Using the same clandestine routes as drug smugglers, male jaguars are crossing into the United States from Mexico.
Four of the elusive cats have been photographed in the last decade -- one as recently as last February -- in the formidable, rugged mountain ranges of southeastern Arizona and southwestern New Mexico.
And while no one knows exactly how many jaguars are here, or how long they hang around before sneaking back to their breeding grounds in Mexico, their presence has set off repercussions on both sides of the border.
At least 10 organizations are working to protect the jaguar in one or both countries. Conservationists are developing incentives to stop bounty hunters in Mexico from killing the big cats. Cameras have been set up near the border to monitor jaguar comings and goings and, inadvertently, the movements of ''mules,'' or drug runners.
Some environmentalists are pressing federal officials to declare parts of Arizona and New Mexico critical habitat for jaguars. But local ranchers and many jaguar experts say such a move is unnecessary because the animals show no signs of breeding here.
And then there is the fence. If the Border Patrol builds a 700-mile barrier in the region to deter illegal immigration, the natural corridors used by jaguars and other migratory wildlife will be cut off.
Jaguars are the largest native American cat. They once roamed much of the Southwest, but when ranchers took cattle to the region in the last century, the jaguars were trapped and hunted to extinction in the United States. The last known resident female was killed in 1963 near the Grand Canyon.
Jaguars were thought to be gone from the Southwest until Warner Glenn, a cattle rancher and mountain lion hunter, saw a live one in the Peloncillos Mountains, near the New Mexico border with Mexico, on March 7, 1996.
''I thought the dogs had treed a lion, but when I went to look, it was a jaguar,'' Mr. Glenn said in an interview at his Malpai Ranch near Douglas, Ariz., where his office is decorated with poster-size photos of jaguars and the occasional plastic jaguar figurine.
Mr. Glenn raced back to his mule, pulled out a camera and snapped what seem to be the first photographs of a live jaguar in the United States, ever. Other known photographs show jaguars that were already dead. Sadly, the same cat was killed a few months later by a federal police officer in Mexico, 30 miles from the United States border.
Working with conservation groups, Mr. Glenn helped place trip cameras in the Peloncillos in 2001. But within a few years, he said, four of the cameras disappeared, apparently taken by drug runners. ''And we never got a picture of another jaguar,'' he said.
That changed last Feb. 20, when Mr. Glenn photographed his second jaguar while on a lion hunt in the Animas Mountains.
Shortly after lunch one of the hunting dogs, Powder, disappeared. ''Then one of the cowboys found him and said he has a huge hole in his neck and shoulder,'' Mr. Glenn said. ''Something had pounded the pudding out of Powder. I thought it might be a feral hog or a boar javelina. It couldn't be a lion. They don't mess with dogs.''
Picking up a scent, Mr. Glenn's five other hounds took off. He gave chase and soon got to within a hundred yards of the commotion. ''I looked up and in the shade of a big cedar tree, I could see a big cat, dark in the shade. I thought, they have a big tom lion,'' he said. ''I moved in closer. The cat charged the dogs. They scattered like quail. Then I saw it was a jaguar.''
Mr. Glenn grabbed his camera and started shooting.
The jaguar caught another dog, Copper, bit him on the back and released him. When the hound Rietta moved in, the cat grabbed him with one paw, then another, and delivered two quick bites in the rump.
''The jaguar could have easily killed the dogs,'' Mr. Glenn said. ''One bite to the head and they'd be gone. But he let them go on purpose.'' They were not seriously hurt.
Moments later, the rest of the hunting party arrived, helped gather the dogs and looked on as the jaguar looked back at them, struck a trot and left.
''He did not run,'' Mr. Glenn said. ''He was not afraid of anything.'' Later he estimated that the jaguar, by the look of his teeth, was eight or nine years old and weighed nearly 200 pounds. Mr. Glenn named the cat Border King.
