Northern Red Legged Frog

Adult R. aurora range from 4.4 to 8.4 cm in length, with females significantly larger than males (Stebbins 1985; Hayes and Miyamoto 1984). The abdomen and underside of hind legs is characterized by a red or pinkish color, often set on a yellowish ground color (Stebbins 1985). Back is a brown, gray, olive, or reddish color, often with may small black flecks and irregular dark splotches, in some individuals forming a network of black lines (Stebbins 1985). Dorsolateral folds are prominent. Tadpoles range in length from 14 to 80 mm, and are a dark brown or olive, marked with darker spots (Storer 1925).R. aurora differs from its close relative, the California red legged frog, R. draytonii in several ways. Adult R. draytonii are 35 to 40 millimeters longer than adult R. aurora (Hayes and Miyamoto 1984). The dorsal spots of R. draytonii are more numerous, and usually have light centers (Stebbins 1985). R. draytonii also has rougher skin, shorter limbs and smaller eyes than R. aurora (Stebbins 1985). R. draytonii has paired vocal sacs and typically calls from the air , while R. aurora lacks vocal sacks ( Hayes and Krempels 1986; Licht 1969). Egg masses in R. draytonii are deposited such that the mass floats at the surface (Hayes and Miyamoto 1984), whereas R. aurora submerge the mass in deeper water (Licht 1969; Storm 1960). R. draytonii breed from November to April (Storer 1925), while R. aurora breeds from January to March (Nussbaum et al 1983). See other subspecies accounts at www.californiaherps.com: R. a. aurora (http://www.californiaherps.com/frogs/pages/r.a.aurora.html) and R. a. draytonii (http://www.californiaherps.com/frogs/pages/r.a.draytonii.html).

R. aurora ranges from Vancouver Island, British Columbia, Canada, south along the Pacific coast, west of the Cascade Ranges to Northern California. R. aurora live in areas subject to temporal and spatial changes, and therefore make use of a variety of habits, consisting of both aquatic, upland and riparian (Draft Recovery Plan). Adults breed in ponds or deep pools in slow-moving creeks. Where ponds are seasonal in nature, thickets and logjams along riparian corridors provide important non-breeding habitat. Populations are most likely to persist in areas with multiple breeding sites surrounded by suitable non-breeding habitat (N. Scott and G. Rathbun in litt. 1998).

Many factors are contributing to the decline of R. aurora populations, the main being habitat destruction and degradation. Introduced predators and perhaps disease have also contributed to the decline of this species (Fellers et al in press) Rana aurora is of particular conservation concern in the Pacific Northwest, where their population has decreased in abundance and in site occupancy in recent years. Nelson et al. (2017), used passive acoustic monitoring devices to show that the species breeds longer than previously thought (see Life History) and demonstrated the importance of continued acoustic monitoring for this sensitive species, especially because conservation strategies often rely on occupancy and detection surveys.

R. aurora breed during a 1-2 week period between January and March, depending on locality (Stebbins 1985; Nussbaum et al. 1983). Nelson et al. (2017), using passive acoustic monitoring devices, have shed light on this cryptically breeding ranid dispelling previous notions of their natural history. They found that red-legged frogs actually breed over the course of at least 32 days instead of a brief two week window as previously thought. Most interestingly, they report that underwater chorus length lasted nearly 8 hours on average, with a maximum chorus length of 14 hours over the course of a single day! Their acoustic analyses also showed calling effort was affected by temperature specifically reduced by cold snaps. Egg masses consist of between 300 and 5,000 eggs (Recovery Plan for the California Red-legged Frog 2002; Storer 1925; Fellers unpubl.). Egg masses are nearly always attached to emergent vegetation, submerged beneath the surface in the deepest water available (Hayes and Miyamoto 1984; Licht 1969; Storm 1960). Eggs hatch after 6 to 14 days depending on water temperature ( Jennings 1988). Larvae typically metamorphose in 3.5 to 7 months ( Storer 1925; Wright and Wright 1949), but some overwinter and transform after more than 12 months in the larval stage (Fellers et al. in press). Males may attain sexual maturity at 2 years, females at 3 (Jennings and Hayes 1985), and adult frogs may live 8 to 10 years. (Jennings et al in lit 1992). Larvae are thought to be algal grazers ( Jennings et al in lit 1992), and the adult diet consists mostly of invertebrates. Pacific Tree Fogs (Hyla regilla) and California mice (Peromyscus californicus are occasionally consumed by adult frogs ( Hayes and Tennant 1985). Juvenile frogs may be active both nocturnally and diurnally, whereas adult frogs are primarily active nocturnally ( Hayes and Tennant 1985). The primary predators on R. aurora include garter snakes (Thamnophis spp.), raccoons (Procyon lotor), and great blue herons (Ardea herodias). Less frequently, red-legged frogs are eaten by American bitterns (Botaurus lentiginosus), black-crowned night herons(Nycticorax nycticorax), and rarely by red-shouldered hawks (Buteo lineatus)(Jennings and Hayes 1990; Rathbun and Murphy 1996). Other introduced species such as the bullfrog (Rana catesbeiana) and non-native fish also prey on the frog.

