Photograph by Justin Guariglia, National Geographic

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The brain region for smell has moved in Chihuahuas and other short-snouted dogs due to breeding, a new study says.

Photograph by Justin Guariglia, National Geographic

Dogs' Brains Reorganized by Breeding

For thousands of years humans have changed dogs' looks through selective breeding. Now it seems we've actually reordered many breeds' brains in the process.

For thousands of years humans have changed the sizes, shapes, colors, and coats of dogs through selective breeding. Now it seems we've actually reordered many breeds' brains in the process.

A new brain-imaging study examined 11 carcasses from 11 different dog breeds, both long-snouted, such as the greyhound and Jack Russell terrier, and short-snouted, such as the mastiff and pug.

The team found that the brains of many short-snouted breeds have rotated forward by as much as 15 degrees.

Furthermore, in these breeds the brain region for smell, called the olfactory bulb, has drifted downward toward the base of the skull, perhaps significantly altering the dogs' all-important source of smell, researchers say.

Since the first wolf was domesticated an estimated 12,000 years ago, "selective breeding has produced a lot of [anatomical] variation, but probably the most dramatic is in terms of skull shape," said study co-author Michael Valenzuela, a neuroscientist at the University of New South Wales in Australia.

"Dogs are very unique in having such a massive diversity of skull shapes," Valenzuela added, "more than any other species really."

Dog Brain Changes Equal Changed Behavior?

It's unclear whether the brain rotation and olfactory bulb movement of short-snouted, or brachycephalic, dogs has affected their ability to smell, but Valenzuela and his colleagues note that short-snouted dogs are usually not used for scent work.

"We think of dogs living in a world of smell—but this finding strongly suggests that one dog's world of smell may be very different to another's," study co-author Paul McGreevy of the University of Sydney said in a statement.

(See National Geographic fans' dog pictures.)

One way the brain changes could have changed the dogs' olfactory sense is by affecting a pathway in the brain called the rostral migratory stream, or RMS, the team speculates. Other studies have suggested the RMS is important for a normal sense of smell.

"The RMS starts very deep in the middle of the brain and traces a very predictable path to the olfactory bulb," study co-author Valenzuela told National Geographic News.

"Since the olfactory bulb has moved in brachycephalic dogs, you'd expect to see a change in the course of the RMS, or it may be disregulated" and dysfunctional, he added.

Investigating these potential RMS deviations, the authors write, would "be of intense interest for future research."

Valenzuela's team plans to undertake at least some of that future research and hope to uncover whether—or how—breeding-based brain changes have changed the sense of smell in dogs. "It is a fascinating question that our group will be looking into more."

The research is detailed in the online journal PLoS One.