The dog is one of the world’s most destructive mammals. Brazil proves it.

RIO DE JANEIRO - High above this Brazilian city, in a jungle blanketing a mountain, the turtles were out and the scene was hopeful.

Scientists were reintroducing 15 mud-caked tortoises to this urban forest where they had once been plentiful. Children were running around. People were oohing and aahing. A stern-looking security guard briefly appeared to smile.

But not government biologist Katyucha Silva. She was thinking about dogs.

What would they do to these turtles? What were they doing to Brazil?

It's a question more researchers are beginning to ask in a country where there are more dogs than children - and where dogs are quickly becoming the most destructive predator. They're invading nature preserves and national parks. They're forming packs, some 15 dogs strong, and are hunting wild prey. They've muscled out native predators such as foxes and big cats in nature preserves, outnumbering pumas 25 to 1 and ocelots 85 to 1.

Every year, they become still more plentiful, spreading diseases, disrupting natural environments, goosing scientists who set up elaborate camera systems to photograph wild animals, only to come away with pictures of curious canines.

"It's a difficult thing for people to hear," said Isadora Lessa, a Rio de Janeiro biologist who wrote her doctoral dissertation on domestic dogs causing environmental mayhem. "They love dogs too much."

How the dog became one of the world's most harmful invasive mammalian predators is as much a global story as a Brazilian one. Over the last century, as the human population exploded, so did the dog population, growing to an estimated 1 billion.

That has been great for people - and even better for dogs - but less so for nature, according to a growing body of academic research implicating canines, particularly the free-roaming ones, in environmental destruction.

"The global impacts of domestic dogs on wildlife are grossly underestimated," researchers concluded in a 2017 study published in the journal Biological Conservation. The researchers, based in Australia, convicted dogs in the extinction of 11 species and declared them the third-most-damaging mammal, behind only cats and rodents.

The International Union for Conservation of Nature maintains a list of animals whose numbers dogs are culling. There are 191, and more than half are classified as either endangered or vulnerable. They range from lowly iguanas to the famed Tasmanian devil, from doves to monkeys, a diversity of animals with nothing in common beyond the fact that dogs enjoy killing them. In New Zealand, the organization reported, a single German shepherd once did in as many as 500 kiwis - and that was the conservative estimate.

"Unfortunately, we have a big problem," said Piero Genovesi, chair of the agency's invasive species unit. "There is a growing number of dogs."

People all over the world are - begrudgingly - beginning to take note.

In Chile, stray dogs were the top concern among city dwellers surveyed this year, topping deteriorating sidewalks and theft. In New Zealand, some communities moved last year to restrict the movement of dogs in a gambit to save little blue penguins. In India, farmers are complaining about stray dogs killing their livestock, just as other predators once had.

And in Brazil, atop a mountain outside of Rio de Janeiro, 15 tortoises were nestling into the forest floor, oblivious to the danger of the forest's leading predator.

Brazil is home to an estimated 52 million dogs, according to the most recent government statistics - more than anywhere in Latin America - but their lives vary widely. In a nation defined by inequality, where the rich fly in helicopters overthe poor in the favelas below, the dog has become one more way of understanding the divide.

In wealthy cities, the dog is everywhere, strolling through fancy shopping malls, sitting in the laps of restaurant patrons, even riding paddle boards out on the surf. Some people wheel their dogs around in little strollers.

"The dog brings to Brazilians some things that Brazilians appreciate in themselves," said Alexandre Rossi, a television personality more commonly known as Dr. Pet. "To be friendly, to want to socialize with everyone . . . and be there and be close to your family. These are perceived as very good Brazilian qualities."

On the streets of trendy Ipanema one recent afternoon, few people could believe that a dog - or at least their dog - could be much of predator.

"The dog is a friend!" sputtered Philipe Soares, the furball Bobby at his feet. "No, I've never thought of him that way."

"Difficult to imagine," said Carlos Alberto Vicente, peering down at his own pooch.

"In her case," said Flavio Vilela, a shirtless man striding through a park with a small mutt named Nicoli, "they'd hunt her."

The problem, researchers say, isn't these dogs, who lead the coddled lives of European or American pets.

The problem is the dogs in poorer and more rural communities, where the life of the dog is more frequently the life of hunger. They prowl the streets day and night with neither a collar nor an owner, looking for food wherever it can be found - in trash heaps, alongside roads, and in forests and fields, where they form packs to hunt and kill.

"It's a very complex problem," said Silva, the government biologist.

A stunning discovery

Ana Maria Paschoal, a researcher at the Federal University of Minas Gerais, remembers when she first started thinking about the dog differently. She was out in the Atlantic Forest in Southeast Brazil around a decade ago when she noticed there were an awful lot of them.

She wondered: How many dogs are using the protected areas? Are these feral or domestic dogs? Is their presence changing the occurrence of wild species?

So she set up cameras across 2,400 acres of forest to find out. What she discovered, published in 2012 in the scientific journal Mammalia, stunned her. The dog wasn't just the most-recorded carnivore; it was the most-recorded animal of the 17 mammals the cameras captured.

"The presence of the domestic dog is a threat," Paschoal and her co-authors concluded.

The research, subsequently confirmed in a larger survey, laid the groundwork for a growing field of study here. One researcher linked Brazil's dogs to the spread of diseases. Another accused the dogs in the National Park of Brasilia, where they hunted in massive packs, of scaring off natural predators. It was found that the closer humans lived to a nature preserve, the more likely dogs had penetrated it.

But perhaps most striking? The dogs were neither feral nor domestic, but somewhere in between.

"All the dogs we detected had an 'owner' or a person that the animal has a bond with," Paschoal said. "The species population increases following human populations, exacerbating their potential impact on wildlife."

It was something Fernando Fernandez, an ecology professor at the University of Rio de Janeiro, learned the hard way. For the last decade, he has been reintroducing native animals to the Tijuca forest, one of the world's largest urban woodlands, which spills across Rio de Janeiro's mountains.

First came the agouti, a squirrel-like rodent. Then followed a problem: "Dogs."

They started killing the agouti, and not for food. It was just for fun.

Fernandez and Silva wanted to learn more. They set up cameras and discovered dozens of dogs in the forest. They estimated more than 100 dogs were in the park - not residents so much as frequent visitors, tracking in from nearby favelas.

"These are people who are very poor," said Silva, who has six dogs at home. "They don't have money to build walls . . . When the owners leave for work, the dog leaves, too, and only returns when the owner comes back to the house from work."

The owners often have no idea what their dogs are up to. Even if they were told, Rob Young said, they almost certainly wouldn't believe it.

Young, chairman of wildlife conservation at the University of Salford in Britain, witnessed the psychology at work after seeing dogs kill flightless birds in the state of Minas Gerais.

"We'd do interviews with the farmers: 'Have you seen these dogs?'

"And they'd say, 'Yeah, but my dogs aren't the problem; it's my neighbor's dogs.'

"Every farmer would say the same thing."

These factors - inability to see aggression in dogs, intractable inequality, the rapid expansion of humanity - left Silva feeling apprehensive as she watched the tortoises being reintroduced into the Tijuca forest.

In the long term, she didn't know how the problem of dogs laying waste to the world's environments would realistically improve.

And in the short term: Could dogs kill these tortoises, just as they'd dispatched a few agouti?

“Yes,” she said. “They could.”