Does the Mexican wolf belong in the US Southwest?

On a dry Arizona afternoon in early-May, Allison Greenleaf found herself in the middle of the Gila National Forest, holding a warm, chaotic ball of newborn wolf pups.

“It can be stressful,” she later recalled. “I get pretty mission-driven and want to get in and out as soon as possible.”

The mission was to deliver two new pups to a wild mother wolf — part of a federal program of “cross-fostering,” or taking captive-born wolves and sneaking them into dens with wild litters while the mother is out hunting.

“The idea is to make all the puppies smell the same,” said Greenleaf, a researcher with the U.S. Fish and Wildlife Service.

She does this by making the new and wild pups urinate and defecate all over each other. When they are placed back into the den, mama wolf cannot smell the difference. And suddenly, six pups become eight.

Since 2014, the Fish and Wildlife Service has made cross-fostering a priority for the Mexican wolf, one of five federally-recognized subspecies of the gray wolf. With less than 150 living in the wild, the Mexican wolf — also called “el lobo” and Canis lupus baileyi — is the most endangered gray wolf in North America.

For 20 years, federal and state agencies have tried to recover wild Mexican wolves, which once numbered in the thousands until unrestricted hunting reduced the population to single digits. While significant gains have been made, the majority of recovery now takes place in captivity, where about 300 wolves live — most without plans to be released.

In the wild, however, population growth has stagnated for reasons not yet understood by scientists.

Biologist Jim Heffelfinger believes the question is not how to recover the wolves, but where. His view is controversial and reveals the tangle of policy, politics, and genetics that govern the lives of Mexican wolves and their North American cousins.

“There’s just a ton of high-quality wolf habitat in Mexico,” said Heffelfinger, who co-authored a paper on the recovery in the journal Biological Conservation earlier this year.

According to Heffelfinger, recovery efforts — now concentrated in southern Arizona and New Mexico — should shift focus and expand south of the border to the Sierra Madre, where many of the wolves once roamed.

“The Mexican wolf has 90 percent of its historic range in Mexico,” he said. “You’re looking at factors that affect how well a wolf can persist and survive on the landscape.”

The paper, which did not receive funding from any agency or non-governmental organization, claims that the wolf is genetically adapted to Mexico’s environment. It is smaller than other gray wolf subspecies, and it hunts in packs of as little as two, making the Sierra Madre’s deer and rodents easier prey than the massive elk and caribou found in the U.S.

Some scientists say Heffelfinger — who primarily studies deer and elk for the Arizona Game and Fish Department — is peddling politics, not science.

“His position is to stymie Mexican wolf recovery,” said Phil Hedrick, professor emeritus at Arizona State University’s School of Life Sciences and the lead author of an unfunded response paper published in the same journal.

“It’s not clear to me that it’s going to be a successful operation in Mexico,” he said. “There were always problems.”

Today, Mexico is home to about 30 wild wolves. Officials there began reintroductions in 2011, funded in part by the U.S. Within the first two months of the program, four wolves were poisoned, and since then, around 20 have been killed, mostly by people.

Hedrick argues that part of the problem is that much of the land is privately-held by ranchers willing to defend their cattle with lethal — and preemptive — force. Researchers have also voiced concerns about drug cartels, which control portions of the habitat. (Heffelfinger says the cartels scare people away, which is a good thing for wolves.)

Hedrick would instead like to see recovery efforts expanded north to Utah and the southern Rocky Mountains in Colorado, noting that gray wolves once roamed throughout the continent. “This whole idea of limitation is so artificial. Wolves lived from Canada to Mexico,” he said. “There was no border.”

But a recovery farther north raises other concerns.

“If you think there are not going to be consequences,” said Eric Odell of Colorado Parks and Wildlife, “you’re wrong.”

Odell, who co-authored the paper with Heffelfinger as well as and an additional response to Hedrick, fears that northwestern wolves — a separate, and larger, gray wolf subspecies — could “genetically swamp” the Mexican wolf. In other words, if the two subspecies mate, the Mexican wolf’s genes would be “diluted.”

That may have policy implications, said Odell. The Fish and Wildlife Service requires the establishment of a stable Mexican wolf population before it can be delisted from the Endangered Species Act. Hybrids don’t count.

Hedrick, a geneticist, disagrees with Odell’s assessment and said that officials have taken a softer approach to species designation in the past. “That’s what Fish and Wildlife have done for Florida panthers,” he said.

In 1995, U.S. officials released eight cougars into the Everglades to help repopulate the endangered and inbred panther population. Today, their offspring are counted as panthers, not hybrids.

The Mexican wolf faces a similar situation. By 1980, after hunters and ranchers decimated the wolves, only seven survived, and inbreeding levels took off. Wild Mexican wolves are now about as genetically similar as brother and sister.

That can lead to smaller litters and lower survival rates. Biologists say it may also lead to genetic defects, though that has not yet been seen in the Mexican wolves.

Hedrick thinks crossbreeding could encourage what is called adaptive introgression: moving “good” genetic material from one species or subspecies into another through a selective process that improves the overall health of the offspring. Mexican wolves would keep the traits that make them “Mexican wolves,” while also improving their genetic diversity. The phenomenon, which has been the focus of an increasing number of scientific studies, has been documented in other species, including Florida panthers and humans.

Wolf specialist Dave Mech, a scientist with the U.S. Geological Survey and the University of Minnesota who was not involved with the journal papers, agreed that crossbreeding is natural for wolves.

“There’s no line that distinguishes between adjacent subspecies,” he said. “A wolf is a wolf is a wolf.”

Mech added that, even among other animals, the concept of subspecies is a “nebulous,” human-imposed distinction.

The U.S. officially designated the Mexican wolf as a subspecies in 1976. But in March, the Fish and Wildlife Service, acting on behalf of Congress, asked the National Academy of Sciences to reassess the taxonomic status of the Mexican wolf. Its conclusion, expected sometime next year, could affect future policy.

No matter where the Mexican wolves go — and no matter their taxonomical distinction — there may never be a guaranteed safe recovery. Ranchers in Arizona and New Mexico see wolves as a nuisance and threat to their livestock. And since 1998, at least 93 wild wolves have been killed by people, making up about 70 percent of all Mexican wolf deaths in the U.S, according to the Fish and Wildlife Service.

As one biologist put it, the wolf problem “is not at all about the wolves.”

Originally published at on October 22, 2018.



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Dean Russell

Dean Russell


Reporting fellow for Columbia Journalism Investigations. Writes about science and nature. Bylines: NPR, The Guardian, Grist.