On a remote stretch of the Guaporé River, straddling the border of Brazil and Bolivia, scientists have documented a wildlife spectacle of staggering scale. Over 41,000 giant South American river turtles gathered here during the nesting season, creating what researchers say is the largest known aggregation of freshwater turtles in the world, according to the University of Florida.

The discovery was made possible through an innovative combination of drone technology and statistical modeling. Researchers from UF and the Wildlife Conservation Society (WCS) used drones to capture thousands of overlapping aerial photographs, then stitched them into high-resolution orthomosaics. As Discover Wildlife reports, this allowed them to identify individual turtles without disturbing the nesting process.

Over 41,000 giant South American river turtles gathered to nest along the Guaporé River.

Precision Counting in a Moving Crowd

Counting turtles in a mass nesting event is far from simple. Traditional ground-based surveys often miss individuals or count the same animal multiple times. Even orthomosaic images, while detailed, can mislead because the animals move in and out of frame.

To tackle this, scientists painted white marks on the shells of 1,187 turtles and monitored the site for 12 consecutive days. Drones made four passes daily, capturing about 1,500 images per flight, IFLScience reports. By noting which turtles were marked and whether they were walking or nesting, researchers built models that accounted for movement and detection errors.

When results were compared, the differences were striking. Ground observers estimated around 16,000 turtles, while unadjusted drone counts approached 79,000. The refined method produced the 41,000 figure — likely the most accurate yet.

A Crucial Species Under Threat

The giant South American river turtle (*Podocnemis expansa*) can reach 90 centimeters in length and weigh up to 80 kilograms. Once abundant across the Amazon and Orinoco basins, the species has declined sharply due to poaching and habitat loss, Discover Wildlife reports. Females gather in huge numbers each July and August to lay eggs on sandy riverbanks, a behavior that makes them vulnerable to human exploitation.

These turtles are vital to the Amazon ecosystem, dispersing seeds and providing food for predators. Protecting nesting sites is critical, and accurate population counts help conservationists target their efforts effectively.

The site is the largest known freshwater turtle nesting ground in the world.

Looking Beyond the Amazon

The technique used on the Guaporé River is not limited to turtles. Similar marking-and-modeling approaches have been applied to seals, elk, and mountain goats, and could aid surveys of other species that gather in large numbers.

For WCS and its partners, the next step is to expand monitoring to other turtle nesting areas in Colombia, and potentially Peru and Venezuela, to build a clearer picture of population trends, UF News reports.

The work echoes drone-based surveys used elsewhere, such as counts of 64,000 green sea turtles at Australia’s Raine Island, which also relied on white shell markings to improve accuracy, Smithsonian Magazine reports. In both cases, the marriage of technology and field biology has given scientists an unprecedented view into the lives of these ancient reptiles — and a better chance to ensure they survive for generations to come.