The Endangered Mountain Yellow-Legged Frog

(July 16) A critically endangered species inhabiting the San Bernardino Mountains is one of the two types of  mountain yellow-legged frogs, Rana muscosa.
Another type of yellow-legged frog, which is also endangered, is the  and Rana sierra, which does not live in the San Bernardino Mountains but does inhabit the Sierra Nevada range.
Yellow legged frongs have existed in the mountains of California and Nevada for millions of years, moving up and down in elevation as glaciers advanced and retreated, over time adapting to alpine lake habitats too cold for other amphibians and devoid of predators.
These frogs have been on the decline since the mid-19th century, when settlers began to stock these naturally-fishless habitats with trout.
While they exist elsewhere, populations of Rana muscosa were found in the San Gabriel, San Bernardino, and San Jacinto Mountains, and on Palomar Mountain. In these ranges, R. muscosa was found primarily in fast-flowing streams.
Reproduction in mountain yellow-legged frogs begins soon after lakes, ponds, and streams become ice-free, ranging from April at low elevations to June or July at high elevations. Females lay 40-300 eggs in a compact cluster that resembles a small bunch of grapes. Eggs are similar to those of other frogs, with the embryo being encased in a thick protective jelly coat. Individual eggs, including the embryo and jelly coat, are 3/8″-1/2″ in diameter and the entire egg mass is often the size of a tennis ball. Eggs are often attached to submerged vegetation, undercut banks, or near-shore rocks. Lake-dwelling frogs often lay their eggs in small streams entering or leaving the lake.
The frogs are in part responsible for their own endangerment in that eggs are often preyed on by mountain yellow-legged frog tadpoles. The eggs are also vulnerable to invertebrates.
After 2-3 weeks when the embryos are approximately 1/4″ long and are sufficiently developed to swim, they wriggle their way out of the egg mass and start life as a tadpole.
Mountain yellow-legged frog tadpoles are among the largest of any frog in North America, reaching sizes of more than 3″ (76 mm). Dorsal coloration varies from drab olive to dark chocolate brown, and the belly is black with gold flecks.
One of the most unique aspects of the natural history of the mountain yellow-legged frog is that tadpoles usually overwinter 2-3 times before transforming (“metamorphosing”) into young frogs Most anurans (the group that includes frogs and toads) complete the transformation from egg to froglet or toadlet in a single summer. Since mountain yellow-legged frogs are typically at high elevations where water temperatures are cold, tadpoles grow very slowly and are not ready to metamophose into young frogs by the end of their first summer. They spend at least one winter beneath the ice and generally don’t metamorphose until their third or fourth year.
Throughout the summer, tadpoles of all ages congregate in the warm shallows near shore where they feed on algae. In the months prior to metamorphosis, the tadpoles begin to grow legs and, during their final weeks as tadpoles, they reabsorb their tails, replace their gills with lungs, and finally hop onto land.
Reborn as frogs, they remain as juveniles for up to four years before reaching adulthood and sexual maturity. Adult mountain yellow-legged frogs typically range in size from 2-3″. Color patterns are highly variable across the range of the mountain yellow-legged frog. Dorsal surfaces have a light- to medium-brown background color that is heavily flecked with tan and dark-brown spots. This color pattern provides excellent camouflage against a wide variety of backgrounds. Muscosa means “mossy” in Latin. Undersides of both species range in color from cream to brilliant yellow. Males tend to be smaller than females.
Juvenile and adult mountain yellow-legged frogs are highly aquatic, and are rarely found more than a few hops from water. On warm days, they bask at the water’s edge, often aggregating in dense clumps that allow frogs to maximize heat intake while minimizing water loss. By selecting particular basking sites, frogs are able to raise their body temperatures well above the ambient air temperature. Frogs feed opportunistically on aquatic and terrestrial invertebrates and occasionally on other amphibians. With the arrival of cooler temperatures in the fall, frogs retreat into deep-water habitats where they spend the entire winter. Mountain yellow-legged frogs are long-lived, with adults likely reaching ages of 15-20 years.
During the active season, mountain yellow-legged frogs often move hundreds of meters between breeding, feeding, and overwintering habitats. When moving between these habitats, frogs often follow lake shores and streams, but will also move short distances across dry land. Over their lifetimes, individual frogs typically show high fidelity to particular lakes or ponds, and dispersal from natal areas is characteristic only of juveniles.
During the spring breeding season, male mountain yellow-legged frogs attract females with their distinctive calls Rana muscosa lack the vocal sacs that many frogs and toads use to produce calls, but are able to produce a relatively loud call nonetheless. The call is rarely heard because it is made from underwater. To initiate mating, a male grasps a female with his powerful front legs (“amplexus”), positioning him to fertilize the eggs as a female lays them. Single males often attempt to break apart amplexing pairs, and if successful often pair with the now-single female. Some mountain yellow-legged frogs smell strongly of garlic during the breeding season, but the source of the smell is unknown.
While these frogs have been around for eons,  the same adaptations that served the mountain yellow-legged frog so well  have also made it extraordinarily vulnerable to the nonnative trout that were stocked into many of these habitats during the past century. Ninety percent of Rana muscosa populations have disappeared during the past century. Mountain Yellow-Legged Frog are an integral part of the ecosystem: the tadpoles feed on algae, but after the metamorphosis into frogs they feed primarily on aquatic insects. The frogs are an important prey for birds, garter snakes, coyotes and bears. In some areas the disappearance of mountain yellow-legged frogs has led to a t decline in garter snake populations.

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