Rana aurora Baird and Girard, 1852(b)
Christopher A. Pearl1
1. Historical versus Current Distribution. Biochemical, ecological, and
life-history differences between northern red-legged frogs (historically known as
Rana aurora aurora) and California red-legged frogs
(historically known as Rana aurora draytonii) support
recognition of these taxa as distinct species (Hayes and Miyamoto, 1984; H.B. Shaffer and
G.M. Fellers, in preparation). Northern red-legged frogs occur
along the California coast north of Elk Creek (Mendocino County) into southwestern
Oregon, where their historical range broadens eastward through the Rogue River drainage
into the lower elevations of the Cascade Range (Fitch, 1936; Dunlap, 1955; Dumas, 1966;
Nussbaum et al., 1983; Jennings and Hayes, 1994a; G.M. Fellers, personal
communication). Northern red-legged frogs utilize wetlands between sea level and
1,200 m in elevation, west of the Cascade crest through northwestern Oregon, western
Washington, and southwestern British Columbia. Northernmost populations occur near
the northern end of Vancouver Island and Sullivan Bay, British Columbia (Green and
Campbell, 1984; Stebbins, 1985). Oregon’s Willamette Valley (and potentially
the Rogue Valley) appear to be the most reduced and fragmented portions of the range,
potentially the result of intensive land use and establishment of a variety of non-native
predators (Nussbaum et al., 1983; St. John, 1987; Blaustein and Wake, 1990).
2. Historical versus Current Abundance. Several herpetologists have suggested
that the abundance of northern red-legged frogs in Oregon’s Willamette Valley has
declined (Nussbaum et al., 1983; St. John, 1987; Blaustein and Wake, 1990). Recent
surveys suggest reproductive populations remain on much of the valley floor (C. A. P.,
Oregon Department of Fish and Wildlife, unpublished data). Northern red-legged
frogs are relatively widespread in portions of western Washington (K.R. McAllister et
al., 1993; Richter and Azous, 1995; Adams et al., 1999), although analysis of present
occurrence at historical sites has not been conducted. They also were widespread
historically in southwestern British Columbia and remain common in at least segments of
that range (R. Haycock, S. Orchard, personal communications).
3. Life History Features.
Reproduction is aquatic.
i. Breeding migrations. Northern red-legged frogs often make extensive movements
to breeding wetlands from summer habitats (Nussbaum et al., 1983; Hayes et al.,
2001). Males generally reach breeding sites before females, sometimes as early as
October, but usually arrive in larger numbers in November–December in Oregon and
northern California (Storm and Pimentel, 1954; Storm, 1960; Twedt, 1993).
ii. Breeding habitat. Oviposition generally occurs in vegetated shallows of
wetlands with little flow (Storm, 1960; Licht, 1971), but egg masses can be deposited in
water up to 5 m (Calef, 1973b). Breeding sites can be permanent or temporary, with
inundation usually necessary into June for successful metamorphosis in the Willamette
Valley (Storm, 1960; Nussbaum et al., 1983). Breeding is initiated when water
temperatures exceed 6–7 ˚C (usually in January), and can extend through March
(Storm, 1960; personal observations).
i. Egg deposition sites. Egg masses are usually attached to herbaceous vegetation
in areas with little or no flow (Storm, 1960; Calef, 1973b).
ii. Clutch size. Females deposit an average of 530–830 eggs/mass, with a
range between 200–1,100 (Storm, 1960; Calef, 1973a; Licht, 1974).
i. Length of larval stage. In Oregon and Washington, eggs generally hatch after
about 30–45 d (often in March–April) and reach metamorphosis 11–14 wk
later in June–July (Storm, 1960; Calef, 1973a; Licht, 1974; H.A. Brown, 1975b).
ii. Larval requirements.
a. Food. Larvae consume a variety of epiphytic algae and can alter species
composition and standing crop in laboratory conditions (Dickman, 1968).
b. Cover. Larvae appear to utilize relatively dense vegetation as cover (Nussbaum
et al., 1983; personal observations). Weins (1970) found that larvae became
conditioned to prefer microhabitats possessing complex background patterns in laboratory
trials, potentially suggesting an affinity for vegetated areas.
iii. Larval polymorphisms. Not reported.
iv. Features of metamorphosis. Approximately 5% of embryos survived to
metamorphosis at two British Columbia breeding sites (Calef, 1973a; Licht, 1974).
