In creating modern dog breeds, humans have sculpted canines into physical specimens perfectly suited for a wide variety of tasks. Bernese Mountain Dogs have strong, muscular bodies capable of pulling heavy loads, while Greyhounds have lean, streamlined frames that are ideal for hunting deer. The compact Jack Russell terrier can easily squeeze into fox or badger dens.
Now a large study, published in Cell on Thursday, suggests behavior, not just appearance, helped qualify these dogs for the job. Breeds that were bred for similar roles – whether herding sheep or chasing birds through the air – tend to cluster into distinct genetic lines, which can be characterized by different combinations of behavioral tendencies. , the researchers found.
“Much of modern breeding has focused primarily on how dogs look,” said Evan MacLean, a canine cognition expert at the University of Arizona who was not involved in the study. , in an email. But, he pointed out, “long before we bred dogs for looks, we bred them for behavioral traits.”
The study also found that many of the genetic variants that distinguish these lineages from each other appear to regulate brain development, and many appear to predate modern races. Together, the findings suggest that people may have created today’s amazing assortment of breeds, in part by exploiting 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 accomplish particular tasks,” said Elaine Ostrander, canine genomics expert at the National Human Genome Research Institute and lead author. of the study. “You are going to modify a gene, you are going to modify it.”
In an email, 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 genomics and canine behavior. We know it’s complicated. This study not only gives us hope, it will be seen as a source of inspiration for all in the field.
Still, major questions remain, some scientists say, including whether humans deliberately decided to create breeds with specific behavioral tendencies. “We don’t have a ton of evidence for intentional selection,” said Elinor Karlsson, a canine genomics expert at the University of Massachusetts Chan Medical School, who was not involved in the research.
But she praised the study, noting that the results were consistent with her own research, which also concluded that many genetic factors that shape modern dog behaviors have their roots in canine history.
“They’re really taking advantage of this very complex history of dog breeds and these relatively subtle but real behavioral differences to explore how genetics and genetic variation can actually shape these behavioral traits,” she said.
Behavioral breeding
The researchers studied the genomes of more than 4,000 canids, including samples from more than 200 different dog breeds, as well as mixed-breed dogs, semi-wild village dogs and wild canids, such as wolves. and coyotes.
Scientists have used computer tools to map the genetic trajectories by which ancient dogs became, for example, generic sheepdogs and then distinct breeds, such as Border collies.
They discovered that domestic dogs could be divided into 10 distinct bloodlines, which generally included breeds developed to perform similar tasks. The terrier line included breeds designed to hunt vermin, for example, while the scent hound line included breeds that tracked game using their sense of smell, rather than eagle-eye vision or speed.
Although some of the lineages have defining physical characteristics, these characteristics alone cannot fully explain this sorting, the researchers noted. “If you look at the lineage of scent hounds, there are breeds that have short legs or long legs or different tail shapes or different coat colors,” said Emily Dutrow, postdoctoral fellow at the National Institute for Animal Research. human genome and the first author of the study. (The research team also included James Serpell, professor emeritus at the University of Pennsylvania School of Veterinary Medicine.)
To identify the behavioral traits that best define each line, researchers analyzed behavioral surveys conducted by owners of more than 46,000 purebred dogs.
Although there was a lot of overlap – no one breed has a monopoly on trainability – in general, breeds bred for similar jobs tended to have similar behavioral traits. And each lineage was characterized by its own pattern of behavioral tendencies.
For example, Sheepdogs, Terriers, and Scent Dogs all displayed relatively high levels of what is called “unsocial fear,” such as fear of loud noises or strange objects. This predisposition could indicate an increased sensitivity to environmental stimuli that could be useful in all three types of canine work, according to the researchers.
Still, there were differences: Terriers showed higher levels of predator hunting than sheepdogs, while sheepdogs scored higher on measures of trainability, the researchers found.
“There is significant behavioral diversification in dogs,” Dr. Dutrow said.
(The scent hound line, alas, scored low on trainability. But this characteristic, the researchers diplomatically noted, is actually “consistent with the selection of advantageous traits for a working style. independent focused on following instincts rather than looking for human clues.”
To identify the genetic underpinnings of these lineage-defining traits, the researchers conducted 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 were found in DNA segments that do not code for proteins but rather regulate the expression of protein-coding genes. Many appeared to regulate genes involved in brain development.
“When we look at the genes involved in differentiating canine lineages, much of the action is in genes related to neurodevelopment, suggesting that selection for cognitive and behavioral traits was likely very important,” said said Dr. MacLean.
For example, herding breeds of sheep were characterized by genetic variants associated with a neurodevelopmental process known as axon guidance, which helps ensure that neurons are properly wired together. Some of these variants were specifically associated with genes that have been linked to anxiety and maternal behaviors, including retrieving pups in mice.
One hypothesis — as yet unproven, the scientists note — is that a sheepdog’s drive to herd is the product of the same anxiety-related neural pathway that motivates animal mothers to care for their young.
“When you look at these mice, these mothers herding their young, it’s like looking at a flock of Border Collie sheep,” Dr Ostrander said. “And so you could run a hypothesis that maybe it was the ancestral behavior that was co-opted.”
(Dr. Ostrander, who once had a Border Collie, saw this herding firsthand. “I used to be able to bring mine to the lab and she could round people up for lab meetings,” said she said.)
Yet many variants closely associated with specific lineages have occurred, at lower levels, in other lineages or even in gray wolves, suggesting that they predate the creation of modern breeds.
And just because there are differences, overall, between canine lines doesn’t mean breed is a behavioral fate, Dr. Karlsson noted.
“That doesn’t mean that every retriever is going to retrieve a ball or that every shepherd is going to be completely different from every retriever,” she said. “Many dogs will not match our expectations based on their breed. And, you know, that’s great, because that’s why they’re such fun to have as pets.
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