Working for a decade, almost entirely out of public view, staffers at the Maryland Zoo in Baltimore have made the institution the nation’s largest breeder and shipper of the endangered Panamanian golden frog.

On Tuesday, 25 of the tiny, yellow-and-black amphibians were packed with wet paper towels in 13 pint-sized deli cups. The cups were set into a comfy nest of crumpled newspapers inside a Styrofoam box labeled “Live Amphibians!” The frogs were then driven to BWI-Marshall Airport for a 10-hour trip to the Fort Worth Zoo, in two hops on Delta Airlines jets.

There’s another shipment of 20 frogs flying out later this week to the Oakland Zoo in California. And 10 more shipments are planned as the zoo works to save a species that is now likely extinct in its native Panamanian cloud forests, a victim of a deadly fungus.

The Maryland Zoo’s work is part of a wider effort, coordinated by the Association of Zoos and Aquariums, to propagate the frogs in captivity so that their descendants might one day be returned to the wild in Panama.

Since 2001 several thousand Baltimore-bred golden frogs have been sent off to live and breed in other institutions.

Last December alone, more than 600 young were hatched in tanks in the zoo’s hospital building; 317 of them were produced by a single pair of frogs. The more typical hatch rates are 10 to 70 per pair.

“We’re still trying to figure out the mystery of how that pair was able to hatch that many,” said keeper Kat Mantzouris.

All the mating pairs were kept under the same conditions, she said. But “it appears that more of their eggs were successful. So we’re going to repeat it again this year and see how we do.”

Hundreds of juvenile “morphs” have spent the last six months growing in racks of small terrariums, hopping on beds of soggy sheet moss, gobbling up springtails and crickets, and hiding under pottery shards and tents of live pothos leaves. About 250 adults are housed in larger tanks nearby.

Mantzouris has clearly been charmed. “They have so much personality,” she said. “I love the waving. I love the calls. And they’re really pretty.”

The “waving” is a language of “hand” motions the frogs evolved to communicate amid the din of falling water in Panama that drowns out their chirps.

Zoo visitors have been charmed, too. Children like to gather around to watch and count the 16 frogs housed in a permanent public exhibit that opened earlier this year in the Chimp Forest building.

Visitors can also find the golden frogs at the National Aquarium in its Upland Tropical Rainforest. The aquarium participates in Project Golden Frog, a conservation, research, and education initiative.

Maryland Zoo veterinarian Allison Wack said the Panamanian golden frogs once were abundant in Panama. A national icon and a symbol of good luck, their image appears on the country’s lottery tickets, in hotels and gift shops.

But today they’re absent in the wild, reduced by habitat loss, toxins and over-collection, then wiped out by the deadly chytrid fungus that has affected 30 percent of the world’s amphibian species.

The Maryland Zoo, led by Anthony Wisnieski, then curator of reptiles and amphibians, began working with other zoos and universities in the 1990s to study the frogs before the chytrid fungus reached Panama. They later obtained the U.S. and Panamanian permits needed to import and breed the golden frogs in captivity.

The Maryland Zoo received its first frogs in 2001. And, under terms of the U.S. permits barring their sale or transfer, the zoo still holds title to all but a handful of those now held legally in the U.S. and Canada, said assistant curator of amphibians, Kevin Murphy.

“Golden frogs are an extremely challenging species to deal with,” Murphy said. Although zoos in Detroit, San Diego, New York and elsewhere have breeding programs, none has had success like Baltimore’s.

“One of the biggest changes we made was not putting the males and females together until the females were really big with eggs,” Wack said. Familiarity, it seemed, dulled the frogs’ interest in mating, probably a defense against in-breeding.

Now we keep them all in separate cages, male and female cages, until it’s time to mate,” Wack said.

They also discovered the frogs like running water in their tanks. So the adult tanks have a steady trickle of water falling through the lid. Temperatures are kept near 70 degrees, like Panama’s mountain forests.

The frogs breed only once a year, still in synch with their reproductive cycle in Panama. “They have some sort of biological cue,” Mantzouris said. “Around this time of year, you hear them start to call, and the females build up eggs.”

Soon, the males and females will be brought together in honeymoon tanks.

“They’ll go underneath a rock, way back where it’s dark and protected,” Mantzouris said. The females will release up to 700 eggs; the males will release their sperm, and if all goes well the fertilized eggs will hatch tadpoles 10 days later. And in three months the young frogs will climb onto a solid surface.

“We do not have everything nailed,” Murphy said. “We still struggle with keeping all the tadpoles alive. … But we definitely have made some great strides and continue to change things as we go.”

To protect the captive frogs’ genetic diversity, the AZA keeps a “studbook.” Every adult golden frog is given a number, has its skin markings photographed, and carries with it a genetic pedigree and health record.

Kevin Barrett, manager for the zoo’s golden frog program, explained that some frogs shipped from Baltimore are for display purposes only. They’re not approved for breeding.

But as he plucked Fort Worth-bound frogs from their tanks and placed them in the deli cups for shipment, he said, “These are … part of a species survival program, so these animals will be going to Fort Worth for future breeding.”

Before any of the frogs are shipped out, they undergo a health exam. Wack checked out the Fort Worth group late last week.

One by one, Mantzouris removed a frog from its tank, and held it gently between her fingertips. “They can hop out of my hand,” she said, “but I’ve had a lot of practice.”

Wack focused her ophthalmoscope’s bright light on one frog. Through the eyepiece, she examined its skin, its corneas, and deep inside its eyes looking for retinal problems.

She looked for skin blemishes and irritation, but found only a slight pink flush that she said indicates only agitation at being handled. The animal’s belly skin is translucent, giving Wack a view of some internal organs and, if it’s a female, its developing egg mass.

She checked for missing digits or other malformations. Sometimes a radiograph will reveal bone density problems, and fecal studies will turn up intestinal parasites. But if they pass her inspection, the frogs are given health certificates that travel with them.

“This frog looks healthy and good for shipment,” she concluded, and moved on to the next.

Murphy said the frogs’ story should be a wake-up call to anyone who has listened to the tree frogs and spring peepers in their own neighborhood, and cares about healthy ecosystems.

“If people know about the golden frogs, and know what kinds of things have happened, and think about the amphibians in their own backyard,” he said, “they might care about them and might protect them in some way.”

The zoo is not breeding other amphibian species threatened by the chytrid fungus. But Murphy hopes Maryland’s success will one day enable the zoo to supply healthy frogs to a breeding program in Panama, and help conserve declining Maryland species such as the barking tree frog.

Scientists elsewhere are also at work looking for ways to combat the chytrid fungus, or to identify and exploit natural resistance factors found in some surviving species. “Those are going to be the future,” Murphy said.

On Tuesday, after the Maryland Zoo’s frogs were packed, it was Bill Walters’ job to get them on their flight to Fort Worth.

Walters, 61, took the 5-pound box of frogs from Barrett, tucked it behind the passenger seat of the zoo’s white panel truck, and headed for BWI-Marshall Airport.

“I do most of the shipping,” he said. He hasn’t lost a frog yet. He’s also helped arrange transport for penguins, exotic ducks, herons, cranes, lions, chimpanzees and many more species. The folks at Worldwide Flight Services at BWI know him well.

“More frogs!” Walters announced as he set the frog box on the counter of the shipper’s cinder-block office.

“They ain’t going to leap out on me are they?” asked a wary female employee, as office manager Dave Balderson opened the box for the mandatory freight inspection.

They didn’t. Balderson sealed and labeled the box. And soon the Panamanian golden frogs were aboard Delta passenger flight 6539 to Memphis, the first leg on their journey to a new life in Texas.