European tree
frog
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.

European tree
frog, Hyla arborea
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. They
are the loudest frogs of Europe, because of their large larynx
(about one-fifth of the body length). 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).
There are three or four species and many subspecies:
- The Common or European tree
frog, Hyla
arborea (Linnaeus, 1758)
- The Mediterranean tree frog or
stripeless tree frog, Hyla meridionalis (Boettger,
1874)
- The Italian tree frog, Hyla intermedia (Boulenger, 1882), not
always considered a species)
- The Sardinian tree frog, Hyla sarda (De Betta, 1853)
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
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 croaking sounds like a loud and rithmic
'creck-creck-creck...' and resemble the call of the Baillon's Crake
(Porzana pusilla) and the
Little Crake (Porzana
parva).
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,
and Menorca.


European tree frogs
Issue #69