Do Monkeys Make "Friends" with Other Animals?—From Adoption to Kidnapping, Strange Inter-Species Interactions

In the forest, a monkey is meticulously parting the fur of a dog. Elsewhere, a young monkey repeatedly approaches a pig, trying to climb onto its back. A Japanese macaque riding on a deer's back, a langur gently stroking a giant squirrel, and a capuchin nurturing a baby of a completely different species.

If you only look at these scenes, you might be inclined to think, "Animals have friendships too," or "Monkeys cherish pets."

However, inter-species interactions do not always result in heartwarming stories. There are reports of rough handling, cases of young individuals being taken from their parents, and instances where the other party ended up losing their life.

Why do monkeys approach animals different from themselves? Is it affection, play, curiosity, or is it just that humans want to label it as "friendship" or "adoption"?

"Cross-Species Sociability" Revealed from 427 Records

In July 2026, an international research team published a study in the journal 'Primates' that systematically organized the social interactions between primates and other species.

The research team gathered cases from scientific papers, reports and videos, and survey results from primate researchers worldwide. The 427 interactions involved 88 species of primates and 127 species of other animals.

The counterparts were not only other species of monkeys and apes. They included dogs, pigs, squirrels, deer, birds, mice, reptiles, amphibians, and even insects.

The most frequently observed behaviors were "play" and "grooming," with 139 and 136 instances, respectively. Other recorded behaviors included holding, carrying, cuddling, and sharing food.

Particularly interesting was the variation in tendencies based on the age and sex of the individuals involved.

Young monkeys often played with animals of other species. Mature females were more likely to exhibit grooming or caregiving behaviors. In contrast, mature males generally had fewer friendly inter-species interactions.

This means not all monkeys approach other animals for the same reasons. For young individuals, it might be a way to learn about the world, while for females, it could be an extension of nurturing behavior. Individual personality, social status, and living environment are also thought to have a significant impact.

1. Macaque Grooming a Dog

In Thailand, members of the pig-tailed macaque group have been observed grooming domestic dogs.

In monkey society, grooming is not merely a hygienic behavior. It is also an important form of communication that alleviates tension among group members, maintains relationships, and confirms trust.

The fact that this behavior is directed toward dogs suggests that monkeys may not judge others solely based on whether they are the same species.

For monkeys living near humans, dogs are a familiar presence. Through repeated interactions, they may learn that dogs are neither dangerous competitors nor prey, and accept them as social contacts.

However, just because monkeys groom dogs does not necessarily mean a pet-owner relationship like that between humans and dogs is established.

It is possible that familiarity from living in the same area and a strong motivation for grooming in monkeys manifest beyond species differences.

2. Langur Gently Stroking a Giant Squirrel

In Sri Lanka, members of the purple-faced langur group have been filmed gently touching large squirrels.

The sight of a monkey's hand touching a squirrel's body might resemble a human doting on a small animal. However, it cannot be immediately concluded as "affection."

Monkeys use their hands to investigate objects. To determine if something is edible, safe, or how it reacts, they may touch unknown subjects.

On the other hand, if grooming or contact lasts a long time and the other party accepts it without fleeing, it might indicate social interaction beyond mere investigative behavior.

What complicates matters further is that among the animals monkeys interact with amicably, some could potentially become prey depending on the situation.

Animal relationships are not simply "friend" or "foe." Even among the same species, relationships can change based on food availability, individual age, hunger levels, and past experiences.

The one being petted today might be chased away in a different situation. Relationships in the natural world are more fluid than humans imagine.

3. Young Monkey Trying to Climb on a Pig's Back

In China, young Yunnan snub-nosed monkeys have been observed approaching domestic pigs and attempting to climb onto their backs.

This behavior reflects the strong curiosity of young monkeys.

For young animals, play is not just a pastime. It is also training to learn how to move their bodies, gauge strength, maintain distance from others, and respond to danger. Even if the other party is not the same species, if they respond, they can become playmates.

Research has confirmed that younger primates are more likely to engage in play with other species.

Mature individuals face practical challenges such as food, hierarchy, reproduction, and safety. In contrast, younger individuals have the leeway to try things that do not immediately yield benefits.

This "tendency to approach things of uncertain utility" can also drive the discovery of new foods, tools, and behaviors.

Inter-species play may demonstrate not only the social flexibility of monkeys but also their intelligence in learning from unknown subjects.

4. Japanese Macaque Riding on a Deer's Back

In Yakushima, Japan, it is common to see Japanese macaques and Yakushima deer moving around together.

There is a food-related aspect to their relationship, such as deer eating fruits and leftovers dropped by monkeys from trees. Deer benefit from being around monkeys as they can easily access fallen food.

However, their interactions are not limited to that. Monkeys riding on the backs of deer have also been recorded.

Sometimes it appears to be used as a means of transportation, while other times it may involve play or sexual behavior. The fact that deer do not strongly reject monkeys is likely one reason the interaction continues.

When we see monkeys riding deer, we might want to understand it as a miniature version of humans riding horses. However, it is not necessarily the case that monkeys recognize deer as "vehicles."

What is important is that different species of animals repeatedly interact in the same space and become able to predict each other's behavior to some extent.

Long-term coexistence may evolve from a simple food-related relationship to a more complex one involving play and contact.

5. Apes and Monkeys Holding Mice and Chickens

In zoos, members of the white-handed gibbon group have been observed holding and petting mice that entered their enclosure.

