Baby monkeys cling to each other primarily for comfort and security, especially when a mother isn’t available. This behavior serves as a substitute for the physical contact they would normally get from their mothers, helping them regulate stress and develop socially. But clinging also serves practical survival purposes, from staying warm to building the social bonds they’ll need as adults.
Comfort and Stress Relief Drive the Behavior
Infant monkeys are born with a powerful need for physical contact. In the wild, that need is met by clinging to their mother’s body, often for weeks or months after birth. When mothers are absent, whether in captivity, after separation, or due to death, baby monkeys will turn to the next available warm body: each other.
The clinging triggers real physiological changes. Oxytocin, a hormone involved in bonding and social connection, plays a central role. Research from the National Institutes of Health found that rhesus monkeys exposed to higher oxytocin levels showed lower cortisol (the body’s primary stress hormone) in their saliva. They were also more socially engaged, more likely to look at caregivers, and more communicative through facial gestures. Physical contact between infant monkeys likely stimulates this same oxytocin response naturally, creating a feedback loop: clinging reduces anxiety, which encourages more clinging.
For a baby monkey without its mother, another infant’s body becomes the closest thing to safety. The contact isn’t random or incidental. It’s a direct response to fear and uncertainty, and it produces measurable calm.
Harlow’s Experiments Revealed How Deep the Need Goes
The most famous research on this topic came from psychologist Harry Harlow’s experiments with rhesus monkeys in the 1950s and 1960s. Harlow separated infant monkeys from their mothers and gave them two artificial surrogates: one made of wire that dispensed milk, and one covered in soft cloth that provided no food. The infants overwhelmingly preferred the cloth surrogate, spending hours clinging to it and only briefly visiting the wire one to feed. Contact comfort, Harlow concluded, mattered more than food.
But the story didn’t end there. Monkeys raised on cloth surrogates grew into psychologically damaged adults. They couldn’t interact normally with other monkeys, showed no interest in mating, and females who did become mothers were often abusive toward their young. A soft surface to cling to wasn’t enough. The monkeys lacked what Harlow called elementary social skills.
This led Harlow to study what he termed “affectional systems,” the distinct relationships monkeys form with mothers, peers, mates, and eventually their own offspring. He discovered that peer relationships were enormously important for healthy development, potentially even more so than the mother-child bond alone. Baby monkeys who could cling to and interact with each other developed social competence that surrogate-raised monkeys never achieved.
Peer Clinging Has a Downside Too
While clinging to peers helps baby monkeys cope, it’s not a perfect substitute for maternal care. Research on rhesus macaques raised exclusively with same-age peers (rather than with mothers) reveals a tradeoff. Peer-reared monkeys show excessive social clinging, more fearfulness, and higher rates of abnormal repetitive behaviors compared to monkeys raised with both surrogates and peers. They cling harder and longer because they have no secure base to return to, no adult figure that signals safety.
Monkeys raised with a surrogate plus peer access strike a better balance. They show less clinging, less fear, less aggression, and more exploration and play. The surrogate provides a stable anchor, freeing the infant to engage with peers in healthier ways. This mirrors what developmental scientists observe across primate species: secure attachment to a caregiver gives infants the confidence to explore their social world rather than desperately holding onto whatever contact they can get.
Staying Warm Is Part of the Equation
Clinging and huddling aren’t purely emotional. They also help baby monkeys survive physically. Infant primates are small, with high surface-area-to-body-mass ratios, which means they lose heat quickly. Huddling with other bodies is a straightforward solution.
Studies of Japanese macaques show that as temperatures drop, monkeys form larger and tighter clusters. Colder conditions reliably increase the likelihood that an individual will join a huddle. Among species like Yunnan and Sichuan snub-nosed monkeys, which live at high altitudes with harsh winters, huddling clusters grow larger and more cohesive on cold nights. Females and juveniles are especially likely to participate. The behavior serves triple duty in these species: conserving heat, reducing predation risk, and reinforcing social bonds.
For baby monkeys specifically, the thermoregulatory benefit of clinging to another warm body can be the difference between maintaining a stable core temperature and dangerous heat loss, particularly at night or in exposed environments.
Building Social Skills for Adulthood
Beyond immediate comfort and warmth, clinging to peers lays the groundwork for the social relationships monkeys will need as adults. Primate societies run on alliances, hierarchies, and cooperation. The physical closeness of infancy is where monkeys first learn to read social signals, tolerate proximity to others, and negotiate conflict.
Play behavior, which often emerges directly from clinging and huddling, teaches young monkeys how to calibrate their strength, take turns, and respond to another individual’s emotional state. Monkeys deprived of peer contact during infancy struggle with all of these skills later. Same-age peers raised in isolation have even been observed acting aggressively toward socially inexperienced monkeys, suggesting that the social rules learned through early physical contact are not easily acquired later in life.
Interestingly, the presence of infants also reshapes adult social dynamics. Research on squirrel monkeys found that when infants were removed from a group, the distances between adults decreased and affiliative interactions among adults increased by more than 100%. Infants, it turns out, had been absorbing much of the group’s social energy. Their clinging and proximity-seeking behavior draws adults into caregiving roles that can temporarily suppress other social interactions, highlighting just how central infant contact-seeking is to the structure of primate social life.