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.
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