Morphological variation between Hyla arborea and Hyla orientalis from Bulgaria

The species status of Hyla arborea and H. orientalis is well supported by genetic data, however, there is still a lack of studies that could differentiate between these taxa based on other characteristics. During the mating season in the period 2021–2024, we measured a total of 196 tree frogs (144 male and 52 female) from six sites – three within the range of H. arborea and three within the range of H. orienatalis in Bulgaria. All frogs were anesthetised, measured, photographed and released at the site of capture after they were fully recovered. We assessed eight morphometric traits and corresponding indices, and used statistical comparisons to examine species- and population-level variation. While some indices showed significant differences, results did not support clear-cut morphological separation between species. Notably, one population (Zaychino), genetically identified as H. orientalis, grouped morphometrically with H. arborea. Our results suggest that interspecific morphological differences are subtle and may be more influenced by local environmental adaptation or historical introgression. Although not sufficient for a direct species identification in the field, the results contribute towards a better understanding of the morphological differences between these two congeneric species.