This blog post is provided by Diana Sousa-Guedes and tells the #StoryBehindThePaper for the article “The effects of warming on loggerhead turtle nesting counts”, which was recently published in the Journal of Animal Ecology. In their study, the authors find that while warming sands are leading to increased female and nesting numbers in some locations, sands may be getting too warm, reducing egg viability.

Warmer beaches mean more nesting sea turtles—but for how long?

Every year, female loggerhead sea turtles (Caretta caretta) embark on a journey unchanged for millions of years, returning to the very beaches where they hatched to lay their eggs. But these rookeries are also ground zero for one of the most profound challenges of our time: a rapidly warming planet.

Sea turtles have an intriguing biological quirk: their sex is determined by the temperature of the sand in which their eggs incubate. Warmer sands produce more females, while cooler sands yield more males. This phenomenon, called temperature-dependent sex determination (TSD), ties the fate of their populations to the climate. Our study started with a simple question: If sand temperatures shape sea turtle populations, what does a warming world mean for their future?

To explore this, we examined nest counts over 40 years (1979–2023) at 35 globally significant nesting sites. Our goal was to uncover whether rising temperatures are driving changes in nesting activity and to explore how these trends vary across regions. The results revealed a complicated story.

What we found

From Brazil to the Mediterranean, warming trends were evident at 33 of the 35 sites. Nesting counts increased at 23 of these locations, suggesting that warmer sands may be boosting female hatchling numbers—potentially leading to more females returning to nest. At first glance, it seemed like good news: warmer beaches appear to have a silver lining for some populations, at least for now.

But the story is not entirely positive. The same warming trends that might temporarily increase nesting activity could pose long-term risks. Several sites, particularly in the Caribbean, Cabo Verde, and northern Brazil, are nearing thermal thresholds where egg viability or balanced sex ratio may be compromised. On Masirah Island in Oman, nesting numbers are already declining despite warming trends, suggesting temperatures may have already crossed critical limits.

And then there were the unexpected findings: regions with rising nesting counts but no significant warming, likely benefiting from conservation efforts, or other environmental changes and others, like Florida and Greece, showing stable trends despite warming—a hint that phenological shifts or local factors may play a buffering role.

Why it matters

Our findings offer insights into how loggerhead turtles—and other species with temperature-dependent sex determination—might thrive in a warming world. While some populations may benefit in the short term from female-biased sex ratios, the long-term consequences of skewed ratios and rising temperatures could jeopardize population viability.

Regional differences are key: while higher-latitude nesting sites may mitigate warming effects through phenological shifts, lower-latitude sites, such as those in Cabo Verde, Brazil, and Oman, are more vulnerable. Even small temperature increases in these regions could disrupt the balance between male and female hatchlings—or reduce hatching success altogether. These findings underscore the need for proactive measures, even in regions currently experiencing nesting increases.

Looking forward

While some populations are thriving in the short term, the question remains: how long can these trends last? Long-term monitoring remains vital for understanding how warming impacts sex ratios, hatchling viability, and population dynamics. As climate change continues to test the resilience of marine turtle populations, the answers may lie in the sands they nest on.

Read the paper

Read the full paper here: Sousa-Guedes, D., Campos, J. C., Bessa, F.; Lasala, J. A., Marco, A., & Sillero, N. (2025). The effects of warming on loggerhead turtle nesting counts. Journal of Animal Ecology, 00, 1–16. https://doi.org/10.1111/1365-2656.14242