European tree-frog | Hyla arborea | Greek herpetofauna

Adult female (C) Charis Kouelis

Introduction

The European tree frog (Hyla arborea) is one of the most widespread and easily recognizable species of frog across Europe. With its beautiful appearance, high color variability and photogenic nature among other traits, it is a favorite among herpers. It is a typical member of the Hylidae family and therefore highly arboreal, spending a lot, if not most of its time climbing in vegetation and other vertical surfaces. Its binomial name is derived from the ancient Greek “hylē”, meaning “forest” or “woods” and “arborea” which loosely translates to “of the trees”, and while it often inhabits wooded areas, it is quite adaptable and can also be found in other habitat types as well. It ranges from as West as Normandy to as far North as Denmark, south to the island of Crete and as far east as Thrace. The binomial name Hyla arborea was until recently used to describe all tree frogs (mainly) in Europe and the Middle East, however, a phylogenetic study by Stöck et. al. shed light on the existence of several distinct genetic lineages in the aforementioned regions, which led to Hyla arborea being split into several different species. In Greece, two members of the species complex can be found, with Hyla arborea occurring in most of Greece and H. orientalis replacing it in Evros prefecture as well as the Anatolian islands (see map in distribution). 

Description

Up to 5cm, females typically larger than males. Very distinct coloration; usually bright green but varies according to substrate coloration, temperature or mood, with yellow, brown and grey individuals being common. A brown lateral stripe extends from the nostril to the hind legs, where it forms a distinct inguinal loop. Τhe ventral side has a granular texture and is lighter than the dorsal side, with darker or lighter shades of an off-white coloration. The inside of the legs as well as some of the toes are yellow-orange. Dorsal skin smooth, with long limbs and toes ending in disc-shaped adhesive toe pads. Eyes large and prominent, with a horizontal pupil. Tympanum size is half of the eye diameter. Males differ from females in having a golden-light brownish vocal sac, which is visible in the form of wrinkles when deflated. Tadpoles up to 5cm, with a golden-olive coloration; spiracle sinister and slightly pointed upwards, upper tail fin reaching as far as the eyes, tail-tip pointed. Eggs small and yellowish, deposited in clutches of 200-1,000 eggs. 

Adult male showing its inflated vocal sac (C) Charis Kouelis

Adult female (C) Charis Kouelis

Adult female, grey coloration (C) Charis Kouelis

Adult male (C) Stamatis Kalogiannis

Adult, spotted grey morph (C) Plato Stefanopoulos

Adult female (C) Plato Stefanopoulos

Adult male (C) Charis Kouelis

Adult female (C) Charis Kouelis

Adult in typical sleeping position (C) Plato Stefanopoulos

Egg clump (C) Plato Stefanopoulos

Adult male, bronze morph (C) Stamatis Kalogiannis

Call

A very loud, fast series of peep-peep-peep-peep-peep croaks, usually heard in chorus.

Biology 

The European tree-frog generally seems to be active throughout the entire year, but it will hibernate on land in sites such as burrows, hollow logs or rock piles in freezing conditions. Activity seems to be relatively low in the summer before the first rains of the fall as well. It can be heard calling throughout the year, even in the summer and winter months, though less frequently and for shorter intervals than during the reproductive season. It breeds from late winter until late spring (February to May), though the breeding season differs across localities. Calling begins to increase while the frogs are migrating towards the breeding sites, and upon arriving, the males will gather in large numbers and call in unison, creating an unmistakable deafening symphony, all in an effort to attract females. Once in amplexus, females will lay their eggs in several clumps of about 100 eggs each, for a total clutch of up to 1,000 eggs. These clumps are often attached to aquatic vegetation. The larvae soon hatch and metamorphose in summer, though it is possible for tadpoles to overwinter as well. The calling of adult males winds down in late spring when they leave the breeding grounds and return to their foraging sites. Like most tree frogs, it spends most of its time on branches, leaves and reeds. It is most often active at night, though daytime activity is not exceedingly rare. Hyla arborea is a formidable predator of insects, and it is highly adaptable, changing its diet according to seasonal availability of prey items. Its high agility and fast leaps across leaves and branches allow it to catch fast flying insects. It has been documented to eat numerous types of arthropods, including but not limited to spiders, beetles, butterflies and moths. It can often be seen crossing roads, often in relatively high densities in very humid or rainy nights in search of new foraging sites. It will usually also sleep in vegetation during the day, though this can also occur on anthropogenic structures it can climb on as well. To stay camouflaged in the daytime, its color can change drastically so that it matches its habitat and allow it to stay hidden, but the frogs can also change color for other reasons, such as to aid thermoregulation or even attract mates. Color change is one of the European tree frog’s most characteristic traits and outside of the substrate color, individual coloration can also be influenced by factors such as temperature and light intensity. Like most frogs, its main defense from predators, apart from fleeing, is secreting a milky toxin from the skin. This makes the frog foul-tasting which successfully deters many predators. The substance can be irritating if it comes in contact with the eyes, mucous membranes or open wounds, and can perhaps be harmful to humans if ingested. 

Habitat

The habitat of the species varies; it can be found in hardwood/mixed forests, thicket-type vegetation (especially comprising of reeds, Juncus spp. and Rubus spp.), coastal wetlands, floodplains, the riparian zone of streams, lakes and rivers, but also in more open habitat such as cultivations and even gardens. Since it can thrive in several types of vegetation, its presence in an area seems to be more dependent on the proximity of breeding sites rather than the flora. This is especially evident in Southern Greece, where water sources can often be scarce. It breeds in shallow, stagnant waters that are preferably surrounded by riparian vegetation and have some aquatic vegetation such as ponds, swamps, vernal pools and even ditches and puddles.

Floodland – habitat of Hyla arborea

Reedline and Tamarix forest along the shores of a reservoir – habitat of Hyla arborea

Seasonal marsh surrounded by thick vegetation cover – habitat of Hyla arborea

Shrubland bordering a large wetland – habitat of Hyla arborea

Wetland area – habitat of Hyla arborea 

Distribution

The European tree-frog is one of the most widespread amphibian species in Greece; It is widely distributed throughout the Greek mainland and occurs on the Peloponnese peninsula and several islands (although absent from Evros prefecture in NE Greece as well some islands of the NE Aegean: Rhodes, Lesvos, Kos, Samos and Chios, where it is replaced by the closely related Hyla orientalis); occurs on Euboea and numerous other Aegean and Ionian islands, such as Corfu, Kefallonia, Lefkada, Zakynthos, Andros, probably Tinos, Samothrace, Thasos, Skiathos and Crete. The validity of the subspecies kretensis in Crete remains unconfirmed.

Approximate distribution of the Hyla arborea complex in Greece