How Loving Your Pet is Like a Virus – or a Song
And why humans may be the only animals who keep pets.
By Hal Herzog
In a previous post, I argued that of all the animals, only humans bring members of other species into their lives purely for enjoyment. True, there is one documented instance in which a group of capuchin monkeys in a Brazilian jungle raised a baby marmoset. This case, however, is the rare exception that proves the general rule that in nature, non-human animals don’t live with pets.
Why don’t even intelligent apes such as chimps, bonobos and gorillas sometimes take on a pet? After all, they frequently run across baby animals they could fall in love with. And, as shown by the capuchins in Brazil, at least some non-human primates have the ability to feed and care for members of other species.
Further, inter-species odd-couples are common in captivity. Witness the gorilla Koko and her well-known pet kitten. Back in the 1970s, an ethologist named Bill Mason conducted an experiment in which he paired rhesus monkeys and adult dogs. 
It only took a couple of hours for the monkeys and dogs to become fast friends. And when offered a choice several months later between playing with another monkey or their dog, the monkeys overwhelmingly opted to hang out with their canine pals. These relationships lasted more than three years!
Does pet-keeping require culture?
My tentative explanation of why wild animals don’t keep pets around is that pet-keeping requires a mix of genes and culture that is only found in Homo sapiens. The genetic part is the instinctive attachment system which predisposes grownups to fall in love with creatures with cute features and big eyes. The cultural part is living in a society which approves of bringing a member of an alien species into your life and treating it like your best friend.
All mammals (well, all female mammals, as well as the males in some species) are born with an innate attachment system designed by evolution to facilitate parental care. What non-human animals don’t possess is the cultural part of the equation. True, lots of species possess the rudiments of culture. For example, chimp societies differ in the types of tools they make and in the brutality of their carnivory. (The chimps of the Cote d’Ivoire disembowel their prey, while Gombe chimpanzees prefer to rip the limbs off their victims.) But as Michael Tomasello of the Max Planck Institute for Evolutionary Anthropology has shown, young humans far outstrip even the smartest chimp when it comes to soaking up sophisticated cultural knowledge like how to read, which sneakers are cool, or if a dog should eat with the family at the dinner table or be eaten by the family at the dinner table.
Pets as Brain Worms
I am arguing that it is no accident that the only animal on earth that keeps pets is also the most facile copy cat. I am, of course, tossing around the idea that pet-keeping spreads across human cultures via the hypothetical entities that the evolutionary biologist Richard Dawkins calls “memes.” Memes are mental analogs of genes – units of culture transmitted from human mind to human mind via imitation. Religions spread through memes. So do snatches of songs you can’t get out of your head. A meme is like a virus that commandeers your computer and uses it as a vehicle to infect other hard drives via the Internet. Dawkins calls memes “viruses of the mind” and the philosopher Daniel Dennett calls memes “brain worms.”
But, you ask, what evidence do I have to support the completely non-intuitive idea that our love of pets is the result of thought contagion? First, human cultures differ widely in the presence, frequency and form of pet-keeping. My Kenyan anthropologist friend Nyaga Mwaniki tells me that his native language does not even have a word for pet, and while people in his village keep mongrels around as guard dogs, they never let the dogs come inside their homes or play with them. Indeed, in a soon to be published study in the journal Anthrozoös, Peter Gray and Sharon Young surveyed pet-keeping practices in 60 cultures. The societies differed dramatically in the frequency and form of pet keeping. Dogs, for example, were clearly considered pets on only 22 of them. In some of cultures, pet animals are routinely killed, while in others, lactating women breast feed them. (Breastfed pets included piglets, puppies, monkeys and bear cubs.)
And, as revealed by the demographics of pet ownership in Sri Lanka, some memes can facilitate or inhibit infection by pet memes. In Colombo, the capital city, your chances of being infected may depend on your religion: 90 percent of Buddhists homes include a pet, but only 5 percent of Muslim households do.
Unlike genes, which take a generation to replicate themselves, memes can spread through a culture at warp speed. When I was kid, there was a short-lived craze for pet baby turtles. I had one and so did all my friends. Who knew that 85 percent of them carried salmonella? In 19th century Japan, ornamental mice were the hot pets; today the pets of choice for Japanese children include giant stag beetles.
My wife, Mary Jean, is not keen on the idea that our love of pets is governed by the same facile consumer psychology that determines whether nose rings are “in” or “out” and which shoes are cool. Neither does out cat, Tilly. That’s OK, Sweetie. I love you even though you are a mental virus, like a song that I can’t get out of my head; like Patti Paige crooning “How much is that kitty in the window….”
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Hal Herzog is a psychologist at Western Carolina University whose research focuses on our attitudes towards and interactions with other species. His book Some We Love, Some We Hate, Some We Eat: Why It’s So Hard to Think Straight About Animals is published by HarperCollins.