Brains, Brawn, or Both: What drove the creation of modern dog breeds?

In creating modern dog breeds, humans sculpted canines into physical specimens perfectly suited for a wide variety of tasks. Bernese mountain dogs have solid, muscular bodies capable of pulling heavy loads, while greyhounds have lean, aerodynamic frames, ideal for chasing down deer. The compact Jack Russell terrier can easily shimmy into fox or badger dens. Now, a large study, published in Cell on Thursday, suggests that behavior, not just appearance, has helped qualify these dogs for their jobs. Breeds that were created for similar roles — whether rounding up sheep or flushing birds into the air — tend to cluster into distinct genetic lineages, which can be characterized by different combinations of behavioral tendencies, the researchers found.

“Much of modern breeding has been focused predominantly on what dogs look like,” Evan MacLean, an expert on canine cognition at the University of Arizona who was not involved in the study, said in an email. But, he emphasized, “Long before we were breeding dogs for their appearances, we were breeding them for behavioral traits.”

The study also found that many of the genetic variants that set these lineages apart from each other appear to regulate brain development, and many seem to predate modern breeds. Together, the results suggest that people may have created today’s stunning assortment of breeds, in part, by harnessing and preserving desirable behavioral traits that already existed in ancient dogs, the researchers said.

“Dogs have fundamentally the same blueprint, but now you’ve got to emphasize certain things to get particular tasks done,” said Elaine Ostrander, a dog genomics expert at the National Human Genome Research Institute and the senior author of the study. “You’re going to tweak a gene up, you’re going to tweak it down.”

Bridgett vonHoldt, an evolutionary biologist at Princeton University who was not involved in the research, called the new paper “a major landmark in the field of dog genomics and behavior. We know it is complicated. This study not only gives us hope, it will be viewed as an inspiration for all in the field.”

Still, major questions remained, some scientists said, including whether humans deliberately set out to create breeds with specific behavioral tendencies. “We don’t have a ton of evidence for intentional selection,” said Elinor Karlsson, an expert in dog genomics at the University of Massachusetts Chan Medical School who was not involved in the research.

But she praised the study, noting that the findings were consistent with her own research, which also concluded that many of the genetic factors that shape the behaviors of modern dogs originated deep in canine history. “They’re really taking advantage of this really complex history of the dog breeds, and these relatively subtle but real differences in behavior, to explore how genetics and genetic variation can actually shape these behavioral traits,” she said.

Although there was plenty of overlap — no single breed has a monopoly on trainability — in general, breeds created for similar jobs tended to have similar behavioral traits. And each lineage was characterized by its own pattern of behavioral tendencies.