Kupferberg (1997) studied the invasion of the American Bullfrog (Rana catesbeiana) into a northern California river system (in the Angelo Coast Range Reserve, Mendocino County, California) where it is not native. Native Foothill Yellow-legged Frogs (Rana boylii) were nearly an order of magnitude less abundant in reaches where R. catesbeiana was well established. Competition experiments indicated that R. catesbeiana tadpoles caused a significant reduction in both survivorship and growth of R. boylii. Competition appeared to be mediated by competition for algal resources on which both species feed.There was no evidence of behavioral or chemical interference.
Kupferberg et al. (2009) report on outbreaks of a parasitic copepod, the Anchor Worm (Lernaea cyprinacea), on Foothill Yellow-legged Frogs in a northern California river in 2006 and 2008. Lernaea cyprinacea is originally a Eurasian species, but is now far more widespread. These copepods were most frequently found embedded near a hind limb or the cloaca. Infestation rates were much higher downstream. In both years of the study, recently metamorphosed frogs with copepods were, on average, slightly smaller than those not infested and, at least for frogs parasitized early in development, it appeared that morphological abnormalities may result from infestation. Copepod outbreaks were associated with three unusual conditions: high water temperature (in 2006 and 2008); drought induced low flow (2008); and high densities of tadpoles. Kupferberg et al. suggest that increasing summer water temperatures, decreased daily discharge, or a combination of both promote outbreaks of this non-native parasite on this native host and could present a threat to the long-term conservation of R. boylii under the flow regime scenarios predicted by climate change models.
Rana boylii is primarily diurnal (Behler and King 1979). The voice of Rana boylii is rarely heard. It is a gutteral, grating sound on one pitch or with rising inflection, a single croak lasting 1/2 to 3/4 seconds. Four or five croaks may be delivered in rapid succession followed by a rattling sound, the entire sequence lasting about 2 1/2 seconds. The inconspicuous vocal sac is evident on each side of the throat, in front of the forelimbs. (Stebbins 1985) Recordings of the calls can be heard here.
The endemic river-dwelling Foothill Yellow-legged Frog (Rana boylii) of California and Oregon (U.S.A.) has disappeared from over 50% of historically occupied localities, with absences more common in close proximity to large dams. Persisting populations are small relative to those in rivers without reservoirs and are harmed by poorly timed flow releases and habitat fragmentation. A general population model developed for this species indicates that many R. boylii populations are at risk of extirpation by virtue of low abundance, even before hydrologic stressors and non-native predators (such as bull frogs or bass) are considered. (Kupferberg et al. 2009 (see report here); Sarah Kupferberg, in litt. August 2010)
Recent radiotelemetry and genetic research shows that these frogs use entire watersheds, sometimes migrating many kilometers to mate and lay eggs at breeding sites that are used year after year. To be successful, their unique survival strategy requires use of all habitats from small creeks to big rivers within a basin. Tadpoles cannot mature into frogs without access to sunlit channels with abundant algal foods in the summer, while juveniles and adults cannot survive winter floods without access to refugia such as small tributaries. During the past 150 years, humans have permanently and drastically transformed riverscapes in the Sierran foothills and the Pacific coast-ranges. This human appropriation of rivers is likely to intensify as we search for carbon-neutral methods to produce electricity (e.g., harnessing hydropower), provide drinking water to urban centers, and deliver irrigation to agricultural users. Current listing status as a California Species of Special Concern does not provide adequate protection for these frogs. (Sarah Kupferberg, in litt. August 2010)
Rana boylii ranges from western Oregon to southern California (Los Angeles County near the coast and Kern County inland, absent from the Central Valley) (Behler and King 1979). It is found west of the crest of the Cascade Mountains in Oregon, south in the coastal mountains of California to the San Gabriel River (Los Angeles County); in the Sierra Nevada foothills to about 1830 meters; and in other isolated populations in California. (Stebbins 1985)
Rana boylii is a frog of woodland, chaparral, and forest. It is usually found near water, especially near riffles where there are rocks and sunny banks. When frightened, it dives to the bottom and takes refuge among stones, silt, or vegetation. It occurs from sea level to around 2130 meters. (Stebbins 1985)
The Red-legged Frog (Rana aurora) has red on the underside of its hind legs, usually has a dark mask, has well defined dorsolateral folds, and has smooth eardrums. Relative to R. boylii, the Mountain Yellow-legged Frog (Rana muscosa) has smoother skin, generally heavier spotting and mottling dorsally, usually lacks a snout patch, and often has dark toe tips. The Tailed Frog (Ascaphus truei) has eyes with vertical pupils (horizontal in R. boylii) and 5th (outer) hind toe broadest (not enlarged in R. boylii). (Stebbins 1985)
The Foothill Yellow-legged Frog (Rana boylii) may be gray, brown, reddish, or olive above. It is sometimes plain-colored, but more often spotted and mottled. Colors are usually well matched to the local rocks and soil. This frog has truly yellow legs, with the yellow extending from the underside of the hind legs onto the lower abdomen (on young frogs, yellow on the hind legs may be faint or absent). The snout has a triangular, usually buff-colored patch from its tip to a line connecting the eyelids. No mask is present. The throat and chest often have dark spots. The skin, including the eardrums, is noticeably granular. There are indistinct dorsolateral folds. The male has a swollen and darkened thumb base. (Stebbins 1985)
Rana boylii breeds from mid-March to early June, after high water of streams subsides (Stebbins 1985). Egg clusters are attached to the downstream side of submerged rocks (Behler and King 1979). Females are able to find low-flow spots to protect their eggs even when the ambient flow velocity is very swift (Kupferberg 1996; Sarah Kupferberg, in litt. August 2010).
Rana boylii adults are around 3.5 to 7 cm long (Stebbins 1985).
Davidson (2004) found a strong statistical association between declines in populations of Rana boylii (and several related species) and total upwind pesticide use. Davidson suggests that the common perception that field levels of pesticides are extremely low may be a sampling artifact since at any one location the bulk of pesticide applications occurs during a few short time periods. Thus, animals may experience much higher short-term exposure than insufficiently frequent sampling might indicate.
Davidson et al. (2007) studied the effects of the pathogenic chytrid fungus Batrachochytrium dendrobatidis on Rana boylii. They found that chytrid infection reduced growth of newly metamorphosed frogs by around half, but did not significantly increase mortality. Interestingly, Rana boylii skin peptides strongly inhibited chytrid growth in vitro, which may explain why chytrid exposure did not increase mortality. However, skin peptide defenses were significantly reduced after exposure to a sub-lethal dose of the pesticide carbaryl, suggesting that pesticides may inhibit this innate immune defense and increase susceptibility to disease, including chytrid infection. The authors found no statistical interaction between chytrid and pesticide exposure on mortality or growth (i.e., under the conditions of these experiments, there was no evidence of a synergistic effect of pesticide and chytrid exposure on mortality). However, they suggest that this could be because for this well-defended anuran species, even the reduced level of skin peptides present after a single exposure to carbaryl was still sufficient to ward off the lethal effects of chytrid infection. In the field, animals may receive multiple exposures to carbaryl or other pesticides, which might make intensify their effect in increasing susceptibility to chytrid infection. It is also possible that while carbaryl suppressed skin peptide levels, other aspects of the immune system may have protected R. boylii against chytrid infection.
