Two American zoos have helped to save an African amphibian from extinction. The Kihansi spray toad (Nectophrynoides asperginis) was declared extinct in the wild in 2009 after its only habitat, the waterfalls of Kihansi Gorge in Tanzania, dried up following the establishment of a nearby hydroelectric dam. But this month 2,000 toads returned to Kihansi, courtesy of scientists at the Wildlife Conservation Society's (WCS) Bronx Zoo and at the Toledo Zoo. The achievement marks the first time that an extinct-in-the-wild amphibian has been returned to its native habitat.

The tiny yellow toads—which reach a length of 2.9 centimeters at their largest—were first described by scientists in 1999, the year before the Kihansi Dam went online. The hydroelectric facility now generates nearly a quarter of Tanzania's electric supply, but it also reduced the flow of water going over the Kihansi Gorge waterfalls by 90 percent. Before that happened, the waterfalls produced a mist zone that created the toads' unique habitat, just 20,000 square meters of wet vegetative territory—the smallest known range for any known vertebrate species. After the dam was completed, the waterfall no longer produced the mists on which the toads depended.

As the mist zone disappeared, so did the toads, which were also starting to suffer from the dreaded chytrid fungus, the rapidly spreading blight that has already been responsible for dozens of amphibian extinctions around the globe. The last wild toads were observed in 2005, and the species was declared extinct in the wild in 2009.

Luckily scientists saw this disappearance coming. In 2000, at the invitation of the Tanzanian government, the Bronx Zoo collected 499 toads from the gorge to create a captive assurance population. Some of those toads were later transferred to the Toledo Zoo. After more than a decade of breeding efforts, each zoo contributed 1,000 toads toward this month's reintroduction effort.

Getting to this point was a long journey that required work in Tanzania and in the U.S. One challenge was repairing the toads' habitat at Kihansi Gorge. The World Bank funded the installation and refinement of a gravity-fed misting system that replicated the original habitat conditions. The Tanzania Electric Supply Company Limited (TANESCO), which operates the dam, monitors the system to make sure it receives the water it needs, and the Tanzanian government has committed to maintaining the misting system.

The other challenge was getting the frogs to breed and thrive in the U.S. "No facilities were keeping spray toads in captivity—and this is a species with very specific needs—so Bronx Zoo staff had to figure it out," says Don Boyer, the zoo's herpetology curator. Both zoos built bio-secure facilities that replicated the unique microhabitat at Kihansi. "The best husbandry practices including physical enclosure design, spray systems, water quality, health monitoring, disease-preventative measures, diet and stocking densities all had to be established." Boyer says the team faced many challenges along the way, such as making sure the toads had the right food and, amazingly, the right diseases. "The wild toads did have parasites and the goal of the captive colony was not to eliminate all disease and thereby make them vulnerable when they were repatriated but to maintain parasite levels comparable to the wild populations." Each zoo now has several thousand toads in their collections.

The first 100 Kihansi spray toads from the U.S. captive assurance population returned to Tanzania in 2010, where the University of Dar es Salaam has now created its own assurance population. A few dozen of those toads made their way to Kihansi Gorge earlier this year in a closely monitored situation to see if they would be safe in the revived habitat. "Experiments were done to see if the captive toads would be a source for chytrid infections, and could captive toads survive if exposed to water, plant and amphibians still living in the gorge," Boyer says. "Those results were very promising and indicated there was a very low risk. The next stage was actual soft-release experiments where captive-born spray toads were placed into screened enclosures in the upper-gorge wetlands." The results of that experiment were encouraging enough to move forward with the full-scale reintroduction.

That reintroduction, which took place on December 11, was a full-scale event, attended by representatives from the WCS, Toledo Zoo, World Bank, International Union for Conservation of Nature (IUCN), Tanzania Wildlife Research Institute, the University of Dar Es Salaam, Sokoine University of Agriculture, and local Tanzanian villagers.

In a press release, WCS president and CEO Cristián Samper called the reintroduction "a momentous achievement in conservation. It clearly shows how zoos can play an important role in conservation. This has been a truly global effort to save a species."

Now that the toads have returned, Tanzanian biologists will take up the task of monitoring the population. "They will continue to visit the wetlands on a regular basis and conduct survey work," Boyer says. Meanwhile, the populations at the Bronx and Toledo zoos continue to grow, as does the collection at the University of Dar es Salaam, which might also be used for future reintroductions.

Photos by Alyssa Borek courtesy of the Wildlife Conservation Society