To Bridgett von Holdt's 11-month-old English sheepdog Marla, the entire world is a friend she has yet to meet.

“She’s hypersocial. I even had her genotyped,” von Holdt admits, somewhat sheepishly (sorry).

Von Holdt’s interest is no casual curiosity. The Princeton evolutionary biologist and colleagues have spent the last three years studying the underlying genetic basis for social behavior in dogs and wolves. (Read why dogs are even more like us than we thought.)

Studies have shown that dogs are more sociable than wolves raised in similar circumstances, generally paying more attention to humans and following our directions and commands more effectively. (See "Can Dogs Feel Our Emotions? Yawn Study Suggests Yes.")

Von Holdt’s background in evolutionary genetics made her wonder about the potential genetic basis for these differences.

Their July 19 study in Science Advances provides an intriguing clue: Hypersocial dogs like Marla carry variants of two genes called GTF2I and GTF2IRD1. Deletion of those genes in people causes Williams syndrome, which is characterized by elfin facial features, cognitive difficulties, and a tendency to love everyone.

Von Holdt suspects that the gene variants in dogs inhibit their normal function, leading to the same issues seen in humans with Williams syndrome.

“We may have bred a behavioral syndrome into a companion animal,” she says.

'Sexy' Question

View Images A border collie poses for the camera. Domestic dogs are much more likely to respond to humans than wolves raised in similar circumstances. Photograph by Vincent J. Musi, Nat Geo Image Collection

Since evolving from a shared ancestor with wolves at least ten thousand years ago, domestic dogs have helped us find food and protected us from becoming dinner ourselves, all while providing a friendly face and wagging tail. (Read more about how dogs evolved in National Geographic magazine.)

Understanding how our best friends, from Chihuahua to mastiff, became what they are today is a "sexy question,” according to Karen Overall, a canine behavior expert at the University of Pennsylvania who wasn't involved in the new study.

In 2010, in collaboration with Monique Udell, an animal behaviorist at Oregon State University, von Holdt searched the dog and wolf genomes and identified alterations in the WBSCR17 gene that occurred during dog domestication, results they published in Nature. (See "Dog and Human Genomes Evolved Together.")

Their project lay dormant until 2014, when von Holdt and Udell secured funding to set up a new set of experiments with 18 dogs of various breeds—including dachshunds, Jack Russell terriers, and Bernese mountain dogs—and 10 wolves habituated to humans.

The scientists trained all of the animals to open a box that contained a piece of sausage. Then they asked the canines to open the box while in three separate situations: with a familiar human present; with an unfamiliar human; and alone, without a person at all.

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In all three scenarios, the wolves outperformed the dogs by a large margin. That margin got even larger when the dogs had to open the box in the presence of people.

“It’s not that they couldn’t solve the puzzle, they were just too busy looking at the human to do it,” von Holdt says.(See "Opinion: We Didn’t Domesticate Dogs. They Domesticated Us.")

Dogs Still Evolving

For the new study, Von Holdt conducted additional genetic analysis of the part of the genome surrounding the altered WBSCR17 gene in a larger sample of dogs and wolves.

Besides confirming her initial findings that WBSCR17 varied in dogs and wolves, she found two nearby genes, GTF2I and GTF2IRD1, were also different.

The combination of the genetic and behavioral data told von Holdt that changes to this region of the genome helped turn wolves into human-loving dogs. (See "Ancient Dog Skull Shows Early Pet Domestication.")

The University of Pennsylvania's Overall cautions that the study size was small, which limits the strength of the findings. But she praised the strength of the genetic analysis.

We’re now selecting for dogs that are easy keepers, that can spend long periods of time in small apartments," Overall notes. (See dog-evolution pictures.)