Amphibians
Japanese Tree Frog
Dryophytes japonicus · also called Japanese tree frog, amagaeru
A Japanese tree frog is a thumbnail-sized green frog that turns grey-brown when it moves off a leaf, and its night chorus from the flooded rice paddies is one of the defining sounds of the East Asian countryside. It has also, unexpectedly, helped engineers: the way rival males space out their calls so as not to overlap has been turned into algorithms for keeping wireless signals from colliding.
Overview
The Japanese tree frog, Dryophytes japonicus, is a small (under 4 cm) tree frog of rice paddies, marshes, ponds, ditches, and gardens across Japan, Korea, northeastern China, and the Russian Far East. It is a night-hunting insectivore of flies, mosquitoes, moths, and aphids, with sticky toe pads for climbing smooth stems and glass. In Japan it is the amagaeru, the “rain frog”, and its calling marks the start of the rainy season.
Colour, chorus, and code
It changes colour between green, grey, and brown to match its surroundings. Males chorus loudly through summer nights, and researchers found that neighbours shift their call timing to avoid overlapping, so a female can tell them apart — a natural solution that has been borrowed as a model for spacing out signals in wireless sensor networks.
Built for the cold
Unlike most tree frogs, it ranges into Hokkaido and Siberia, overwintering buried in soil and tolerating partial freezing by flooding its tissues with glucose antifreeze, the same trick used by the wood frog and gray tree frog.
Status
Least Concern and stable — one of the most common and adaptable amphibians in its range, thriving in the rice-farming landscape. Its close relative the Suweon tree frog of Korea, only recently split off, is Endangered, confined to a few wetlands and threatened by development and hybridisation with the common species.
Japanese Tree Frog: common questions
What does a Japanese tree frog sound like?
The male call is a loud, rapid, rasping series of notes, often written as kekekeke or gepgepgep, given in bouts through the night from flooded rice fields, pond edges, and wet ditches from about May through July. A single frog is fairly loud for its size, and because males gather in large numbers the combined chorus can be a wall of sound audible from a considerable distance. In Japanese the frog is amagaeru, the rain frog, and its calling is associated with humid weather and the arrival of the rainy season, and the sound is a common motif in classical poetry as a sign of early summer.
How did studying these frogs help wireless technology?
When male Japanese tree frogs call in a chorus, close neighbours do not simply shout over each other. Instead each male shifts the timing of his calls so that his notes fall into the gaps between a rival's notes, producing an alternating pattern that keeps each male's signal distinct so females can evaluate them. Engineers studying how to make many small wireless devices share a communication channel without their signals colliding realised this was the same problem the frogs had solved, and they built control algorithms based on the frogs' rule of adjusting your own timing in response to your neighbours. The approach has been applied to sensor networks and to spreading out data transmissions and traffic-light timing.
Can Japanese tree frogs really survive freezing?
They tolerate their bodies partially freezing, which is unusual for a tree frog and lets them live much farther north than most. In autumn the frog burrows into soil and leaf litter rather than hibernating underwater, and as temperatures drop below freezing its liver releases large amounts of glucose into the blood and tissues, which acts as a natural antifreeze that limits ice formation to safe spaces outside the cells. In this state the frog stops breathing and its heart stops, and both restart when it thaws in spring. This freeze tolerance, shared with the North American wood frog and gray tree frog, is why the Japanese tree frog can occupy Hokkaido and Siberia.
Are Japanese tree frogs kept as pets?
They are kept, mainly by hobbyists in their native countries and by tree frog enthusiasts elsewhere, and they are hardy and undemanding, needing a planted vivarium with climbing surfaces, moderate humidity, a seasonal cool period, and small live insects. They are small, usually under 4 centimetres, and quite bold once settled, often perching in view. As with all amphibians they absorb substances through their skin, so handling should be brief with clean wet hands, and captive-bred frogs are preferable. Their mild skin secretions can irritate the eyes, so hands should be washed after contact.