Behind a heavy metal door at The Maryland Zoo, some of the world’s few remaining Panamanian golden frogs luxuriate in a paradise of aphrodisiacs — at least for amphibians.

Waterfalls trickle through plastic vines, thickening the air with humidity. Moisture beads on shaggy moss. Fruit flies, crickets and other delicacies abound.

Snuggled in moss on a winter morning, Frog 9134 balances Frog 9030 on her back, the beginning of a mating ritual that will end with her ejecting a mass of eggs for him to fertilize. If all goes well, tadpoles will squiggle away in about two weeks, carrying their species’ hope for survival in their tiny bodies.

For more than two decades, The Maryland Zoo in Baltimore has functioned as a sort of Noah’s Ark for the bright-yellow-and-black tree frogs, which were wiped out in the wild by a fungus that raced across the globe in the early 2000s.

As the fungus approached Panamanian cloud forests, scientists sent golden frogs and other threatened amphibians to zoos around the world, hoping the creatures would breed in captivity and their descendants could one day repopulate their native home.

The Maryland Zoo has been so successful in breeding frogs that it now hosts the world’s largest population of golden frogs outside of Panama, according to Heidi Ross, who leads the project at Panama’s El Valle Amphibian Conservation Center, or EVACC.

This year, for the first time, zoo veterinarians plan to send some of Baltimore’s 300 golden frogs back to Panama to live in a facility where they will be exposed to the elements but protected from the fungus.

Scientists hope the frogs — who were born in captivity and have never been outside — will be able to feel the sun and rain on their tender skin.

“The plan was never to keep them for us,” said Ellen Bronson, the zoo’s head veterinarian. “The plan was always to safekeep them and send them back to Panama.”

If the experiment fails, the fungus will slip into the facility and kill off the frogs, which are about the size and weight of a dollop of whipped cream.

But if scientists succeed, they will be one step closer to releasing the frogs back into the forests of their ancestors — and charting a pathway to saving other vulnerable species from extinction.

‘It changed the sound of the forest’

When Ross first came to Panama’s El Valle region around 2000, golden frogs were so plentiful she had to be careful not to step on them when exploring the area’s misty forests.

“You could see hundreds,” said Ross, describing male frogs trilling along streambeds during mating season. “They almost look fake, these orange-yellow gorgeous little frogs.”

The frogs’ population was already in decline due to habitat destruction. But their greatest enemy was spreading invisibly.

Scientists aren’t certain just where and when the Batrachochytrium dendrobatidis fungus, commonly known as chytrid, emerged, but the first outbreaks were observed in 1998 in Australia and Central America.

“There had been reports from all over the world about frogs mysteriously disappearing,” said Vance Vredenburg, a San Francisco State University professor who was studying yellow-legged frogs in the Sierra Nevada mountains when the fungus swept through. “I saw tens of thousands of frogs die at our study site.”

For frogs, skin is not just a protective layer but a membrane that enables them to breathe and also exchange chemicals in water. The chytrid fungus causes the skin to thicken and become less porous, essentially suffocating the frog.

At least 500 of Earth’s 9,000 species of amphibians have been severely harmed by the fungus, and more than 200 species have been driven to extinction or near extinction, Vredenburg said.

“This has been the worst disease caused by a pathogen we’ve ever seen in recorded history for vertebrates,” Vredenburg said. “We’re just lucky it doesn’t affect mammals.”

Amphibians in cloud and rainforests are especially susceptible.

“It changed the sound of the forest,” said Ross, of Panama’s EVACC. “You used to go out at night and hear the frogs calling. Now you don’t hear anything.”

In the past, there was little that scientists could do to protect an animal facing extinction. Veterinarians watched helplessly as the last passenger pigeon died in 1914 at the Cincinnati Zoo.

By the early 2000s, ecologists and biologists had adopted strategies to protect creatures at risk. As the chytrid fungus encroached upon Panama’s cloud forests, scientists filled two hotel rooms with vulnerable amphibians.

Ross and her husband, Edgardo Griffith, EVACC’s founder, initially attempted to protect some 60 frog species, but eventually decided to focus on the golden frog.

They maintain about 600 frogs at EVACC’s headquarters in the mountains of El Valle and have sent others to zoos in Denver, Atlanta, and Rochester, New York.

When Maryland Zoo veterinarians brought golden frogs back with them in 2001, they faced a daunting task: keeping them alive and breeding them in an environment far from home.

A frog stud book

On a recent morning, the zoo’s chimpanzees rushed into their indoor play area, hooting, banging on windows and swinging on vines.

Across from them, a half-dozen golden frogs clung to mossy rocks and tree branches inside a glass enclosure. A portion of each zoo ticket sold goes to efforts to preserve the frogs.

When zoo workers first brought golden frogs to Maryland in 2001, scientists knew little about the creatures’ needs, Bronson said.

“There was a lot that needed to be learned so we could breed them successfully and keep them well and healthy,” she said.

The zoo’s veterinary hospital, a brick building set apart from visitor areas, is home to most of the frogs. The population of a few dozen frogs originally brought back from Panama has now swelled to about 300, Bronson said.

Since the chytrid fungus has spread throughout the world — including here in Baltimore — workers must dip their shoes in a disinfectant solution before entering the frogs’ quarters.

The zoo maintains three separate subpopulations of the frogs, whose ancestors came from different regions in El Valle. Workers keep a detailed “stud book” with each frog’s lineage to determine how to mate frogs for the most genetic diversity.

“A stud book is Match.com for breeding animals,” Bronson said. “We know the parents, the grandparents, the lineage. They can be repatriated back to Panama in a genetically sound way.”

Through trial and error, zoo workers have roughly re-created the frogs’ native habitat. Timed UV lights mimic sunlight. Mechanical waterfalls trickle through the terrariums; the vibrating water seems to be a mating cue, Bronson said.

One major difference from the frogs’ lives in the wild is diet. In the cloud forest, the frogs accumulate toxins from insects, making them poisonous. The zoo frogs eat harmless insects, enabling them to be safely handled.

When a female’s eggs are approaching maturity, zoo workers will place her in a terrarium with a specially selected male, said Jenny Egan, the zoo’s curator of reptiles and amphibians.

The male then hops onto the female’s back, where he will stay for about a week, in a process called amplexus. “She goes where she wants and he also goes where she wants,” Egan said.

The frogs take about two years to become fully mature. While it’s unclear how long they live in the wild, some have lived more than 20 years in captivity, Bronson said.

‘If I were a frog, I would rather live there’

Back in the cloud forests of central Panama, Ross and her team have been constructing a space that they hope will expose frogs to the elements while protecting them from chytrid.

Mosquito netting should keep out predators and other amphibians who could spread the fungus. But the sun and rain will pour down.

“There’s a lot more moss, more plants,” Ross said. “If I were a frog, I would rather live there.”

The Maryland Zoo team is eagerly awaiting approval from the United States Fish and Wildlife Service to send the frogs to Panama. Then the team will fly the frogs south.

“It’s the first step towards reintroducing them into the wild,” Bronson said.

Vredenburg, the expert on frogs and chytrid, said it’s possible that the fungus has grown less virulent over time and some of the frogs could now survive exposure.

It will likely take several trials before the frogs can be successfully reintroduced to the wild, Vredenburg said. He’s hopeful that scientists will ultimately be able to keep the species alive.

“Amphibians have been around for over 390 million years,” he said. “They made it through the last four mass extinctions on Earth. They are resilient in the right circumstances.”