Amphibians
European Tree Frog
Hyla arborea · also called common tree frog, European treefrog
On a warm spring night in a French or German marsh, the noise can be startling: a rolling, rasping roar from hundreds of throats, each belonging to a frog small enough to sit on a coin. By day the same frogs are almost invisible, bright green discs pressed flat against sunlit leaves. This is the European tree frog, the only tree frog of mainland Europe north of the Mediterranean.
Overview
Hyla arborea is a small climbing frog, usually 3 to 5 cm long, with smooth skin, a dark stripe running from the nose along each flank, and round sticky pads on its fingers and toes. Its colour is usually bright leaf-green but can shift to brown, grey, or yellow within minutes. It lives across continental Europe in damp, sunny, scrubby country near fish-free ponds, and it hunts insects at night in the bushes and reeds.
A small frog with a big voice
In spring, males gather at breeding ponds and each inflates a throat sac larger than its head, broadcasting a fast, harsh, repeated call. A full chorus carries more than a kilometre. Females choose the loudest, most persistent males. For centuries people kept these frogs in jars with a tiny ladder as “weather frogs”, believing they climbed in fair weather.
Losing ground in the north-west
Across the Netherlands, Denmark, and the Baltic, the tree frog has vanished from large areas as small ponds were filled in and farmland intensified. Several countries now dig clusters of new ponds and, in places, breed and release frogs, and some local populations have bounced back.
Status
The European tree frog is Least Concern globally, common through central, eastern, and southern Europe, but declining and protected across much of its north-western range.
European Tree Frog: common questions
Do European tree frogs live in Britain?
Not as a native species. Britain has native common frogs, common toads, and natterjack toads, but no native tree frog. Small populations of European tree frogs have been established in southern England several times from deliberate or accidental releases, most famously in the New Forest, and a few of these have persisted for decades, but they are introductions rather than a natural part of the British fauna. The frog is native to the near Continent, reaching as far north-west as the Netherlands and northern Germany, and it may once have occurred naturally in south-east England thousands of years ago before the land bridge was lost.
How does a tree frog climb so well?
The tips of its fingers and toes are widened into flat, round discs. Each disc has a surface of tightly packed columnar cells separated by fine channels, and a layer of mucus that the frog secretes onto them. The mucus creates a wet adhesion, like a damp glass sticking to a table, spread over many tiny contact points, and the channels drain away excess fluid so the grip does not simply slide. The frog can also press the soft pads flat to increase contact area. Together this lets a frog weighing only a few grams hang from a vertical leaf or the underside of a branch, and even climb smooth glass.
Why do male tree frogs call so loudly?
The call is a competitive advertisement aimed at females and rival males. On spring nights, males gather at a breeding pond and each one inflates a large vocal sac under the chin, using it as a resonating chamber to broadcast a rapid, rasping call. Females move toward the loudest, most vigorous, most persistent callers, which tend to be the largest and healthiest males, so calling hard improves a male's chances of mating. Because many males call at once from a small pond, the combined chorus becomes extremely loud for the size of the animals, carrying over a kilometre on a still night and helping other frogs locate the breeding site.
Is the European tree frog endangered?
Globally it is listed as Least Concern, because it remains common and widespread across central, eastern, and southern Europe. The concern is regional. In the north-west of the range, in the Netherlands, Denmark, northern Germany, and the Baltic countries, the frog has disappeared from large areas as small farmland ponds were filled in or stocked with fish, wetlands were drained, and hedgerows and rough ground were removed. Several of these countries now run active conservation programmes, digging clusters of new shallow ponds and, in some cases, breeding and releasing frogs, and a number of local populations have recovered as a result.