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Wolf in dog’s clothing? Study points to twist in fur color

Feb. 6, 2009
Courtesy Stanford University
and World Science staff

Slip­ping amid trees or across snow, the wolf has glid­ed in­to leg­end on paws of white, gray or—in North Amer­i­ca—black. 

This last group owes an un­ex­pected debt to the cousins of the do­mes­tic dog, ge­neti­cists say: in an un­usu­al ev­o­lu­tion­ary twist, dogs that bred with wolves thou­sands of years ago passed a muta­t­ion for dark fur to their form­er an­ces­tors. As a re­sult, the Gray Wolf, or Ca­nis lu­pus, is not always gray.

Two wolf pups from Yel­low­stone Na­tion­al Park born to the same par­ents, a black fe­male and gray ma­le. (Im­age cour­te­sy of Dan­iel Stahler/NPS)


The ef­fect was more than just cos­met­ic, they add: the re­sulting black wolves, in­hab­i­tants of North Amer­i­ca, seem to have an ev­o­lu­tion­ary ad­van­tage over lighter-col­ored wolves in for­ests.

It’s a rare in­stance of do­mes­tic an­i­mals con­tri­but­ing to the ge­net­ic va­ri­e­ty of their wild coun­ter­parts in a way that af­fects the re­cip­i­ents’ ap­pear­ance and sur­viv­al, ac­cord­ing to the re­search­ers. In this case, the do­mes­tic an­i­mals would have probably been the first Na­tive Amer­i­cans’ dogs.

“We usu­ally think of do­mes­tica­t­ion as some­thing that is car­ried out to ben­e­fit hu­mans,” said ge­net­icist Greg Barsh of Stan­ford Uni­ver­s­ity in Cal­i­for­nia. 

“So we were really sur­prised to find that do­mes­tic an­i­mals can serve as a ge­net­ic res­er­voir that can ben­e­fit the nat­u­ral popula­t­ions from which they were de­rived. It’s al­so fas­ci­nat­ing to think that a por­tion of the first Na­tive Amer­i­can dogs, which are now ex­tinct, may live on in wolves.” Ca­nine ge­neti­cists gen­er­ally agree that North Amer­i­can dogs to­day de­scend from Eu­ro­pe­an dogs.

Barsh and Stan­ford grad­u­ate stu­dent Tovi An­der­son joined many oth­er sci­en­tists to con­duct the stu­dy, pub­lished on Feb. 5 in the re­search jour­nal Sci­ence. The group com­pared DNA col­lect­ed from 41 black, white and gray wolves in the Ca­na­di­an Arc­tic and 224 black and gray wolves in Yel­low­stone Na­tional Park in the North­west U.S. with that of do­mes­tic dogs and gray and black coy­otes. 

“We ex­pected this to be a short re­search proj­ect to con­firm that wolves and dogs shared the same ge­net­ic path­way that de­ter­mines black coat col­or,” said An­der­son. “The sto­ry got much more in­ter­est­ing when we ex­pand­ed our re­search and be­gan ask­ing about the or­i­gin of the muta­t­ion in wolves.”

Black wolves are much more com­mon in Ca­na­di­an Arc­tic for­ests than in the icy tun­dra—62 per­cent vs. 7 per­cent of the to­tal popula­t­ion, re­spec­tive­ly. Bi­ol­o­gists have long sus­pected black fur is some­how ad­van­tageous to wood­land wolves spe­cif­ic­ally, but weren’t sure why. Be­cause black wolves gray with age, it seemed the root cause might be deeper than just coat col­or.

Barsh’s lab­o­r­a­to­ry, which stud­ies genes af­fect­ing coat col­or and oth­er bi­o­log­i­cal path­ways in mam­mals, found in 2007 that the gene for black fur in dogs, called beta-defen­sin, be­longs to a family of genes pre­vi­ously be­lieved to be in­volved in fight­ing in­fec­tion. One ver­sion of the gene pro­duces light or yellow-col­ored dogs and wolves; a mu­tant ver­sion pro­duces black an­i­mals.

“Wildlife bi­ol­o­gists don’t really think that wolves rely much on cam­ou­flage,” said Barsh. “It’s pos­si­ble there is some­thing else go­ing on he­re. For ex­am­ple, the pro­tein re­spon­si­ble for the coat col­or dif­fer­ence has been im­pli­cat­ed, in hu­mans, in in­flamma­t­ion and in­fec­tion, and the­refore might give black an­i­mals an ad­van­tage that is dis­tinct from its ef­fect on pig­menta­t­ion.”

Al­though the “why” re­mains a mys­tery, the “how” is be­com­ing clear­er, ac­cord­ing to the in­ves­ti­ga­tors. An­der­son said she con­firmed that the black-coat gene shows ev­i­dence of pos­i­tive se­lec­tion in for­est wolves, sug­gest­ing ev­o­lu­tion­ary pres­sures have fa­vored an­i­mals with the gene. She al­so found that the gene is dom­i­nant, mean­ing an an­i­mal with only one copy of the gene would still have a black coat.

