In creating modern dog breeds, humans sculpted canines into physical specimens that are perfect for a variety of tasks. Bernese Mountain Dogs have solid, muscular bodies that can pull heavy loads, while Greyhounds have lean, aerodynamic bodies that are ideal for hunting 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, helped qualify these dogs for their jobs. Breeds created for similar roles — whether it be rounding up sheep or blowing up birds — tend to group themselves into distinct genetic lineages that may be characterized by different combinations of behavioral tendencies, the researchers found.
“A lot of modern breeding is largely focused on what dogs look like,” Evan MacLean, a canine cognition expert at the University of Arizona who wasn’t involved with the study, said in an email. But he emphasized, “Long before we bred dogs for their looks, we bred them for behavioral traits.”
The study also found that many of the genetic variants that set these lineages apart appear to regulate brain development, and many appear to predate modern races. Taken together, the results suggest that humans may have created today’s amazing breed selection in part by utilizing and preserving desirable behavioral traits that already existed in ancient dogs, the researchers said.
“Dogs basically have the same blueprint, but now you have to emphasize certain things to do certain tasks,” said Elaine Ostrander, a canine genomics expert at the National Human Genome Research Institute and the study’s senior author. “You’re going to optimize a gene up, you’re going to optimize it down.”
In an email, Bridgett von Holdt, an evolutionary biologist at Princeton University who was not involved with the research, called the new paper “an important milestone in the field of canine genomics and behavior. We know it’s complicated. Not only does this study give us hope, it will be seen as an inspiration to all in this field.”
Still, important questions remained, some scientists said, including whether humans intentionally create races with specific behavioral tendencies. “We don’t have much evidence of intentional selection,” said Elinor Karlsson, a canine genomics expert at the University of Massachusetts Chan Medical School who was not involved with the research.
But she praised the study, noting that the results were consistent with her own research, which also concluded that many of the genetic factors that shape modern dog behavior have origins deep in canine history.
“They’re really exploiting this really complex history of 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.
The researchers examined the genomes of more than 4,000 canids, including samples from more than 200 different dog breeds, as well as mongrel dogs, semi-feral village dogs and wild canids such as wolves and coyotes.
The scientists used computer tools to map the genetic pathways that went from ancient dogs to, for example, generic herding dogs and then to distinct breeds like border collies.
They found that domestic dogs can be classified into 10 different lineages, which generally included breeds developed for similar tasks. The terrier line included breeds developed to hunt vermin, for example, while the scent hound line included breeds that track game with their sense of smell rather than eagle eyes or speed.
Although some of the lineages have defining physical traits, those traits alone cannot fully explain this sorting, the researchers noted. “If you look at scent hound ancestry, there are breeds everywhere with short legs or long legs or different tail shapes or different coat colors,” said Emily Dutrow, a postdoctoral researcher at the National Human Genome Research Institute and the study’s first author. (The research team also included James Serpell, a professor emeritus at the University of Pennsylvania School of Veterinary Medicine.)
To identify the behavioral traits that best define each lineage, the researchers analyzed behavioral surveys conducted by the owners of more than 46,000 purebred dogs.
Although there was much overlap—no single race has a monopoly on trainability—breeds created for similar jobs generally shared similar behavioral traits. And each lineage was characterized by its own pattern of behavioral tendencies.
For example, herding dogs, terriers, and tracker dogs all showed relatively high levels of so-called “nonsocial anxiety,” such as fear of loud noises or strange objects. This predisposition could indicate an increased sensitivity to environmental stimuli, which could be useful in all three types of dog work, the researchers say.
Still, there were differences: terriers exhibited higher levels of predatory hunting than herding dogs, while herding dogs scored higher on measures of trainability, the researchers found.
“There is meaningful behavioral diversification in dogs,” said Dr. Dutrow.
(The ancestry of hounds unfortunately scored low on trainability. But this trait, the researchers diplomatically state, is actually “consistent with the selection of traits favorable to an independently driven work style that focuses on honing instincts.” follow instead of looking for human clues.”)
To identify the genetic basis of these lineage-defining traits, the researchers performed a genome-wide association study, looking for specific genetic variants that were unusually common in certain lineages.
The vast majority of these lineage-associated variants resided in stretches of DNA that do not code for proteins, but instead regulate the expression of protein-coding genes. Many appear to regulate genes involved in brain development.
“If we look at the genes involved in canine lineage differentiation, much of the action lies in genes associated with neurodevelopment, suggesting that selection for cognitive and behavioral traits was likely very important,” said Dr . MacLean.
For example, the sheepherding breeds were characterized by genetic variants associated with a neurodevelopmental process known as axon guidance, which helps neurons connect properly. Some of these variants have been linked specifically to genes that have been linked to anxiety and maternal behavior, including puppy retrieval in mice.
One hypothesis – yet unproven, the scientists note – is that a shepherd’s herd instinct is a product of the same fear-related neural pathway that motivates animal mothers to care for their young.
“When you watch these mice, these mothers gathering their young, it’s like watching a border collie herd sheep,” said Dr. Ostrander. “And so one could hypothesize that maybe this is ancestral behavior that was co-opted.”
(Dr. Ostrander, who used to have a border collie, has seen this drive instinct firsthand. “I used to be able to take mine to the lab and she could round people up for lab meetings,” she said.)
Nonetheless, many of the variants closely associated with certain lineages occurred at lower levels in other lineages or even in gray wolves, suggesting they predate the creation of modern breeds.
And just because there are overall differences between dog lines doesn’t mean the breed is a behavioral fate, noted Dr. Karlson.
“That doesn’t mean every single retriever is going to fetch a ball or that every single herder is going to be completely different than every single retriever,” she said. “Many dogs will not live up to our expectations because of their breed. And that’s totally fine, because that’s what makes them so much fun to have as pets.”