Species: Red-legged Frog Rana aurora

Family: Ranidae Order: Anura Class: Amphibia

Species note author: Carlos Davidson
Reviewed by: Mark Jennings
Date: November 16, 1993, Status updated May, 1996.

Management Status: California red-legged frog (Rana aurora draytonii) - Federal Threatened species as of May 20th, 1996. Northern red-legged frog (Rana aurora aurora) - was a federal Candidate 2 species before that category was abolished. Both subspecies are California Species of Special Concern.

DISTRIBUTION, ABUNDANCE, AND SEASONALITY

The red-legged frog is the largest native ranid frog in California (Wright and Wright 1949, Jennings et al. 1993), and inhabits still or slow water in streams, marshes, ponds, reservoirs, and canals.

There are two subspecies in California: The northern red-legged frog (Rana aurora aurora) and the California red-legged frog (Rana aurora draytonii). The systematic relationship between the two subspecies is still not clear (Hayes and Miyamoto 1984, Green 1985, Hayes and Kremples 1986). California red-legged frogs are larger than northern red-legged frogs (adults are greater than 100mm snout to vent length), males have paired vocal sacs and call in the air, and eggs are laid in contact with the water surface (Hayes and Miyamoto 1984, Hayes and Kremples 1986). Northern red-legged frogs are smaller, males call underwater, and eggs are typically submerged (Hayes and Miyamoto 1984).

Historically, California red-legged frogs occurred from Point Reyes National Seashore on the coast, and inland from near Redding, Shasta County, south to northern Baja California. Northern red-legged frogs occur north of the range of California red-legged frogs, west of the Cascade crest, north to southern British Columbia. (For current range information see the status section below). The exact geographic boundary between the two subspecies is the subject of current work by Jennings, Hayes, and Green (Jennings pers comm).

This literature review contains information on both subspecies, however, most of the literature cited is on the California subspecies. There are some clear differences in behavior and habitat preferences and one should not assume the information on one subspecies applies to the other (Hayes and Miyamoto 1984, Hayes and Tennant 1985).

SPECIFIC HABITAT REQUIREMENTS

Water: California red-legged frogs are found in ponds and intermittent and permanent streams with slow or still water. Intermittent streams must retain surface water in pools year-round in order for frogs to survive (Jennings et al. 1993). In the Central Valley drainage area Hayes and Jennings (1988) found red-legged frogs about twice as frequently in intermittent aquatic habitats. This is probably due to their exclusion from permanent aquatic habitats by introduced predators (fish and bullfrogs (Rana catesbeiana) that require permanent water (Hayes and Jennings 1988).

Deep pools are necessary for many aspects of red-legged frogs' life cycle. In a study of the Central Valley drainage area Hayes and Jennings (1988) found California red-legged frogs almost exclusively (99%) at sites with some water at least 70cm deep. Licht (1969) reported male northern red-legged frogs usually call underwater at a depth of at least 92 cm. The predator response behavior of northern red-legged frogs is to flee directly into the water and swim to the deepest part of the channel or pool (Gregory 1979), a pattern also observed in California red-legged frogs (Jennings pers comm).

Red-legged frogs require cool water. There are no studies delimiting the critical thermal maximum for the California red-legged frog, but the northern red-legged frog has the lowest upper (21^oC) and lower (4^oC) lethal embryonic temperatures of any North American ranid frog (Licht 1971). Jennings and Hayes (1989) report that adult California red-legged frogs stress when exposed to water temperatures at or above 29^oC, and can die if the exposure is chronic.

In addition to water depth and temperature, salinity may also be an important factor. Jennings and Hayes (1989) report exposure of pre-hatching embryos to salinity greater than 4.5 percent causes 100% mortality.

Feeding: California red-legged frog have a highly variable diet, probably taking any prey they can subdue that is not distasteful (Hayes and Tennant 1985). Other amphibians and small mammals may form a significant portion of their diet (Hayes and Tennant 1985, Baldwin and Stanford 1987).

Reproduction: California red-legged frogs breed from late November to early April (Jennings and Hayes 1989). Timing is probably to ensure that water is cool enough for embryonic survival and that sufficient water exists for larval growth to metamorphosis. Egg masses are typically attached to emergent vegetation with a vertical orientation. Unlike Rana a. aurora (Storm 1960), R. a. draytonii eggs are attached to vegetation at or near the surface of the water (Hayes and Miyamoto 1984).

Northern red-legged frogs usually breed in February and March soon after the ice melts from spawning areas and water temperatures reach at least 6^oC or 7^oC (Licht 1971). Males call underwater at night at a depth from 92 cm to 500 cm (Licht 1971). The female joins the male and eggs are then attached to vegetation at a minimum depth of 18 inches (46 cm) and at least 2 to 3 feet (61­92 cm) from the waters edge (Licht 1971).