The primary impact from humans has been from direct habitat loss, especially the construction of houses, shopping centers, and roads. Much of the range of R. aurora has been historically grazed, both by dairy and by beef cattle. Cattle grazing in riparian zones causes serious damage to the vegetation, stream channel, and water quality. R. aurora may have benefited from beef cattle grazing due to the increased number of stock ponds that are maintained for the cattle. R. aurora are also threatened by a number of introduced (non-native) species, some of which are regularly introduced by humans. These include sunfish, bass, trout, mosquitofish, and bullfrogs (Fellers et al in press).

Maximum longevity: 15 years (wild) Observations: Animals living in the north, also called Northern red-legged frogs, appear to develop slower, probably due to the colder water temperatures, and live longer. In California, maximum lifespan is probably around 10 years. Northern frogs also lay fewer eggs.

Red-legged frogs have a highly variable diet, eating any prey they can subdue that is not distasteful. Adults feed on invertebrates, small mammals and other amphibians like the small tree frog. Larvae are thought to feed on algae.

(Davidson, 1996; Defenders of Wildlife)

Rana aurora occcurs in the state of California in North America. Two subspecies of R. aurora occupy different ranges within the state. The first, Rana aurora aurora, or the Northern Red-legged frog, occupies the extreme northwestern corner of Calfornia, north to southern British Columbia, west of the Cascade crest. The second, Rana aurora draytonii, or the Calfornia Red-legged frog, occupies central and southern portions of the state, west of the Sierran divides and into the mountains of southern California up to an altitude of 4000 feet.

(Grinnell, 1917; Defenders of Wildlife; Davidson, 1996)

Biogeographic Regions: nearctic (Native )

Adult frogs must have emergent riparian vegetation near deep, still or slow-moving ponds or intermittent streams. These well-vegetated areas along the river are needed for escaping from predators, for shade to maintain cool water temperatures, and as shelter especially during the winter. Rana aurora aurora has the lowest upper and lower embryonic temperatures of any North American ranid frog, ranging from 4 to 21 degrees Celsius. Rana aurora draytonii cannot be exposed to water temperatures much higher than 29 degrees Celsius. They are found more often in intermittent than permanent waters because of predators that inhabit permanent waters. Red-legged frogs may move out of riparian zones into nearby upland forest during non-breeding seasons. R. a. draytonii may move seasonally within their aquatic habitats between places where they breed and foraging habitats.

(Cole, 1997; Defenders of Wildlife; Davidson, 1996)

Aquatic Biomes: lakes and ponds; rivers and streams

Average lifespan
Status: captivity: 15 years.

Rana aurora reaches from 2 to 5.25 inches in length. It is reddish brown to gray and contains many poorly defined dark specks and blotches, which are absent on the back and top of its head. A light stripe on its jaw borders its dark mask. Folds are present on its back and sides, and the underside is yellow with red on the lower abdomen and hind legs. Its toes are not fully webbed. Females grow larger than males. Males, however, have enlarged forearms and swollen thumbs. Rana aurora aurora has very smooth and thin skin and an unspotted dorsal surface. Rana aurora draytonii has thick, rough skin, light centered spots on its dorsal surface, and a larger build. Northern red-legged frogs have no vocal sacs while Southern red-legged frogs have paired vocal sacs.

(Green,1996; Hayes, 1986; Thomas,1993; USFW, 1996; Defenders of Wildlife)

Other Physical Features: ectothermic ; bilateral symmetry

The red-legged frog has been used as a resource for fisheries since the the gold rush of 1849. Its frog legs are used by many humans from the region as food. Like many other amphibians, this species can be used to indicate changes in the environment such as the cleanliness of the water and the amount of vegetation in the area. It is the prey of many native animals as well as a predator of many insects and other invertebrates, therefore it is necessary in sustaining an ecological balance in thier environment.

