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Cougars that get too close to humans, like this one in Washington State, are trapped and released into remote areas. Mark Mulligan / The Daily Herald, via Associated Press

The great migration began perhaps 40 years ago. From strongholds in the United States in the Rocky Mountains and Texas, young males headed east, seeking female companionship and new places to settle.

The emigrants were about two meters long, nose to tail, and weighed up to 70 kilograms. They preferred deer, but would eat almost anything that moved: elk, bighorn sheep, wild horses, beaver. Left free for an evening, they were capable of killing a dozen domestic sheep. They were also known to attack humans on occasion.

Long ago the Inca called these animals puma, but today everyone knows them as cougars.

Until relatively recently, they were mainly a memory: They were all but exterminated east of the Rockies by 1900.

But today the cougar is back. That is one of the great success stories in wildlife conservation, but also a source of concern among biologists and other advocates, for their increasing numbers make them harder to manage - and harder for people to tolerate. No reliable estimate exists for the cougar population at its lowest point, before the 1970s, but there are now believed to be more than 30,000 in North America. And as cougars migrate eastward, they are likely to become unwelcome. People in states unaccustomed to these outsize prowlers will have to answer unpleasant questions: How many livestock and game animals are people willing to lose? How dangerous are cougars to pets and children? How much disruption is a community willing to endure?

"A lot of state conservation agencies are looking into how to prepare for recolonization," said Clay Nielsen, the director of scientific research for the Cougar Network. Surveys he conducted in Illinois, North Dakota and Kentucky found "the public more supportive than I would have guessed." But as the big cats become more plentiful, he added, "attitudes are probably going to change."

The center of cougar genetic diversity is in Brazil, but the Western Hemisphere has six robust subspecies in all.

Melanie Culver, a wildlife geneticist at the University of Arizona, says the cougar appears to have evolved about 300,000 years ago from a cheetahlike cat that is now extinct. When Europeans arrived in the Americas, cougars were everywhere, but human predation and the loss of habitat to agriculture took a heavy toll.

Cougars are solitary predators whose hunting ground can vary widely in size, depending on available prey, water supply and cover. They like woodland and high country, but can handle almost any habitat that offers concealment.

Cougar offspring stay with their mothers up to two years. After that the young males tend to disperse, partly to avoid other males in their home territory and partly to lower the odds of inbreeding. After cougars filled up the mountain states and West Texas, the young males began to travel east. (Females also move, but tend to stay closer to home.)

Jw Nuckolls, a rancher in Wyoming, lost 15 sheep one night to a single cougar.

During an aerial survey at the Kofa National Wildlife Refuge in southwestern Arizona in 2000, three cougars were spotted on a stone outcrop, recalled Susanna Henry, the refuge manager.

"In the following years the population of bighorn sheep at the refuge began to decline precipitously, from 800 at the turn of the century to 620 in 2003 and 390 in 2006," Ms. Henry said.

Despite their propensity to wreak havoc on other wildlife and livestock (they will take on animals up to seven times their own size, including elk, horses and steers), cougars are regarded as a manageable nuisance by ranchers and offered a respect that wolves, the West's other legendary marauders, can only dream about.

There is no easy explanation for this. Dr. Nielsen noted that Europeans had no experience with big cats when they arrived in the New World, but had long vilified the "big bad wolf." Wolves, he said, "had a bad rap."

Ogden Driskill, a northeast Wyoming cattle rancher, offered a simpler explanation.

"Cougar are easier to hunt" than wolves "and easier to control," he said. Cougars run from wolves and will run from barking dogs. Hunters use hounds to tree them. They are predictable, while wolves are not.

But if cougars are easier to control now, "things will change," said Harley G. Shaw, a retired wildlife biologist in Arizona. "That time may even be here now."

The New York Times

(China Daily 07/07/2013 page11)