Some like it hot. Apparently, the endangered mountain yellow-legged frog is not among them.
The 3-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 joshingly refer to as "Valentine's Day retreats" in hopes the amphibians will emerge with the urge. To mate, that is.
The big chill at the zoo's Institute for Conservation Research represents one of the nation's most ambitious wildlife reintroduction experiments.
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.
"Will it work? We think so," said Jeffrey Lemm, a zoo research coordinator. "A month from now, there could be tubs of tadpoles all over the place. 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 hundreds of streams cascading down the San Bernardino, San Gabriel 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, including a group in the San Gabriel Mountains' Devils Canyon that survived last year's devastating Station fire.
Their minuscule, scattered population gives 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 availability for amphibian amour 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, let's 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% mature into adult frogs.
"In the laboratory, the hard work comes when we've got a bazillion 2-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 a still-surviving 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.
The recovery effort has been funded by the California Department of Transportation to mitigate for emergency work to stabilize a slope near the frog's habitat on California 330 in the San Bernardino Mountains. It is part of an ongoing collaborative effort of government and nonprofit partners to increase the number of frogs in native habitat and in captive breeding programs.
The Fresno Chaffee Zoo recently received about 100 tadpoles rescued last summer from the Station fire area.
The Los Angeles Zoo and the Living Desert in Palm Desert will each get 10 adult frogs for captive breeding purposes.
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. That includes a remote stretch of Tahquitz Creek in the San Jacinto Wilderness near Idyllwild, where two yellow-legged frogs were discovered last year.
"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."