Description
The mountain yellow-legged frog is a moderate-sized (approximately 40 to 80 millimeters (mm) [1.5 to 3.25 inches] snout-vent length) ranid frog. As is common with ranid frogs, females average slightly larger than males, and males have a swollen, darkened thumb base. Adult coloration is highly variable, with a dorsal pattern ranging from discrete dark spots that can be few and large, to smaller and more numerous spots with a mixture of sizes and shapes. Irregular lichen-like patches, or a poorly defined reticulum, may also be present. The dorsal coloration is usually a mix of brown and yellow, but often with gray, red, or green-brown; some individuals may be a dark brown with little pattern. Dorsolateral folds are present, but not usually prominent. The venter and undersurfaces of the hind limbs are yellow, with ranges in hue from pale lemon yellow to an intense sun yellow. The throat is white or yellow, sometimes with a mottling of dark pigment. This frog lacks vocal sacs, and the tympanum is smoother and the toe tips darker than those of the foothill-yellow legged frog (Rana boylii), with which it may be confused. The mountain yellow-legged frog produces an odiferous secretion when disturbed.
The larvae (tadpoles) of this species are generally mottled brown in dorsal coloration with a golden tint and a faintly-yellow venter. They range to 72 mm (2.8 inches) in total length. There are a maximum of 7 labial tooth rows (2-3 upper and 4 lower). Larvae often take 2 to 4 years or more to reach metamorphosis.
Taxonomy
Distribution
The mountain yellow-legged frog (MYLF), a member of the true frog family Ranidae, consists of two species. The southern mountain yellow-legged frog (Rana muscosa) is endemic to the southern Sierra Nevada and the Transverse Ranges, while the Sierra Nevada mountain yellow-legged frog (R. sierrae) is native to the northern and central Sierra Nevada. MYLF distribution (PDF) is generally restricted to mid- to high-elevation aquatic habitat, with most extant populations occurring on national park and national forest lands.
The mountain yellow-legged frog was once extremely abundant in aquatic ecosystems of the Sierra Nevada (Grinnell and Storer 1924, Storer 1925, Zweifel 1955). It was distributed nearly continuously in high elevation water bodies in the Sierra Nevada Mountains of California and Nevada, from southern Plumas County to southern Tulare County (Jennings and Hayes 1994), at elevations mostly above 1,820 meters (6,000 feet). The historic range of the Sierra Nevada mountain yellow-legged frog encompasses 10 National Forests (Lassen, Plumas, Tahoe, Lake Tahoe Basin Management Unit, Eldorado, Stanislaus, Toiyabe, Inyo, Sierra and Sequoia) and 3 National Parks (Yosemite, Sequoia and Kings Canyon). Since about 1970, mountain yellow-legged frog numbers and populations have undergone a precipitous decline throughout the Sierra Nevada (Sherman and Morton 1993, Jennings and Hayes 1994, Bradford et al. 1994a, Jennings 1996, Drost and Fellers 1996, Matthews and Knapp 1999, Knapp and Matthews 2000). Further declines continue to be documented. Mountain yellow-legged frogs have disappeared from between 70 and 90 percent of their historic localities (Center for Biological Diversity and Pacific Rivers Council 2000). Remaining populations are widely scattered and consist of few breeding adults (Center for Biological Diversity and Pacific Rivers Council 2000). Numerous factors, separately and in combination, have contributed to the species' decline. Introduction of non-native fishes, pesticides, ultraviolet radiation, pathogens, acidification from atmospheric deposition, nitrate deposition, livestock grazing, recreational activities, and drought have all been identified as potential factors impacting this species and its habitat.
Habitat and Life History
Mountain yellow-legged frogs in the Sierra Nevada live in high mountain lakes, ponds, tarns, and streams--largely in areas that were glaciated as recently as 10,000 years ago. Alpine lakes used by mountain yellow-legged frogs usually have open shorelines, margins that are grassy or muddy and have a depth greater than 2.5 meters (greater than 8.2 feet). Adults are typically found sitting on rocks along the shoreline, usually where there is little or no vegetation. Larvae are often distributed in the warm water shallow areas along the shoreline during the daytime. Mountain yellow-legged frogs also use stream habitats, especially in the northern part of their range. Breeding habitat consists of ponds, lakes and streams that do not dry out in summer, are deep enough to prevent freezing to the bottom in winter, and do not contain fish. MYLF tadpoles take two to four years to metamorphose into juvenile frogs, depending on water temperature. In the Sierra Nevada, adult frogs apparently hibernate during the coldest winter months. Larvae and adults generally overwinter under ice and have been found to overwinter up to 9 months in the bottoms of lakes. Mountain yellow-legged frogs emerge from overwintering sites immediately following snowmelt. Adults sometimes travel over snow to reach preferred breeding sites early in the season. Breeding activity begins early in the spring and can range from April at lower elevations to June and July in higher elevations. Oviposition typically occurs in the shallow water of ponds or fast flowing inlet streams. The egg masses are normally attached to rocks, gravel, under banks, or to vegetation. Clutch size varies from 15 to 350 eggs per egg mass and egg hatching times range from 18 to 21 days. The time required to develop from fertilization to metamorphosis is believed to vary between 1 and 3.5 years. The time required to reach reproductive maturity is believed to vary between 3 and 4 years after metamorphosis. Adults tend to move between selected breeding, feeding, and overwintering habitats during the course of the year. Adult mountain yellow-legged frogs are thought to feed preferentially upon terrestrial insects and adult stages of aquatic insects while on the shore and in shallow water. Larvae graze on algae and diatoms along rocky bottoms in streams, lakes, and ponds.