Rana auroraNorthern Red-legged Frog(Also: Red-legged Frog)

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Geographic Range

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)

Habitat

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

Physical Description

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)

Development

Reproduction

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
    AnAge
  • Average age at sexual or reproductive maturity (female)
    Sex: female
    1460 days
    AnAge
  • Average age at sexual or reproductive maturity (male)
    Sex: male
    1095 days
    AnAge

Lifespan/Longevity

  • Average lifespan
    Status: captivity
    15 years
    AnAge

Behavior

The red-legged frog is a solitary, primarily diurnal species. Breeding takes place over very few days. The males appear at breeding sites two to four weeks before females. They give their mating call in small mobile groups between three and seven. California Red-legged frogs call in the air and have paired vocal sacs. They are found in water at least 70 centimeters deep while Northern Red-legged frogs call underwater at a depth of at least 92 centimeters. California Red-legged frogs are quite wary and nocturnal, and juveniles are more active during the day than adults. They are often attacked by wading birds, and seem to use vibrations transmitted along the surrounding vegetation to sense predators. These predators include snakes and raccoons. The predator response of both R.a.draytonii and R. a. aurora is to flee directly into the water, swimming to the deepest part of the water. During periods of flooding, R. a. draytonii conceals itself in small pockets or mammal burrows along the banks of rivers.

(Hayes, 1986; Jennings,1985; Defenders of Wildlife)

Food Habits

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)

Economic Importance for Humans: Positive

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)

Conservation Status

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)

Other Comments

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)

Contributors

Sarita Lynn Brown (author), University of Michigan-Ann Arbor.

Glossary

Nearctic

living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map

bilateral symmetry

having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

metamorphosis

A large change in the shape or structure of an animal that happens as the animal grows. In insects, "incomplete metamorphosis" is when young animals are similar to adults and change gradually into the adult form, and "complete metamorphosis" is when there is a profound change between larval and adult forms. Butterflies have complete metamorphosis, grasshoppers have incomplete metamorphosis.

motile

having the capacity to move from one place to another.

native range

the area in which the animal is naturally found, the region in which it is endemic.

threatened

The term is used in the 1994 IUCN Red List of Threatened Animals to refer collectively to species categorized as Endangered (E), Vulnerable (V), Rare (R), Indeterminate (I), or Insufficiently Known (K) and in the 1996 IUCN Red List of Threatened Animals to refer collectively to species categorized as Critically Endangered (CR), Endangered (EN), or Vulnerable (VU).

References

Camp, Charles Lewis. 1917. Zoology. Notes on the Systematic Status of the Toads and Frogs of California. University of California Press. Berkeley. pp.115-125.

Cole, Elizabeth C. 1997. Journal of Wildlife Management. Response of Amphibians to Clearcutting, Burning, and Glyphosate Application in the Oregon Coast Range. Wildlife Society. pp.656-664.

Green, David M. 1996. Systematic Zoology. Systematics and Evolution of Western North American Frogs Allied to Rana aurora and Rana boylii: Karyological Evidence. Society of Systematic Zoology. pp.272-296.

Grinnell, Joseph. 1917. Zoology. A Distributional List of the Amphibians and Reptiles of California. University of California Press, Berkeley. pp.127-208

Hayes, Marc P. 1986. Vocal Sac Variation Among Frogs of the Genus Rana from Western North America. Copeia. pp.927-936.

Hickman, Cleveland P.Jr. Roberts, Larry S. 1995. Animal Diversity. The McGraw-Hill Companies, Inc.

Jennings, Mark R. 1985.Pre-1900 Overharvest of California Red-legged Frogs (Rana aurora draytonii): The Inducement for Bullfrog (Rana Catesbeiana) Introduction. Herpetologica, pp.94-103.

Thomas, Eric O. 1993. Comparative Histochemistry of the Sexually Dimorphic Skin Glands of Anuran Amphibians. Copeia, pp.133-143

http://www.ucsc.edu/ucsc/nat-reserves/papers/Jd-McCambridge-species.html

Davidson, C. 1996. "Rana aurora - Red-legged Frog" (On-line). Accessed 14 November 2000 at http://ice.ucdavis.edu/Toads/texthtml/aurora.html.

Defenders of Wildlife, "CALIFORNIA RED-LEGGED FROG ((Rana aurora draytonii)" (On-line). Accessed 14 November 2000 at http://www.defenders.org/rada.html.

US Fish and Wildlife Service, 1996. "The California Red-legged Frog: Leaping Towards Recovery?" (On-line). Accessed 14 November 2000 at http://www.r1.fws.gov/sfbnwr/frog~1.html.