Wood Frog. Rana sylvatica

Description: Adult: A smallish to medium-sized frog, with a head and body length of 30-60 mm. The eyes are moderately large and protruding and the tympanum is smaller than the eye. The dorso- lateral folds are prominent and the limbs are relatively short. The skin is relatively smooth, although there may be some tubercles laterally. It is brownish above, often with two light-coloured dorsal stripes. There may also be a whitish vertebral stripe. The lateral surfaces bear dark spots. The venter is white with dark markings laterally, on the throat and occasionally on the chest. There is a dark spot at the base of each foreleg and the hindlimbs are usually barred. There is a prominent dark brown or black mask extending from the snout, through the eye and tympanum, to the insertion of the forelimb. This is bordered below by a white stripe.

Larvae: Tadpoles hatch at about 7-10 mm total length and attain a maximum length of about 50 mm. The eyes are dorsally placed and the labial tooth rows are 2/4 or 3/4. The caudal fin is arched and the tail tip is pointed. The anus is dextral and the spiracle sinistral. Tadpoles are brownish or greenish above and creamy below, with a pinkish tint. A light line occurs along the edge of the mouth.

Variation: The males bear nuptial pads during the breeding season, and have paired vocal sacs, one being positioned over each fore- limb insertion. Females are slightly larger than males, and subtle differences in colouration occur between the sexes.

Natural history: The wood frog is chiefly diurnal and is found in wooded areas or associated with open ponds. It occurs up to 2,500 m elevation and is very cold-tolerant and hibernates on land beneath litter and humus. This species employs physicochemical mechanisms to protect against extreme cold and dryness. It forages widely, often far from water, and eats molluscs, worms, insects and other arthropods. It is largely terrestrial in the non-breeding season. Population densities may be very high. It produces repul- sive skin secretions. Predation by fishes has been observed on adults in Wisconsin.

Reproduction: Breeding begins as soon as the ice begins to melt and occurs for a one to two week period from late April until June (depending upon latitude and/or altitude). After breeding, the adults disperse. They normally breed in shallow, clear ponds or in ephemeral water bodies. Males are non-territorial during the breeding period. They call while floating on the surface, and may call day and night. Amplexus is pectoral. Males clasp any female that is encountered and try to displace other males from amplexus with females. Such struggles are often intense and may result in death. Eggs are laid in globular masses, usually attached to plants, but they may be laid free in the water. Several females may lay their eggs in close proximity to each other. The eggs are somewhat resistant to desiccation. The number of eggs in a clutch varies greatly, but may average about 2,000-3,000. The number laid by females in Alberta is not known. Each egg is about 1.6 mm in diameter. Hatching occurs after three weeks or more. The tadpoles then grow quickly and metamorphosis occurs six to twelve weeks after hatching.

Voice: The voice of the wood frog is similar to that ot Rana pipiens, but the calls are higher pitched, less intense and shorter in duration. Adults emit an escape scream if attacked by a predator.

Distribution: It occurs throughout the province except for short-grass prairie habitats. An isolated record for the Cypress Hills exists, and probably represents an outlying population in a persisting area of suitable habitat. Extralimitally it occurs from northern Alaska and south central B.C. in the west to the Appalachians, Atlantic coastal states and Maritime provinces in the east. Isolated (primarily montane) populations are scattered west of the Mississippi River.

Conservation status: Alberta populations are robust and there is no evidence of decline.

Remarks: The wood frog is the only North American amphibian to cross the Arctic Circle.

Significant references: Banta 1914; Bastien and Leclair, 1992; Bellis 1961,1962; Berven 1981, 1982a, b, 1987, 1988; Berven and Chadra 1988; Berven and Grudzien 1990; Clark 1998; Cochran 1999; de Benedictis 1974; Ferguson 1956; Forester and Lykens 1988; Gamboa et al. 1991; Heatwole 1961; Herreid and Kinney 1966, 1967; Howard 1980; Howe 1899; King and King 1991; Layne, 1992; Layne and Lee 1986; Licht 1991; Lotshaw 1977; Martof 1970; Martof and Humphries 1959; Moore 1951; Moore and Strickland 1955; Patch 1939; Pierce et al. 1987; Pollister and Moore 1937; Roberts and Lewin 1979; Salt 1979; Storey, J.M. and Storey 1985a; Storey, K.B. 1984,1987; Wilbur 1972, 1976,1977.