European tree frog
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The European tree frogs (some members of the genus Hyla) are small frogs that can grow 5 cm in size at most. They are the only members of the wide-spread tree frog family (Hylidae) indigenous to Mainland Europe. Characteristic are the discs on the frog's toes which it uses to climb trees and hedges.
There are three or four species and many subspecies:
- Hyla arborea (Linnaeus, 1758) (common or European tree frog)
- Hyla meridionalis Boettger, 1874 (Mediterranean tree frog or stripeless tree frog)
- Hyla intermedia Boulenger, 1882 (Italian tree frog) (not always considered a species)
- Hyla sarda (De Betta, 1853) (Sardinian tree frog)
The European tree frogs actually don't live in forests, but rather prefer sunny forest edges, bushy heaths, wet dune pans, wet scrubland and extensively used meadows and parks with ponds rich in submerged vegetation without fish nearby. These habitats are increasingly influenced by human activity. Hyla arborea, the common tree frog, is endangered in western Europe (nearly extinct in Belgium) while the more common Mediterranean tree frog lives in wet gardens, treegarths, vineyards, campings, and near pine trees.
Common tree frog
Historically, tree frogs were used as barometers because they respond to approaching rain by croaking. In the breeding season, even when migrating to their mating pools, they croak as well. Depending on subspecies, temperature, humidity, and the frog's 'mood', skin color ranges from bright to olive green, grey, brown and yellow. The head is rounded, the lip drops strongly, the pupil has the shape of a horizontal ellipse and the eardrum is clearly recognizable.
Males can be distinguished from females by their browny-yellowy, large (folded) vocal sacs in the throat region. The amplexus is axillary (in the armpits). Both adult males and females reach sizes up to 30-40 mm, rarily longer than 45 mm. The smooth, shining, usually leaf-green back and the white-yellowish to grey belly are separated by a dark stripe on its flank reaching from the nostrils, over the eye and the eardrum, to the groin, contrasting the green, and forming a dark spot near the hips. The hind legs are much larger and stronger than the fore legs, which enables to jump rapidly.
Calls
Some frog species look so identical the frogs cannot discern the difference between their species and a different species. To overcome this difficulty, each frog species has a specific call that the male innately knows and performs. Thus, the females can recognize their specific species’ mating call and respond to it by following the call. It is always impressive, that such a small froglet have this very loud voice. They are the loudest frogs of Europe, because of their large larynx (about one-fifth of the body length). The croaking sounds like a strong rithmic 'creck-creck-creck-creck...' and resemble the call of the Baillon's Crake (Porzana pusilla) and the Little Crake (Porzana parva). In late afternoon or dusk, H. arborea starts to call sometimes throughout the night. Calling and breeding activity probably begins in warm nights at late April. The call of H. arborea is very characteristic and easy identifable, so this fact helps to determinate these frogs in their natural habitats.
Mating
Between the months of April and August, Squirrel Treefrogs of at least one year old begin to mate. Once the male and female find each other, amplexus, the term for the embrace between the male and the female begins. The male, because of its smaller size, goes on top of the female. Fertilization is external, with the female’s eggs laid during amplexus. Next, the male sheds sperm over the up to 1000 usually individually deposited eggs and fertilization transpires. Females usually lay eggs twice a mating season.
Spawn
Eggs are deposited mostly during May, with the earliest spawn observed at the end of March. The clusters of spawn (each consisting of 10 to 50 eggs) are as big as walnuts and are deposited in shallow places grown with water plants. The top of every egg is brown and the bottom yellowy white. The diameter of an egg lies between 1.5 to 2 millimeter. After the deposition the eggs come together in clusters in order that the animal pole, which is pigmented brown and yellow, points upward and the vegetative pole, which is white and unpigmented, points downward.
The time of development of the clutches depends on the prevailing water temperatures. Clusters that sink to the bottom of the water develop considerably slower than those which are exposed to the sunlight directly under the surface of the water.
Distribution
Widely distributed throughout Europe from the Ukraine and Belarus to the Balkan, Crete, Italy, the Benelux, Germany, most of France and the northwestern Iberian peninsula. Absent in the British Isles, the majority of Scandinavia and Denmark, the Alps and small northern parts of the Netherlands and Germany.
Mediterranean tree frog
Resembles the common tree frog, but is larger (some females up to 65 mm), has longer hindlegs, and the flank stripe only reaches to the front legs (often starting at the eyes, not at the nostrils). The croaking resembles that of the common tree frog, but it is deeper and slower: 'wroar... wroar... wroar'.
Distribution
Southern France, northern Catalonia, southern Portugal and Spain, also in Menorca and Madeira.
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