The ancestral wolves that evolved into domestic dogs may have carried genetic mutations that made them socialize more readily with people. What’s more, the same genes cause excessive sociability in humans.
It was already known that even if wolves have been raised with humans from birth, they never become as close to people or look at them as often as dogs tend to.
Several years ago, Bridgett vonHoldt at Princeton University in New Jersey and her colleagues linked this “hypersociability” to a 28-gene stretch of the dog genome that includes canine versions of the genes responsible for Williams syndrome – a human disorder characterized by extreme sociability. However, they had no direct proof that these genes caused it.
To find out whether they do, vonHoldt and her team tested the behavior of 18 domestic dogs and 10 wolves, all of which had been raised identically with constant human contact. Each animal was scored for its hypersociability towards humans. As expected, the dogs scored higher than the wolves.
The researchers then sequenced the key region of each animal’s genome in fine detail and searched for structural variations – deletions or insertions of genetic material – that seemed to match well with their social behaviour. They found four, including two in genes called GTF2I and GTF2IRD1. These genes are known to cause the hypersociability involved in Williams syndrome in humans, and GTF2I has also been shown to cause hypersociability in mice.
VonHoldt’s team also looked for the same hypersociability gene variants in a larger sample of dogs, non-domesticated wolves and coyotes. They found them in both dogs and wolves, but not in the more distantly related coyotes. This suggests that they were already present in wolves, and then were preferentially selected in the evolution of dogs.
“It seems to make sense that this could be the foundation of the interaction between humans and wolves,” says vonHoldt. If so, gene variants for hypersociability should have become more common during the course of dog evolution, at least until people began developing modern breeds a few hundred years ago, when their selective breeding may have focused on traits other than sociability.
The team’s results may help resolve a long-standing question about the domestication of dogs: did domestication start because people began to use wolves for guarding or hunting, with sociability arising later, or did wolves become friendlier first? Given that some wolves carry the “friendly” mutations, VonHoldt’s study supports the latter explanation.