In the mountains, Tibetan mastiffs dare to tread where other pups would drop their squeaky toys and whimper with exhaustion. The massive, 150-pound animals thrive at high altitudes, and now researchers know why: the mastiffs have a little extra dose of wolf in their genes.
The big, furry dog breed with a lion-like mane may date back as far as 1,100 B.C., when it began its role as a high-altitude guard dog. Tibetan people have used mastiffs guard their flocks of sheep from predators, like wolves, for centuries. The dogs lived alongside their human companions at altitudes of 15,000 feet or higher, heights in which average dogs wouldn’t withstand the lack of oxygen.
Researchers knew that at one point the Tibetan mastiff interbred with a Tibetan subspecies of gray wolf because the two share a genetic mutation that does not appear in the genome of other dog breeds. Rafi Letzer at Live Science reports that it was unclear what the amino acids coded by those genes actually did, but researchers did suspect it was related to their high-altitude superpowers.
In a new study published in the journal Molecular Biology and Evolution, a team examined the mastiff’s hemoglobin architecture, finding that the protein on red blood cells that transports oxygen is about 50 percent more efficient in Tibetan mastiffs than in other dog breeds.
“At altitude, the problem is taking in oxygen, because there’s just less of it,” says the study’s author Tony Signore, a biologist at the University of Nebraska-Lincoln, in a press release. “If you think of hemoglobin like an oxygen magnet, this magnet's just stronger.”
The story of the hemoglobin-boosting gene, however, is a little more complicated than the normal course of natural selection. In the distant past, wolves had the genes for the boosting hemoglobin, but they were pseudo-genes, meaning the weren’t expressed in their genome. So, even though the genes were present, they weren’t active and therefore didn’t give the wolves any high-altitude advantages.
At some point, the hemoglobin pseudo-genes were copied and pasted into one of the wolf’s active genes. That changed the way its hemoglobin worked, helping the Tibetan wolf become a high-altitude specialist, an advantage that was passed down to future generations.
In fact, these changes, along with mitochondrial DNA and other genetic markers have convinced some zoologists that the Tibetan wolf and other wolf populations in the Himalayas should be classified as a separate species from the gray wolf. It’s believed this new Himalayan wolf diverged from other gray wolf subspecies around 700,000 years ago.
At some point, the wolf interbred with a domestic dog breed, passing along the unique mountaineering gene that persists in the Tibetan mastiff to this day.
For the research team, the exciting part is the pseudo-gene coming to life after being dormant for generations. “[The genes] wouldn’t have conferred any benefit under normal circumstances,” senior author Jay Storz, also of the University of Nebraska-Lincoln, says in the release. “It was just [that] this conversion event occurred in an environmental context where the increase in hemoglobin-oxygen affinity would have been beneficial. So mutations that otherwise would have been either neutral or even detrimental actually had a positive fitness effect.”
Today, the breed remains popular in Tibet, but the genetic mutation can’t protect the dogs from the modern world. Tibetan mastiffs became a status symbol in China in the 2000s and early 2010s. (In 2011, one Tibetan mastiff named Big Splash sold for a reported $1.6 million.) But the dog's popularity led to a wave of overbreeding. By 2013, Tibetan mastiffs were considered passé and many were either released into the streets and mountains or sold to processors to make leather and to line winter gloves.