Why are small dogs small? Science thinks it knowsCall it the "yappy" code.
An international team of scientists has identified a piece of dog DNA that seems to be the difference in the tall and the short when it comes to canine stature.
The genetic snippet regulates a particular gene. And the two are found together in all breeds of small dogs, but not in medium and large breeds -- except for rottweilers. More on them later.
The gene itself is found in all dogs -- all mammals, in fact -- and is crucial to pumping out hormonal growth signals from birth through adolescence. It appears likely that the same kind of genetic restriction also plays a role in limiting size in humans.
"Nearly all of what we learn from studying body structure, behavior and disease susceptibility in dogs helps us understand some aspect of human health and biology," said Elaine Ostrander, senior author of the study published today in the journal Science. She is also chief of cancer genetics at the National Human Genome Research Institute.
"By learning how genes control body size in dogs, we are apt to learn something about how skeletal body size is genetically programmed in humans," she added. "We also will increase our data set of genes likely to play a role in diseases such as cancer, in which regulation of cell growth has been lost."
The gene in question is called insulinlike growth factor 1 (IGF-1). The genetic quirk, called a haplotype, is found in all small breeds, from Chihuahuas and to Yorkies and Pekinese.
"All dogs under 20 pounds have this, all of them. That's extraordinary," said Gordon Lark, a biologist at the University of Utah. Lark, with Utah colleague Kevin Chase, got the study rolling with a look at size variation among Portuguese water dogs, which can range in size from about 25 pounds to more than 75 pounds.
The researchers analyzed DNA samples and reviewed X-rays of more than 500 Portuguese to narrow the size regulator down to one stretch of code on dog chromosome 15.
Researchers from other universities and pet-industry labs in the United States and England joined in, bringing different tools. Ultimately, the analysis included more than 3,200 dogs from 143 breeds.
Dogs evolved from wolves roughly 15,000 years ago as humans domesticated them. Because the small-dog coding was found in small breeds that are only distantly related, and in different regions, the researchers figured the variant must be at least 12,000 years old.
"It's as ancient as all small dogs," Lark said. "Dogs are derived from wolves. 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. The small-dog haplotype is not found in wolves today."
Of course, there's an exception to the rule.
Rottweilers, hardly lapdogs, also carry the sequence for smallness. The researchers said that obviously there are other genes involved in size when in comes to rottweilers, and probably other large breeds.
Lark and his colleagues figured that the genetic signal to make small dogs arose either because "a small wolf couldn't survive in nature, but it could survive in company with humans," or because early humans "wanted to domesticate a wolf, and they didn't want to adopt a big sucker" for reasons of safety, economy or crowded conditions in early walled settlements.
It may be simply that the "aww" factor tugged at prehistoric human hearts as much as it does today.
"Everybody treats their dogs like their babies, so it's not surprising they would select for tiny dogs," said Chase, the owner of two toy poodle/Maltese mix mutts. "Tiny dogs are not particularly functional. They don't hunt with you. They don't protect your house. They don't pull carts. They're just small and sweet."
And very often yappy, Chase concedes. "But 'yappy,' we didn't study."
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