Field Guide to Amphibians
of Southern California
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Mountain Yellow-Legged Frog (Rana muscosa)
Photograph (C)Gary Nafis
On the Angeles National Forest, small populations of mountain yellow-legged frogs occur in the San Gabriel Mountains in the following locations: Bear Gulch, Devil's Canyon, Little Rock Creek, South Fork of Big Rock Creek, and Vincent Gulch (67 Federal Register 44382). In the San Jacinto mountains of the San Bernardino National Forest, small populations occur in Fuller Mill Creek and Dark Canyon. Adults were detected in Hall Canyon in 1995, although not during surveys in 1998. In the San Bernardino Mountains, mountain yellow-legged frogs were recently (1998) rediscovered in the East Fork of City Creek (67 Federal Register 44382, U.S. Fish and Wildlife Service 2001). Due to a wildland fire in 2003 and subsequent flooding, this population is believed to be extirpated, although there is still some chance that they may still be present somewhere in the watershed. Two historically occupied sites on Palomar Mountain on the Cleveland National Forest appear to have been extirpated (67 Federal Register 44382, Jennings and Hayes 1994).
The mountain yellow-legged frog has been recognized as a distinctive species since the work of Zweifel in 1955 (Jennings and Hayes 1994). Prior to this time, the mountain yellow-legged frog was considered a subspecies of foothill yellow-legged frog, and the Sierran and southern California populations were considered distinct subspecies, Rana boylii sierrae and Rana boylii muscosa (Zweifel 1955). The southern California population of the mountain yellow-legged frog is listed as endangered under the Endangered Species Act as a distinct vertebrate population segment (67 Federal Register 44382).
Mountain yellow-legged frogs inhabit riverbanks, meadow streams, isolated pools, and lake borders in the Sierra Nevada (Stebbins 1985). In southern California, mountain yellow-legged frogs inhabit perennial mountain streams (i.e. streams that contain plunge pools or backwaters year-round, although not necessarily flowing year-round) with steep gradients—often in the chaparral belt—but may range up into small meadow streams at higher elevations (64 Federal Register 71714, U.S. Fish and Wildlife Service 2001). Mountain yellow-legged frogs are seldom found more than two or three jumps from water (Stebbins 1985). Highly aquatic, they occupy rocky shaded streams with cool waters originating from springs and snowmelt. Where characteristics of mountain streams extend to lower elevations (e.g., Eaton Canyon, San Gabriel River), mountain yellow-legged frogs have been recorded at elevations below 1,500 feet (457 meters) (U.S. Fish and Wildlife Service 2001). Mountain yellow-legged frogs are generally absent from very small creeks, most likely because these have insufficient depth for adequate refuge and overwintering. The species occurs along a variety of shorelines but appears to prefer open stream and lake margins that gently slope to a depth of 2–3 inches (5–8 centimeters) (Jennings and Hayes 1994). Such shorelines are probably necessary for oviposition and thermoregulation of larvae and postmetamorphs, and may provide refuge from predation if fish occur in adjacent deeper water (Jennings and Hayes 1994).
In the high Sierra Nevada, mountain yellow-legged frogs breed May–August, as soon as the ice melts from lakes (Stebbins 1985, Zweifel 1955). In southern California, they breed March–June after high water in streams subsides (Stebbins 1985). Eggs are typically deposited in shallow water (Mullally 1959) and may be attached to undercut banks or vegetation (Zweifel 1955). In streams in rocky canyons, the eggs may be attached to stones on the stream bottom (Zweifel 1955). In the Sierra Nevada, larvae select warm microhabitats (Bradford 1984). The time required to develop from fertilization to metamorphosis reportedly ranges up to 3.5 years, with reproductive maturity reached from 3 to 4 years after metamorphosis (67 Federal Register 44382).
Mountain yellow-legged frogs are diurnal and highly aquatic. The coldest winter months are spent in hibernation, probably under water beneath the ice in lakes and under shelter in or at the edge of streams (Zweifel 1955). Some individuals have been found overwintering in near-shore environments in deep crevices and under ledges (Matthews and Pope 1999). Mountain yellow-legged frogs emerge from overwintering sites in early spring, and breeding soon follows (U.S. Fish and Wildlife Service 2001). Timing of emergence from winter retreats is dependent on local climate. At lower elevations in southern California, most activity occurs from mid-March to October; however, juveniles have been found in November and early January (Zweifel 1955). At high elevations in southern California and the Sierra Nevada the period of activity is shorter, generally from May or June to mid-October (Zweifel 1955). In the high Sierra Nevada, mountain yellow-legged frogs have been documented to move seasonally between over wintering, breeding, and foraging habitats (Pope and Matthews 2001).
