For centuries, the realm McCain monitors has served as a trade route linking Arizona's natives with the Sea of Cortez to the southwest. The Borderlands Jaguar Detection Project study area is riven with trails frequented by smugglers ferrying humans and contraband into the United States. These are not people who welcome the sight of a gringo. Some are ruthlessly violent. "It's an interesting relationship," McCain says. "There are guys coming through this area, carrying heavy loads. We've run into each other on the trail."

 McCain believes the smugglers have come to know who he is and understand that he's looking for evidence of jaguars, not illicit traffic. "We both go to every effort possible to avoid each other," he says. "My attitude is, 'I don't care what you're doing. Don't mind me.' "

 When a camera captures the image of a smuggler or illegal, the startled subject frequently destroys the camera. McCain isn't happy about it - each camera set-up costs about $500 - but he understands the reaction. "When a flash goes off, it's fight or flight. But when I get a picture of someone on one of my cameras, I throw it away or delete it," he says. "They pretty much leave me alone. I have signs (posted in Spanish near his cameras) that explain what I'm doing."

 On two occasions, McCain has discovered the remains of illegals who succumbed to the elements. "One was a 38-year-old woman who had died of exposure the night before," McCain says softly. "She was still beautiful, just lying there.

 "I remember crying, 'What for? What in the world are we doing?' "

 Border Patrol agents who ply this terrain are sensitive to McCain's dilemma. "The Border Patrol knows how vulnerable I am," McCain says. "They don't push me for information."

 But the volume of illegal crossers bodes ill for Macho B and his kin in the U.S. The Homeland Security Department is rushing to construct miles of fencing along the international border, including in areas where jaguars have been detected.

 About 90 miles of fencing is slated to go up in Arizona this year, with a total of 370 miles of pedestrian fencing and 200 miles of new vehicle barriers to be built along the entire U.S.-Mexico border within a year, according to Brad Benson, a Washington, D.C., spokesman for U.S. Customs and Border Protection. Benson says a seven-mile stretch of pedestrian fence at Sasabe, near Macho B's range, will cost $31 million.

 "Anything that will keep people out definitely will keep any animals out," McCain says. "That means we will not have a future with jaguars in the United States."

 Although the Sasabe fence was already under construction, the U.S. Fish and Wildlife Service did not get around to releasing a biological opinion on the project until Sept. 7. If all border-crossing corridors used by jaguars are blocked, the document says, the 12- to 18-foot walls may result in the extirpation of the jaguar in the United States.

 But it does not recommend stopping or delaying the project. The opinion concludes: "The range of the jaguar within the United States is not enough area to provide for conservation (i.e., recovery) ..." It also contends that the jaguar's U.S. habitat "cannot be defended as essential to the conservation of the species."

The opinion notes that jaguars that confront the border fence could choose to walk around it. The opinion concedes that a fence would cause "stress" to the jaguars. "So, jaguars may walk around the fence, but illegal aliens won't?" McCain wryly notes.

 Even if the Fish and Wildlife Service were to object to the border fencing, Homeland Security could trump any effort to derail construction. The federal Real ID Act of 2006 allows the agency's secretary to waive environmental laws in the name of national security.

 Worshipped as a god of power, mystery and stealth by pre-Columbian Aztecs and Mayans, the jaguar is exceeded in size among felids only by tigers and lions and so is the largest wild cat in the Americas. The muscular, low-slung predator possesses a lethally powerful jaw. In the jungles of Central and South America, where prey is plentiful, jaguars can weigh in at 350 pounds. Males in the arid climes of northern Mexico and the U.S., where they prey primarily on javelina, peak at about half that size.

 Jaguars (Panthera onca) now range from the Southwest U.S. all the way past the Amazon Basin. But their habitat once included a good portion of the Southwest United States. The U.S. Fish and Wildlife Service says the jaguar's historic range stretched from California to Louisiana. Conservationists claim the big cat's turf was much broader - as far north as Canada and as far east as the Carolinas.

