Captive-Bred Axolotls Were Successfully Introduced to the Wild. Can This Work for Other Species?

If an animal is born in captivity, can it ever truly thrive in the wild? The answer is complicated. 

As escalating threats, from climate change to development, decimate biodiversity, scientists and conservationists are scrambling to raise certain species in captivity. The goal: Eventually release them back into their habitats to rebuild populations. 

The latest poster child for this approach—known as captive or conservation breeding—may be the axolotl, a wide-headed salamander native to Mexico City. The critically endangered amphibians have drastically declined in past decades due to development, pollution and other human activities. Scientists were recently able to successfully breed and introduce a cohort of axolotls to two wetlands around Mexico City, according to a study published last week. 

However, conservation breeding programs can be deceptively complicated depending on the animal, and they frequently fail. Like humans, animals often have complex social dynamics, habitat preferences and behaviors that can throw a wrench into reintroduction plans. 

Scientists say the most important factor is ensuring that the animals actually have a place to go back to, and are able to avoid the stressors that diminished populations in the first place. 

An Axolotl Case Study: With a seemingly permanent grin and feathery gills, axolotls have charmed people around the world in recent years, fueling a boom in the pet trade. But native populations aren’t doing as well; scientists estimate that less than 1,000 individuals remain in the wild. 

Axolotls are endemic to Lake Xochimilco in the central valley of Mexico, which has been degraded by urbanization and the introduction of invasive species. In 2004, conservationists launched a project to restore parts of this wetland. With the ecosystem now revitalized enough to support axolotls, scientists in 2017 and 2018 introduced a small group of captive-reared individuals to the lake and a nearby artificial pond created in the wake of a decommissioned mine. 

Each individual was tagged and monitored with a tiny tracking device. After more than 40 days, the researchers found that all the animals had survived.

“This is very big, because when you have animals in captivity and you do these reintroduction programs … many projects fail due to mortality in the first days after release,” study author Alejandra Ramos, a science faculty member at the Autonomous University of Baja California, told me. 

Ramos said that it’s not difficult to get the axolotls to breed in captivity because “if the temperature drops, they have babies.” The tough part is ensuring that they can hunt, avoid predators and make a habitat for themselves—parts of life that the animals typically aren’t exposed to in controlled settings. But the pioneer axolotls seemingly figured it out, according to the paper. 

“When we were able to recapture some of them, they had even gained weight, so that means that they were eating,” she said. 

However, just because these captive-bred axolotls may be able to survive in the wild, that doesn’t mean people should start throwing their pet salamanders into nearby wetlands. 

“The ones that you see in people’s homes, like in private aquariums or in research laboratories, are axolotls that were taken out of the wild around over 100 years ago,” she said, explaining that this captive line of axolotls are typically lighter while native groups are mostly dark. “They’re the same species, but the ones that are in captivity now have been in captivity for a lot longer than the ones we have. Ours are definitely wilder, and they have the colors they need to survive.” 

Hit or Miss: There have been mixed results for conservation breeding programs depending on the species. Some have seen similar success as the axolotl, such as programs to bring black-footed ferrets back into their native environments. 

Pierre Comizzoli, a research biologist at the Smithsonian’s National Zoo and Conservation Biology Institute, pointed to the scimitar-horned oryx’s comeback as an example of how this strategy can look at scale. Around two decades ago, the horned antelopes—native to the Sahara and Sahel regions of Northern Africa—were declared “extinct in the wild” after being over-hunted for their meat and leather. 

In 2013, the Environment Agency of Abu Dhabi partnered with the government of Chad and other groups to launch a reintroduction program. Thousands of scimitar-horned oryx had been raised in captivity in zoos or private collections, so researchers were able to transport hundreds of them to a new, large enclosure more similar to their native habitats as a way to introduce them to potential stressors of the wild. Think of it like training wheels.

Eventually, the project coordinators opened the gates to the broader environment, and wild populations have grown to steadier levels since. 

Conservation breeding and release programs are “a long-term process, and you need to introduce a lot of animals,” Comizzoli said. “It’s not a one-time effort. It’s a sustained effort over sometimes multiple months or years.” 

As a result, one of the major keys to this approach is patience, he added. Animals with complex social behaviors such as primates or pandas take time for humans to understand and design programs around, making reintroduction efforts difficult or impossible to carry out at the moment. 

“It’s very complicated to try to make sure that reintroduced animals are going to be able to blend with the existing wild populations, but then also you have the exact opposite, like very solitary animals that need a lot of space,” Comizzoli said. 

Another snag in this strategy is that animals can become habituated to humans, which Ramos learned firsthand while contributing to reintroduction efforts with Mexican populations of endangered California condors. When the program first began, “condors would actually actively look for humans, and they would like to be around humans,” she said. One night, an astronomer working at an observatory in the national park in which the condors had been released called to let her know that one of the birds had managed to get into the facility. 

“He was like, ‘Is this one of your kids?’” Ramos recalled. They were able to remove the condor safely. But “that was a behavioral problem, because you don’t want wild animals to be trying to look for humans and trying to interact with humans,” she said. “And also, you don’t want this condor to teach other condors this behavior.” 

Even if an endangered animal cannot yet be released into the wild, captive populations can be crucial for ensuring that future native populations maintain genetic diversity if their numbers get too low, research shows. However, experts say these programs are not always run ethically. Last year, The New York Times published an investigation on panda captive breeding in China and the U.S. that revealed programs were only breeding certain genetic lines and that few cubs had actually been released into the wild. 

No conservation breeding release program will work if there isn’t support from local communities, both Comizzoli and Ramos agreed. In the case of the axolotls, farmers have been open to using fewer chemicals near the wetlands, but the amphibians still face a lot of the same threats from pollution and development. Meanwhile, Comizzoli said the scimitar-horned oryx program’s progress has a lot to do with support from hunters and local conservationists. 

“This is not only driven by scientists with a white coat,” he said. “No, not at all. And that’s the key of the success, by the way, because you cannot really have a successful reintroduction program or conservation program if you do not take into account the human dimensions.”

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