From the Chihuahua that's tiny enough to fit in a purse to the most massive mastiff, dogs have an incredible, and scientifically baffling, range in size.
"Dogs have the biggest range of sizes of any mammal in existence," said biologist K. Gordon Lark of the University of Utah. "One of the big questions has always been, 'Where does this range of sizes come from?'"
Lark and his colleagues think they've found part of the answer to this question: a piece of doggy DNA that regulates growth to keep small dogs small.
To find the genetic clues to dog size, the researchers enlisted the help of hundreds of Portuguese water dog owners to get DNA samples and body-size measurements of their pooches.
This breed of dogs has a remarkable three-fold range in size, from 25 to 75 pounds.
By analyzing the dogs' DNA, the researchers found the piece of their genetic material that strongly correlated with their size.
The DNA snippet isn't actually a gene — it's called a regulatory sequence. This sequence is next to a gene that regulates a growth-inducing protein hormone that helps humans and other mammals grow from birth to adolescence.
In small dogs, one or more mutations in the regulatory sequence suppress the gene's activity, so it won't produce as much of the hormone, Lark said, effectively preventing any Labrador-sized Chihuahuas.
Medium and large dogs are missing this regulatory sequence, said study team member Kevin Chase, also of the University of Utah. So Great Danes can grow to their normal, intimidating size.
Other genetic material that has yet to be identified also likely contribute to the size of these dogs, he added.
To confirm their findings, detailed in the April 6 issue of the journal Science, the researchers examined 3,241 other dogs from 143 different breeds, including Chihuahuas, pugs, toy poodles, Saint Bernards, Irish wolfhounds and standard poodles.
In all of the small breeds, the same regulatory sequence was found.
"All dogs under 20 pounds have this — all of them," Lark said. "That's extraordinary."
Oddly, Rottweilers also have the sequence, but other genetic factors likely make them big, Chase said.
From wolf to dog
Dogs were domesticated from wolves around 12,000 years ago, and because small dogs from all over the world have this piece of DNA, the researchers think that the genetic instructions to make small dogs must be just as old.
"Since this is found in all small dogs, it either got into dogs when they were first domesticated, or it was a small wolf that dogs descended from," Lark said, noting that the sequence isn't found in wolves today.
The researchers say small dogs likely proliferated because humans saw them as good companions.
"Tiny dogs are not particularly functional," said Chase, who owns a toy-poodle-Maltese mix. "They don't hunt with you. They don't protect your house. They don't pull carts. They're just small and sweet."
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