Additionally, in another facility, young crested black macaques have been reported to accept and protect chickens that wandered in.

To human eyes, these actions appear as if they are "cherishing small pets."

However, interactions that occur in captivity cannot be evaluated in the same way as wild behavior.

In zoos, there may be limited same-species individuals available for contact. Boredom, loneliness, and lack of stimulation may increase interest in other species.

On the other hand, if apes and monkeys continue to adjust their strength, hold, pet, and protect small and weak animals, it suggests that motivations related to caregiving and social contact may be at work.

Even if it is not the same as human pet-keeping, there may be similar psychological tendencies in the preliminary stages. This possibility is precisely what the current study has raised.

6. Capuchin Monkeys Raising a Baby of a Different Species

Among the many reports, the case in Brazil involving tufted capuchins and marmosets particularly stands out as something that could be called "adoption."

In 2004, a small baby clinging to an adult female was found in a wild capuchin monkey troop. However, the baby was not a capuchin but a much smaller marmoset.

Researchers named this individual Fortunata.

When the first caregiving female became pregnant, another female took over the caregiving role. Fortunata moved with the troop, foraged for food, and responded to alarm calls. Young capuchins were said to adjust their play strength to avoid injuring the smaller individual.

Fortunata looked, moved, and ate differently from the troop members. Yet, she was not excluded and was treated as a member of society for an extended period.

This case suggests that rather than ignoring differences in appearance and species, monkeys may have recognized these differences and adjusted their behavior accordingly.

The fact that they changed their play style to match the other's physique is particularly important. It means they did not just mechanically repeat their caregiving behavior but adjusted their approach according to the other's vulnerability.

Whether this can be called "empathy" requires discussion, but at the very least, it indicates flexible social judgment was at play.

7. The Fine Line Between Caregiving and Kidnapping

Not all actions of carrying a baby of another species result in adoption.

In Saudi Arabia, there is a reported case where a male baboon was seen carrying away a puppy and holding it for an extended period.

Even more shocking is the behavior of white-faced capuchins observed on Jicarón Island in Panama.

Researchers had originally set up numerous automatic cameras on the island to observe capuchins using stones as tools. However, in 2022, footage captured a young male capuchin walking with a baby howler monkey on its back.

The individual who first started this behavior was named "Joker."

Upon reviewing the footage, it was found that not only Joker but other young males had started the same behavior. Over approximately 15 months, five capuchins were said to have carried at least 11 howler monkey babies.

The problem was that the capuchins did not adequately care for the babies.

They did not actively attack or eat them. However, because young males who could not nurse separated the babies from their mothers, several individuals likely died from malnutrition.

It is unknown why this behavior spread.

The initial individual may have had a misdirected caregiving motivation. Alternatively, capturing and carrying might have been an exciting game. It is also suggested that living on a predator-scarce island provided leisure, allowing new behaviors to spread among young males like a "trend."

There is an important distinction between adoption and kidnapping.

The appearance of holding and carrying does not alone determine affection. It is necessary to see if they understand the other's needs, provide food, ensure safety, and allow freedom to escape.

"Appearing to hold gently" and "a beneficial relationship for the other" are not the same.

"Inspiration" and "Intervention Needed" Intersect on Social Media

Cross-species interactions tend to garner strong reactions on social media.

Videos of monkeys grooming dogs or holding babies of other species often receive comments like "Animals have affection similar to humans" or "They can become family despite being different species."

On the other hand, a Reddit post about capuchins taking howler monkey babies elicited divided reactions.

Prominent were feelings of sadness and anger over the high likelihood of babies dying after being separated from their mothers. Some suggested that researchers should intervene if it affects endangered populations.

Conversely, questions were raised about whether it is right for humans to intervene in the culture of wild animals. Even if it seems cruel from a human ethical perspective, stopping newly emerged behaviors in nature could be another form of ecological intervention.

There were also speculations like "Boredom led to the behavior, and it spread through imitation," "It's similar to human children carrying dolls," and "Young males might be practicing caregiving."

Additionally, there was a sarcastic remark suggesting that if you want to understand why humans participate in inexplicable trends, you should look at monkeys.

However, comments on social media are not scientific conclusions. They are interpretations by users who have seen limited footage and reports, and some include unfounded assertions.

Nevertheless, these reactions show how much humans project their emotions and ethics when observing animal behavior.

"Cute Videos" Aid Research but Also Cause Misunderstandings

In this study, not only papers and expert surveys but also online photos and videos were used as materials.

Unusual inter-species interactions often end before researchers can arrive on the scene. Records posted on social media and video-sharing sites can sometimes lead to the discovery of new behaviors.

However, there are significant issues with animal footage on the internet.

There is the possibility that the footage is intentionally staged, that the photographer brought the animals together, or that the context before and after is cut out. From the footage alone, it is unclear whether the environment allowed the other party to escape, how long the contact lasted, or what happened after filming.

The research team excluded footage where species and behavior could not be clearly identified, those heavily edited, those suspected of being staged, and those lacking contextual explanation.

Even so, what gains attention online tends to be scenes perceived as "human-like," "cute," or "cruel and shocking," rather than everyday interactions.

Therefore, the number 427 cannot be read as "monkeys frequently keep other animals as pets."

The collected records do not accurately measure how common inter-species interactions are but broadly organize what kinds of behaviors can occur for the first time.

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