![\"The](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/07\/k-picta2.jpg\")
The slender-toed chorus frog is a ground-dwelling frog that lives in burrows. Unlike a tree frog, it does not have expanded toes. Image courtesy of Thomas Brown via iNaturalist<\/a> (CC BY-NC-SA 2.0<\/a>)<\/p>\nIn a recent study<\/a>, published in the journal Hereditary, Brown and his colleagues unravel the evolutionary history of this rare, elusive and \u201coddball\u201d frog, and provide evidence to show that the Leyte chorus frog (K. c. stickeli) isn\u2019t a distinct subspecies of the truncate-toed chorus frog as previously assumed but a hybrid between the two frog species \u2014 K. c. meridionalis and K. picta \u2014 as Brown believed. The researchers posit that this hybrid, the first of its kind observed in narrowmouth frogs, is the result of large-scale deforestation<\/a> in the country that has forced tree frogs to come into close contact with terrestrial frogs and interbreed.<\/p>\n \u201cFiguring out the \u2018stickeli mystery\u2019 was satisfying,\u201d Brown said. \u201cIt\u2019s very important that we understand the impacts of human activities and habitat modification via wholesale forest disturbance \u2026 it can produce disastrous, unnatural outcomes.\u201d<\/p>\n \u201cThis study uses modern genetic analytical techniques to solve a mystery which traditional taxonomy would have likely never been able to solve,\u201d ecologist and herpetologist Seshadri K.S. at the Ashoka Trust for Research in Ecology and the Environment (ATREE), India, told Mongabay by email. The techniques used in this study can help other researchers \u201cexamine species complexes or species that have been thought to be lost with a new lens,\u201d said Seshadri, who wasn\u2019t involved in this study.<\/p>\n Solving an 80-year-old evolutionary mystery<\/p>\n The genus Kaloula comprises a group of 20 frog species spread across South and Southeast Asia, with half of them found in the Philippines. The truncate-toed chorus frog (K. conjuncta) is one of them, which is further divided into four subspecies based on geography. K. c. conjuncta is restricted to Luzon and surrounding islands, K. c. meridionalis is found in Mindanao and surrounding islands, and K. c. negrosensis is found in the Visayan Islands and surroundings.<\/p>\n The Leyte chorus frog, long thought to be the fourth subspecies, is found in the same habitat as the K. c. meridionalis, which inhabits understory trees and shrubs and breeds in pools within undisturbed forests on Mindanao and surrounding islands. It was considered a rare species not seen for nearly half a century since its first description.<\/p>\n \u201cSome conservation biologists had begun to question whether the stickeli shrub frog [Leyte chorus frog] was an endangered species or possibly one that had already gone extinct,\u201d Brown said.<\/p>\n The slender-digit chorus frog, on the other hand, is a burrowing frog widely distributed in highly disturbed open habitats such as rice fields and ditches. \u201cThose are explosive breeders \u2026 they all emerge from underground [during the monsoon season], find temporary pools of water, find mates as quickly as possible and breed so their tadpoles have enough time to transform into frogs by the time the rainy season stops and they have to go back underground,\u201d Brown said.<\/p>\n Turns out, it\u2019s this scurry to breed that causes evolutionary oopsies, resulting in the Leyte chorus frog \u2014 an infertile hybrid.<\/p>\n \u201cIn that mad rush to reproduce, mistakes can be made, and unrelated species can breed and create a \u2018mule\u2019 or a non-viable hybrid that is really a dead-end,\u201d Brown said.<\/p>\n While hybrids are not rare in nature \u2014 there are grolar bears<\/a> (a hybrid between grizzly and polar bears) and African elephant hybrids<\/a> \u2014 this frog hybrid seemed quite unlikely because of the vast differences in the appearance, habitats and behaviors of a ground-dwelling frog and a tree-dwelling one. But when Brown and his colleagues looked at the mitochondrial DNA from Kaloula frogs collected in the Philippines for a previous study, there were surprises.<\/p>\n \u201cOne result of that study<\/a> jumped out at us when we noticed that the morphological intermediate frogs that we had collected back in 1996 appeared to be almost genetically identical to the treefrog K. c. meridionalis, even though they really looked much more similar to K. picta, the ground frog,\u201d Brown said. That was an aha moment \u2014 it struck him that K. c. stickeli might be a hybrid, as he had initially believed. And then he began his genetic investigations, which led to the current findings.<\/p>\n There was also more circumstantial evidence: The hybrids were so rare that scientists have seen them only twice, once in the 1940s and then in 1996, probably because they are infertile and can\u2019t mate and expand their numbers. Both these observations were from areas that had recently been disturbed by deforestation, and the specimens were collected at the edge of a forested area after the forest had been cut down.<\/p>\n When the researchers placed the specimens collected in the 1940s and 1996 side by side and took measurements of their physical characteristics, they were almost a perfect match.