egg cluster

Northern Red-legged Frog

The Northern Red-legged Frog, Rana aurora, is a protected species of amphibian, whose range is the coastal region stretching from Northern California to southwest British Columbia. As a member of the genus Rana, this species is considered a true frog, with characteristic smooth skin and a narrow waist. This frog requires still waters for breeding and is rarely found at any great distance from its breeding ponds or marshes.

Rana aurora adults may attain a length of eight centimeters; they have a dark facial mask and a characteristic light stripe along the jawline. The Northern Red-legged Frog has long, powerful legs well adapted to significant leaps; in fact its proximate species, R. draytonii, is considered to be that depicted by Mark Twain, in the famed tale of the Leaping Frog of Calaveras County. It is one of two amphibian species classified as Red-legged Frog, the other species being termed California Red-legged Frog; however, the latter species is found primarily from Marin County southerly to Baja California. These two genetically distinct species are believed to intergrade in the counties of Marin and Sonoma. In some systems of taxonomy, this species is classified as Rana aurora aurora.


The Northern Red-legged Frog is found in every coastal county of California from Mendocino County northward and including coastal Oregon. While it occurs primarily in the Northern California coastal mountain ranges, it is not found above an elevation of 1200 meters. It also occurs somewhat less commonly in the southern Cascade Range. The species is thought to intergrade with Rana draytonii in Marin County and Sonoma County, California, but has been observed as far south as San Mateo County.


The still waters of ponds, marshes or stream pools are essential for Northern Red-legged Frog breeding habitat; moreover, this species of frog is considered unusually highly oriented to its aquatic habitat, with a clear preference for thickly vegetated shoreline. Rana aurora requires cover, since it is subject to predation by various fish, snakes, birds, mammals and even certain other amphibians. When this frog senses danger, it will quickly plunge to depths of one meter or more to seek safety in the benthic zone of a pond. Adults leave the breeding pond soon after the breeding activity is concluded and may migrate about one half kilometer to its summer location, which is likely to be a riparian zone. In the northern part of its range, adults may hibernate. Juveniles are slower to leave the breeding ponds, but also tend to find cover in riparian areas and may readily migrate about one half kilometer by summertime. Mature Rana aurora prey upon terrestrial insects, but will also take small snails and crustaceans; moreover, they will consume worms, tadpoles, small fish and even small frogs of other species. The larvae of the Northern Red-legged Frog are considered totally herbivorous.


Males and females begin to move to the breeding sites as early as October, and sometimes as late as January depending on latitude, cumulative rainfall for the season and average temperature. Typically stable minimum temperatures of 42 to 44 degrees Fahrenheit are required to induce breeding. Observations of adult migration are best made on moonlit nights with light rains. The male is thought to defend territory (animal), once he is in the breeding pond area, using nocturnal displays. In fact, most activity is in the night-time, especially enhanced by periods of light rain. The actual courtship behaviors commence in January in the California part of the range, and as late as March in northerly regions. In any case, the breeding season terminates in July at the very latest, and earlier in the drier locales.

Each female produces 200 to 1100 eggs per season and attaches the egg cluster to submerged vegetation or rotting logs, characteristically seven to fifteen centimeters below the pond surface. Egg clusters are typically about ten centimeters in diameter and may disperse into an irregular form underwater. Eggs hatch out in 39 to 45 days, and tadpoles require approximately 80 days to attain metamorphasis.

Oviposition generally takes place in densely vegetated shallow portions of wetlands with mimimal current, and in unusual cases egg masses have been observed in water up to 500 centimeters in depth. Breeding sites can be either permanent or temporary, with inundation usually necessary into June for successful metamorphosis. The June date is based upon Oregon conditions; in Northern California late May or early June witnesses metamorphosis. Breeding is initiated when water temperatures exceed about six degrees Celsius (usually in January), but can be as late as March in the extreme northern part of the range.


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