Diet and Foraging
Mountain yellow-legged frogs have been reported to consume ladybugs, dragonfly nymphs, and Yosemite toad (Bufo canorus) tadpoles (Mullally 1953). Juveniles and adults feed on beetles, flies, ants, bees, wasps, true bugs (Jennings and Hayes 1994), and treefrog tadpoles (Matthews and Pope 1999).
Predators of mountain yellow-legged frogs include western terrestrial garter snake (Thamnophis elegans) and introduced predatory fishes such as trout (Bradford 1989, Jennings and others 1992). Other animals known to prey on larvae and postmetamorphs include coyote (Canis latrans), Brewer's blackbird (Euphagus cyanocephalus), and Clark's nutcracker (Nucifraga columbiana) (Camp 1917, Moore 1929, Mullally and Cunningham 1956).
Bradford, D.F. 1984. Temperature modulation in a high-elevation amphibian, Rana muscosa. Copeia 1984 (4): 966-976.
Bradford, D.F.; Graber, D.M.; Tabatabai, F. 1993. Isolation of remaining populations of the native frog, Rana muscosa, by introduced fishes in the Sequoia and Kings Canyon National Parks, California. Conservation Biology 7(4): 882-888.
Camp, C.L. 1917. Notes on the systematic status of the toads and frogs of California. University of California Publications in Zoology 17 (9): 115-125.
Emlen, S.T. 1968. Territoriality in the bullfrog, Rana catesbeiana. Copeia 1968: 240-243.
Jennings, M.R.; Hayes, M.P. 1994. Amphibian and reptile species of special concern in California. Rancho Cordova, CA: California Department of Fish and Game, Inland Fisheries Division.
Jennings, W.B.; Bradford, D.F.; Johnson, D.F. 1992. Dependence of the garter snake Thamnophis elegans on amphibians in the Sierra Nevada of California. Journal of Herpetology 26(4): 503-505.
Martof, B.S. 1953. Territoriality in the green frog, Rana clamitans. Ecology 34: 165-174.
Matthews, K.R.; Pope, K.L. 1999. A telemetric study of the movement patterns and habitat use of Rana muscosa, the mountain yellow-legged frog, in a high-elevation basin in Kings Canyon National Park, California. Journal of Herpetology 33: 615-623.
Moore, R.D. 1929. Canis latrans lestes Merriam feeding on tadpoles and frogs. Journal of Mammalogy 10 (3): 255.
Mullally, D.P. 1953. Observations on the ecology of the toad Bufo canorus. Copeia 1953(3): 182-183.
Mullally, D.P.; Cunningham, J.D. 1956. Ecological relations of Rana muscosa at high elevations in the Sierra Nevada. Herpetologica 12(3): 189-198.
Pope, K.L.; Matthews, K.R. 2001. Movement ecology and seasonal distribution of mountain yellow-legged frogs, Rana muscosa, in a high-elevation Sierra Nevada basin. Copeia 2001: 787-793. Stebbins, R.C. 1985. A field guide to western amphibians and reptiles. 2d ed. New York, NY: Houghton Mifflin Company.
Stephenson, J.R.; Calcarone, G.M. 1999. Southern California mountains and foothills assessment: Habitat and species conservation issues. General Technical Report PSW-GTR-172. Albany, CA: Pacific Southwest Research Station, Forest Service, U.S. Department of Agriculture.
USDA Forest Service. 2002. Pacific Southwest Research Station Web Site. [Online]. Accessed March 14, 2002.
USDA Forest Service. 2002. Mountain yellow-legged frog conservation assessment and strategy Angeles and San Bernardino National Forests. Unpublished report on file.
U.S. Fish and Wildlife Service. 2001. Biological and conference opinions on the continued implementation of land and resource management plans for the four southern California National Forests, as modified by new interim management direction and conservation measures (1-6-00-F-773.2).
Zeiner, D.C.; Laudenslayer, W.F., Jr.; Mayer, K.E., eds. 1988. California's wildlife. Volume I: Amphibians and reptiles. Sacramento, CA: California Department of Fish and Game.
Zweifel, R.G. 1955. Ecology, distribution, and systematics of frogs of the Rana boylei group. University of California Publications in Zoology 54(4): 207-292.
Information gathered from California DFG - California Interagency Wildlife Task Group