An­der­son and col­la­bo­ra­tors used ge­net­ic tests to de­ter­mine that the muta­t­ion was likely in­tro­duced in­to wolves by dogs in the last 10,000 to 15,000 years, about when the first Amer­i­cans were mi­grat­ing across the Ber­ing land bridge in­to the con­ti­nent. These peo­ple probably brought dogs, some of which car­ried the black-coat muta­t­ion es­ti­mat­ed to have aris­en about 50,000 years ago, ac­cord­ing to the re­search­ers.

“It may have been eas­i­er for dogs to in­ter­act with wolves in North Amer­i­ca than in Eu­rope,” said An­der­son. “There was probably a high­er con­centra­t­ion of wolves, and the dogs, like the hu­mans, were more migrato­ry.” It’s un­cer­tain wheth­er there were any black wolves pri­or to the do­mes­tica­t­ion of dogs, ac­cord­ing to the sci­en­tists. 

“This is a muta­t­ion that had been cul­ti­vat­ed by hu­mans in the form of the do­mes­tic dog for thou­sands of years,” said An­der­son. “Now we see that it not only en­tered the wild popula­t­ion, but al­so is ben­e­fiting them.”


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Slipping amid trees or across snow, the wolf has glided into legend on paws of white, gray or—in North America—black. This last group owes an unexpected debt to the cousins of the domestic dog, geneticists say. In an unusual evolutionary twist, dogs that bred with wolves thousands of years ago passed a mutation for dark fur to their former ancestors, the scientists say. As a result, the Gray Wolf, or Canis lupus, is no longer just gray. The effect was more than just cosmetic, they add: the resulting black wolves, inhabitants of North America, seem to have an evolutionary advantage over lighter-colored wolves in forests. It’s a rare instance of domestic animals contributing to the genetic variety of their wild counterparts in a way that affects both the recipients’ appearance and survival, the researchers claim. In this case, the domestic animals would have probably been the first Native Americans’ dogs. “We usually think of domestication as something that is carried out to benefit humans,” said geneticist Greg Barsh of Stanford University in California. “So we were really surprised to find that domestic animals can serve as a genetic reservoir that can benefit the natural populations from which they were derived. It’s also fascinating to think that a portion of the first Native American dogs, which are now extinct, may live on in wolves.” Canine geneticists generally agree that North American dogs today descend from European dogs. Barsh and Stanford graduate student Tovi Anderson joined many other scientists to conduct the study, published on Feb. 5 in the research journal Science. The group compared DNA collected from 41 black, white and gray wolves in the Canadian Arctic and 224 black and gray wolves in Yellowstone National Park in the Northwest U.S. with that of domestic dogs and gray and black coyotes. “We expected this to be a short research project to confirm that wolves and dogs shared the same genetic pathway that determines black coat color,” said Anderson. “The story got much more interesting when we expanded our research and began asking about the origin of the mutation in wolves.” Black wolves are much more common in Canadian Arctic forests than in the icy tundra—62% vs. 7% of the total population, respectively. Biologists have long suspected black fur is somehow advantageous to woodland wolves specifically, but weren’t sure why. Because black wolves gray with age, it seemed the root cause might be deeper than just coat color. Barsh’s laboratory, which studies genes affecting coat color and other biological pathways in mammals, found in 2007 that the gene for black fur in dogs, called beta-defensin, belongs to a family of genes previously believed to be involved in fighting infection. One version of the gene produces light or yellow-colored dogs and wolves; a mutant version produces black animals. “Wildlife biologists don’t really think that wolves rely much on camouflage,” said Barsh. “It’s possible there is something else going on here. For example, the protein responsible for the coat color difference has been implicated, in humans, in inflammation and infection, and therefore might give black animals an advantage that is distinct from its effect on pigmentation.” Although the “why” remains a mystery, the “how” is becoming clearer, according to the investigators. Anderson said she confirmed that the black-coat gene shows evidence of positive selection in forest wolves, suggesting evolutionary pressures have favored animals with the gene. She also found that the gene is dominant, meaning an animal with only one copy of the gene would still have a black coat. Anderson and collaborators used genetic tests to determine that the mutation was likely introduced into wolves by dogs in the last 10,000 to 15,000 years, about when the first Americans were migrating across the Bering land bridge into the continent. These people probably brought dogs, some of which carried the black-coat mutation estimated to have arisen about 50,000 years ago, according to the researchers. “It may have been easier for dogs to interact with wolves in North America than in Europe,” said Anderson. “There was probably a higher concentration of wolves, and the dogs, like the humans, were more migratory.” It’s uncertain whether there were any black wolves prior to the domestication of dogs, according to the scientists. “This is a mutation that had been cultivated by humans in the form of the domestic dog for thousands of years,” said Anderson. “Now we see that it not only entered the wild population, but also is benefiting them.”