- The truncate-toed chorus frog is a widely distributed species in the Philippines archipelago, with four distinct subspecies. One of them, the Leyte chorus frog, first described around 80 years ago, is considered a rare, enigmatic and morphologically distinct subspecies that has not been seen for nearly half a century since its first sighting.
- In a recent study, scientists used genetic analysis to find that the subspecies is, in fact, a hybrid between another subspecies of the truncate-toed chorus frog and a ground-dwelling, closely related slender-digit chorus frog.
- Researchers say this infertile hybrid is a result of large-scale deforestation in the Philippines, which forced tree-dwelling frogs to come on land, where ground-dwelling frogs “ambushed” and mated with them, creating the hybrid.
- Conservationists say this hybrid shows how human-caused disturbances impact biodiversity in the Philippines, where deforestation is rampant due to mining, logging and expanding agriculture.
In 1996, when herpetologist Rafe Brown accompanied a few biologists to the Philippine island of Mindanao to study frogs, they heard a cacophony near their campsite. In a puddle that skirted the forest, hundreds of chubby frogs with big, bright eyes had gathered, looking for mates to breed with. Brown said he remembers hearing two distinct croaks from that frog fracas — one from a tree-dwelling species and another from a burrowing species.
On a closer look, he identified some of them as a subspecies of the truncate-toed chorus frog (Kaloula conjuncta meridionalis, also called the Philippine narrowmouth toad), based on the round, expanded tips — called pads — on their toes, typical of tree frogs. The other was the ground-dwelling slender-digit chorus frog (Kaloula picta, also called the painted narrowmouth toad), which didn’t have expanded toes. But also in the mix was a little-known third kind called the Leyte chorus frog (Kaloula conjuncta stickeli).
“We found several additional frogs that seem to be physically intermediate between these two very different forms,” Brown, who is now at the University of Kansas, U.S., told Mongabay by email. These frogs had toe pads larger than those in ground frogs but smaller than those in tree frogs, and they were slightly bigger in appearance. “That started us thinking about whether these two distinct species sometimes interbred and hybridized,” Brown said.
As new genetic analysis tools and techniques became available, Brown and his colleagues reinvestigated old specimens in the last few years to find a definitive answer. They examined specimens collected during the 1996 field trip and those in museum collections, which were nearly 80 years old, dating back to the 1940s when the frog was first described.
In a recent study, published in the journal Hereditary, Brown and his colleagues unravel the evolutionary history of this rare, elusive and “oddball” frog, and provide evidence to show that the Leyte chorus frog (K. c. stickeli) isn’t a distinct subspecies of the truncate-toed chorus frog as previously assumed but a hybrid between the two frog species — K. c. meridionalis and K. picta — 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 in the country that has forced tree frogs to come into close contact with terrestrial frogs and interbreed.
“Figuring out the ‘stickeli mystery’ was satisfying,” Brown said. “It’s very important that we understand the impacts of human activities and habitat modification via wholesale forest disturbance … it can produce disastrous, unnatural outcomes.”
“This study uses modern genetic analytical techniques to solve a mystery which traditional taxonomy would have likely never been able to solve,” 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 “examine species complexes or species that have been thought to be lost with a new lens,” said Seshadri, who wasn’t involved in this study.
Solving an 80-year-old evolutionary mystery
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.
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.
“Some 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,” Brown said.
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. “Those are explosive breeders … 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,” Brown said.
Turns out, it’s this scurry to breed that causes evolutionary oopsies, resulting in the Leyte chorus frog — an infertile hybrid.
“In that mad rush to reproduce, mistakes can be made, and unrelated species can breed and create a ‘mule’ or a non-viable hybrid that is really a dead-end,” Brown said.
While hybrids are not rare in nature — there are grolar bears (a hybrid between grizzly and polar bears) and African elephant hybrids — 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.
“One result of that study 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,” Brown said. That was an aha moment — 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.
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’t 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.
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.
“At 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,” Brown said. “The question at that point just became: what is this thing?” The current study provides an answer to that long-haunting question.
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.
Hybrid a result of deforestation
Philippine rainforests have been plundered 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’s coast-accessible lowland forests were cut down. With expanding agriculture, forests continue to make way for cropland, their biodiversity disappearing with them. Data from Global Forest Watch shows that between 2002 to 2024, the Philippines lost more than 4% of its remaining primary forest cover.
Years of deforestation have also impacted frogs that call the forest trees home. “Large scale logging doesn’t just remove a few trees from an area, it removes all the trees from an area and leaves no habitat for tree frogs,” Brown said. He posits that’s what led to hybridization.
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. “In this situation, it’s 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,” Brown said, adding that these males were also in the pools, giving out a mixed chorus of frog croaks. “Our genetic data confirmed this.”
Hybrids by themselves aren’t unusual, unnatural or concerning, and sometimes act as precursors to the evolution of well-adapted species. But when deforestation forces it, it can be a source of concern for conservationists.
“Just imagine an entire population—thousands of individuals—emerging with the onset of the monsoon rains, and all the forest is gone. That’s a pretty sad scenario,” Brown said. “In this case, we believe we’ve uncovered pretty convincing data from several different independent sources that all point to an unnatural frog reproduction outcome resulting from deforestation.”
“No 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 into conservation planning becomes a key element,” Seshadri said. “What 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.”
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 (CC BY 4.0)
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Citations:
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. 200–208. doi:10.1038/s41437-025-00748-y
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), 329–341. doi:10.1655/herpetologica-d-12-00094
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