Yellow Legged Frog


The mountain yellow-legged frog is a species of frog living in the San Bernardino Mountains that is now endangered. Also known as the southern mountain yellow-legged frog and its scientific name Rana muscosa, the mountain yellow-legged frog is a species of true frog endemic to California. It occurs in the San Jacinto Mountains, San Bernardino Mountains, and San Gabriel Mountains in Southern California and the Southern Sierra Nevada.
It is a federally listed endangered species.
Rana muscosa is 1.1 inch to to 3.5 inches long. Its color and patterning are variable. It is yellowish, brownish, or olive with black and brown markings. Its species name muscosa is from the Latin meaning “mossy” or “full of moss,” inspired by its coloration. It may have light orange or yellow thighs. When handled, the frog emits a defensive odor reminiscent of garlic.
The frog occurs in mountain creeks, lakes and lakeshores, streams, and pools, preferring sunny areas. It rarely strays far from water, and can remain underwater for a very long time, likely through cutaneous gas exchange. The tadpoles require a permanent water habitat for at least two years while they develop. The frog has been noted at elevations of between about 1,214 and 7,546 feet in Southern California.
The frog emerges from its wintering site soon after snowmelt. Its breeding season begins once the highest meltwater flow is over, around March through May in the southern part of its range, and up to July in higher mountains to the north. Fertilization is external, and the egg cluster is secured to vegetation in a current, or in still waters sometimes left floating free. The juvenile may be a tadpole for 3 to 4 years before undergoing metamorphosis.
The frog lacks a vocal sac. Its call is raspy, rising at the end. During the day, it calls underwater.
This species feeds on insects such as beetles, ants, bees, wasps, flies, and dragonflies. It is also known to eat tadpoles.
The frog is an endangered species under the US Endangered Species Act. The International Union for Conservation of Nature has also listed it as endangered. Its NatureServe conservation status is Imperiled.
Once a common species, Rana muscosa was absent from much of its native range by the 1970s. Over the course of the last hundred years, 90 percent of its populations have been eliminated. The frog was known from 166 locations in the Southern California mountains, and as of 2007, only seven or eight remained. The 2009 discovery of R. muscosa at two locations in the San Bernardino National Forest was newsworthy. The frog is represented in the Sierra Nevada by three or four populations. Its decline is attributed to many factors, including introduced species of fish such as trout, livestock grazing, chytrid fungus, and probably pesticides, drought, and ultraviolet radiation.
Trout were introduced to lakes and streams throughout the Sierra Nevada in the late 1800s to increase recreational fishing in the area. The fish feed on tadpoles, a main prey item. The introduced trout have changed the distribution of several native species in the local ecosystems. After the removal of fish from several lakes, the frog reappeared and its populations increased. It then began to disperse to other suitable habitats nearby.
The decline of the frog from its historic range has been associated with pesticide drift from agricultural areas. Frogs that have been reintroduced to water bodies cleared of fish have failed to survive, and analysis has isolated pesticides in their tissues. Pesticides are considered by some authorities to be a greater threat to the frog than the trout.
This species is one of many amphibians affected by the fungal disease chytridiomycosis.
The first successful captive breeding of the frog occurred in 2009 when three tadpoles were reared at the San Diego Zoo. Conservation workers at the zoo plan to release any more surviving captive-bred frogs in the San Jacinto Mountains, part of their native range.
In 2015 frogs and tadpoles of the species were reintroduced to Fuller Mill Creek in the San Bernardino Mountains and San Bernardino National Forest. They were bred and raised at the Arnold and Mabel Beckman Center for Conservation Research in Escondido, one of the organizations that have partnered with the San Diego Zoo Institute for Conservation Research (ICR) to make an effort to preserve the species.
Recently more than 100 endangered mountain yellow-legged frogs were reintroduced into the San Bernardino Mountains by a team of scientists from San Diego Zoo Global and the U.S. Geological Survey.
The release, at an elevation of 1,500 feet, is in a location where only a few of the rare yellow-legged frogs have been seen since the heavy rains in the winter following the 2003 Old fire, according to Adam R. Backlin, an ecologist with the U.S. Geological Survey.
It is believed that heavy rains on the slopes that were stripped of vegetation as a consequence of the fires near City Creek caused mudslides into the frog’s habitat, killing some of the population and leaving the survivors in peril because the erosion had destroyed the water refuge.
Because biologists anticipated significant habitat loss after that fire, they collected yellow-legged frogs before the rains came. The recently released frogs are descended from frogs that were genetically distinct from other mountain yellow-legged frog populations in Southern California.
The USGS intends to reintroduce more frogs at higher elevation points in the San Bernardino Mountains between April and June, 2017.

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