Radio tags reveal how giant green anacondas hide underwater in the Amazon; researchers spotted them at just 17.7% of tracked locations |

Representative Image of a green anaconda concealed among dense vegetation and riverbanks in the western Amazon rainforest (AI-generated image)

In the western Amazon, finding a green anaconda can depend on more than knowing which river it inhabits. Dense vegetation, submerged debris and concealed riverbanks can make the giant reptile difficult to observe, even when researchers are tracking its movements. A study published in the Journal of Natural History in 2025 has documented how the giant reptiles use waterways and surrounding vegetation in the western Amazon. In Habitat use of cryptic Green Anaconda (Eunectes murinus) in the western Amazon, researchers compiled 180 observations involving at least 50 individual snakes in Peru’s Madre de Dios region. Four adults were fitted with very-high-frequency radio tags and tracked. At just 23 of their recorded radio locations, or 17.7%, were the snakes visible without disturbance. The findings show why the animals’ tendency to remain concealed, emerging into view mainly to bask, matters when researchers attempt to locate them and estimate their populations.

How researchers tracked four green anacondas in Peru

The research brought together habitat records from several collaborating projects in the Madre de Dios region of southeastern Peru. The observations came mainly from three methods: opportunistic encounters, intensive surveys and radio telemetry. The study’s results and supplementary records also include a small number of observations from general fauna surveys. The researchers recorded 180 observations involving at least 50 individual E. murinus, with telemetry tracking accounting for 130 records, or 72.2% of the total. The fieldwork covered a variety of aquatic environments, including rivers, streams, oxbow lakes and aguajales. An aguajal is a wetland dominated by aguaje palms, known scientifically as Mauritia flexuosa. The tagged snakes were tracked along the Los Amigos, Las Piedras and Madre de Dios rivers. The wider dataset included observations from additional river systems and wetland habitats across the region.Radio telemetry allowed researchers to locate tagged snakes even when vegetation, submerged logs or riverbanks obscured them from view. Capturing the larger snakes required substantial personnel and experience. The researchers described using a team of at least four people when handling particularly large individuals. Capture attempts depended on whether suitable personnel were available and whether the circumstances allowed the animal to be handled safely. The four tagged animals were adults. Their tracking records provided detailed information about where the snakes rested, basked and remained concealed across different habitats. However, the 180 observations were not all tracking records, and the results should be understood as a combination of telemetry data and other field observations.

Where green anacondas bask and hide

The study found that green anacondas in the western Amazon were typically exposed when basking, while remaining partly or entirely concealed at most other tracked locations. Their hiding places included fallen logs, accumulated leaf litter and other detritus, submerged vegetation, fallen palm trunks and undercut riverbanks. On one occasion, a tracked snake was found inside a burrow of unknown origin. Basking sites varied by habitat. Along rivers, log jams appeared to be the preferred places for basking. In aguajales and oxbow lakes, the snakes used well-vegetated embankments and fixed or floating mats of vegetation. The researchers also found that the animals rarely occupied streams, although they sometimes used connecting waterways and seasonally flooded areas.Individual tracking records illustrate how much these habitats can vary. One female weighed 100 kg and had a snout-to-vent length of 486 cm, with a further 77 cm measured along her tail. Her records placed her most often in closed-canopy aguajal habitat, while the remaining observations were from grass mats in open water. These records show that the tracked snake used more than one type of aquatic environment. A tracked male was also found in temporarily flooded forest, where he was nearly invisible in shallow pools beneath dense vegetation. In one observation, he was concealed beneath leaf litter and shallow water, with only his nostrils exposed. Such records help explain why a snake’s location may be known through telemetry even when a researcher cannot readily see the animal.The findings also fit with earlier research on anacondas and mineral licks. A separate 2024 study, Does the Southern Green Anaconda, Eunectes murinus, seek areas of high prey concentration in southeastern Peru?, described predation at mineral licks and associations between known mineral-lick locations and three radio-tagged snakes. Its authors suggested that anacondas may target such sites because prey can congregate there. This is related evidence about hunting behaviour, rather than a result that should be attributed solely to the 2025 habitat study.

Why hidden snakes complicate population surveys

The most important implication is for wildlife surveys. If researchers count only the snakes they can see from riverbanks or boats, they may miss animals concealed beneath vegetation, submerged debris or riverbanks. The finding that tagged snakes were visible without disturbance at only 17.7% of their radio locations shows how rarely these four adults were observed in the open at their recorded positions. The authors identify basking behaviour as an important factor when evaluating population densities from survey detections. They report that, in most of the habitats surveyed, anacondas were easily detected and best positioned for safe capture when basking. The study nevertheless has limits. Only four adult snakes were radio-tagged, and the work focused on one region of Peru. The results cannot establish that all green anacondas, including juveniles and animals in other parts of South America, use habitats in precisely the same way.Taxonomy is another consideration. A 2024 genetic study, Disentangling the Anacondas: Revealing a New Green Species and Rethinking Yellows, proposed recognising the northern green anaconda as Eunectes akayima, distinct from the southern green anaconda, Eunectes murinus. The 2025 study concerns E. murinus in Peru and should not automatically be taken as describing the behaviour of both proposed species. The central finding is straightforward: green anacondas can occupy waterways and wetlands while remaining difficult to detect, even when researchers know their locations. By combining radio tracking with other field observations, the study provides a clearer account of how these giant snakes use their habitats and why finding them in the wild remains a challenge.