Tiny Transmitters for Mountain Yellow-Legged Frogs: Freshwater Species of the Week

This captive-bred mountain yellow-legged frog was released in Southern California with a radio transmitter. Photography by Ken Bohn, San Diego Zoo Global

On Wednesday, Frank Santana, a researcher at the San Diego Zoo Institute for Conservation Research, released 65 froglets into a Southern California creek. The small amphibians represent new hope for an endangered species, the mountain yellow-legged frog. (We wrote about how specimens of this frog were refrigerated for preservation in 2010, part of the restoration effort.)

freshwater species of the week

Mountain yellow-legged frogs live in perennial streams in the San Gabriel, San Bernardino, and San Jacinto mountains. They were listed on the U.S. endangered species list in 2002. By 2003, scientists estimated that there were fewer than 200 of the frogs alive.

Conservation biologists started a captive breeding program, and this week Santana and colleagues released 65 of the animals into Indian Creek, a stream at an elevation of 5,500 feet (1,700 meters). Santana told Water Currents that the creek is surrounded by lush greenery, including ferns and azaleas, and bordered by spruce and pine forest.

The creek teems with dragonflies, butterflies, moths, beetles, and other insects. That’s good, according to Santana, because the mountain yellow-legged frog “eats any invertebrate they can get in their mouth.”

He added that, before their decline, the frogs played an important role in controlling insect populations, and that they also served as an important prey item for snakes, birds, and other predators.

About their decline, Santana said scientists aren’t sure of the exact causes, though he suggested the usual suspects of habitat loss, pollution, and the chytrid fungus. Introduced species have also been a problem for the frog, especially brown and rainbow trout, which eat the eggs, tadpoles, and juveniles.

Wildlife officials have been removing invasive species in the frog’s historic range, Santana said, and Indian Creek is free of invaders. The stream is also on land protected by the University of California. Mountain yellow-legged frogs had been seen there as recently as the 1990s, he added. “So we know the habitat is suitable.”

Tiny Transmitters

In a first for the species, several of the released froglets were fitted with tiny radio transmitters, so the scientists can track their movements. The effort is part of ongoing monitoring and research.

Santana has been working on breeding mountain yellow-legged frogs for the past seven years. The ones released this week were hatched in captivity about a year ago.

Santana said there are a few more releases planned for Indian Creek this summer. The team also tested different release methods. One group of the froglets was let out of their cages as soon as they were carried to water’s edge, but some were kept in an acclimation cage in the stream for 30 days first. Another group was kept in a cage for 15 days.

Santana said the goal is seeing if scientists can make the transition to the wild easier on the amphibians, which tend to be sensitive to changes.

Researcher Frank Santana releases mountain yellow-legged froglets into in Indian Creek in southern California
Researcher Frank Santana releases mountain yellow-legged froglets into in Indian Creek in southern California. Photograph by Ken Bohn, San Diego Zoo Global

Breeding Frogs

Santana added that it took a few years of trial and error before the scientists could successfully breed the frogs in captivity. The key, he said, was inducing them to hibernation by lowering the temperature. Normally, the frogs hibernate during winter by hiding in rock crevasses at the bottom of streams. Unlike most frogs, however, they are primarily active during the day.

In 2011, Santana and team released mountain yellow-legged tadpoles into Indian Creek. A year later, the scientists couldn’t find any trace of them. But after they released the 65 froglets this week, they saw two adult frogs that had grown up from the released tadpoles.

“This was exciting, it indicated this is a well suited site and that our reintroduction efforts are working,” said Santana.

In a statement, the zoo’s Institute for Conservation Research said, “Watched over by a team of federal, state, and private scientists, the mountain yellow-legged frog continues to maintain a perilous toehold in the mountains of Southern California.”

mountain yellow legged frog with transmitter Santana hand
Another view of a mountain yellow-legged froglet with transmitter. Photograph by Ken Bohn, San Diego Zoo Global


Brian Clark Howard covers the environment for National Geographic. He previously served as an editor for TheDailyGreen.com and E/The Environmental Magazine, and has written for Popular Science, TheAtlantic.com, FastCompany.com, PopularMechanics.com, Yahoo!, MSN, and elsewhere. He is the co-author of six books, including Geothermal HVACGreen LightingBuild Your Own Small Wind Power System, and Rock Your Ugly Christmas Sweater.

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