Reproduction and Eggs
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" (10-13 mm) in diameter and the entire egg mass is often the size of a tennis ball (Zweifel 1955; Stebbins 2003). 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. Eggs are often preyed on by invertebrates and mountain yellow-legged frog tadpoles (Vredenburg 2000). After 2-3 weeks when the embryos are approximately 1/4" (6 mm) 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 (Zweifel 1955; Vredenburg et al. 2005). Most anurans (the group that includes frogs and toads) complete the transformation from egg to froglet or toadlet in a single summer. However, habitats occupied by mountain yellow-legged frogs are typically at high elevations where water temperatures are cold. As a consequence, tadpoles grow very slowly and are not ready to metamophose into young frogs by the end of their first summer. Instead, the tadpoles spend at least one winter beneath the ice (Bradford 1983) and generally don't metamorphose until their third or fourth year when they are 2.5-3.7" (65-95 mm) in length.
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.
Juveniles and Adults
Following metamorphosis from the tadpole stage, frogs 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" (snout-vent length; 50-80 mm). 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, and is the basis for the scientific name of the southern yellow-legged frog, Rana muscosa; muscosa means "mossy" in Latin. Undersides of both species range in color from cream to brilliant yellow (Stebbins 2003). Males can be distinguished from females by the large nuptial pads on the "thumbs" of front legs of males. Males also 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 (Vredenburg et al. 2005). 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 (Bradford 1984). Frogs feed opportunistically on aquatic and terrestrial invertebrates (Finlay and Vredenburg 2007) and occasionally on other amphibians (Pope and Matthews 2002). 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 (Matthews and Miaud 2007), 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 (Pope and Matthews 2001). When moving between these habitats, frogs often follow lake shores and streams, but will also move short distances across dry land (Matthews and Pope 1999). Over their lifetimes, individual frogs typically show high fidelity to particular lakes or ponds (Matthews and Preisler 2010), 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 (Vredenburg et al. 2005). Rana muscosa and Rana sierrae 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 hear this vocalization, click here. 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.