Escalating habitat warming in Amazonian rivers shifting turtle hatchlings sex-ratios towards feminization

Amazonia is experiencing more frequent and severe weather extremes. Freshwater species, like the endangered Giant Amazonian turtles, Podocnemis expansa , are key for assessing how climate change influences Amazonian rivers and biodiversity. Extreme habitat temperatures can increase turtle embryo mortality; however, other non-lethal effects are expected due to their temperature-dependent sex determination, which can eventually drive populations to become predominantly female-biased. Using in-situ and satellite-based data from 1998 to 2024, we show escalating warming trends, up to + 0.3ºC a year, in the water and sandbanks of three major Amazon Basin rivers (Purus, Trombetas, and Tapajós) used by turtles for nesting. We then estimated the sex-ratio of all hatchlings bourn at these sandbanks monitored by local communities. We observed that, as warming intensifies, cohorts unlikely reach balanced sex-ratios, with the magnitude of the bias progressively shifting towards more females, while males comprise less than 15% of offspring. Our results emphasize the need for long-term nest-temperature observations and monitoring as a critical conservation effort to access ongoing population changes under future warming scenarios.