Another jaguar is now being tracked and photographed in southern Arizona by Jack Childs, a rancher and lion hunter from Tucson. Mr. Childs first videotaped the animal, which he nicknamed Macho B, in August 1996 in the Baboquivari Mountains. It left the region that year but came back in 2004, where it now wanders along the border.
Like all jaguars, Macho B has distinct black rosettes on his golden fur. A spot on his right rib cage looks like Pinocchio and one on his left rib looks like Betty Boop, Mr. Childs said.
With support from the Arizona Game and Fish Department and other organizations, Mr. Childs now has nearly 50 trip cameras along the Arizona border with Mexico south of Tucson. Macho B has been photographed 52 times in the last two years. ''We're tracking him now,'' Mr. Childs said. ''He is at least 12 years old. We don't know what he does in Mexico, but we know for sure he goes. We have photographed him at the fence.''
A second cat, Macho A, showed up in 2001, was photographed first, hung around for three years and left about the same time Macho B came back, Mr. Childs said. No one knows what happened to him.
As in New Mexico, drug runners in Arizona use the same mountain routes where Macho A and Macho B have been photographed. Fourteen trip cameras have been smashed or taken in recent years, while six others have been lost to bears or flooding.
As for what Macho B does in Mexico, it is likely that he travels back to a jaguar breeding area 130 miles south of the border in a remote region of Sonora. In 2003, a Mexican conservation group, Naturalia, bought a 10,000-acre ranch called Los Pavos, put up a fence, removed cattle, hired jaguar guardians and established the first protected habitat for jaguars in northern Mexico. Naturalia estimates there are 100 to 150 jaguars in the region, including females and cubs.
Females tend to stay local, whereas male jaguars have wanderlust, said Dr. Alan Rabinowitz, a leading jaguar expert at the Wildlife Conservation Society in New York City. Males will migrate up to 500 miles, he said, spreading their genes as they go. Recent DNA analysis shows that because of such genetic mixing, jaguars from Patagonia to Mexico are a single species.
But the jaguars in northern Mexico are at the utmost edge of the animal's natural range, Dr. Rabinowitz said. The ones coming into the United States look like transients, which means it would be ''foolish'' to call them a resident population, he said.
The Northern Jaguar Project, based in Tucson, is a nonprofit organization helping Naturalia increase such jaguar protection in Mexico. The group hopes to raise $2 million to buy a 33,000 -acre ranch next to Los Pavos. The current owner unapologetically believes in killing jaguars.
The jaguar project is now in the process of setting up trip cameras in many more adjacent ranches. Ranch owners and their cowboys will be paid handsomely for each photo of a jaguar.
''Jaguar poaching is a serious problem in Mexico where laws are not enforced,'' said Peter Warshall, a biologist with the project. More than 20 jaguars have been killed in the past three years between the Los Pavos sanctuary and the United States border, he said, adding, ''We hope people can make more money off live jaguars than dead ones.''
In another approach, some Mexican ranchers in the area have formed hunting zones where jaguars are protected and deer are hunted by Americans happy to pay for the adventure.
Moreover, other wealthy Americans are buying ranches on the Mexican side of the border with the aim of protecting the natural corridors used by jaguars and other wildlife in entering the United States.
In all, almost a million acres in Mexico have come under some level of jaguar protection in the last couple of years, Mr. Warshall said, in a patchwork of conservation ranching.
And in New Mexico, Mr. Glenn and his neighbors formed the Malpai Borderlands Group, which has withdrawn an additional million acres from development through conservation easements and other agreements. The reason two jaguars were seen there, he said, is that the land is protected.
An environmental group based in Tucson, however, the Center for Biological Diversity, does not think enough is being done to protect the jaguar. When the United States Fish and Wildlife Service announced in July that it would not declare parts of New Mexico and Arizona critical habitat for jaguars -- arguing that the animals do not breed there -- the group filed an intent to sue. The matter rests in federal court.
Of course, if the Border Patrol built an effective barrier in the mountains where jaguars cross into the United States, ''it'd be all over,'' said Jon Schwedler of the Northern Jaguar Project. ''You could kiss the jaguar goodbye.''