We conducted field observations of spawning sites of the Japanese forest green tree frog (Zhangixalus arboreus: Rhacophoridae) at four ponds for two years. In 2019, a total of 50 foam nests were made on trees (77%), whereas 15 foam nests were made on the ground (23%). In 2020, 65 nests were arboreal (97%) and there were only two ground nests (3%). About 70% of all ground nests deteriorated or disappeared without their eggs hatching, mostly because of predation. In contrast, only about 4% of all arboreal nests died because of desiccation and 27% died mainly of outside spawning and disappearance before egg hatching for any reason. The relative proportions of arboreal versus ground spawning sites differed significantly between 2019 and 2020. Ground spawning was scarcely observed in 2020; in that year the amount of precipitation during the reproductive period was about 1.5 times that in 2019. Our field experiment using paper-clay models confirmed that arboreal nests were more susceptible to desiccation than ground nests. These results suggest that ground spawning may be of some advantage in resistance to desiccation, whereas arboreal spawning may be less susceptible to predation of foam nests.

1. Temperature is essential for the survival and development of eggs. Some anurans have evolved and developed foam nesting traits, with thermal insulation considered to be among their functions. Foam nesting frogs tend to exhibit reproductive plasticity. For example, they oviposit on both trees and the ground. How such plasticity affects foam nest function is of major relevance and is likely related to the adaptation of foam nesting frogs. However, this has not been well studied. 2. In this study, we studied the interaction between foam nest site, foam nest function, and egg fate using the Japanese green tree frog, Zhangixalus arboreus, and analysed how nest site differences (arboreal or terrestrial) affect the thermal function of foam nests. 3. We compared the thermal functions of foam nests between arboreal and terrestrial oviposition sites of Z. arboreus. We artificially replaced half of the arboreal nests to terrestrial environments and recorded temperature in and outside of the experimental terrestrial nest and original arboreal nests. We also examined egg survival and hatching rate for all the nests. 4. The results indicated superior heat insulation in terrestrial nests, with warmer temperatures inside than outside the nests, especially at night, which led to a high egg survival rate. Therefore, terrestrial ovipositing should be valid under cold weather conditions. This may be related to the evolutionary history of oviposition site plasticity of this genus, which originally had an arboreal oviposition trait but evolved into terrestrial site use owing to global cooling. 5. Our novel insights into the evolution and adaptivity of foam nesting and oviposition site use in Z. arboreus make a significant contribution to animal ecology.

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