Phenological shifts of low-latitude frogs over 15 years revealed by passive acoustic monitoring

Abstract Phenological responses to climate warming are well-documented, yet limited data come from low latitudes. Because phenological sensitivity may vary with latitude, studies in low-latitude regions are essential to understand global patterns. To fill this gap, we analysed acoustic recordings collected daily over 15 years on Iriomote Island, in Japan's transitional zone between subtropical and tropical biomes. We applied the BirdNET deep learning model to quantify the breeding phenology of seven frog species. Seasonal calling patterns differed among species, with peaks occurring in various seasons while one species called nearly year-round. Among six species analysed for phenological trends, a winter-breeding tree-frog (Zhangixalus owstoni) showed a statistically significant shift in median calling date (3.8 days later per decade), whereas the other species showed no significant shifts of varying magnitude and direction. Sliding time window analysis indicated late-summer temperature—which increased during the study period—best predicted median calling dates in Z. owstoni, suggesting that climate warming delayed the phenology of a winter breeder even where year-round reproduction is possible. The absence of significant phenological advancement in our dataset contrasts with spring advancement commonly reported in temperate regions. These findings reinforce the importance of examining long-term phenological shifts across more sites in low-latitude regions.