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Red-legged Frog (Rana Aurora)

Update
COSEWIC Status Report
on the
Red-legged Frog
Rana aurora
in Canada
2004

Species Information

Name and Classification

The Red-legged Frog belongs to the large, nearly cosmopolitan family of Ranidae or “true frogs” (Amphibia: Anura: Ranidae: Rana: Rana aurora Baird and Girard, 1852). The genus Rana consists of over 250 described species distributed throughout the world, with the exception of southern South America, Australia, and Antarctica (Duellman and Trueb 1996). As currently composed, the genus Rana may be polyphyletic, and its systematics require further investigation (Crother 2000).

Six species of frogs of the genus Rana are native to the west coast of North America: Rana aurora, R. boylii (Foothill Yellow-legged Frog), R. cascadae (Cascades Frog), R. muscosa (Mountain Yellow-legged Frog), R. pretiosa (Oregon Spotted Frog), and R. luteiventris (Columbia Spotted Frog). These species form the R. boylii species group, which molecular evidence suggests is a well-defined, monophyletic group, about 8 million years old (Macey et al. 2001). Relationships within the R. boylii group are incompletely understood, but recent mitochondrial DNA sequencing suggests that R. aurora, R. cascadae, and R. muscosa might be closely related (Macey et al. 2001).

Rana aurora is divided into two geographically separated subspecies: R. a. aurora (the Northern Red-legged Frog), which occurs from British Columbia south to northern California, and R. a. draytonii (the California Red-legged Frog), which occurs from northern California to Baja California, Mexico. The two forms differ in their morphology, behaviour, and genetics (Hayes and Miyamoto 1984), but whether they are sufficiently distinct to warrant the recognition of two separate species is still to be resolved (Crother 2000).

 

Description

The Northern Red-legged Frog is a moderate-sized frog with snout-vent length of adults usually from about 50 to over 70 mm (Green and Campbell 1984); females attain a somewhat larger body size than do males and may be up to about 100 mm long (Nussbaum et al. 1983). As is typical of most other North America ranids, these frogs have a smooth to somewhat rugose skin, a dorsolateral fold along each side of the body extending from near the eye to near the groin, relatively long legs when compared to other groups of frogs, and webbed feet. The back of the Red-legged Frog is brownish, flecked with small black spots with indistinct edges; the dorsal surface of the limbs is often banded with black (Figure 1a). A dark mask typically extends from the eye to the jaw line and is bordered from below by a cream-coloured band. The throat and chest are gray or white with black flecking, whereas the undersides of the hind legs and the lower portion of the trunk are reddish, giving the species its common name. The brightness of the red varies both geographically and with ontogeny (Altig and Dumas 1972); small juveniles may lack the red colour altogether or show only a faint reddish or yellowish tint on the underside of the legs.

Figure 1:  Red-legged Frog, Rana aurora: a) adult (Rocky Point, Vancouver Island, B.C.), b) Egg-mass (Vedder Creek, B.C.), c) Tadpole (Vedder Creek, B.C.)

Figure 1.  Red-legged Frog, Rana aurora: a. adult (Rocky Point, Vancouver Island, B.C.), b. Egg-mass (Vedder Creek, B.C.), c. Tadpole (Vedder Creek, B.C.).

Photographs by Kristiina Ovaska.

Tadpoles are tan or greenish brown, and the trunk, tail, and fins are typically covered with gold- or brass-coloured flecking or blotches; the white underside often has a pinkish tinge. They can attain a relatively large size (up to about 70 – 80 mm) immediately before metamorphosis. The tail is relatively short (about 1.5 times or less the length of the body), and the dorsal fin is relatively tall (taller than the tail musculature at its widest point), resulting in a stubby appearance (Figure 1c; Corkran and Thoms 1996).

In British Columbia, the Red-legged Frog may be confused with the Oregon and Columbia spotted frogs, which have a similar body form and reddish underside of the hind limbs and the lower portion of the trunk; the two spotted frogs cannot be reliably distinguished from each other based on morphology. The Red-legged Frog is sympatric with the Oregon Spotted Frog in the lower Fraser Valley, whereas it is largely allopatric with the Columbia Spotted Frog. Potential for overlap with the Columbia Spotted Frog exists at the southeastern and northern distributional limits of the Red-legged Frog on the mainland (see section on Canadian Distribution), and specimens from these areas should be examined carefully. Adults of the Red-legged Frog can be distinguished from the spotted frogs by the presence of greenish and black mottling in the groin area, longer legs (the heel extends beyond the snout when pressed against the body), less extensive webbing on the hind feet, lateral rather than upward orientation of the eyes, and pronounced dorsolateral folds (Nussbaum et al. 1983, Leonard et al. 1993). The tadpoles of the Red-legged Frog have a shorter tail and taller dorsal fin than those of the spotted frogs; the dorsal fin of the Red-legged Frog typically has distinct gold-coloured flecks, which are usually absent from tadpoles of the spotted frogs (Corkran and Thoms 1996).

More detailed information on the appearance of this species and its distinguishing characteristics can be found in Altig and Dumas (1972) and various field-guides (Nussbaum et al. 1983, Green and Campbell 1984, Stebbins 1985, Leonard et al. 1993, Corkran and Thoms 1996).