Why do humans enjoy keeping pets?
A new international study suggests that the answer may reach much further back in evolution than scientists once believed.
Researchers have found that many monkeys and apes naturally form friendly relationships with animals from completely different species, hinting that the roots of human-animal companionship may have existed long before people began keeping pets.
The findings were published in the journal Primates.
The research was led by scientists from the University of Oxford, who collected the largest record so far of positive interactions between primates and other animals.
The team documented 427 examples involving 88 species of primates and 127 different partner species, including 55 species that were not primates.
Their information came from scientific studies, reports in the media and surveys completed by primate experts around the world.
The examples were surprisingly varied. In Thailand, stump-tailed macaques were seen grooming domestic dogs. In Sri Lanka, tufted gray langurs gently stroked giant squirrels.
In China, young black-and-white snub-nosed monkeys played with domestic pigs, while in Madagascar, ring-tailed lemurs groomed bamboo lemurs.
One of the most remarkable cases came from Brazil, where wild capuchin monkeys adopted a baby marmoset and cared for it over several months.
The researchers found that play and grooming were the most common types of interaction. Young primates were especially likely to play with animals from other species, showing curiosity and a willingness to explore new social relationships.
Adult females were more likely to groom other animals, while adult males were much less likely to take part in these friendly behaviors.
Most of the time, primates formed relationships with species that were closely related to themselves. However, many also interacted with much more distant animals, including dogs, birds, squirrels and even reptiles. In most cases, it was the primates that started the interaction rather than the other animals.
Unlike many mixed-species groups in nature, these friendships did not appear to provide obvious survival benefits such as finding food or avoiding predators.
Instead, the animals often played together, groomed one another, carried each other or simply rested side by side. These behaviors suggest that primates possess a natural curiosity and social flexibility that allows them to form bonds beyond their own species.
The researchers believe these behaviors may represent some of the evolutionary foundations that eventually allowed humans to develop close relationships with animals. While monkeys and apes are not keeping pets in the same way people do today, their actions may reflect early forms of caregiving and cross-species companionship.
Not every interaction had a happy ending, however. Curiosity sometimes became rough handling, and the study recorded 13 cases in which the partner animal died.
In one example, a male baboon in Saudi Arabia repeatedly carried around a domestic puppy in what researchers described as an apparent kidnapping. These cases show that cross-species relationships can sometimes be stressful or harmful for the animals involved.
Beyond helping explain the origins of pet-keeping, the findings may also benefit conservation and animal care. Understanding why some species naturally form positive relationships with others could help improve the management of mixed-species groups in zoos and wildlife sanctuaries.
The researchers hope future field studies will pay closer attention to these unusual friendships. By recording them more systematically, scientists may gain an even better understanding of how empathy, curiosity and social behavior evolved—not only in humans, but across the primate family.