Northern red-legged frogs generally transform at 20–25 mm SVL (Storm, 1960; Calef,
1973a). At a northwestern Washington site, developmental periods for larvae
averaged 110 d; mean size at metamorphosis was 28.7 mm (H.A. Brown, 1975b).
v. Post-metamorphic migrations. Juveniles often remain around edges of breeding
ponds for short periods (days to weeks) before dispersing, but cues for emigration are
not well known (Licht, 1986; Twedt, 1993). Dispersal distances of newly
transformed juveniles appear to be related to size at metamorphosis (N. Chelgren, Oregon
State University, unpublished data). In the fall, juveniles have been observed in
riparian areas > 0.5 km from nearest known breeding site (C.A.P. and M. Hayes,
Habitat. After dispersing from breeding habitats, juveniles tend to occupy
relatively moist, densely vegetated riparian microhabitats during the summer (Twedt,
1993; personal observations). Movements away from these microenvironments may be
related to elevated moisture levels (Licht, 1986). Hayes and Hayes (2003) have
recently documented juvenile growth rates.
Habitat. Most northern red-legged frog adults appear to leave breeding sites
relatively soon after the breeding period and may move substantial distances (> 300 m)
from breeding pools in mesic forests and riparian areas (Nussbaum et al., 1983; Licht,
1986; Gomez and Anthony, 1996; Hayes et al., 2001). Summer habitats of adults in
the mid elevations of the Oregon Cascade range include streambanks and moist riparian
areas (Hayes et al., 2001; personal observations). At one northern California
breeding lagoon, adults tended to use microhabitats adjacent to standing water rather
than remaining in standing water (Twedt, 1993).
Greater numbers of
adult northern red-legged frogs have been trapped in coniferous stands of moderate
moisture than in drier stands in the Oregon and Washington Cascades (Aubry and Hall,
1991; Bury et al., 1991). One study found red-legged frog adults more frequently in
older managed forest stands (Aubry, 2000), but other terrestrial studies have not
documented clear preferences for any stand age in managed and unmanaged forests (Aubry
and Hall, 1991; Bury et al., 1991; Bosakowski, 1999). Conclusions of the
aforementioned terrestrial studies are limited by low captures in pitfall traps and
variable juxtaposition of sampled stands relative to breeding sites.
F. Home Range
Size. Unknown, but adults are wide ranging (see "Adult Habitat" and
"Breeding migrations" above).
Territories. Northern red-legged frogs generally are not considered territorial,
but males can act aggressively toward one another at breeding sites, and egg masses are
often deposited in a dispersed fashion, in contrast to other northwestern lentic ranid
frogs (Calef, 1973b; Nussbaum et al., 1983; personal observation).
Aestivation/Avoiding Dessication. Not documented.
Migrations. See "Breeding migrations" and "Post-metamorphic
(Hibernation). Unknown, but likely in northern ranges and higher elevations.
Adults in southern and coastal ranges can remain active through winter (Nussbaum et al.,
1983; Twedt, 1993).
Associations/Exclusions. Northern red-legged frogs often share breeding sites with
northwestern salamanders (Ambystoma gracile), long-toed salamanders
(A. macrodactylum), Pacific chorus frogs (Hyla
regilla), rough-skinned newts (Taricha granulosa), and
introduced American bullfrogs (Rana catesbeiana). In moderate
elevations of the Cascade Range (about 600–1000 m), northern red-legged frogs may
share breeding ponds and co-occur along streams with Cascade frogs (Rana
cascadae; Nussbaum et al., 1983; Hayes, 1996; B. Bury and D. Major,
unpublished data). Historically, northern red-legged frogs occurred syntopically
with Oregon spotted frogs (Rana pretiosa) in lowland western Oregon,
Washington, and southwestern British Columbia (Licht, 1971, 1974, 1986; Nussbaum et al.,
1983; K.R. McAllister et al., 1993). Negative interactions have been suggested
between northern red-legged frogs and introduced bullfrogs and sport-fish in Oregon
(Hayes and Jennings, 1986; Kiesecker and Blaustein, 1997a, 1998).
L. Age/Size at
Reproductive Maturity. Males can become sexually mature the breeding season
following metamorphosis, but the majority breed only after 2 yr of age (Licht, 1974; see
also Hayes and Hayes, 2003). Females usually reproduce after they reach 3 yr old,
although a small portion may be able to breed in the second season after transforming
Poorly known in field situations, but reportedly can exceed 10 yr in captivity (Cowan,
Behavior. Northern red-legged frogs consume a variety of small insects, arachnids,
and mollusks (Fitch, 1936; Licht, 1986). Larger adults are able to take larger food
items, including juvenile conspecifics and salamanders (Licht, 1986; Rabinowe et al.,
2002). Young frogs are believed to forage in moist areas close to water, pursuing
food farther away only during wet periods (Licht, 1986).
Larval northern red-legged frogs are eaten by fish, rough-skinned newts, northwestern
salamanders, giant water bugs (Belostomatidae), larval diving beetles (Dytiscidae), and
anisopteran odonates (Calef, 1973a; Licht, 1974). Garter snakes (particularly
common garter snakes [Thamnophis sirtalis]) and adult northern
red-legged frogs consume juvenile frogs, and herons and raccoons also prey on adults
(Licht, 1974, 1986; Gregory, 1979; personal observations). Predation by introduced
game-fish and American bullfrogs may represent important threats to northern red-legged
frogs throughout their range (Hayes and Jennings, 1986; Twedt, 1993; Kiesecker and
Blaustein, 1997a, 1998).
