Scientists studying the world’s largest river turtles, a South American species that grows to a length of nearly a meter, or 3 feet, have found the largest nesting aggregation ever recorded.
Using drones to conduct a population survey in the western Brazilian Amazon, researchers recorded a nesting area of the endangered giant South American river turtle (Podocnemis expansa) with roughly 41,000 adult female turtles. The nesting site is on the largest sandbank of the Guaporé River, which forms part of the border between Brazil and Bolivia.
“We knew it was an important area, but we didn’t have the full picture of the size,” Camila Ferrara, one of the study co-authors and a turtle specialist at the Wildlife Conservation Society, told Mongabay by phone.
Ferrara said the survey was an undercount of the full region’s turtle population since five other smaller nearby beaches were not included, nor were young or male turtles counted.
The turtle population in the area has been growing since at least 2014, a win that Ferrara attributes to the work of the Brazilian government’s 40-year-old Amazon River Turtle conservation program in collaboration with local communities.
For this study, scientists tested three different methods of counting turtles. Using only on-the-ground counting, they logged around 16,000 turtles. And using only drones, the scientists counted nearly 79,000, with some turtles likely double- or triple-counted as they moved through the sand.
The scientists say a third approach they developed produced a more accurate estimate of 41,000. They collected data using both on-the-ground tracking, marking some of the turtles to track movement, and zig-zag drone surveys. They then processed the data with mathematical models that account for the movement of turtles.
The results of the study are useful for scientists who track fluctuations in turtle populations, allowing them to understand if threats are mounting or if conservation efforts are working.
“These populations are highly threatened by illegal commerce,” Ferrara said, adding that two recent police operations in Brazil’s Amazonas state detained wildlife traffickers with more than 450 turtles in August 2025. “There is a huge difference between removing one or two turtles from the river to eat, and the illegal commerce.”
The decline of river turtle populations, she added, threatens food security for riverine communities. Turtle meat has been an important part of local diets for thousands of years. A 2015 archaeological study in the region found that turtle meat was more prominent in Amazonian diets 7,000 years ago than mammals, coming second only to fish.
“We know that if we don’t do something, these animals will disappear because in many areas their numbers are already diminishing,” Ferrara added.
Banner image: Giant South American river turtles nesting on a sandbank in the Amazon’s Guaporé River. Image courtesy of Omar Torrico/Wildlife Conservation Society.