<\/p>\n \u201cAt that point we were certain that the frogs I had collected in the mixed chorus on Mindanao in 1996 were the same thing as the frogs collected on Leyte island [in] 1943,\u201d Brown said. \u201cThe question at that point just became: what is this thing?\u201d The current study provides an answer to that long-haunting question.<\/p>\n Based on their findings, the researchers argue that, despite its unique appearance, the hybrid must not be classified as a subspecies. Instead, they suggest changing its taxonomy from its subspecies name to K. C. meridionalis x K. picta, indicating it is a hybrid between the two.<\/p>\n Hybrid a result of deforestation<\/p>\n Philippine rainforests have been plundered<\/a> since Spanish colonization in the 16th century. However, in the early 20th century, deforestation reached new heights during the U.S. and Japanese occupation, when researchers estimate that more than half of the country\u2019s coast-accessible lowland forests were cut down. With expanding agriculture, forests continue<\/a> to make way for cropland, their biodiversity disappearing with them. Data from Global Forest Watch<\/a> shows that between 2002 to 2024, the Philippines lost more than 4% of its remaining primary forest cover.<\/p>\n Years of deforestation have also impacted frogs that call the forest trees home. \u201cLarge scale logging doesn\u2019t just remove a few trees from an area, it removes all the trees from an area and leaves no habitat for tree frogs,\u201d Brown said. He posits that\u2019s what led to hybridization.<\/p>\n In a forest, tree frogs like K. c. meridionalis would approach breeding puddles by climbing through the vegetation. However, without trees, these frogs had to traverse open fields to get to breeding pools. \u201cIn this situation, it\u2019s clear that the females of the tree frogs (K. c. meridionalis) were ambushed by the males of the ground frogs (K. picta) while attempting to approach calling males of their own species,\u201d Brown said, adding that these males were also in the pools, giving out a mixed chorus of frog croaks. \u201cOur genetic data confirmed this.\u201d<\/p>\n Hybrids by themselves aren\u2019t unusual, unnatural or concerning, and sometimes act as precursors to the evolution<\/a> of well-adapted species. But when deforestation forces it, it can be a source of concern for conservationists.<\/p>\n \u201cJust imagine an entire population\u2014thousands of individuals\u2014emerging with the onset of the monsoon rains, and all the forest is gone. That\u2019s a pretty sad scenario,\u201d Brown said. \u201cIn this case, we believe we\u2019ve uncovered pretty convincing data from several different independent sources that all point to an unnatural frog reproduction outcome resulting from deforestation.\u201d<\/p>\n \u201cNo ecosystem is now left untouched by humans and we knowingly or unknowingly shape the world around us. Acknowledging this upfront and bringing in systems thinking<\/a> into conservation planning becomes a key element,\u201d Seshadri said. \u201cWhat this work underscores is how little we know and how it is absolutely essential to monitor populations, something that is lacking in much of South and Southeast Asia where there is an incredible diversity of biodiversity.\u201d<\/p>\n Banner image: The truncate-toed chorus frog is a tree-dwelling species endemic to the Philippines with four recognized subspecies. They have expanded toe discs, called pads, on their toes. Image courtesy of Wouter Beukema via iNaturalist<\/a> (CC BY 4.0<\/a>)<\/p>\n Scientists describe three new frog species from Peruvian Andes<\/a><\/p>\n Citations:<\/p>\n Chan, K. O., Hime, P. M., & Brown, R. M. (2025). Deforestation-induced hybridization in Philippine frogs creates a distinct phenotype with an inviable genotype. Heredity, 134. 200\u2013208. doi:10.1038\/s41437-025-00748-y<\/a><\/p>\n Chan, K. O., Blackburn, D. C., Murphy, R. W., Stuart, B. L., Emmett, D. A., Ho, C. T., & Brown, R. M. (2013). A new species of narrow-mouthed frog of the genus Kaloula from eastern Indochina. Herpetologica, 69(3), 329\u2013341. doi:10.1655\/herpetologica-d-12-00094<\/a><\/p>\n![\"Newly](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/07\/USAID_Measuring_Impact_Conservation_Enterprise_Retrospective_Philippines_Nagkakaisang_Tribu_ng_Palaw.jpeg\")
Newly cleared forest for agriculture in the Philippines. Expanding agriculture, logging and mining has meant the Philippines has lost more than 4% of its remaining primary forest between 2002 and 2024. Image courtesy of USAID Biodiversity & Forestry \u2013 USAID Measuring Impact Conservation Enterprise Retrospective via Wikimedia Commons<\/a> (Public Domain).<\/p>\n![\"The](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/07\/k-picta.jpg\")
The researchers say the large-scale deforestation has meant the tree-living truncate-toed chorus frog females have to approach breeding males by traversing open fields, where they get ambushed by the males of the ground frogs (K. picta), shown here, leading to an infertile hybrid. Image courtesy of Will Flaxington via iNaturalist<\/a> (CC BY-NC 4.0<\/a>)<\/p>\n