This frog is a member of the mountain yellow-legged frog complex, which is comprised of two species: Rana muscosa and Rana sierrae. Both species are highly aquatic and are always found within a meter or two from the edge of water. Like Rana sierrae, Rana muscosa is yellowish or reddish brown from above, with black or brown spots or lichen-like markings. Toe tips are usually dusky. Underside of hind legs and sometimes entire belly is yellow or slightly orange, usually more opaque than in the Foothill Yellow-legged Frog, Rana boylii. Yellow coloration often extends forward to level of forelimbs. Dorsolateral folds present but frequently indistinct. The tadpoles are black or dark brown and are large (total length often exceeds 10 cm) and metamorphose in 1-4 years depending on the elevation.
Rana muscosa differs from Rana sierrae in having relatively longer legs. When a leg is folded against the body the tibio-tarsal joint typically extends beyond the external nares. The mating call of R. muscosa is significantly different from that of R. sierrae in that they lack transitions between pulsed and noted sounds. Both species call underwater. Males can be heard above water but only from a short distance away (<2 meters). The two species also differ in mitochondrial DNA. The mitochondrial DNA, male advertisement calls, and morphology datasets are geographically concordant (Vredenburg et al. 2007).
Distribution and Habitat
Country distribution from AmphibiaWeb's database: United States
U.S. state distribution from AmphibiaWeb's database: California
Rana muscosa is endemic to California, U.S.A. The Southern Mountain Yellow-legged Frog once ranged from Palomar Mountain in San Diego County through the San Jacinto, San Bernardino and San Gabriel Mountains of Riverside, San Bernardino and Los Angeles counties in southern California. These formed four isolated clusters of montane populations. In addition the species occurred as an isolated cluster of populations on Breckenridge Mountain, south of the Kern River in Kern County, and in the Sierra Nevada mountains in Tulare, Inyo, and Fresno counties, extending north to Mather Pass. The distribution of Rana muscosa in the Sierra Nevada is bordered by the crest of Sierra Nevada. No populations occur east of the crest. The mountain ridges that separate the headwaters of the South Fork Kings River from the Middle Fork Kings River, from Mather Pass on the John Muir Trail to the Monarch Divide, form the northern border of the range. R. muscosa is extinct on Palomar and Breckenridge mountains.
In summer 2009 a population of Rana muscosa was discovered in the San Jacinto Mountains by a team of USGS and San Diego Natural History Museum biologists (Salzberg 2009; Lewis 2009). The frogs were found at two localities in Tahquitz Creek and one of its tributaries, Willow Creek, about 2.5 miles apart (Salzberg 2009; Lewis 2009). The newly discovered San Jacinto population appears to be occupying a larger area than the other known populations, although the extent and population number are not yet known (Salzberg 2009; Lewis 2009). A total of eight other populations are known from the San Jacinto, San Bernardino, and San Gabriel mountain ranges, each occupying less than half a mile of stream (Salzberg 2009; Lewis 2009).
Life History, Abundance, Activity, and Special Behaviors
Similar to R. sierrae, breeding begins soon after ice-melt or early in spring and can range from April at lower elevations to June and July in higher elevations
(Wright and Wright 1949; Stebbins 1951; Zweifel 1955). Eggs are deposited underwater in clusters attached to rocks, gravel, and under banks, or to vegetation in streams or lakes
(Wright and Wright 1949; Stebbins 1951; Zweifel 1955). Livezey and Wright
(1945) report an average of 233 eggs per mass (n=6, range 100-350) for R. sierrae, but egg counts per egg mass appear similar for R. muscosa (Vredenburg, unpublished data). Eggs contain a vitelline capsule, and three gelatinous envelopes, all clear and transparent
(see illustrations in Stebbins 2003). In laboratory breeding experiments egg hatching times ranged from 18-21+ days at temperatures ranging from 5-13.5 degrees C
The length of the larval stage depends upon the elevation. At lower elevations where the summers are longer, tadpoles are able to grow to metamorphosis in a single season
(Storer 1925). At higher elevations where the growing season can be as short as three months, tadpoles must overwinter at least once and may take two to four years of growth before they are large enough to transform
(Wright and Wright 1949; Zweifel 1955).
Trends and Threats
Rana muscosa have declined dramatically despite the fact that most of the habitat is protected in National Parks and National Forest lands. A study that compared recent surveys (1995-2005) to historical localities (1899-1994; specimens from the Museum of Vertebrate Zoology and the California Academy of Sciences) found that 96.2% of populations had gone extinct, with only 3 remaining out of 79 resurveyed sites (Vredenburg et al. 2007).
The two most important factors leading to declines in R. sierrae and R. muscosa are introduced predators and disease.