Vegetation: Adult California red-legged frogs require dense riparian vegetation that is in contact with, or close to, deep water (greater than 0.7 m) (Hayes and Jennings 1988). Vegetation structure is important for escape cover from predators and possibly also as shading to maintain cool water temperature (Hayes and Jennings 1988). Vegetation often includes, but is not limited to cattails (Typha spp.), bulrushes (Scirpus spp.) and willows (Salix spp.). At sites with adult California red-legged frogs, vegetation typically shades a substantial portion of water surface area with a dense matrix right at or near water level (Hayes and Jennings 1988). Vegetation is often sufficiently dense to prevent the entry of predators such as birds and racoons (Procyon lotor) (Jennings pers comm).

SPECIES LIFE HISTORY

Reproduction: Male California red-legged frogs appear at breeding sites from two to four weeks before females (Storer 1925, Jennings et al. 1993). Females typically attach egg masses to stems of emergent vegetation such as bulrushes (Scirpus spp.) or cattails (Typha spp). Egg masses contain from 2,000 to 5,000 dark brown eggs from 2.0 to 2.8mm in diameter (Wright and Wright 1949, Hayes and Miyamoto 1984, Jennings et al. 1993). Time to hatching is 6-14 days (Jennings et al. 1993). Larvae metamorph between July and September, 3.5 to 7 months after egg laying (Jennings et al. 1993). Males probably reproduce after 3 years and females after fours years of age (Jennings and Hayes 1985). Jennings and Hayes (1989) estimate minimum life spans of 8 years for males and 10 years for females.

Northern red-legged frogs egg masses contain 500-1100 eggs (Storm 1960, Nussbaum et al. 1983), averaging 3.03 mm in diameter (Licht 1971). Egg development from spawning to hatching is slow compared to other species of Rana due to the cold water temperatures required for development (Licht 1971). Cowan (1941) reported captive northern red-legged frogs living approximately 12 to 15 years.

Movement/Migration: Northern red-legged frogs may move out of riparian zones into adjacent upland forests during the non-breeding season. Nussbaum et al. (1983) report northern red-legged frogs 200-300m from water. Recent telemetry studies indicate that California red-legged frogs also leave riparian zones (Jennings pers comm). California red-legged frogs may move seasonally within aquatic habitats between breeding sites and foraging habitat (Jennings et al. 1993).

Niche: Wading birds, particularly Bitterns (Botaurus lentiginosus) and Black-crowned night herons (Nycticorax nycticorax) are likely significant predators on adult California red-legged frogs (Jennings and Hayes 1989). Juveniles, which are more active during the day than adults, are preyed upon by garter snakes (Thamnophis atratus, T. elegans, T. hammondii and T. sirtalis tetrataenia) (Fitch 1940, Fox 1951, 1952, Barry 1978). The fact that California red-legged frogs larvae seldom co-occur with non-native predatory fish suggest the fish likely prey upon the larva (Hayes and Jennings 1986,1988). Similarly, Hayes and Jennings (1988) found a negative correlation with bullfrog presence and the presence of California red-legged frogs, although it is not clear if bullfrogs are significant predators or competitors of California red-legged frogs (Hayes and Jennings 1986).

Threats: Current threats to California red-legged frogs include:

Dam construction, which destroys suitable habitat and fragments remaining habitat, results in isolated populations. Reservoirs act as barriers because they are unsuitable frog habitat and they are suitable habitat for introduced aquatic predators.

Livestock grazing, even in moderate levels, may have a severe impact on California red-legged frog habitat (Jennings 1988). Cattle can destroy crucial riparian vegetation, especially in drought years. Cattle also trample banks creating erosion leading to a wider and shallower stream channel and the filling of pools (Jennings 1988). A shallower stream channel raises water temperatures favoring a number of introduced predatory fish (Jennings 1988, Jennings et al. 1993).

Introduction of non­native fishes and bullfrogs is probably responsible for the absence of R. a. draytonii from perennial stream habitats over most of its former range (Hayes and Jennings 1988, Jennings et al. 1993). In a study of historic California red-legged frog sites in the Central Valley drainage area, Hayes and Jennings (1988) found R. a. draytonii probably extant at 81 percent of sites lacking bullfrogs and probably extinct at all sites with bullfrogs. California ranids have evolved under conditions of limited fish predation since California possesses only a small number of native fish species that prey on vertebrates (Miller 1958, Moyle 1976). A majority of the over 50 exotic and transplanted species are known to prey on frogs or their pre­metamorphic stages (See Pearse 1915­1916, Ball 1948, Heman et al. 1969, Grubb 1972, and Cochran 1983 in Hayes and Jennings 1986).

Status: California red-legged frogs have disappeared from 75 percent of their historic range (Jennings et al. 1993). Historically, declines are likely due to four factors: habitat loss, habitat fragmentation, introduction of exotic predators and overexploitation (Jennings et al. 1993). Although declines began with human hunting for food beginning with the Gold Rush in 1849 (Jennings and Hayes 1985), over half of the reduction in range has probably taken place in the last 25 years (Jennings et al. 1993). The small coastal drainages between Point Reyes National Seashore in Marin County and Carpenteria in Santa Barbara County are the only remaining areas with significant numbers of California red-legged frogs. There are currently only three known California red-legged frog populations with greater than 350 adults, all near the coast in the San Francisco Bay Region. The California red-legged frog was extirpated from the floor of the Central Valley by 1960 and is now extremely rare in the Sierra foothills (Jennings et al. 1993). Jennings et al. (1993) report only six small drainages in the Coast Range slope of the Central Valley that still may contain California red-legged frogs. In southern California (Ventura County and south), R. a. draytonii is only known from four locations (Jennings pers comm).

