Taking the High Toad: Denver Zoo Releases Boreal Toads to Help Near-Endangered Species

Behind a gate at the edge of the Denver Zoo Conservation Alliance in City Park is the Boreal Toad Conservation Center, a building colloquially known by zoo staff as the "Toad Barn." Inside, awkward tadpoles with tails and legs swim in a tank while tiny toads about the size of a fingertip perch on rocks and leaves. Across the aisle, adult toads — some of them up to seventeen years old — laze in their own tanks.

For several months each year, during their brumation (toad hibernation) period, the boreal toads in the Toad Barn live in a refrigerator. In May, they are brought out to the tanks — labeled with names like "The Disco Suite" and "The Space Suite" — to lay eggs. Earlier this month, about 300 of the recently hatched toads were released in the Northern Sawatch Range, where the zoo hopes the amphibians will survive and keep their species going.

Because the sad truth is, boreal toads are nearly extinct in Colorado.

The species as a whole is not on the federal endangered species list (if it were, revival efforts would receive federal funding) because there are still abundant and healthy populations in Alaska and parts of the Pacific Northwest, says Stefan Ekernas, director of Colorado field conservation at the zoo.

"But in Colorado, it's tanking," Ekernas adds. "They've gone locally extinct in New Mexico, and it's close to it in Colorado. There are some numbers out there from Colorado Parks and Wildlife that there are as few as 800 adult toads in our state. For comparison, there are 300,000 elk in Colorado."

Why are the toads dying in Colorado? Chytrid, a pathogenic fungus that is affecting toads worldwide. "What it does is it gets into their skin and breaks down the keratin and messes up their ability to osmoregulate properly," says animal care specialist Derek Cossaboon. "They ultimately die of a heart attack. It's very contagious."

Chytrid spores are spread through mud, and studies show the fungus likely came out of Korea in the 1950s. "What was happening in Korea in the '50s? The Korean War," Ekernas explains. "Where mud was being transported everywhere, from boots and tanks and whatever else, all the stuff that happens in a war, where you get mud everywhere and then you're bringing it around the world."

At this point, chytrid has spread around the world and into Colorado's mountains. According to Cossaboon, it's responsible for why amphibians in general are one of the most critically endangered species right now. Boreal toads in Utah are evolving resistance so that they can live with chytrid; working in collaboration with CPW, the zoo hopes to help Colorado's boreal toad population bounce back and evolve that resistance, too.
The Colorado project started in 2021, after the zoo successfully bred boreal toads from Utah in 2019. CPW, which runs the Native Aquatic Species Restoration Facility in Alamosa, gave the zoo around 100 adult toads. "We started this program to breed and release them into the wild," Ekernas says.

By now, the zoo has released around 5,000 toads in Utah and Colorado, according to Cossaboon. But it could take decades to know if the efforts are working.

"We're trying to recover a species that's on the brink of extinction in the state," Ekernas says. "That's not a five-year project or a ten-year project. If we're lucky, it's a twenty-year project."

Volunteers work in collaboration with the zoo and CPW to spot boreal toads in the wild, but other than that, currently there are no other ways to keep tabs on the toads released in Colorado, since they are released too young to be tagged. However, the zoo held back some of the Utah toads until they were old enough to be tagged before being released. "We've recaptured several, so we know they are making it," Cossaboon says, adding that the project might do that with Colorado toads in the future, too.

Boreal toads have a long lifespan and don't reach breeding age until they are five to seven years old. They are also Colorado's only high-elevation toad, living at about 8,000 to 12,000 feet above sea level. "They are ectothermic, so they're incapable of producing any of their own body heat," Cossaboon says. "To be able to live at the elevation these guys live and not produce any of their own body heat makes them super special."

To stimulate the cold during the winter brumation period, the adult toads at the zoo are put in a fridge in October, and the temperature is slowly brought down to 36 degrees. "They have to have that cooldown period for the females to produce the ova and the males to produce viable sperm," Cossaboon explains. "We slowly bring them up to temperature in the spring. We increase humidity, try to stimulate spring rains, and then our veterinary team will dose hormones to help females lay the eggs. It's pretty complex, honestly."

The toads lay eggs about a week to ten days after they are brought back to the tanks in May; the eggs hatch within four days and develop within five to six weeks.

Toads lay thousands of eggs at a time. In the wild, 99.9 percent of those eggs are eaten. "They're snacks for fish, birds, salamanders, mice — you name it," Ekernas says. "But at the zoo, there's no predation in those tanks, so we can go from almost no survival to 99 percent survival, and that's a lot of raw genetics."

Boreal toads aren't important to Colorado's ecosystem just because they provide high-elevation snacks for other animals: They're also an indicator species. "All amphibians, including boreal toads, are sponges," Ekernas says. "They soak up everything through their skin, and they're the first thing that will go wrong in a wetland. So the fact that it is going wrong is an indicator that Colorado's wetlands aren't as healthy as they ought to be."
While the initial toads came from CPW and the zoo works closely with the agency, funding for the boreal toad project is covered by the zoo's budget, not the state's. The zoo spends about $2 million a year on field conservation, half in Colorado and half internationally, according to Ekernas. In Colorado, it works on projects to help near-endangered species like the boreal toads, as well as pika and bison. The zoo also partners with Native American communities on bison and bighorn sheep projects and grassland restoration, and handles other issues that fall between the gaps of state agencies, like building wildlife crossings over I-70 and keeping mountain goats looking for salt out of parking lots.

For Cossaboon, releasing the toads is the best part of the project. Zoo staff and regional biologists pack the baby toads into buckets with moist moss, go out to the Northern Sawatch Range and hike for about four and a half miles into the high country.

"We make sure to bring boots that we sterilize before we go into the wetland habitat and we sterilize them after to make sure we're not spreading the chytrid fungus around. We put them on the edge of the bog and let them do their thing," Cossaboon says. "To be part of a collaborative effort that is actually making a difference in the wild is really fulfilling.

And hopping out into the wild is very instinctual for the toads, which immediately start eating and settling in for the next big task ahead: survival.