An endangered wolf went in search of a mate. The border wall blocked him.

In late 2021 an endangered Mexican gray wolf set out on an epic journey.

Known as Mr. Goodbar, the male had months earlier left his pack in eastern Arizona in search of his own territory and a mate. He headed south and east, through the Chihuahuan Desert, a vast, biodiverse expanse of grasslands and shrublands interspersed with mountain ranges and valleys.

The lanky canine, sporting a mix of silvery brown fur and not yet two years old, passed by the outskirts of Las Cruces, New Mexico, on November 22. The land is wide open and speckled with creosote, yucca, and cacti. Before him were distant peaks, including now extinct volcanoes and craters of the East Potrillo Mountains, the southern tip of which nearly reaches the Mexican border. Guided by instinct through the ancient territory of his kind, he headed that direction.

But he soon found himself at a perplexing impasse: The U.S.-Mexico border. Just one year prior, the land was open, except for a short vehicle barrier, a type of low porous fence meant to stop cars and trucks from illegal crossings. But now he found it blocked by a 30-foot-high wall, composed of massive steel beams separated by four-inch gaps, admitting only the tiniest of animals. (Learn more: Arizona’s border wall will include openings too small for many animals.)

Most of New Mexico’s border is now flanked by this fence, built from 2018 to 2020 under Donald Trump’s administration, a fact lost on Mr. Goodbar, who simply kept moving west. In all, he spent nearly five days migrating along the wall, sometimes briefly switching directions, presumably trying to head south around the obstacle. Eventually, about 23 miles west of where he encountered it, he gave up and headed back north.

The wolf’s path, tracked by a GPS collar affixed by the U.S. Fish and Wildlife Service, is among the first bits of concrete proof that the wall alters the movement of free-ranging wildlife, says Michael Robinson, a wolf advocate with the Center for Biological Diversity, an environmental group based in Arizona.

“I wasn’t surprised that it happened, because we’d predicted it,” Robinson says. “But I was bummed out.”

The travails of Mr. Goodbar confirm what conservationists and scientists have been warning about for years: That movements of all large animals will be disrupted by the border wall. That includes not just wolves but endangered Sonoran pronghorn, jaguars, ocelots, and bighorn sheep, and more common species such as mountain lions, bobcats, mule deer, and many more.

The wolf’s case study “is an extremely important data point,” says Myles Traphagen, a biologist with the Wildlands Network, a nonpartisan group dedicated to preserving wildlife corridors. First, it shows “the border wall is placing the recovery of an endangered species at risk.

“And think of all the other animals [it affects], and daily events that occur that we can’t see,” he says.

The Fish and Wildlife Service maintains that the Mexican wolf can recover without individuals traveling back and forth between the two countries, Aislinn Maestas, a FWS agency spokesperson, told National Geographic.

Biological models employed by the agency assume there will be “limited connectivity between the populations in the U.S. and Mexico due to the distance between the populations, the presence of an international border, and the higher mortality rate observed with dispersing wolves.” Still, scientists estimate that one wolf could disperse to the other population once every 12 to 16 years—but those calculations were done before the wall was planned or built.

“Successful dispersal between the populations has the potential to benefit population genetics, but the recovery of the Mexican wolf can be achieved without successful dispersal as we are utilizing releases from captivity to address genetic needs,” Maestas says. The service regularly introduces newborn wolves, born in captivity—such as Mr. Goodbar—into wild wolf dens, a strategy with its critics and supporters.

Back from the brink

Mexican gray wolves, a subspecies of gray wolf, are protected under the U.S. Endangered Species Act. These predators are slightly smaller than gray wolves and once ranged widely throughout Arizona, New Mexico, Texas, and northern Mexico. But the U.S. government targeted the animal on behalf of the livestock industry. The last breeding Mexican wolves were eliminated from the United States by the 1930s. In 1976, they were listed under the Endangered Species Act, which reversed long-standing efforts to eliminate them. 

The next year, the government hired a wolf trapper, Roy McBride, who had previously killed the animals, to capture alive the last wolves in northern Mexico. Three animals he caught and four others already in captivity, totaling seven, were bred in captivity. Beginning in 1998, their descendants were released into the wilds of eastern Arizona and western New Mexico. As of March 2021, an estimated 186 animals live in the two states, a 14 percent increase over the year prior. Another couple dozen wolves live in a small population in northern Mexico. Still the population’s genetic diversity is dangerously low, Robinson says.

Ideally, wolves from the two populations could naturally reach other and interbreed, which would benefit the species as a whole by widening the gene pool. To make such a journey, however, they face many obstacles, such as Interstate 10, which Mr. Goodbar has now crossed twice. (A Mexican wolf was killed by a car along the interstate in February 2021.)

But the newly-built border wall allows no such passage. That’s harmful to wide-ranging animals such as wolves, whose ability to migrate is “crucial to their long-term genetic viability,” says John Linnell, a biologist with the Norwegian Institute for Nature Research who studies interactions between predators such as wolves and humans.

Since the wall directly interferes with Mexican wolf movement, it would normally violate the Endangered Species Act. But the 2005 Real ID Act gives the head of the Department of Homeland Security the authority to override this law and dozens of others. U.S. Customs and Border Protection did not reply to National Geographic’s request for comment.

What’s more, New Mexico’s 105 miles of new wall are only a small part of the 450-plus miles of wall constructed under the Trump administration. More than 220 miles of new wall stretch along the Arizona border, 120 miles in California, and 17 miles in Texas, Traphagen says. (Learn more: Sacred Arizona spring drying up as border wall construction continues.)

Overt border wall construction has ceased, though Customs and Border Protection announced on December 20 that it will be “closing small gaps” in the wall, though what that entails is unclear. Several environmental and tribal groups have sued the federal government to stop construction, attempts which have mostly failed.

Lonely journey

Mr. Goodbar is now in Gila National Forest, near where he started his journey. It’s unknown if he’ll strike out again for new territory or try to establish a place near his home range.

In 2017, two wolves crossed into the United States from Mexico. One passed without issue across the border in nearly the exact spot that Mr. Goodbar tried and failed to get through. Another was Mr. Goodbar’s mother: The female made her way north near Douglas, Arizona, past San Bernardino Valley, a world hotspot for bee diversity that’s also cut through with new wall. She was captured in Arizona based on complaints from a rancher and gave birth in captivity to Mr. Goodbar at Sedgwick County Zoo in Kansas, which gave the wolf his name. 

Conservationists and scientists urge more action to address the plight of borderland animals like these wolves, emphasizing that the wall seriously fragments populations and blocks ancient migration routes. “This is an event that I believe needs a lot more attention,” Traphagen says. “This is just the beginning.”

Robinson once loved visiting the area where Mr. Goodbar tried to pass, loping in the footsteps of countless animals present and past.

“It’s a sublime landscape, affecting, full of life,” he says. But the 30-foot barrier cut through the landscape changed his experience.

“It breaks my heart to go down to the wall,” he says.