats. 

Tactile communication via grooming and body contact helps soothe infants. Females and young adults also groom each other and males socially, reinforcing their group bonds. Juveniles might comfort each other by engaging in social suckling, and females may rub their chests against each other, all serving as ways to increase social cohesion in their troops.