Mechanisms. Tadpoles reduce activity when exposed to chemical cues of injured
conspecifics in lab trials (Wilson and Lefcort, 1993), and ammonium (NH4+) may be a
component of exudates from disturbed larvae (Kiesecker et al., 1999).
red-legged frogs reduced activity and distanced themselves from native odonate predators
in lab trials and were larger at metamorphosis when the predator was present (Barnett
and Richardson, 2002). A laboratory study found that northern red-legged frog
tadpoles reared in the presence of predators fed conspecifics or cues of injured
conspecifics transformed earlier and smaller than controls (Kiesecker et al.,
2002). One field study did not detect northern red-legged frog tadpole avoidance of
injured conspecifics (Adams and Claeson, 1998).
Adults possess long
rear legs and exceptional leaping ability, which, in tandem with cryptic coloration and
stationary evasion behavior, can make juveniles and adults difficult for terrestrial
predators to apprehend (Gregory, 1979).
An iridovirus has been reported from northern California (Mao et al., 1999).
Lefcort and Blaustein (1995) reported altered behavior when larvae were exposed to the
yeast Candida humicola, which may be transmitted through water and fecal
material (Richards, 1958).
The trematodes Megalodiscus microphagus and Prosthopycoides
lynchi have been documented in the digestive tracts of Oregon northern
red-legged frogs (Macy, 1960; Martin, 1966a). Johnson et al. (2002) report
moderate levels of morphological abnormalities in juvenile northern red-legged frogs
relative to other Pacific northwestern amphibians. These malformations potentially
are associated with infection by the trematode Ribeiroia ondatrae
(Johnson et al., 2002).
4. Conservation. Thorough field studies documenting declines in northern
red-legged frogs are lacking, but a broad array of potential stressors may affect this
species. Non-native American bullfrogs are established throughout much of the
lowland range of northern red-legged frogs west of the Cascades, and bullfrogs have been
hypothesized as displacing northern red-legged frogs here (Nussbaum et al., 1983; St.
John, 1987; Kiesecker and Blaustein, 1997a, 1998). In Oregon, northern red-legged
frog larvae have been found to compete poorly with bullfrog larvae when food resources
are concentrated (Kiesecker and Blaustein, 1998; Kiesecker et al., 2001b). However,
other studies conducted in Washington have failed to identify direct effects of
competition on northern red-legged frog larvae or exclusion from wetlands supporting
bullfrogs (Richter and Azous, 1995; Adams, 2000). Negative associations of nonnative
fish appear to be of greater importance (Adams, 1999, 2000), and the interactive effects
of fishes and bullfrogs may be greater than either separately (Kiesecker and Blaustein,
degradation and altered hydrological regimes associated with agricultural and urban land
uses are also of concern in the Puget Trough and Willamette Valley (Nussbaum et al.,
1983; Platin, 1994; Richter and Azous, 1995; De Solla et al., 2002b). However,
northern red-legged frogs persist in some urbanized habitats in the region (Richter and
Azous, 1995, 2000). Laboratory investigations of nitrogenous by-products from
agricultural fertilizers suggest northern red-legged frog larvae are intermediate among
Willamette Valley amphibians in their sensitivity to ammonium sulfate, but may be
susceptible to ammonium ions derived from related compounds (Schuytema and Nebeker,
1999b; Nebeker and Schuytema, 2000). Larval northern red-legged frogs are
relatively susceptible to nitrite, a form that is usually short-lived in aerobic field
conditions (Marco et al., 1999). De Solla et al (2002a) found organochlorine
pesticides and PCB residues in eggs of northern red-legged frogs, but levels were not
high relative to reference sites. Hatching rates of northern red-legged frogs in
their lowland range do not appear to be reduced by ambient UV-B radiation, although
embryos may be susceptible to future increases in UV-B irradiance (Blaustein et al.,
1996; Ovaska et al., 1997; Belden and Blaustein, 2002a). Modeling studies suggest
stressors that impact juvenile red-legged frogs have the greatest potential to influence
population fluctuations (Biek et al., 2002).
frogs are considered a Species of Special Concern in California (California Department of
Fish and Game, 1999), Sensitive-Vulnerable in Oregon’s Willamette Valley, and
Sensitive-Unknown elsewhere in Oregon (Oregon Natural Heritage Program, 1995).
1Christopher A. Pearl
USGS Forest and Rangeland Ecosystem Science Center
3200 SW Jefferson Way
Corvallis, Oregon 97331
Literature references for Amphibian Declines: The Conservation Status of United States Species, edited by Michael Lannoo, are here.
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