Possible Habitat Management Objectives for the California red-legged frog

Objective #1: Maintain dense stands of native riparian vegetation to maintain cool water temperatures and ensure hiding cover.
Measurement: Greater than 25 percent of riparian area thickly vegetated. Ideally vegetation at ground level should be sufficiently thick to prevent entry of Racoons (Procyon lotor) and predatory wading birds such as Bitterns (Botaurus lentiginosus) and Black-crowned night herons (Nycticorax nycticorax).

Objective #2: Maintain emergent vegetation in slow or still pools for egg laying and hiding cover.
Measurement: Greater than 25 percent of pool area thickly vegetated with cattails, bullrushes or other emergent vegetation.

Objective #3: Maintain water depth greater than 28 inches (70cm). California red-legged frogs are found in association with deep water, although it is currently unclear if water depth helps maintain microclimate, is more likely to contain hiding cover, or serves other functions.
Measurement: measure water depth.

Objective #4: Maximize the isolation of California red-legged frog populations from all introduced aquatic predators (bullfrogs and exotic fish). Ideally, keep introduced aquatic predators out of drainages with red-legged frog populations.
Measurements: 1. Survey for the presence/absence of introduced fish and bullfrogs in ponds and slow moving streams. 2. Manage spacing of newly constructed water impoundments to ensure that all new impoundments are greater than one mile from existing impoundments with bullfrogs.

Literature Cited

Baldwin, K. S. and R. A. Stanford. 1987. Life history notes: Ambystoma tigrinum californiense (California tiger salamander): predation. Herpetological Review 18 (2): 33.

Barry, S. J. 1978. Status of the San Francisco garter snake. Special Publication 78­2. California Department of Fish and Game, Inland Fisheries Endangered Species Program. p. 1-21.

Cowan, I. M. 1941. Longevity of the Red­legged frog (Rana a. aurora). Copeia (1): 48.

Fitch, H. S. 1940. A biogeographical study of the ordinoides Artankreis of garter snakes (genus Thamnophis). University of California Publications in Zoology 44(1): 1­150.

Fox, W. 1952. Notes on feeding habits of Pacific coast garter snakes. Herpetologica 8 (1): 4­8.

­­­. 1951. Relationships among the garter snakes of the Thamnophis elegans Rassenkrais. 50(5). University of California Publications in Zoology, pp. 485­530.

Green, D. M. 1985. Differentiation in amount of cantromaric heterochromatin between subspecies of the red­legged frog, Rana aurora. Copeia 4: 1071­1074.

Gregory, P. T. 1979. Predator avoidance behavior of the red­legged frog (Rana aurora). Herpetologica 35 (2): 175­184.

Hayes, M. P. and Kremples. 1986. Vocal sac variation among frogs of the genus Rana from western North America. Copeia 4: 927­936.

Hayes, M. P. and M. M. Miyamoto. 1984. Biochemical, behavioral, and body size differences between Rana aurora aurora and Rana aurora draytonii. Copeia 1018­1022.

Hayes, M. P. and M. R. Tennant. 1985. Diet and feeding behavior of the California red­legged frog Rana aurora draytonii (Ranidae). The Southwestern Naturalist 30 (4): 601­605.

Hayes, M. P. and M. R. Jennings. 1986. Decline of ranid frog species in western North America: are bullfrogs (Rana catesbeiana) responsible? Journal of Herpetology 20 (4): 490­509.

­­­. 1988. Habitat correlates of distribution of the California red­legged frog (Rana aurora draytonii) and the foothill yellow­legged frog (Rana boylii): implications for management. In Management of amphibians, reptiles, and small mammals in North America. eds R. C. Szaro, K. E. Severson and D. R. Patton. Gen. Tech. Rep. RM­166. USDA Forest Service, Rocky Mountain Forest and Range Experiment Station, Fort Collins, CO. pp. 144­158.

Jennings, M. R. and M. P. Hayes. 1989. Final report of the status of the California red­legged frog (Rana aurora draytonii) in the Pescadero Marsh Natural Preserve. Contract No. 4­823­9018. California Academy of Sciences, pp. 56.

­­­. 1985. Pre­1900 overharvest of California red­legged frogs (Rana aurora draytonii): the inducement for bullfrogs (Rana catesbeiana) introduction. Herpetologica 41 (1): 94­103.

Jennings, M. R. 1988. Natural history and decline of native ranids in California. Proceedings of the Conference on California Herpetology. eds H. F. De Lisle, P. R. Brown, B. Kaufman and B. M. McGurty. Southwestern Herpetologists Society,

Jennings, M. R., M. P. Hayes and D. C. Holland. 1993. A petition to the U. S. Fish and Wildlife Service to place the California Red­legged Frog (Rana aurora draytonii) and the Western Pond Turtle (Clemmys marmorata) on the list of endangered and threatened wildlife and plants.