Moor Frog, Rana arvalis

The Moor Frog (Rana arvalis) is a slim, reddish brown colored, and semiaquatic amphibian that is native to Europe and Asia. It belongs to the Ranidae family, or true frogs.

Ranidae, the family that the Moor Frog belongs to, is a wide group covering about 605 species. The family is like a “catch-all” for ranoid frogs that don’t belong to any other families. Seeing that this is the case, the traits that define them are more general, and the frogs are found all throughout the world, on every continent except Antarctica.

This frog is of small size, characterized by an unspotted belly, a large, dark colored ear spot, and — often, but not always — a pale stripe down the center of the back. They are usually described as a reddish brown, but can also be yellow, gray, or light olive. Their bellies are white or yellow and they feature a “bandit-like” black stripe going from their nose to their ears. They vary from 5.5 to 6.0 centimeters longs, but can reach up to 7 centimeters in length, and their heads are more tapered than those of the European common frog Rana temporaria. The skin on their flanks and thighs is smooth, and the posterior part of their tongues is forked and free. They have horizontal pupils, their feet are especially webbed, and their back legs are shorter than those of other species of frogs. The male individuals are different from the females due to their nuptial pads on their first fingers and their paired guttural vocal sacs. These frogs hibernate sometime between September and June, depending on the latitude of the location.

The mating season occurs between March and June right after the end of hibernation. The males form breeding choruses, and their songs sound much like those of the agile frog, Rana dalmatina. The males can also develop bright blue coloration for a couple days during the season. Spawning happens very quickly and is accomplished in three to twenty-eight days. The spawn of each frog is laid in one or two clusters of 500 to 3000 eggs in warm and shallow waters in ponds.

Metamorphosis occurs between June and October. The larvae are about 45 millimeters long and colored dark with small metallic dots. When they become tadpoles, they consume algae and small invertebrates. The adult frogs’ feeding is put on hold during the breeding season, but their diets are made up of insects and a variety of invertebrates.

Image Caption: Moor frog (Rana arvalis); younger female. Credit: Christian Fischer/Wikipedia (CC BY-SA 3.0)