People on a series of redeye flights from San Francisco to Puerto Rico in late October "had no idea they were flying with 4,000 highly endangered toads," says Adam Fink, the zoological manager of the Oakland Zoo.

And rather than take up seat space or overhead bins, the tadpoles were stashed away in an insulated container in the plane's hold.

The tadpoles represent hope for the critically endangered Puerto Rican crested toad (Peltophryne lemur), the only toad native to Puerto Rico, and a unique species that had been thought extinct for much of the 20th century. Preferring arid or semi-arid areas of Karst limestone, the toads are identifiable by their unique head crests.

On October 20, the tadpoles were unpacked and safely released into ponds in two protected national forests in Puerto Rico. (Learn about a mass die-off of the bizarre scrotum frog.)

"This release will help bolster the numbers of the species significantly," says Fink, who leads reptile and amphibian breeding efforts at the Oakland Zoo.

The Puerto Rican crested toad has declined due to introduction to Puerto Rico of the marine toad and other invasive species, which eat the toads and tadpoles and outcompete them for resources. The crested toad has also experienced loss of habitat from development and intensive sugarcane production.

Scientists thought the species had gone extinct by the 1930s, until a small population was rediscovered in the '60s. A few individuals were taken into captivity, and they have been bred ever since at a number of zoos around the U.S. and Canada. Several of those zoos have been involved in a coordinated breeding program (facilitated by the American Zoo and Aquarium Association), which seeks to maximize the remaining genetic diversity of the species.

In October, a total of about 11,000 of the tadpoles were released on the island. They were bred at the Oakland Zoo and three other zoos: the North Carolina Zoo, Omaha Zoo, and Sedgwick County Zoo in Wichita, Kansas.

Growing Toads

The Oakland Zoo has a special facility for breeding the toads, a species that was chosen because of its rareness and high priority among the scientific community (the zoo also works with several endangered California species). The zoo has 19 adult Puerto Rican crested toads on hand.

When it's time for breeding—based on a schedule coordinated across participating zoos—Fink places a few carefully selected individuals in a wine refrigerator. That readily available equipment has a fine level of temperature control, so the cold-blooded toads can be gradually cooled. When their body temperatures get to the mid 60s, they begin to hibernate.

After a few weeks, the toads are gradually warmed back up, causing them to wake up. Next, they are placed in a "rain chamber," a terrarium with plastic plants and a soil substrate. Regular dousing with imitation rain puts the amphibians in the mood.

The toads are paired with potential mates, which are typically sent from other zoos on loan, in a plan to maximize genetic diversity. If they hit it off, the males fertilize the females' eggs. Within a few days, those eggs develop into tadpoles, which can be carefully scooped up.

To count the little wigglers, Fink places the tadpoles into shallow pools and takes their picture. With a Sharpie, he counts each one on the image—recently he counted 4,096, a big increase over last year's 732. The tadpoles are then packed up and driven to the airport.

"We've had great luck shipping them via commercial airlines," Fink says.

Hopeful Future

Fink hopes the toad will begin to rebound and re-establish itself on its own over time, although challenges with invasive species and limited habitat remain. Still, wildlife managers are now more aware of the threats.

And the species does not seem to be susceptible to the invasive chytrid fungus that is attacking so many other amphibians around the world.

Frogs and toads play important roles in their ecosystems, Fink adds, from keeping insects in check to serving as a food source for larger animals.

A number of rare frogs and toads have been immortalized in the photos of Joel Sartore through his Photo Ark project, which is supported by the National Geographic Society.