07/25/2012 05:48 pm ET Updated Sep 24, 2012

Are Humans The Only Animal That Keep Pets?

Wild animals don't keep pets.

Oh, I can already hear the howls of objections. What about Koko's Kitten, you ask, referring to the well-known case of the American Sign Language-trained gorilla who fell in love with a kitty cat? What about Owen, the 600 pound baby hippo who became fast friends with Mzee, a 160 year old giant tortoise in a Kenyan game preserve? How about Tarra, the Asian elephant, at the Elephant Sanctuary in the hills of Tennessee, whose BFF for eight years was a dog named Bella?

You are right. There are scads of examples of long-term attachments between animals of different species. The problem is that all these cases have occurred among captive or semi-captive animals in zoos, wildlife parks, or research labs. True, there is the occasional article in primatology journals which describe instances in which wild chimpanzees "played" with small animals like hyraxes. But in each case, the relationship soon went south when the chimps killed their new pals and proceeded to toss their corpses around like rag dolls.

In his book Stumbling On Happiness, Harvard's Dan Gilbert claims that every psychologist who puts pen to paper takes a vow to someday write a sentence that begins, "The human being is the only animal that...." I was so convinced that pet-keeping did not occur in other species that I took up Gilbert's challenge by confidently writing in my book on human-animal relationships, Some We Love, Some We Hate, Some We Eat: Why It's So Hard To Think Straight About Animals, "The human being is the only animal that keeps members of other species for extended periods of time purely for enjoyment."

The Exception That Proves the Rule?

But then, just days after I sent the last copy-editing changes to my publisher, I received an e-mail from my friend James Serpell, director of the University of Pennsylvania's Center for the Interaction of Animals and Society. James, who knows that I view pet keeping as a uniquely human phenomenon, cryptically wrote, "Hal, I came across this and thought you'd be interested." Attached was an article from the American Journal of Primatology.

Arggggh. It was bad news for my only-humans-keep-pets theory. I could almost hear James chuckling. The article described a group of a dozen or so bearded capuchin monkeys who were caring for a baby marmoset, another species of monkey. One of the authors of the article was Dorothy Fragaszy, a University of Georgia primatologist who was studying the capuchins at a private nature preserve in Brazil. The article also included a series of photographs by Jeanne Shirley, a California hospital epidemiologist and a serious amateur naturalist who regularly visits the tropics to photograph the wild things. Jeanne came across the group of capuchins by accident and was astounded to see them carrying around and even giving food to the little marmoset.

The researchers found that the capuchins treated the marmoset, who they named Fortunata, just like an infant capuchin. They regularly fed the baby monkey and talked to her in capuchinese. They cradled Fortunata, carried the monkey around and let her ride on their backs during the day. And when they played with their little friend, they carefully adjusted the force of their movements so they wouldn't injure the much smaller marmoset.

Most importantly, the friendship between Fortunata and the capuchins was not just a transient hook-up. The monkeys raised the marmoset from infancy to about the age she would have been an adult. One day, however, she suddenly disappeared, and Dorothy does not know if she left the capuchin group voluntarily or was killed by a predator.

Why I Am Sticking To My Guns...

So, has this case caused me to throw in the towel and abandon the theory that humans are the only species to keep pets? I have to admit that the capuchin-marmoset relationship has caused me moments of doubt. I am not, however, ready to give up the idea for a couple of reasons. First, while the capuchins were not confined, the situation was not completely natural as they were given food every day as part of a program designed to promote ecotourism at the research site. Second, it is unclear whether this is pet-keeping or adoption. In their article, the researchers called it adoption, but in an e-mail Dorothy agreed that there is an obvious parallel between the relationship that the capuchins had with Fortunata and that I have with my cat Tilly, who I also play with, feed, and converse with in baby-talk.

Finally, Fortunata may be the exception that proves the more general rule that non-human animals don't keep pets. Capuchins are among the smartest of monkeys and have been referred to as "the New World Chimpanzee." Like chimps, they live in complex societies, use tools, eat meat, and have large brains in comparison to their body size. But, if capuchins can manage to bring a stranger into their lives and keep it as a pet for well over a year, why don't chimps?

James Serpell has defined pets as animals we live with that have no obvious function. I like this definition because it excludes "mutualistic" relationships such as seen between some ant and aphid species. I suspect that human-style pet love requires a combination anthropomorphism and learned cultural values found in only one species -- ours. Anthropomorphism enables us to empathize with non-human creatures, and cultural values give us permission to fall in love with some types of animals. (The form and frequency of pet-keeping vary tremendously between cultures. Some societies are completely devoid of pets.)

Whatever the reason, I don't know of any other animal whose members commonly form deep emotional relationships with members of other species with no apparent payoff. If you do, let me know!

Hal Herzog is professor of psychology at Western Carolina University and author of Some We Love, Some We Hate, Some We Eat: Why It's So Hard To Think Straight About Animals.