Guwahati: In the dense rainforests of Arunachal Pradesh’s Namdapha landscape, scientists are discovering that sometimes the best way to find frogs is not by looking for them—but by listening.
A new study by researchers N.V. Rajiv of the Wildlife Conservation Society and Abhijit Das of the Wildlife Institute of India has found that passive acoustic monitoring, which uses automated sound recorders placed in forests, can reveal elusive frog species that often escape traditional field surveys.
The findings could transform how amphibians are monitored in some of the world’s most inaccessible biodiversity hotspots.
Published in the journal Herpetozoa, the study is the first to compare passive acoustic monitoring and visual encounter surveys for frogs in the Indo-Burma biodiversity hotspot and is described as the first anuran-focused acoustic study from Southeast Asia. It also helps fill a major research gap.
A recent global review found no published passive acoustic monitoring studies on frogs from Asia between 2013 and 2023, despite the continent harbouring more than a quarter of the world’s frog species.
The researchers conducted their work in and around Namdapha Tiger Reserve in Arunachal Pradesh, a region regarded as the world’s northernmost lowland tropical wet evergreen rainforest and one of India’s richest centres of amphibian diversity. The landscape’s steep terrain, dense vegetation and remoteness make conventional biodiversity surveys difficult, particularly at night when many amphibians are most active.
Using autonomous recording units placed in forests, streams, swamps and ponds, the team recorded frog calls during the night and compared the results with conventional visual surveys conducted by researchers on foot.
Both methods detected 19 amphibian species, but the microphones proved particularly effective at finding species that are difficult to spot, including cryptic, arboreal and fossorial frogs that spend much of their lives hidden in vegetation, tree canopies or beneath leaf litter.
Among the species detected were several endemic frogs found only in India, including Gracixalus patkaiensis, Microhyla eos, Nidirana noadihing and Rhacophorus namdaphaensis. The study recorded 11 endemic species through acoustic monitoring alone.
The recordings did more than document known frogs. In some cases, they captured calls that initially could not be identified and were later linked to species formally described as new to science, including the mud-nesting fanged frog Limnonectes motijheel and the cryptic bush frog Raorchestes nasuta.
The finding highlights the potential of acoustic monitoring not only for conservation but also for discovering species in poorly explored tropical forests.
The biggest surprise came when researchers combined both methods. Together, they documented 23 species—about 21 per cent more than either method could detect on its own.
The findings suggest that relying on a single survey technique may underestimate amphibian diversity in tropical rainforests. While microphones excelled at detecting hidden, tree-dwelling and burrowing frogs, visual surveys remained important for finding non-calling species and frogs living near noisy streams where flowing water can mask calls.
“Passive acoustic monitoring is a reliable method for estimating amphibian species richness, especially in remote and inaccessible regions,” the researchers wrote, while recommending a combined approach for comprehensive biodiversity assessments.
The study comes at a time when amphibians are facing unprecedented global declines. Frogs and other amphibians are considered among the world’s most threatened vertebrates, while scientists continue to discover new species at a rapid pace.
The findings also underscore a major gap in conservation monitoring. Although amphibians are regarded as the most threatened group of vertebrates, there is no monitoring protocol for them in any protected areas of India. Climate change-related threats may wipe out their population without being noticed… thus, such passive monitoring may be useful,” said Abhijit Das of the Wildlife Institute of India.
Researchers also found that nearly half the species recorded during the survey were detected at only one or two sites, underscoring the remarkable habitat diversity of the Namdapha landscape and hinting at how much remains undocumented.
According to the researchers, automated acoustic recorders could provide a practical and cost-effective way to monitor amphibian populations over long periods, especially in remote and rugged landscapes where regular field surveys are difficult.
The rainforest still holds many secrets. Species accumulation analyses suggested that additional amphibian species remain to be discovered in the landscape and that a much larger survey effort would be needed to build a comprehensive inventory. For the researchers, the message is clear: in Northeast India’s rainforests, microphones may soon become as important as binoculars and field notebooks.
Scientists are now finding that some of the rainforest’s secrets can be heard before they are seen.
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