Especially observant visitors to the Donau-Auen National Park may often come across the clay-coloured Agile Frog – but usually only in the very moment in which it escapes by big leaps into the thick underbrush!
Description
These slim frogs are clay-coloured to medium brown and rarely have dark markings on their upperparts. The somewhat darker bands around the upper and lower hind legs are however characteristic. Yet the most obvious point of differentiation from other native Rana species is the large eardrum: its size corresponds to that of eye diameter; the distance from the eye is always less than half the eardrum diameter. The underside is whitish and without spots. Like all "brown frogs", a dark patch stretches from the eye to the top of the foreleg.
Distribution
The Agile Frog may be found from France to the Black Sea, only avoiding high Alpine areas. It is the most common amphibian in the Donau-Auen National Park.
Endangerment and Conservation Status
Endangered species in Austria; protected all over Europe.
Behaviour
The Agile Frog's conspicuous spawn clumps may be found in spring in nearly all stagnant waters. Attached to underwater vegetation or branches, later floating on the water's surface, several hundred clutches may be sighted in favourable spawning waters. In fact, the Agile Frog utilizes as many suitable areas as possible in the early springtime to deposit its eggs. Receptive females lay spawn clumps consisting of around 1000 eggs. The clutches reveal Mother Nature's hidden games. Small one-celled algae penetrate the gelatinous capsules and move as close as possible to the embryo, which requires oxygen and, like all animals, exhales carbon dioxide. Using sunlight, algae process this basic material of plant life to form all those chemical bonds required to build up bodily substance. This microcosm also provides sufficient amounts of the required water and some salts.
But the embryo also exploits the algae's settlement: algae produce oxygen, which the embryo desperately needs. It has been scientifically proven that even the innermost eggs found in algae-rich clutches receive sufficient oxygen, while in algae-poor clutches, deficiencies may occur which result in retardation of development or even premature death of the nuclei. The gelatine in the clutches consists of highly-absorbent material which swells up with water to reach its substantial volume only after it is outside of the womb.
The embryo itself is not directly enclosed in this gelatine. A small water-filled capsule in the middle of gelatinous layers serves as a kind of "private pool" in which the embryo slowly develops. Yet how do the tadpoles release themselves from the robust gelatinous layers and reach open water? Mother Nature has equipped them with special glands. These secrete a substance which softens the gelatinous material, making it penetrable for even the most delicate larvae.
Special Characteristics
The Agile Frog is the first amphibian to spawn each year. At this time of year, night frosts are common and cause the waters to freeze. This often leads to the uppermost layer of eggs to be cased in ice. The clutch may survive short periods of frost without significant damage, yet if the period is long or especially cold, the uppermost layer may die off completely.