For the better part of two decades, Casey and Berani have enthralled and educated visitors to the Audubon Zoo by doing the sorts of things gorillas and orangutans do: sitting, sleeping, snacking, lounging, playing and otherwise whiling away the hours in their habitats.

But later this month, the two great apes will set forth on a journey designed to encourage them to fulfill a vital role that the public likely does not think much about: protecting the genetic code of their critically endangered species.

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Casey, a male western silverback gorilla, will leave for the Louisville Zoo on June 12, while Berani, a male Sumatran orangutan, will head for the Denver Zoo by the end of the month. 
 
Each will take with them zookeepers' hope that a new environment and potential new mate will encourage them to breed, passing on their healthy genes and helping to ensure the survival of their species.

“Great apes are in danger of extinction all around the world, due to habitat loss from unsustainable agricultural and mining practices,’’ said Courtney Eparvier, the zoo’s curator of primates.

Audubon is inviting the public to visit the pair before they leave and to sign farewell banners created by Zoo Camp children.

The banners will be put to use as "enrichment" — a term of art for everyday items keepers use to stimulate natural behaviors in captive animals — for Casey and Berani before they leave.

Audubon will be getting a new silverback gorilla later this year and a new male orangutan in 2018, with the hope that each will be able to breed in their new homes.

The rotation is part of a complex process administered by the Association of Zoos and Aquariums, which reviews the genetic makeup of animals throughout its accredited facilities, including Audubon, and makes recommendations about which animals should be moved where, given their genes and personalities and the needs of and potential mates at other zoos.

Each species has an advisory board of experts who use the parentage and genetic information of every animal in the system to figure out which animals would make the best match at this particular moment, given the current gene pool. The system seeks to maintain a healthy amount of the genetic variation that a species would get, ideally, in the wild.

It also considers factors from one zoo to the next. For example, if a zoo has a newborn or an adolescent animal, does the transfer candidate have experience with the young of its species?

Participating zoos are free to reject the recommendation if they think something has been overlooked and it’s not a good fit, but generally they work toward the common goal.

“This isn’t all about Audubon; we’re just part of the puzzle,” Eparvier said.

Audubon in recent years has been the beneficiary of this process, known as the Species Survival Plan, getting Bonnie, a female Southern white rhino; two mandrills, Mapema and Jinx; and Bumi, its Malayan tiger. This year, the recommendations came down in the other direction.

It was a day that Eparvier and her primate team knew could come.

“We haven’t had a major great ape transfer in a long time,” she said. “But within AZA, it happens all the time. It hasn’t been our turn yet.”

Berani, who is 23, fathered a female, Menari, with his Audubon Zoo mate, Feliz, in 2009.

Casey, who will turn 35 this month and remains biologically capable of reproducing, has not produced offspring despite being paired with four females during his 15 years at Audubon Zoo. The last gorilla born at the zoo was Praline, 21 years ago.

“We are not contributing right now, and we need to start contributing,” Eparvier said Tuesday with a matter-of-factness that belies how personally difficult she and her team know it will be to lose the primates they’ve worked with so closely for years.

“These two boys, they’re a big presence in our life, so when they go, it’s going to leave a huge hole. And it’s going to be weird not having them around,” she said, pausing for a moment. “But it’s for the greater good.”

Most animals in zoos are not brought in from the wild, nor are they destined to return there. They are bred and live in captivity and serve as a parallel track to their counterparts in the wild, a healthy and genetically diverse population. It's an ark, of sorts, to protect against total extinction.

There are 350 gorillas in North American zoos, with 12 born in North America last year, according to Audubon. Many gorilla populations have declined or completely disappeared over the past few decades. The western lowland gorilla is the most widespread, numbering approximately 100,000 animals.

Both species of orangutan — Bornean and Sumatran — are critically endangered, and the North American population, which includes both species and hybrids of the two, stands at 222 animals at 53 institutions, Audubon said.

Eparvier said the mining of coltan, a metallic ore used in cellphones, and deforestation in pursuit of palm oil, which has become ubiquitous in consumer products, are destroying the natural habitats of both species.

Zoo officials hope Casey and Berani can serve as an inspiration for people to make little changes, like paying attention to what goes into the things they buy and recycling their cellphones.

“It’s those little things that everyone can do in their lives that can actually help these animals,” Eparvier said. “I think sometimes the world is a bad place and people think, 'I can’t handle it; there’s nothing I can do,' but there are actually things we can do at home that can help these guys.”

It’s important, Eparvier said, because the unthinkable is not inconceivable.

“The next step for them is extinction in the wild, which is terrifying to me,” she said. “I’m honored to work with them. It’s a privilege. I don’t want to tell my grandchildren, 'Grandma used to work with these (animals) but they’re not here anymore.’ ”

Curators from the Louisville and Denver zoos have been working closely with Audubon officials on Casey and Berani’s departures and will work alongside Eparvier and her staff for the animals' final week. Some of her team members will go along on the trip and spend a few days with them as they settle in before returning to their work at Audubon.

“Then we’ll just kind of fade out,” she said.

Eparvier said the staff keeps in mind that every departure is an arrival — every addition someone else’s loss.

“There’s two sides to each story,” she said, noting the recent arrivals of the mandrills. “It’s exciting that we got these two animals, but you have to think about where Mapema and Jinx came from. (Those facilities) lost them.”

Eparvier said gorillas and orangutans are “completely different,” with gorillas living in complex social groups while “orangs” prefer solitude.

As individuals, she said, Casey is “cool, calm and collected.” He’ll show excitement from time to time in his eyes, but he tends to be stoic. He loves yellow mustard, celery and sitting on top of things, no matter how small. “If it fits, he sits,” Eparvier said.

Berani, on the other hand, “is just a goofball. He’s 23 years old, but he acts like a kid. He’s always watching. He’s very observant and he learns quickly. Orangutans are the MacGyvers of the great ape world. They turn nothing into something for a tool.”

“I love gorillas,” she admitted with a smile, “but orangutans are my jam.”

Eparvier said it was a shock to see Casey's and Berani’s names come up in March for transfer, but that quickly dissolved into the realization that there was a lot of work to do.

“It seems like it’s been forever ago,” she said.

Eparvier started as a sea lion trainer, having fallen in love with marine mammals during a trip to Sea World in the fifth grade. Eleven years ago, she began working with the great apes.

“We hang out with these guys all day long,” she said. “Some of the things that we’re able to do with them, be it training or getting a really cool enrichment thing going for them, the relationship that we build with them, it’s just really special.”

Follow Chad Calder on Twitter, @Chad_Calder.