Dryophytes japonicus, with frequently used synonym Hyla japonica, commonly known as the Japanese tree frog, is a species of anuran native to Japan, China, and Korea. It is unique in its ability to withstand extreme cold, with some individuals showing cold resistance at temperatures as low as −30 °C for up to 120 days. Japanese tree frogs are not currently facing any notable risk of extinction and are classified by the IUCN as a species of "least concern". Notably, it has been sent to space in a study that explored the effect of microgravity on Japanese tree frogs. Some consider that Dryophytes japonicus is synonymous with Hyla japonica. However, a 2025 study treated that this species as Dryophytes japonicus again, and separated northern species as Dryophytes leopardus.
The Japanese tree frog lives in a variety of habitats such as wetlands, forests, rivers, and mountains. They are generally located near vegetation near water sources and forests. They are carnivores that prey on insects and spiders. Their average clutch size is around 340–1,500 eggs, and their lifespan is usually around six years. There is an estimated 100 million of these frogs in Japan, but the accuracy is limited due to difficulty in counting.
Taxonomy Some authorities use the scientific name, Hyla japonica, in reference to the Japanese tree frog. The binomial name, Dryophytes japonicus, is also sometimes used. Studies have characterized the relationship between H. suweonensis and H. japonica. H. suweonensis is a closely related species to H. japonica. In general, H. suweonensis is smaller and more slender than H. japonica. The distance between nostril and upper lip (NL), distance between posterior corners of eyes (EPD), distance between semi-minor axis of the upper eye (LILe), angle between the two lines that connect the posterior corner of the eyes and ipsilateral nostrils (αEPD-N), and the angle between the two lines that connect the anterior corner of the eyes and the ipsilateral nostrils (αEAD-N) can all be used to differentiate between H. suweonensis and H. japonica. In 2025, populations in Japan east of Kansai and Sakhalin were reclassified as .
Description Japanese tree frogs are on average 32.81±0.96 mm in length. They have an average skull width of 12.02±0.36 mm and an average skull length of 9.38±0.14 mm. The dorsal body of Japanese tree frog is green/brown and the ventral body is white. It is also characterized by a dark spot on the upper lip below the eye. Female Japanese tree frogs, on average, are larger in size compared to male. It has a dark vocal sac.
Abnormal coloration Some Japanese tree frogs are abnormally colored.Maslova, Irina, et al. "Colour variants in the Japanese Treefrog (Dryophytes japonicus) from Russia and Korea." Herpetology Notes 11 (2018): 1007–1008. Frogs observed in South Korea were found to be entirely blue, while others yellow, with green dorsal patterns. Another frog found in Russia was observed to be fully blue, and was captured for observation, where it ultimately returned to a green/brown color. Specific reasons behind such observations in color are currently unexplained, but mutations and maladaptations have been put forth by scientists as possible explanations. Further work must be conducted in order to elucidate the mechanisms behind these color changes.
Habitat and distribution Japanese tree frogs are found in many parts of Asia, specifically in Japan, China, Korea, Mongolia, and Russia. It inhabits forest-like environments, bushlands, meadows, swamps, and river valleys. Japanese tree frog, like most frog species, inhabit locations with both aquatic and terrestrial features. This is due to the necessity of the frog life cycle for both water and land.
Changes in availability of native Japanese tree frog habitats have resulted in rice paddies serving as lodging for it. It seems to be able to inhabit these rice paddies successfully and have a demonstrated preference for sites high in vegetation.
Behavior
The behavior of Japanese tree frog when exposed to microgravity has been experimentally investigated. These frogs, under such microgravity conditions, would bend their neck backwards. These frogs would also walk backwards, an observation consistent with the behavior of sick frogs. The combination of neck backwards movement and backward walking could be indicators of motion sickness in the frogs. They were shown to adapt to the microgravity and were able to improve their jumping and perching activity over time. Japanese tree frogs, under micro-gravitational conditions, were also observed to attempt to eat but were unable to ingest the food. All the frogs that were sent to space were safely recovered and were observed to resume normal function after 2.5 hours back under normal gravity.
Diet Japanese tree frog forages in both breeding and non-breeding seasons. They are known to be opportunistic predators. This feature of this species was discovered through analysis that showed a strong correlation between the relative abundance of organisms in a given environment and the prey composition for that environment. Its diet consists of ants, beetles, caterpillars, flies, and spiders. There does not appear to be a significant difference in the diet composition between the two sexes of this species. However, during the breeding season, males have a higher chance of having an empty stomach due to the heightened energetic cost imposed by breeding.
