Scientists hope big chill will get frogs off thin ice

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By Louis Sahagun
Washington Post Staff Writer
Tuesday, March 30, 2010

Some like it hot. Apparently, the endangered mountain yellow-legged frog is not among them.

The three-inch-long amphibians much prefer it cold as melting snow. So conservationists at the San Diego Zoo have placed two dozen of the nearly extinct frogs in refrigerators they jokingly refer to as "Valentine's Day retreats" in hopes the animals will emerge with the urge to mate.

The big chill at the zoo's Institute for Conservation Research represents one of the most ambitious wildlife reintroduction experiments in the nation. If it is successful, the frogs could produce upward of 6,000 tadpoles next month, all of them scheduled for a spring homecoming in a remote San Jacinto Mountains stream from which they have been absent for a decade.

Scientists hope many of those tadpoles will mature and produce new generations in the wild, paving the way for the Rana muscosa population to reestablish residency in Southern California and grow exponentially.

"A month from now, there could be tubs of tadpoles all over the place," said Jeffrey Lemm, a zoo research coordinator. "Eventually, we may have thousands of adult frogs in self-sustaining populations for the first time in half a century."

Mountain yellow-legged frogs thrived for thousands of years in the streams cascading down the San Gabriel, San Bernardino and San Jacinto mountains. Since the 1960s, the species has been decimated by an array of threats: fires, mudslides, pesticides, fungal infections, loss of habitat as a result of development, and the appetites of nonnative trout, bullfrogs and crayfish.

Today, fewer than 200 of their descendants are believed to exist in nine isolated wild populations, giving mountain yellow-legged frogs the distinction of being one of the most endangered amphibians on the planet.

The most intimate details of their mating behavior are the focus of a master's thesis project being conducted at the institute by research technician Frank Santana.

In their native habitat, the frogs flock to streams gushing with spring snowmelt. Males announce their presence with a low-pitched underwater bark. Parental discretion is advised for what follows: "A male gets a good grip of a female with his forearms, and the female, if she's in the mood, lets him," Santana said. "Then the male thrusts his whole body to stimulate the release of her eggs. The female goes into contractions as both arch their backs to line up their cloacae." Sperm and eggs are released simultaneously. Tadpoles emerge from the eggs about three weeks later. In the wild, only 3 to 5 percent mature into adult frogs.

"In the laboratory, the hard work comes when we've got a bazillion two-millimeter-long tadpoles on our hands in need of daily water changes, and meals of frozen lettuce and fish food," Santana said.

The zoo's recovery program was launched in the summer of 2006, with 82 tadpoles rescued from a drying creek in the San Bernardino National Forest.

Two years later, institute researchers discovered a clutch of 200 eggs in one of their tanks. However, the frogs were younger than is typical for breeding, and only a handful of the eggs were fertile. The institute became the first to breed a yellow-legged frog in captivity when one of those eggs produced a tadpole that matured into an adult.

Now the institute has 61 frogs, including the 16 females in the refrigerator -- each one of them, Lemm said, "looking nice and healthy and bulging with 200 to 300 eggs." All the tadpoles produced in the laboratory will be reintroduced into a mountain stream that U.S. Geological Survey biologists have determined is free of predators.

In the meantime, federal wildlife authorities are developing measures to reduce the effect of human activities in areas where the yellow-legged frog is still found and may be reintroduced.

"A few years ago, there wasn't even a captive breeding program for these frogs," Santana said. "Now, we are hoping to reestablish populations by mimicking their natural cycles. For these frogs, that means winter hibernation, spring thaw and lots of tadpoles. Hopefully."

-- Los Angeles Times


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