29 • 05 • 2019
Have You Already Heard Frogs Sing?
It is easy to recognize tree frog “wedding songs”. It is a very specific and fascinating sound.
Hylidae or tree frogs belong to the Anura order. It is a widespread family - tree frogs dwell on all continents except Antarctica. However, there is only one species of tree frog living in Latvia - the European tree frog. They live in individual areas.
With suction cups and an active tongue
The body of the European tree frog (Hyla arborea) is oval, the head is rounded. The amphibian is characterised by slender hind legs, which help making relatively long jumps. The tips of its toes are “equipped” with disc-shaped suction cups that help to stay on a hard surface.
The length of an adult tree frog is small, usually less than 5 cm. Males, as with the majority of amphibians, are smaller than females.
The animal's skin on the back is almost smooth, but uneven and rough on the abdomen. This amphibian can and sometimes changes the colour of the back of its body.
All of our tree frogs spend most of their lives on the soil and coastal plants, as well as in low tree foliage. In Latvia, they usually live in shrubs, deciduous forest edges, also in populated areas, most often in the vicinity of water bodies surrounded by rich vegetation. Amphibians also like to reproduce in such environment, namely, in shallow, miry, overgrown water bodies, usually in standing waters.
Tree frogs move most actively in the dusk and in the dark. They feed on invertebrates, mainly on flying insects and various insect larvae. The binocular vision helps them to jump and successfully hit their flying game - their eyes are designed so that their gaze is directed straight forward.
Tree frogs spend winters on the ground - in caves, tree root cavities, under stumps, in the foundations of buildings, in thick soil. In Latvia, they start wintering in late September, October. The end of the wintering season depends on the meteorological conditions. Since spring arrived early this year, tree frogs could be spotted already at the beginning of April.
It is quite a challenging task to notice the camouflaged, just a few centimetres long creatures in lush, greening summer vegetation, therefore they are most often to be observed in spring. It is quite simple to discover their presence at some point when, at the end of April and May, tree frog males arrive at their spawning grounds (most often in ponds) and transmit their acoustic territorial signals and “wedding songs”.
Spawning songs
The spawning songs start in the evening. The first “singer” will soon be joined by the next, then, more and more ... The number is gradually increasing to a real frog choir.
Male tree frogs have a very well developed internal resonator (sound amplifier), so in favourable conditions people can be lucky to hear the noise made by a spawning choir even from a distance of several kilometres.
Of course, in the life of tree frog males, this singing is mostly related to reproduction and usually they are most enthusiastic in the spawning sites in the twilight and in the dark. However, it can often also be a reaction to some deliberate or accidental acoustic provocation and be heard during the daylight, quite far away from water, even in autumn. Examples of such provocation include a real-life audio recording from a man-made device, rhythmic beating of a dry tree branch against another tree branch, sounds made by hand saws, breaking of dry branches under feet, a crack made by engine of an equipment.
Eggs spawned by tree frog females fall into the bottom of the body of water, making it difficult to see and explore them. However, it has been found that after the development of about two-weeks (depending on the water temperature) light, large-tailed larvae or tadpoles hatch.
The tadpoles live in the water for about three months, feeding, getting ready for changes and growing up to a length of about 5 cm. It is the end of July or August when metamorphosis or transformation occurs. As a result, miniature tree frogs leave water bodies. Some of them may be lucky and live for more than ten years.
Male animals reach sexual maturity already in their second year of life, but female frogs - in their third year of life.
What is the connection between tree frogs, beavers and agriculture?
The European tree frog area is broad: it includes most of Europe and also spreads to Asia. However, the species has been declared protected, included in the list of endangered amphibians and reptiles in Europe, and also in the Latvian Red Book.
Although there is some information about the observation of the tree frog in Latvia even in the first half of the 20th century, it is likely that since the end of the 19th century the species had disappeared. The main cause of extinction is the complete destruction (hunting) of beavers and the intensive development of agriculture.
Beaver activity areas are recognized as the most suitable habitats for spawning in Latvia. Earlier, beavers were constantly providing an environment suitable for the spawning of tree frogs - shallow ponds with lush vegetation. The drainage and use of land for agricultural purposes destroyed large areas suitable for tree frog spawning and life. Thus, beaver hunting and the boom of agriculture led to shrinking of the tree frog population until its complete destruction.
With the help of people, beavers returned to us. They were successfully reintroduced and they managed to stabilise their population. In the 1980s, the activity of these large rodents in many parts of Latvia contributed to the regeneration of water bodies suitable for tree frog spawning. Agriculture of the occupation period dwindled. The time had come to bring tree frogs back.
In 1987, implementing the idea of senior research fellow Juris Zvirgzds, the Ecology Laboratory was established at Riga Zoo; its aim was to perform artificial reproduction of rare and disappearing amphibians (first of all, the tree frog).
Juris Zvirgzds became the first head of this laboratory. He managed the laboratory until 1995, when he was promoted to the position of deputy director of the zoo for scientific work.
Thousands of tree frog babies have been cultivated and grown at the zoo since 1988 and then let out to live in their natural environment. Most of them got into the natural surroundings in Kalvene and Embūte, where there is a nature reserve called “Blažģa ezers” (Lake Blažģis). A good amount of the frogs cultivated in the laboratory was released near Ēdole.
31 years have passed since the start of the European tree frog restoration project. Now the amphibians once again take a stable position in the Latvian nature. It is known that these amphibians live in a significant part of Kurzeme, including places far away from the sites where the animal was once reintroduced.
We have successfully restored an animal species that was once extinct here. However, there is not enough reliable information on the extent to which it inhabits our territory. Employees of Riga Zoo have decided that it is necessary to identify the current distribution of tree frogs, therefore they call on everyone to report any tree frogs that have been heard and seen, writing to the e-mail address
