How many lemurs live in a forest? Answering this question is critical for their safety, yet directly counting them in a rainforest is quite difficult. A new study published in the scientific journal Conservation Science and Practice and coordinated by researchers at the University of Turin demonstrates that the density of the indri (Indri indri), also known as the "singing lemur", can be estimated by listening to their vocalizations with automatic recorders. From February 2022 to March 2023, a network of 26 autonomous acoustic recorders in the Maromizaha forest in eastern Madagascar acquired almost 20,000 hours of recordings over an area of approximately 390 hectares. Indris are well-known for their strong and unusual choral sounds, which are used by groups to alert neighbors of their presence. To transform the noises into density estimations, the researchers used a method previously developed for the aquatic environment. "We have demonstrated that an approach originally developed for the ocean can also work in a rainforest, one of the most complex environments for sound propagation", the researchers state. The indri is listed as "Critically Endangered" on the International Union for Conservation of Nature's Red List, and its population is dropping owing to loss of habitat, hunting, and climate change. From 1973 to 2014, the rainforests of eastern Madagascar lost around 37% of their native forest cover. This circumstance necessitates more accurate data on the species' abundance and density. This work is the first to use this strategy on non-human primates in a terrestrial context, and it might be applied to other species with loud and identifiable sounds, such as other primates or birds.