There are hundreds of dog breeds around the world, from the teensy chihuahua to the massive Saint Bernard—all thanks to centuries of selective breeding by humans. With such a wide range of canine sizes and temperaments, it’s no surprise that, in the process, we have reshaped their brains as well as their bodies.

A new study performed MRI scans on 33 breeds and discovered how a dog was bred is reflected in their brain structure. (Read “How to build a dog” in National Geographic magazine.)

For instance, dogs bred to be small—say the lhasa apso—have round heads with similarly round brains that take up most of their skull. A larger breed like a golden retriever has a long, narrow head, and thus a more elongated brain that doesn't fill all of the skull space.

“The biggest wow moment for me was just looking at the scans,” says study leader Erin E. Hecht, an evolutionary neuroscientist at Harvard University. “It’s really cool in science where you have a result where you don’t have to do any fancy statistics to be able to tell there’s something going on.” (Read more how humans have reordered dog brains.)

This fresh look inside the mind of dogs offers a better understanding of how breeds are hardwired, which in turn helps potential dog owners choose the right breed for their home, adds Hecht, whose study was published today in the journal Neurosci. (See our fun photo gallery of pet dogs.)

Dogs on the brain

For the study, Hecht and colleagues recruited 62 individual pet dogs in American homes, including breeds such as beagles, Yorkshire terriers, Doberman pinschers, boxers, and more.

After noting the differences in brain size and shape, the team then further analyzed differences within the brain, observing how how certain regions varied across breeds with certain behavioral traits.

A History of Dogs 101

Bulldogs, for example, were originally bred to fight captive bulls but later were bred to be loving family pets, putting them both in the “sport fighting” and “explicit companionship” groups. The study team used the American Kennel Club website for data on the breeds’ original roles.

The scientists then mapped out six brain networks that could be discerned by a dog's behavior, like scent hunting or companionship.

For example, in the part of the brain called the prefrontal cortex, one area associated with group size and social interaction had the same variation among dogs bred for herding; police, military, and war work; vermin control; bird flushing and retrieving; and sport fighting.

That makes sense, since these breeds serve roles that are “cognitively complex and demanding, so they might require greater support from the prefrontal cortex,” Hecht says.

Daniel Horschler, a Ph.D. student in at the University of Arizona who studies brain anatomy in dogs, praised the study’s approach. (See dog-evolution pictures.)

“They didn’t try to divide the brain into regions themselves, which is I think a really good approach because we don't yet know a lot about how dogs’ brains are organized,” says Horschler, who wasn’t involved in the research.

It was smart, he said, that the team noted areas in which dogs’ brains tended to change in the same ways, then related those changes to breed-specific traits.

“It’s really exciting,” he adds. “Dogs are such a great model for this sort of thing and no one has really explored this before.”

Science’s best friend?

Though domestic dogs were once dissed by scientists as “a fake animal,” Horschler says, not worthy of scientific inquiry, they’ve become a more common study subject—in particular as part of the study of emotion and cognition. For instance, 20,000 years of cohabitation has made our pets finely tuned interpreters of human emotion—possibly more so than any other species. (See 19 ways dogs tell us what they want.)

Study author Hecht and colleagues also performed a statistic analysis that shows the brain variations occurred more recently in the dog family tree, rather than deep in the past—suggesting that “dog brain evolution has happened quickly,” Hecht says.