Dogs are a weird bunch. They range in size from bearish behemoths like Newfoundlands to pipsqueaks like Milly, a Chihuahua who measures less than four inches tall and holds the Guinness record for World's Shortest Dog.

Some breeds are fast runners. Others are jumpers, swimmers or diggers. Bloodhounds specialize in sniffing, while greyhounds hunt primarily by sight. Border collies excel at herding, Jack Russell terriers at flushing foxes from dens.

Over at least 15,000 years, and especially since a Victorian-era dog-creation craze, selective breeding by humans has resulted in a single species with more physical variation than almost any other in the animal kingdom.

And now, scientists have provided the first evidence that all of this selective tweaking hasn't just changed dogs' sizes, shapes, colors and behaviors - it's also altered the way their brains are built.

Their research, published Monday in The Journal of Neuroscience, began with MRI scans from 62 dogs that had visited the University of Georgia Veterinary Teaching Hospital for neurological evaluations. All the dogs, representing 33 breeds, were discharged with clean bills of brain health. But their scans provided the scientists with a treasure trove of data.

"The first question we wanted to ask was, are the brains of different breeds of dogs different?" said Erin Hecht, a neuroscientist studying dog cognition at Harvard University and lead author of the study.

Indeed, from Dachshunds to Dobermans, the scientists found well-defined differences between dog brains, even after accounting for things such as the pooches' overall size and shape.

By looking at the areas of the dogs' brains that varied most across the breeds, the scientists were able to create maps of six neural networks linked to certain functions, such as the sense of smell or movement. The team found the shape of these networks "correlated significantly" with common traits associated with each breed, as described by the American Kennel Club.

"Brain anatomy varies across dog breeds," Hecht said, "and it appears that at least some of this variation is due to selective breeding for particular behaviors like hunting, herding and guarding."

In other words, not only do the shapes and sizes of canine brains vary by breed, the structures within those brains also are different. This discovery helps explain what makes a Maltese act like a Maltese, or a boxer like a boxer.

Jeffrey Stevens, director of the Canine Cognition and Human Interaction Lab at the University of Nebraska at Lincoln, called the study's use of existing MRI data "clever" and its premise "exciting." However, he offered some words of caution.

"The one thing that I think there's a bit of disagreement on in the literature and in people's views is how useful it is to map behaviors to breeds," Stevens said. "There's often a lot of variation within a breed, across individuals."

Stevens also noted the MRI scans weren't performed as the dogs were performing breed-specific tasks, making it difficult to draw big conclusions linking breed to behavior.

"This is very well known in the human neuroimaging field, where you want to be really careful drawing any inferences about cognitive processes that are based on brain activity that you're not directly testing," Stevens said.

But this raises another intriguing question. Most dogs today do not actively fill the roles for which their breed was created.

In fact, all 63 study dogs were house pets, not working dogs. So even though they may be the descendants of great herders or hunters, they probably don't perform those tasks in any serious capacity. That could make a big difference.

"It's not like [your brain] gets a new wrinkle every time you learn something," Hecht said. "But there have been lots of studies that show your brain changes as you learn a new language or as you learn a new motor skill."

So it's quite possible that a Labrador retriever that does the job its kind was bred to do - retrieving birds shot by hunters - might have a brain that looks different from a Lab that retrieves popcorn stuck between couch cushions.

Stevens said he viewed this as a hint that the researchers might be onto something. If they've managed to find such significant variation in pets, he said, imagine what might be discovered in the brains of working dogs.

"The correlations actually could be stronger if you used animals that were still bred for those purposes," Stevens said.

Daniel Horschler, a PhD student at the University of Arizona's Canine Cognition Center, said the variation found across dog breeds could prove to be an important model for understanding how brains work in general.

Previous studies have investigated neurological differences between species, said Horschler, but such animals "obviously have a lot more differences in terms of their ecology and their behavior and their environment, so being able to do this in a single species is really interesting."

Hecht said her team is trying to better understand why and to what extent variations within a breed occur - or, as she put it, the differences between "high-skill performers and low-skill performers."

"For example, border collies who are winning herding competitions out in the real world and siblings of those dogs who, for whatever reason, would rather just sit on the couch," she said.

Hecht said she also hopes to be able to build on the study's findings by next investigating other facets of a dog's brain. For instance, skull shape and size may also have an impact on the anatomy of ventricles, the spaces that hold cerebrospinal fluid, which is critical for brain health. One day, this line of research could even help tailor veterinary treatment for different breeds.

One study has already found border collies and Siberian huskies respond differently when given the hormone oxytocin, which means there's reason to believe dog breeds could also have varied responses to medicines, anesthesia or other interventions.

Hecht isn't just in it for the science. Her fascination with dogs started at home, she said, with her own miniature Australian shepherds, Lefty and Izzy.

"We're just individual organisms at this one point and time, but we're also a result of all of our ancestors and the selection pressures that they underwent," she said.

"So my dogs are here with me, and they are the way they are because of what my ancestors did to their ancestors, which I think is sort of profound and weird and kind of cool."

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