Habitat use of cryptic Green Anaconda (Eunectes murinus) in the western Amazon

Knowledge of Eunectes murinus autecology in the humid, lowland Amazon rainforest of Peru to date is limited. We aggregated and reviewed habitat records from several collaborating projects in the Madre de Dios region of Peru. The records originated from opportunistic sampling, intensive surveying, and radio telemetry tracking. We include details on detection, capture, and processing methodology. Our efforts yielded 180 observations of 50 individual E. murinus, four of which were tagged with very high-frequency radio tags and tracked. We report the first presence of E. murinus for several river tributaries and field sites, and provide insight into the species’ habitat use. In the western Amazon, E. murinus is cryptic and typically exposed only when basking. Of all radiolocations of tagged individuals, in only 17.7% (n = 23) were individuals observable without disturbance. Well-vegetated embankments were used as basking habitats in aguajales, oxbow lakes, streams, and rivers. On rivers, log jams appeared to be the preferred basking habitat. In oxbow lakes and lakes of aguajales, E. murinus basked on fixed and freefloating vegetation mats. When not basking, radiotracked E. murinus were almost always partially or entirely concealed by fallen logs, detritus, submerged vegetation, inside fallen Mauritia flexuosa trunks, beneath undercut banks, and on one occasion within a burrow of unknown origin. Based on our field observations, basking behaviour should be an important factor for evaluating population densities of this species using survey detection methods in Amazon rainforest systems.