 In any case, the arrival of European settlers squeezed the cats' habitat and marked them as threats to livestock. Government programs that employed hunters and a lethal array of poisons hastened the demise of the jaguar and other now-endangered species such as the grizzly, three species of wolf and the lynx, says Michael Robinson, a conservation advocate for the Tucson-based Center for Biological Diversity. Although they posed no threats to livestock, black-footed ferrets and California condors were indirectly decimated by poisoning programs, says Robinson, whose 2005 book, Predatory Bureaucracy: The Extermination of Wolves and the Transformation of the West, definitively plumbs the extent of the government's 92-year-old predator-control effort.

 A recent study indicated that jaguars occupy less than half of the range they inhabited in 1900, with an estimated 10,000 jaguars believed to survive. Fish and Wildlife acknowledged in 2006 that five "transient male jaguars" - Macho A and Macho B included - had been documented in the United States in the preceding decade.

 McCain says the jaguar's range will no longer include the United States if fences are built at the pace and to the extent he fears. "Any single segment of the border fence, such as the current seven-mile section near Sasabe, should not have devastating effects on the jaguar by themselves," says McCain via e-mail. "However, when combined with the resulting redirected immigrant and law enforcement traffic into the remaining wild corridors, these projects will have huge negative impacts on jaguars."

 And, he says, fence construction is unlikely to stop. Once the Department of Homeland Security realizes illegals are simply skirting the new fence, the barriers may be expanded farther into the rugged mountains bisected by the border.

 "This pattern could eventually fence the entire border and completely partition the range of the jaguar and prevent gene flow in an already small population," McCain says. "Since no known breeding has occurred in the U.S. since the early 1900s, and no females have been documented since 1963, this will be the end of naturally occurring jaguars in the United States."

 Daniel Patterson is Southwest director and ecologist for Public Employees for Environmental Responsibility, a private group that advocates for professionals who work for government agencies charged with enforcing environmental laws. Patterson calls the Fish and Wildlife opinion "very disturbing. The border walls would be a huge problem. People have largely been celebrating the slow return of the jaguar to Arizona and New Mexico. The political people at the Fish and Wildlife Service seem to be willing to write the jaguar off. The Fish and Wildlife Service once again is not enforcing the Endangered Species Act."

 Patterson, a former Bureau of Land Management biologist who lives in Tucson, contends that morale at Fish and Wildlife and other land and wildlife agencies is dismal due to politicization of enforcement and ignorance of the law. But Fish and Wildlife Service spokesman Jeff Humphrey says Homeland Security is concerned about the fate of the jaguar, noting that the opinion lists conservation measures DHS will take to "offset" the fence's impact on jaguars and other species. Among them is the installation of additional cameras and sensors in areas adjacent to the new fencing. The cameras would monitor any traffic - illegal crossers or jaguars.

 Additional "specific jaguar conservation measures" will be developed over the next four months, says Humphrey, adding that the government and jaguar aficionados alike "stand to learn a lot" from fence construction. "The concern is more than just the direct effect of that fencing, but the flow of foot traffic around the ends of the fences and into pristine habitat," Humphrey says. "Is the fence funneling people and wildlife into a more confined area where there's going to be more interaction?"

 McCain and Childs say that's already happening and is certain to worsen. After the temporary vehicle barriers replaced a common four-strand barbed-wire fence along some border stretches, the researchers noticed a surge in human traffic in more remote areas spanning the boundary - areas used by Macho B. Vegetation is trampled and the formerly pristine areas are increasingly littered with trash and human waste. More of the border jaguar project's cameras have been stolen or destroyed, Childs says.

 As for the mood of government biologists, Humphrey says, "Natural resource managers are always wishing we could do more with less. What the Endangered Species Act and Congress charged us with is to make a determination on whether or not a project jeopardizes a specific species. In this case, we have a species that occurs intermittently in Arizona, but its range extends all the way to Argentina. In our evaluation, the fence project did not exceed the jeopardy threshold for jaguar."

 Brad Benson, a spokesman for U.S. Customs and Border Protection, insists that fears of uninterrupted fencing are unfounded. "The mountain ranges where I think the jaguars are spending most of their time are still going to be available to them" for border crossing, Benson says.

He says the plan is to leave more remote areas unfenced and allow new cameras and ground radars to monitor wilder areas and identify what's coming across. If it's people, the gadgetry will allow them to be tracked and intercepted without necessarily sending agents and their vehicles into environmentally sensitive areas. If it's jaguars or other wild animals, he says, the information will be shared with Fish and Wildlife.