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RENEE MONTAGNE, host:

Okay, so you don't find people the size of giraffes or insects. Humans are roughly the same size. That's true of most species. There's not a huge difference between the biggest and the smallest. One exception is the dog. There are very big dogs and teeny, tiny dogs. Scientists have been taking canine cheek swabs to find out why. Their report appears today in the journal Science.

NPR's David Kestenbaum reports.

DAVID KESTENBAUM: Domestic dogs are the offspring of the offspring of the offspring thousands of years ago of gray wolves. But while there are lots of kinds of wolves, they're all, ballpark, the same size. So why are dogs different?

What's the largest dog you've ever seen?

Mr. NATHAN SUTTER (Geneticist, National Institutes of Health): An Irish wolfhound named Merlin who won at Westminster. He's enormous. He's a size of a horse.

KESTENBAUM: This is Nathan Sutter, a geneticist at the National Institutes of Health.

And the smallest dog you've ever seen?

Mr. SUTTER: Frenchie might take the cake, a Chihuahua that we photographed last Sunday. She's two pounds as an adult, and she's not going to get any bigger.

KESTENBAUM: Merlin is some 80 times heavier than little Frenchie, relatively speaking, like a human being 50 feet tall. To find out how the dog genome generates such large and small animals, Sutter and other researchers studied the Portuguese water dog.

Ms. ELAINE OSTRANDER (Researcher, National Institutes of Health): They were actually used by the fishermen to send messages between boats.

KESTENBAUM: Elaine Ostrander runs the genetics lab at the NIH that did some of the analysis.

Ms. OSTRANDER: They would herd the fish into the nets. They could retrieve fish or articles from the water, and they were also used to guard the fishing boats, and they could be used to help in bring in the nets.

KESTENBAUM: There are big ones and small ones. Most dogs bred for competition these days 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; larger Portuguese water dogs, different versions.

But, you know, maybe that was just the Portuguese water dog. So for two years, the researchers went to dog shows - anywhere there were dogs, really - to collect dog DNA. Owners were very helpful, said things like: test my dog, my dog wants to be part of your study. The researchers took blood samples from Chihuahuas, Pekingese, Mastiffs, Great Danes, lots of blood samples and cheek swabs.

Dogs happy to offer a cheek swab?

Ms. OSTRANDER: You know what, they didn't care. Especially if they were going to get a treat or there was a tennis ball in our other hand.

KESTENBAUM: The results came in, and just as with the Portuguese water dogs, the small breeds...

(Soundbite of yelping dog)

KESTENBAUM: had one variant of the gene. Big dogs...

(Soundbite of growling dog)

KESTENBAUM: Had different variants. Ostrander says it's surprising that a single gene plays such a prominent role in so many dogs.

Ms. OSTRANDER: Because when you look at the different dog breeds, and you look at their histories and they've come from all over the world and they've been bred to do such different things, I mean, there's herding dogs and there's draft dogs, and there's hunting dogs and there's sight hounds, and there's scent hounds and there's these tiny little companion dogs, and there's racing dogs and there's these great, big, huge dogs. It just seemed to us that the story had to be more complex.

KESTENBAUM: But it wasn't. 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 evolutionary force was us.

There's no evidence that wolves had the genetic variant for smallness. So maybe 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.

Mr. PAUL JONES (Waltham Pet Center): We really, really don't know. There's lots of theories.

KESTENBAUM: This is Paul Jones from the Waltham Pet Center in England who worked on the project.

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

KESTENBAUM: Why couldn't we have big dogs protecting our food?

Mr. JONES: We could have big dogs protecting our food, but they're not quite as good ratters.

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

(Soundbite of growling dog)

KESTENBAUM: David Kestenbaum, NPR News.

MONTAGNE: Get a look at some of the world's smallest and largest dogs at npr.org.

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