Researchers Identify Gene Affecting Dog Size

An large, black Afghan and a small, white Chihuahua stand side by side. i i

Dogs exhibit the greatest size disparity of all mammals. Above, an Afghan and a Chihuahua/Poodle mix illustrate the possible difference in size. Edouard Cadieu/NHGRI hide caption

itoggle caption Edouard Cadieu/NHGRI
An large, black Afghan and a small, white Chihuahua stand side by side.

Dogs exhibit the greatest size disparity of all mammals. Above, an Afghan and a Chihuahua/Poodle mix illustrate the possible difference in size.

Edouard Cadieu/NHGRI
A large, gray Irish Wolfhound and a small, white Chihuahua run side by side. i i

Researchers have been trying to learn why dogs can vary so greatly in size from other members of their species. Above are a Chihuahua (left) and an Irish Wolfhound. Tyrone Spady/NHGRI hide caption

itoggle caption Tyrone Spady/NHGRI
A large, gray Irish Wolfhound and a small, white Chihuahua run side by side.

Researchers have been trying to learn why dogs can vary so greatly in size from other members of their species. Above are a Chihuahua (left) and an Irish Wolfhound.

Tyrone Spady/NHGRI

You don't find people the size of insects or giraffes; humans are all roughly the same size. The same is true of most species. There isn't a huge difference between the biggest and the smallest.

One exception to the rule may be lying at your feet right now: the dog. There are very, very big dogs and teeny, tiny dogs.

Modern dogs are the offspring of the offspring of the offspring of gray wolves the ancestry goes back thousands of years. But while there are many kinds of wolves, they're all approximately the same size. So why are dogs different?

Scientists have been taking canine cheek swabs to find out. Their report appears this week in the journal Science.

Nathan Sutter is a geneticist at the National Institutes of Health. He says the largest dog ever seen was an Irish wolfhound named Merlin.

"[He] won at Westminster," Sutter says. "He's enormous. He's the size of a horse."

Sutter says the smallest dog ever might have been a Chihuahua named Frenchie, just two pounds as an adult. Merlin is some 80 times heavier than little Frenchie.

To find out how the dog genome generates such large and small animals, Sutter and other researchers studied the Portuguese water dog.

Elaine Ostrander runs the genetics lab at NIH that performed some of the analysis.

"[Portuguese water dogs] were actually used by the fisherman to send messages between boats," Ostrander says. "They would herd the fish into nets. They could retrieve fish or articles from the water. They were also used to guard the fishing boats, and they could be used to help bring in the nets."

Portuguese water dogs come in both big and small sizes. Today, most dogs bred for competition have to fall into narrow size ranges, but the rules for Portuguese water dogs happen to be looser.

The researchers analyzed Portuguese water dog DNA and found a single gene what Ostrander calls a master regulator that seems to account for a big part of the size difference. Small Portuguese water dogs had one version, while larger Portuguese water dogs had different versions.

But was this just the case with the Portuguese water dog?

For two years, the researchers went to dog shows and anywhere they could find dogs to collect dog DNA. They took blood samples from Chihuahuas, Pekingese, Mastiffs, Great Danes many blood samples and cheek swabs.

Were dogs happy to offer a cheek swab?

"They didn't care," Ostrander says, "especially if they were going to get a treat or if there was a tennis ball in our other hand."

The results came in. And just as with the Portuguese water dogs, the small breeds had one variant of the gene, while big dogs had different variants.

Ostrander says it is surprising that a single gene plays such a prominent role in all dogs.

"When you look at the different dog breeds," Ostrander says, "and you look at their histories, and they've come from all over world, and they've been bred to do such different things it just seemed to us that the story had to be more complex."

But it wasn't more complex. So you have to wonder, why and when did these variants evolve? You can see why big dogs might thrive, but what evolutionary force made it beneficial to be tiny?

One possibility is that humans were the evolutionary force. There is no evidence that wolves had the genetic variant for small size. It is possible that when humans started to domesticate dogs, a bit of DNA didn't get copied right, and a small dog appeared in a litter.

We kept it, protected it, bred it. Maybe we thought it was cute, or more likely, useful.

"We really, really don't know," says Paul Jones from the Waltham Pet Center, in England, who worked on the project.

"It was just a very, very lucky event," Jones says. "And it's probably lucky for man as well. When you think about humans, when they actually first started farming barley, wheat and everything, they actually started gathering those food stores together. As you know, you need to protect those food stores from mice and rats — and the ideal dog to do that is a small, terrier-like dog."

Jones hopes study of dog genomes may lead to healthier pets. Dog may be man's best friend, but there's this sad truth: Humans can live 80 years, but dogs, barely 15.

The Canine Spectrum

Danka Kordak, a 5.4 in tall, white Chihuahua stands beside a box of matches.

Danka Kordak Slovakia, a long-haired Chihuahua, was the world's smallest dog until his death in September 2006. He was 5.4 inches tall. Igor Kvetko/AFP/Getty Images hide caption

itoggle caption Igor Kvetko/AFP/Getty Images
Gibson, a 170 lb harlequin Great Dane sits in front of a brick wall.

Gibson, a harlequin Great Dane, is currently the tallest dog in the world. When standing on his hind legs, he is over 7 feet tall. Courtesy of Sandy Hall hide caption

itoggle caption Courtesy of Sandy Hall

Danka Kordak Slovakia, a long-haired Chihuahua from Revuca, Slovakia, held the record of world's smallest dog until his death in September 2006. Danka was 5.4 inches tall and 7.4 inches long. Weighing just 27 ounces, he ate 2.5 ounces of dog food a day. Since his death, no one has put forth a paw to claim the title of world's smallest dog.

While Danka was the tiniest dog in terms of height, Heaven Sent Brandy, a female Chihuahua from Florida, currently holds the record lengthwise; she is 6 inches from her nose to the tip of her tail. She is not allowed on her owner's couch, for fear that she will injure herself jumping off.

On the other end of the spectrum, the tallest living dog is Gibson, a harlequin Great Dane, who is 43 inches tall. He weighs in at 170 pounds and is over 7 feet tall when standing on his hind legs. Gibson has been measured against several professional basketball players. He lives with his owner, Sandy Hall, in Grass Valley, Calif.

Though the Guinness Book of World Records does not currently list the world's heaviest dog, as of 2001, the heaviest living dog was a 284-pound English mastiff named Hercules. According to his owner, he eats a pound of dry dog food each day.

On average, small dogs (8 inches and under) tend to live longer than large dogs (24 inches and up). Small dogs usually live 12 to 15 years, while large dogs have a life expectancy of eight to 10 years.

Sources: The Guinness Book of World Records; CNN; St. Petersburg Times; Paul Jones; and Sandy Hall.

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