IBERÁ NATIONAL PARK, Argentina — They had a big job to do, drafted as the first few jaguars to be reintroduced to Argentina’s wetlands after more than seven decades of absence.
But they were a troubled bunch.
Tobuna came from an Argentine zoo and was fat and lethargic, in the twilight of her reproductive life. Her daughter, Tania, had been hidden from view in the same zoo because a tiger had mauled one of her legs as a cub.
Nahuel required custom dental work to ease the exasperating toothache that made him constantly grumpy, and never in the mood to mate.
Then there was Jatobazinho, who had stumbled into a rural school in neighboring Brazil in 2017, severely dehydrated and famished, having lost the ability to fend for himself in a region where farmland gobbles up more jungle canopy each year.
“They all had pretty traumatic stories,” said Sebastián Di Martino, a biologist who oversees conservation projects at the Rewilding Argentina Foundation, an initiative to restore the health of the country’s ecosystems by reintroducing species wiped out by human activity.
But in the tough business of rewilding, where obtaining breedable animals is often costly and logistically vexing, beggars can’t be choosers.
So Mr. Di Martino was thrilled to get any and all jaguars for the most challenging phase of a yearslong effort to create vast wildlife sanctuaries across Chile and Argentina.
For these imperfect jaguars, most of whom came from zoos, the splendor of their new home, Iberá National Park, must have seemed like paradise, filled with quarry.
On a recent visit, packs of howler monkeys looked like acrobats as they swung from tree to tree, yelping loudly. Swamp deer and capybaras grazed placidly as storks cruised overhead.
The jaguars aren’t the park’s only meat eaters. When kayakers glide down narrow streams, they must navigate around stoic caimans, absorbing the waning afternoon rays.
The idea of rewilding jaguars grew out of a project of Kristine and Douglas Tompkins, who ran the outdoor equipment and clothing companies Patagonia and the North Face before turning full time to environmental causes.
In the 1990s, they began snapping up millions of dollars worth of land in the Southern Cone of South America. The goal of the American couple (Mr. Tomkins died in 2015) was to acquire the first building blocks of what would eventually become national parks.
But it struck them early on that simply halting the degradation of forests felt insufficient.
“Landscape without wildlife is just scenery,” Ms. Tompkins heard someone say soon after she and her husband bought an old cattle ranch in Argentina’s Corrientes province in 1998, which later became part of the Iberá park, tucked in the country’s northeast corner. “For us it was an epiphany and an opportunity.”
Across the Southern Cone, which includes Brazil, ecosystems are perishing at a staggering rate. Loggers, miners and farmers raze vast areas of the Amazon and other biomes each year, turning more and more emerald green canopy into grasslands.
The enormous scale of destruction across the region can make even Iberá, and its some 5,000 square miles of swamps and lakes, feel like a very small-scale utopia in comparison.
And bringing back jaguars to this bucolic landscape seems like just a tiny advance against an overbearing current.
The difficulty of making a difference is not lost on the conservationists who spend their days and nights at the remote sanctuary obsessing over how to get the jaguars, giant river otters and giant anteaters to mate — and ultimately to survive on their own.
But it’s a challenge they’re willing to accept.
“We can’t just be in the trenches resisting,” Mr. Di Martino said. “Now more than ever we have to go beyond conservation and restore, which means going to battle.”
The battlegrounds the Tompkinses picked have at times been rather hostile. As they began acquiring land, they were often greeted with suspicion.
In Corrientes province, some warned that the American couple would bottle up the area’s spring water and leave behind a parched wasteland.
“There were rumors they were going to take all the water all the way to the United States,” said Diana Frete, a deputy mayor of Colonia Carlos Pellegrini, a tiny town that serves as a gateway for the wetlands. “There were a lot of doubts and distrust.”
But the naysayers were proved wrong as the conservation efforts at Iberá, and the buzz surrounding the jaguars’ return, transformed the park into an emerging tourist destination.
“This was a town where everyone used to leave,” said Ms. Frete, noting that now some 80 percent of her constituents work in tourism. “Today, we’re better off tying our fate to protecting nature.”
Jaguars were a dominant predator in North and South America for millenniums, and played a vital role in keeping ecosystems in harmony.
But the conversion of land to agriculture over the past two centuries drove jaguars to extinction in several of their former domains, including Corrientes province. With just 200 wild jaguars estimated to be remaining elsewhere in Argentina, the majestic cats are critically endangered.
That vulnerability contrasts starkly with the way potential prey feels in the presence of an individual jaguar. When at ease, the animals move in assured, bouncy steps that feel a bit like a dance. But a swoosh of the claws and a guttural roar inspire terror.
“I feel so tiny around them, and I like that feeling,” Ms. Tompkins said. “I love feeling that I’m not at the top of the food chain, it’s almost like a shuddering in my chest.”
Ms. Tompkins said that by reintroducing the imposing cats alongside giant river otters, giant anteaters and red and green macaws, she wants to demonstrate this form of conservation is not only possible but scalable.
Yet it takes plenty of human meddling to begin to return places like Iberá to a state closer to what they were like before humans spoiled them.
Mr. Di Martino leads a group of dozens of biologists, veterinarians and volunteers who have spent untold hours in recent years coaxing the jaguars to mate — from a safe and respectful distance.
Before they are fully released back into the wild, the jaguars are being kept in large, enclosed pens where their hunting skills, and sexual arousal, are tracked by a web of security cameras.
Setting the right mood for sex between jaguars requires a long and complicated courtship.
Females in heat are placed in pens adjoining a male prospect, allowing the biologists to glean whether their body language is conveying aggression or desire.
“When there is interest, the female starts rolling on the ground and scratching the earth,” said Magalí Longo, a biologist who monitors the animals on television screens showing live surveillance footage. “That’s when you know she’s game.”
The first major reproductive breakthrough came in 2018 when Tania, the jaguar missing a leg, gave birth to two cubs. Along with Jatobazinho, the Brazilian cub who nearly starved, they’re among the five jaguars whose ability to hunt on their own is being assessed by biologists.
This assessment requires releasing the animals into ever larger pens, where instead of slabs of meat, they’re given live prey — including wild boar and capybaras — to sharpen their survival skills. If all goes as planned, the project hopes to release the first jaguars fully into the wild later this year or early in 2021.
If they do well, Iberá could be home to a population of some 100 jaguars in the next few decades. The prospect makes Ms. Longo beam.
“We’re fixing the damage we’ve done, and it feels great to start seeing results,” she said. “We’re working to make our jobs extinct, but that’s a good thing.”