California red-legged frogs are the largest frogs native to California, reaching sizes upwards of 4.5 inches in body length. Adult frogs are variable in color but are often characterized by the rich red coloration of the lower sides of their bodies and the under-surfaces of their hind limbs. Upper portions of red-legged frogs are red-pink to green-brown, with irregular black mottling on dorsal surfaces of the back and thighs. There are dorsolateral folds extending from the hips to eyes on both sides of the body.
Red-legged frog tadpoles are brown, often with a pinkish sheen on their undersides, and commonly reach 3 inches in total length. Tadpoles may be mottled with irregular dark spots, but they do not have the pencil-point black dots typical of bullfrog tadpoles. Juveniles are generally less than an inch in body length at metamorphosis and more brown-green than red.
Eggs are laid in loose clusters, generally in shallow water. These rough egg masses are clear to yellow brown or grey in color, with a dark developing embryo in each individual egg.
Historically, California red-legged frogs were found throughout California from Mendocino County in the north to Baja California in the south, and scattered through the eastern foothills of the Sierra Nevada. This range has considerably reduced, particularly in southern and eastern areas of California, where the California red-legged frog has all but disappeared. A related subspecies (Rana aurora aurora) persists in northern California and ranges north into British Columbia. Recent genetic analyses have proposed splitting these two subspecies of red-legged frogs into two separate species: Rana aurora (northern red-legged frog) and Rana draytonii (California red-legged frog).
Red-legged frogs are typically associated with still, permanent bodies of freshwater, such as ponds, lakes, marshes or the slow flowing sections of creeks and streams. Red-legged frogs are known to occur in, or at least traverse through, grasslands, riparian woodlands, oak woodlands, and coniferous forests. Dispersal almost always occurs at night, often during rainstorms. Seasonal bodies of water are frequently occupied by red-legged frogs and, in some areas, these water bodies may be critical for their persistence.
Community relationships and behavior
Adult and juvenile red-legged frogs are generally crepuscular or nocturnal. They feed on a wide range of invertebrates and small vertebrates including aquatic and terrestrial insects, snails, crustaceans, fishes, worms, tadpoles, small mammals, and smaller frogs (including members of their own species). During the daylight hours, red-legged frogs are fairly inactive, typically sitting in a covered location. They will, however, move in the daytime if threatened or if presented suitable prey. Most hunting occurs at night, at which time the frogs tend to leave their hiding places and position themselves in areas where they are more likely to encounter prey. The tadpoles are primarily herbivorous.
California red-legged frogs are prey for a number of species, including bullfrogs, largemouth bass, snakes, raccoons, dogs, foxes, coyotes, cats, herons, and egrets. Crayfish are also thought to prey upon red-legged frog eggs and tadpoles. Newts may eat red-legged frog eggs. Late season heavy rains also wash away egg masses and young tadpoles.
Reproduction generally begins in late January and lasts through March. Minimum breeding age appears to be 2 years in males and 3 years in females. Females lay 750-4,000 eggs in clusters attached to aquatic vegetation, 2 to 6 inches below the water surface. Eggs hatch in 2 to 3 weeks. Once hatched, the tadpoles generally take between 11-20 weeks to metamorphose, doing so between May and August. In some locations red-legged frog tadpoles overwinter and do not metamorphose until their second spring or summer.
Loss of habitat, the introduction of non-native species, the wide-spread use of biocides and fertilizers, and the spread of pathogens are current major threats to red-legged frog survival and have led to widespread declines. Rana aurora draytonii was listed as a threatened species by the U.S. Fish and Wildlife Service in 1996.
California red-legged frogs at Stanford
California red-legged frogs have been monitored annually on Stanford lands since 1997. These surveys have documented two distinct populations: one along Matadero and Deer creeks and the other along San Francisquito Creek. Prior to the construction of Highway 280 and the general suburban buildup of the area, it is likely that these two populations were part of a single, more widespread population.
California red-legged frogs have been recorded in Los Trancos Creek. However, most of the recently observed frogs were found well upstream of Stanford and there is only a single recent record of a California red-legged frog from Stanford’s portion of Los Trancos Creek; in 1995, a single frog was repeatedly observed in the roots of a large bay tree located just downstream of the Los Trancos Diversion facility. This individual apparently left the site when a fish ladder was added to the diversion facility, causing the downstream pool to shift away from the bay tree.
There have been other sporadic records of California red-legged frogs in the San Francisquito watershed. There are unsubstantiated records from the 1970s of red-legged frogs in San Francisquito Creek immediately south of the golf course, near the non-Stanford residences along Bishop Lane (a reach some 3 to 4 kilometers downstream from the frogís current distribution). A large red-legged frog was found in January 2000 as a road-kill along Junipero Serra Boulevard, opposite Frenchmanís Road (approximately 1.5 kilometers from the nearest creek site known to support frogs). In 2006, two red-legged frogs were reported from an area between SLAC and Sand Hill Road. Multiple subsequent surveys at the site failed to observe any California red-legged frogs, but given the location (across Sand Hill Road from a fairly large concentration of red-legged frogs in an area north of Stanford), transient individuals are not unexpected. Other historic records of California red-legged frogs at Stanford indicate that in the early- and mid-part of the last century, they were occasionally found in Lagunita and in the goldfish pond of a campus apartment building (Kingscote). No California red-legged frogs have been observed at these central campus locations for many decades.
The local populations of red-legged frogs have probably declined considerably during the last 50 years. Anecdotal accounts and collection locations of museum specimens indicate that red-legged frogs were more widespread and abundant in many locations where the frog is now absent. Most likely, no single major reason for this decline exists, but rather the decline is the result of long-term changes to the area that have occurred with increased urbanization.
References (PDF, 36 KB)
Click here to view images of the California red-legged frog.
Click here to view a map of the California red-legged frog on Stanford lands.