Beyond Humans: Evidence Suggests Primates Also Keep Pets

Two young eastern chimpanzees, Ishe and Dembe, were swinging through the canopy of a Ugandan forest reserve when researchers observed them approach a red‑tailed monkey. Instead of the typical hostile chimpanzee response, they gently stroked the monkey and began grooming its tail.

Such an encounter may hint at the nascent stages of pet‑keeping among our primate relatives, according to a recent study in the journal *Primates*. The findings indicate that Homo sapiens are not unique in caring for other species, suggesting that the impulse to keep animal companions could be rooted deep in our evolutionary history—much like the famous case of Koko the gorilla nurturing her kitten, All Ball.

Keeping pets presents an evolutionary puzzle for scientists. Providing food, shelter, grooming, dental care, and affection demands considerable resources, which at first glance seems to offer little direct benefit.

“Investing resources in species with which we share no evolutionary stake appears to defy evolutionary logic,” noted Hal Herzog, a psychologist and animal expert at Western Carolina University who was not part of the study. Nevertheless, pet ownership is widespread and deeply valued across cultures worldwide.

Earlier work has shown that motivations for pet ownership are as diverse as the animals themselves—spanning companionship, social bonding, novelty seeking, and status. Illustrative examples include Nicolas Cage’s two‑headed snake and Pablo Escobar’s private zoo, which housed hippos, elephants, and giraffes. Twin studies point to a genetic component in the desire for animal companions, yet the precise origins of this propensity remain unclear.

“Some scholars argue that pet‑keeping originated from the practical benefits animals provided in hunting or defense, while others contend it stems from an innate human drive to nurture and bond with other living creatures,” explained Zoë Goldsborough, a behavioral ecologist at the Max Planck Institute of Animal Behavior, who was not involved in the study.

The oldest archaeological indication of pet keeping dates to a 14,000‑year‑old burial in Germany where a man and woman were interred with their cherished dog. However, the practice probably predates this find. As humans domesticated animals, our bond with them shifted from chiefly utilitarian functions to the more affectionate relationships we recognize today.

“Regrettably, we cannot determine how long humans have kept animals as pets,” Herzog remarked. “Without a time machine, this question may remain unanswered.”

The impetus to examine pet‑like behavior in other primates arose when Cyril Grueter, a biological anthropologist and primatologist at the University of Oxford and lead author of the new study, was a master’s student visiting the Shanghai Zoo. There he observed a female gibbon capture a small mouse, cradle it gently, and stroke it. “She continued to care for that little mouse meticulously,” Grueter recalled, “and the mouse appeared quite at ease in her hand.”

Grueter suggested that such cross‑species interactions could illuminate why humans keep furry companions, yet these observations rarely appear in peer‑reviewed literature. “If you witness a gorilla hugging a bush baby,” he noted, “someone might snap a photo for social media, but scientists seldom formalize it into a research paper.”

To transcend anecdotal reports, Grueter and his team combed through media coverage, scientific publications, and surveyed primatologists for comparable instances of primates interacting with other species. Their search yielded 427 records spanning 88 primate species, revealing that every primate taxon exhibited at least one behavior suggestive of pet‑keeping.

Grueter cited one example in which a macaque forged a strong social bond with a chicken that had wandered into its enclosure, and another where a troop of capuchin monkeys adopted a baby marmoset, nursing and caring for it for several months. Such patterns appear to be widespread among both wild and captive primates.

Upon closer video analysis, the researchers documented monkeys and apes engaging with a variety of unrelated species—lizards, birds, deer, dogs, squirrels, and even other primates. The predominant behaviors were play and grooming, mirroring the primates’ own intra‑species interactions: juveniles tended to play, adult females often nursed adoptees, while adult males showed indifference or occasional aggression. Most encounters were brief, though a minority resembled sustained adoption.

What could primates gain from these transient encounters with unrelated animals? Grueter posits that loneliness may play a role. Just as humans seek comfort when lonely, primates might satisfy a craving for closeness by grooming or briefly huddling with another warm body. Thus, whereas a person might adopt a puppy, a howler monkey could capture and tenderly hold a nearby mouse.

Grueter hopes the findings will assist zoos and sanctuaries managing mixed‑species exhibits in selecting combinations of animals that can coexist peacefully.

Nevertheless, Grueter emphasizes that the observed behaviors fall far short of the pet‑keeping practices seen with domestic dogs or tigers in human households. “There is no true analogue of pet‑keeping among nonhuman primates,” he stated, “but we can investigate the underlying ‘ingredients’ that may have paved the way for its evolution.”

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