Sea turtles maintain their preferences for the sites where they lay their eggs each season, which could influence the sex of the offspring, according to international research conducted in Costa Rica by Universidad Complutense de Madrid (UCM), the Mediterranean Institute for Advanced Studies (IMEDEA) of the CSIC, and the NGO the Leatherback Trust.
During the nesting season, which occurs every three or four years, the turtles emerge from the water to find the perfect spot – the area on the beach where they dig a hole in the sand. There they lay several dozen eggs, cover them over and return to the sea. Two months later, when they hatch, the offspring head from the sand into the water.
The study, published in Climatic Change, was begun by Lucía Heredero as her Final Master’s Degree Project at the UCM’s Institute of Knowledge Technology (ITC). It demonstrates that although years may have passed, turtles continue to choose the same areas of the beach as in previous years, which has an impact on the future of the offspring.
“Since the sex of the future turtles is determined by the temperature at which the eggs buried in the sand incubate, the fact that their mothers reveal a preference for a nesting site means that this could have an indirect influence on the sex of their offspring”, explains Pilar Santidrián, IMEDEA researcher and co-director of the research.
Males are produced at lower temperatures than females. Turtles that always nest beneath trees could make it more likely that their offspring will be male, as the shade lowers the temperature.
“Bearing in mind the threat of climate change, one would expect nesting beneath trees to become increasingly more frequent as the temperature rises, leading to more males being born,” Santidrián warns.
Social science methodology in a biological study
To conduct the research, they used data gathered since 2011 by the organisation the Leatherback Trust in Cabuyal, on the Northern Pacific coast of Costa Rica.
In the egg-laying season (October to March) the identity of each turtle is logged, indicating where they make their nest, using sensors to log their temperature every hour during the 60 days that the incubation period can last.
The novel aspect of this research lies not only in the results obtained, but also the methodology employed. “Artificial intelligence algorithms were used, specifically unsupervised learning, serving to analyse the strategies followed by the turtles in choosing the area of the beach where they dig their nests”, explains Luis Jáñez, a researcher at the UCM’s ITC.
Meanwhile, Lucía Heredero, who is currently researching as a biologist at the Leatherback Trust, explains that for the study they used spatial correlations to measure the tendency of turtles to nest close to their previous nest. This methodology is normally used in social sciences.
“In this study we would stress the huge importance of combining such theoretically diverse disciplines as methodology applied to social sciences and conservation biology”, she adds.
The next step in the study, reveals Jáñez, is to identify the biological mechanisms that allow turtles to return to the same area of the same beach several years later, having covered thousands of kilometres across the ocean. “This information will help to identify the most appropriate actions to be taken by national and international bodies wishing to protect these species”, he concludes.
Method of Research
Subject of Research
Nest-site selection influences offspring sex ratio in green turtles, a species with temperature-dependent sex determination. Climatic Change
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