(Jennings, 1985; USFW 1996)

Development - Life Cycle: metamorphosis

Rana aurora draytonii has been declared a threatened species by the Fish and Wildlife Services following a year-long Congressional moratorium on listings which began April of 1995 and was lifted the next April by President Clinton. Rana aurora draytonii was harvested at the turn of the century for its prized frog legs. As a result of this overharvesting, populations declined drastically. Bullfrogs that eat the eggs of R. a. draytonii were introduced as a substitute for the red-legged frogs, along with non-native fish, and replaced them in habitat. Exotic plant species have also taken over the riparian habitat of R. a. draytonii. Dam construction also poses a threat to the frogs, it destroys and fragments its habitat and reservoirs favor aquatic predators. Road-building has also put silt into pools that the frogs dwell in, and flood projects along with livestock grazing along streams destroy emergent vegetation. Pollution from garbage and sewage contaminate the clean waters that provide a habitat for the frogs. R. a. draytonii has disappeared from 75 percent of its historic range. A few things are being done to help this subspecies recover. Dense stands of riparian vegetation in slow or still waters are being maintained along with sufficient water depth for these frogs. Silt removal is being proposed during fall months so as not to disturb the breeding season. Exotic plant species are being removed and red-legged frog tadpoles are being introduced. Fences and buffer zones around frog habitats are also being proposed to cattle ranchers. Results of studies on clear-cutting have shown that R.aurora was found more frequently in riparian environments than upslope areas.

(Cole, 1997; Davidson, 1996; USFW 1996, Defenders of Wildlife)

US Federal List: threatened

IUCN Red List of Threatened Species: least concern

The frog caught in a swamp near Angels Camp in Mark Twain's "The Notorious Jumping Frog of Calveras Country" was probably a red-legged frog.

The subspecies R. a. draytonii and R. a. aurora are placed together as a single species because of the recognition of hybrids from the coast region of northern California.

In the past, over 80,000 individuals were harvested every year for their legs.

(Camp, 1917; Jennings,1985; USFW 1996)

Reproduction in Northern Red-legged frogs occurs from late November to early April to ensure cool water, six or seven degrees Celsius. These conditions ensure embroyonic survival and sufficient water for larval growth and metamorphosis. Red-legged frogs breed via external fertilization. The male grasps the female in a process call amplexus, and while the female lays her eggs, he fertilizes them with a fluid containing sperm. The female lays egg masses (ranging from 2000 to 5000 eggs in R. a. draytonii and 500-1100 in R. a. aurora) in permanent bodies of water that contain extensive vegetation, consisting of cattails and tules or bulrushes growing in the water with a vertical orientation. R. a. draytonii eggs are attached at or near the surface of the water while R. a. aurora eggs are attached at a minimum depth of eighteen inches and at least two to three feet from the water's edge. The eggs are dark brown and range from 2.0 to 2.8 millimeters in diameter. The eggs hatch anywhere from six to fourteen days between July and September into brown tadpoles that can reach around three inches long within four to seven months. They then grow legs, lose their tales, and change into a juvenile form of the adult frog with dark masks on their faces and bright orange folds on their backs. As they grow into adults, the froglets move from shallow water to knee-deep water to hide from larger predators. Males can probably reproduce after three years of age while females reproduce after four. Life spans of the California Red-legged frog are about eight years for males and ten for females, while Northern red-legged frogs live twelve to fifteen years.

(Hickman and Roberts, 1995; Jennings, 1985; Davidson, 1996; USFW 1996; Defenders of Wildlife)

Key Reproductive Features: gonochoric/gonochoristic/dioecious (sexes separate)

Average number of offspring: 2000.

Average age at sexual or reproductive maturity (male)
Sex: male: 1095 days.

Average age at sexual or reproductive maturity (female)
Sex: female: 1460 days.

The northern red-legged frog (Rana aurora) is a species of amphibian whose range is the coastal region stretching from southwest British Columbia to southern Mendocino County in Northern California, and is protected in Oregon and California. As a member of the genus Rana, this species is considered a true frog, with characteristic smooth skin and a narrow waist. This frog requires still waters for breeding, and is rarely found at any great distance from its breeding ponds or marshes.

Northern red-legged frog adults may attain a length of 8 cm (3.1 in); they have dark facial masks and single characteristic light stripes along their jawlines. The northern red-legged frog has long, powerful legs well adapted to jumping. It is one of two amphibian species classified as red-legged frog, the other species being the California red-legged frog (Rana draytonii); however, the latter species is found from southern Mendocino County southerly to Baja California.