Introduced trout prey on R. sierrae
(Needham and Vestal, 1938; Mullally and Cunningham, 1956) and have been implicated in a number of studies as one of the sources of decline in both R. sierrae and R. muscosa (Bradford 1989; Bradford et al. 1993; Jennings 1994; Knapp 1996; Drost and Fellers 1996; Knapp and Matthews 2000). Whole lake field experiments have shown that when non-native trout are removed, both Rana sierrae and Rana muscosa populations rebound
(Vredenburg 2004; Knapp et al. 2007).
While it is clear that introduced trout negatively affect R. sierrae and R. muscosa mainly through predation on tadpoles, trout also compete for resources with adult frogs. A food web study that used stable isotopes to trace energy through food webs in Sierran lakes concluded that introduced trout are superior competitors and suppress the availability of large aquatic insects that make up a major portion of the diets of adult frogs
(Finlay and Vredenburg 2007). Trout removal by the California Department of Fish and Game has reduced fish populations in the Little Rock Creek in the Angeles National Forest, resulting in increased numbers of Rana muscosa (Salzberg 2009; Lewis 2009).
A lethal disease, chytridiomycosis, caused by an aquatic fungal pathogen Batrachochytrium dendrobatidis
(Berger et al. 1998) has caused population extinctions in R. muscosa and R. sierrae in the Sierra Nevada
(Rachowicz et al. 2006). Long-term studies reveal that infection intensity is key; once a critical threshold of Bd fungal infection is reached, death ensues (Vredenburg et al. 2010). Population extirpation is the most common outcome, but a few mountain yellow-legged frog (Rana sierrae and Rana muscosa) populations have survived in low numbers. Modeling shows that chytriodiomycosis outcome at the population level (extirpation vs. persistence) can result solely from density-dependent host-pathogen dynamics, which may hold for other wildlife diseases as well (Briggs et al. 2010).
A new study conducted during an outbreak of chytridiomycosis in the Sierra Nevada has shown that electrolyte depletion (sodium and potassium) for heavily infected wild mountain yellow-legged frogs is even more extensive than studies done in captivity have suggested, and is accompanied by severe dehydration despite the frogs' aquatic environment (Voyles et al. 2012). See also the NSF commentary. In an effort to rescue the last surviving mountain yellow-legged frogs, the Vredenburg lab is treating adult frogs in the field with anti-fungal medication; frogs are bathed for five minutes daily over the course of a week (Lubick 2010). Electrolyte supplementation may also be a way to help save individual frogs (Voyles et al. 2012).
Other possible causes for decline in R. muscosa include air pollution from pesticide drift
(Davidson et al. 2002; Davidson 2004), UV-B radiation, and long term changes in weather patterns, especially concerning the severity and duration of droughts. Acidification from atmospheric deposition has been suggested as another cause, but Bradford et al.
(1994) found no evidence to support this hypothesis.
The San Diego's Institute for Conservation Research has instituted a captive breeding program for this species (Salzberg 2009; Lewis 2009). In 2006, tadpoles were rescued from a drying creek in the San Jacinto Wilderness and were reared in captivity (Salzberg 2009; Lewis 2009). In December 2008 a pair of these frogs laid a clutch of 200 eggs in captivity; only a handful of these eggs were fertile, due to the young parental age, and a single offspring has survived to maturity (Salzberg 2009; Lewis 2009). The most recent breeding season, however (December 2009-March 2010) was very successful and biologists have just reintroduced about 500 eggs into the wild, as of April 23, 2010, into deep permanent pools at the University of California Riverside’s James San Jacinto Mountains Reserve. Tadpoles that hatch from these eggs will take about two years to mature into adults. The adults are expected to stay within the reserve since they do not migrate. The tadpole rescue and frog breeding effort has been funded by Caltrans, as part of mitigation for emergency work necessary to stabilize a slope and reopen State Route 330 near Rana muscosa habitat in the San Bernardino Mountains (Salzberg 2009; Lewis 2009).
Relation to Humans
Mountain yellow-legged frogs (the amphibian species complex including Rana muscosa and Rana sierrae) were once the most common vertebrates in the high elevation Sierra
Nevada. Documented historical accounts go back to the turn of the last century (1915) from surveys conducted by Joseph Grinnell and Tracy Storer (published in 1924) from the University of California's Museum of Vertebrate Zoology.
Joseph Grinnell was instrumental in the foundation of Yosemite National Park, one of the crown jewels of the United States National Park Service.
Possible reasons for amphibian decline
Long-distance pesticides, toxins, and pollutants
Predators (natural or introduced)
Southern California populations Endangered
The southern California populations of the species were formally recognized as an Endangered distinct population segment as of July 2, 2002. For details please see the U.S. Federal Register at:
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Written by Vance T. Vredenburg (vancev AT sfsu.edu), San Francisco State University
First submitted 1999-02-24
Edited by Kellie Whittaker (2012-04-30)
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