Mating
Mating system Male Japanese tree frogs are observed to congregate in leks in an attempt to mate with female. A lek is an area where males will congregate in order to perform courtship displays in order to mate with females. Male leks seem to form preferentially at spots with significant water resources. Female distribution appears to be skewed towards male lekking sites. These lekking sites were identified by their extremely high male density. Female distribution does not seem to be explained by other factors like water availability, vegetation, or herbicide levels. The lek model that seems to fit the lekking exhibited by this species is the environmental hotspot model. This is because the sites that had the highest male density were those that had significantly high female encounter rates. Thus, there seems to be some bias of lek location towards areas with high female densities. Females need water for oviposition and the preference of male leks to form near water could be a mediating factor in choosing spots close to females.
Calling Males will call to signal their presence to females and to compete with other males. Notes of this species calls are made up of fine pulses, and exist mainly at the frequency of 1.7 kHz. Japanese tree frog was observed to make the majority, if not all, of their calls at night. It also seemed to call when they were located on the banks of rice paddies. Note length and note interval were observed to decrease in males when temperature increased.
Preference Japanese tree frogs are observed to prefer more shallow and smaller bodies of water for breeding. They prefer bodies of water termed oxbow lakes, likely due to their freestanding nature and higher chance of being refilled. Oxbow lakes are likely preferred due to the inability of tadpoles to swim along or against strong currents.
Infection effects on calling
Japanese tree frog is susceptible to infection by Batrachochytrium dendrobatidis. Batrachochytrium dendrobatidis infection causes a disease termed Chytridiomycosis. Chytridiomycosis is an amphibian disease that has devastated many amphibian populations across the world. It seems to be susceptible to Chytridiomycosis, however the disease does not appear to pose a high burden to this species. In fact, it has not been observed to suffer from increased morbidity or mortality from Chytridiomycosis This species in Korea seem to have Batrachochytrium dendrobatidis infection rates ranging from 10.6 to16.2%.
Chytridiomycosis has been observed to affect the calling of Japanese tree frogs in a multitude of different ways. Number of pulses per note and note duration were both observed to be significantly higher in infected ones compared to uninfected ones.
The increased effort devoted to reproductive efforts by infected specimens is an interesting result that warrants further research. Two hypotheses have been proposed to explain the observed behavior. First, this increased investment towards reproduction might be a result of Batrachochytrium dendrobatidis driving increased reproduction in order to increase spread of infection. Another hypothesis is that it increases its reproductive effort in the event that they die earlier due to Chytridiomycosis. This behavior would increase the chance of reproductive success by propagating their genes before they die.
Heterospecific amplexus This species have been observed with Pelophylax chosenicus in amplexus.Koo, Kyo Soung, et al. "First record of heterospecific amplexus behaviour between Pelophylax chosenicus (Okada, 1931) and Dryophytes japonicus (Günther, 1859) In Paju, Republic of Korea." Herpetology Notes 14 (2021): 1225–1226. Both species inhabit rice paddies and this shared habitat is a possible explanation for the observed interspecies copulation. Mating between different, but closely related species can sometimes result in hybridization. Further work is required to uncover the extent of heterospecific amplexus between this species and Pelophylax chosenicus.
Predators
Predation by the American bullfrog Lithobates catesbeianus, colloquially known as the American bullfrog, is an exotic predator of Japanese tree frogs. Predation by L. catesbeianus has been shown to significantly decrease the bone mineral density of these tree frogs. Because bone mineral density can be used as a proxy for food intake, the conclusion that L. catesbeianus predation of this species exerted a predation pressure that reduced food intake of it can be drawn. Predation by L. catesbeianus was not observed to induce any morphological changes in Japanese tree frogs.
Physiology
Predator defense by toxic peptide secretion Japanese tree frogs have evolved against predation in arboreal environments by producing special Anntoxin-like neurotoxins from their skin. Anntoxin is a 60-residue toxic peptide that inhibits ion channels such as tetrodotoxin-sensitive voltage-gated sodium channels. While these peptides display analgesic properties after binding onto ion channels, they can harm and kill predators after frog skin consumption. Such a mechanism deters predators from further frog hunting.
Cold resistance Japanese tree frogs demonstrates the remarkable ability to withstand extremely cold temperatures. It is able to survive temperatures as low as −35 °C. The majority of individuals in a population from the Amur River were shown to withstand multiple rounds of exposure to −30 °C. These specimens were shown to survive at −30 °C for up to 120 days. Other frog species, at such temperatures, will accumulate ice, a phenomenon that proves lethal. This accumulation of ice was not observed in this species.
During the exposure to cold, Japanese tree frogs seems to produce glycerol. This production of glycerol increases as temperature decreases. It is thought that this glycerol production plays a role in the cold-resistance of this species. However, other frog species have similar glycerol production, but do not have cold resistance to the extent of this frog. Thus, the biochemical mechanism for the cold resistance of Japanese tree frogs is yet to be fully determined.
Bone mineral density