From 1977 to 1980, Roy McBride combed pine and oak forests high in the Sierra Madre Occidental range of northern Mexico, searching for the last remaining wild Mexican gray wolves.

He had been there before, hunting wolves for ranchers, even patiently outwitting one of the most notorious wolves in local folklore. It had thwarted his traps for months, until finally falling prey to one he disguised in ashes and baited with a skunk hide.

But this time, McBride’s client wanted wolves alive.

The U.S. Fish and Wildlife Service wanted to breed more endangered Mexican gray wolves in captivity to save them from extinction. Eradication campaigns had eliminated them from the wild in the United States, so the agency hired McBride to capture wolves across the border.

He caught three of the seven wolves in whose DNA the fate of the entire Mexican gray wolf subspecies would depend. But he was shocked when he saw some of the other founding wolves in the captive breeding program. They looked like dogs to him.

“The real reason that many of the … animals look like dogs is because that is what they are,” McBride later wrote in a 1997 letter to David Parsons, the agency’s Mexican Wolf Recovery Coordinator at the time.

Anecdotal accounts of genetic impurities among these seven founders have long been a cannon for arguments against recovering this gray wolf subspecies. A contaminated bloodline could undermine efforts to restore the wolf to its historical habitat.

Fish and Wildlife Service officials have rejected the accusations, but new research into the Mexican gray wolf bloodline found what advocates say is ample evidence to support findings that the bloodline is pure.

A University of Arizona study examined the DNA of decedents from each lineage the seven founding wolves created.

Since the 1980s, the captive breeding program, an international effort between the United States and Mexico, has grown to around 250 wolves, said John Bradley, a spokesman for the Fish and Wildlife Service.

At least 114 wolves roamed wild in Arizona and New Mexico at the time of the latest annual survey. And in February, Mexican officials reported 37 wild wolves south of the border.

Wolves without a dog in the fight

The recovery program's opponents have argued that Mexican gray wolves already went extinct, accusing program managers of raising a wolf bloodline contaminated by dogs or coyotes.

But the University of Arizona study found no evidence that any of the founding wolves were wolf-dog hybrids or that dogs had recently hybridized with Mexican gray wolves.

“Our work is the largest genetic study, in sample size, of Mexican wolves to date,” the researchers wrote in their findings. They analyzed 87 Mexican gray wolves, comparing DNA samples to that of dogs and other gray wolves.

ALSO:Mexican gray wolf recovery plan criticized for doing too much, too little

It’s still an open question if wolves have interbred with coyotes and to what extent, but genetic work by the Fish and Wildlife Service suggests they have not, said Robert Fitak, one of the study’s authors.

He and the other researchers focused their attention on dogs over coyotes, he said, because it was a higher-priority question.

Their research could have dealt a blow to the recovery program if it had found wolf-dog hybridization.

Federal agencies were asking, “'Do we need to put more resources to protecting this animal if it’s just a hybrid?'” Fitak said.

'Forever cloud the issue'

McBride warned Parsons in his letter that questions of impurities would always haunt the Mexican gray wolf bloodline.

“You may put dog blood in the wolves, but you will never take it out. And you will forever cloud the issue of what it is you have released into the wild,” McBride wrote in his letter.

Recovery program officials have tried to dispel this misperception, but it’s been around since recovery efforts started and repeatedly comes up at public meetings, according to an agency wolf biologist in a 2016 report by U.S. Department of Interior investigators.

In 2006, a rancher and a Catron County employee in New Mexico suspected that two animals the rancher had killed were wolf-dog hybrids, according to the 2016 Interior Department investigation.

The county employee said he gave blood and tissue samples from the animals to a biologist at the recovery program to test, the investigators noted. But county commissioners later accused the Mexican Gray Wolf Recovery Project of destroying the samples.

MORE: Mexican gray wolves: Ranchers want fewer. Environmentalists want more.

This accusation and more against the recovery project sparked the investigation by the department’s Office of Inspector General. Among their other claims, the Catron County commissioners more broadly alleged that Mexican gray wolveshadseverely interbred with dogs or coyotes.

Apache County also passed an ordinance in 2013 that questioned the legality of releasing Mexican gray wolves in light of the county’s suspicion that the subspecies was really a “wolf-dog hybrid.”

Although Fitak said he and the other researchers found no evidence of wolf-dog hybridization, wolves and dogs could still interbreed.

It has happened in the past, but the pups were euthanized in their den, the federal wolf biologist told investigators looking into the Catron County commissioners’ accusations.

And the recovery program tests every wolf and coyote they catch for genetic purity, she added.

'The more urgent takeaway'

Beyond disproving wolf-dog hybridization, Fitak and the other researchers found that Mexican gray wolves’ genetic variety is deteriorating, which could limit their ability to evolve and adapt to their environment.

“In general this loss of variation makes (Mexican gray wolves) much more susceptible to things in the future, like disease,” Fitak said.

What we don’t know, he said, is whether management practices are increasing or decreasing the speed at which variation is decreasing.

Genetic issues, like “loss of adaptive potential,” are a top threat to these wolves, according to the Mexican gray wolf recovery plan.

The plan aims to increase gene diversity by releasing more captive wolves into the wild and moving wild wolves around on the landscape to breed.

Mexican gray wolves will remain on the endangered species list until officials release 22 in the U.S. and 37 in Mexico that survive to reach breeding age at 2 years old. Wolf populations must also average 320 in the United States and 200 in Mexico over eight years.

The study’s main findings — that wolves are not hybridized with dogs — are important, said Michael Robinson, a wolf advocate at the Center for Biological Diversity. “It should put it to rest.”

But the decline in genetic variety concerns him more, he said.

“That is the more urgent takeaway from this study,” he said.

Environmental coverage on azcentral.com and in The Arizona Republic is supported by a grant from the Nina Mason Pulliam Charitable Trust. Follow the azcentral and Arizona Republic environmental reporting team at OurGrandAZ on Facebook, Twitter and Instagram.

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