A loggerhead turtle nesting. (Credit:
Mollie Rickwood)
In a nutshell
- Green turtles are shifting their nesting timing in response to climate change, advancing by 6.47 days for every 1°C increase in sea temperature.
- Individual-level plasticity accounts for about 30% of the observed advancement in nesting dates, with the remaining change explained by factors like breeding experience and clutch frequency.
- While this behavioral flexibility provides some resilience against warming oceans, researchers caution there are limits to how far this adaptation can take them as climate change accelerates.
CORNWALL, England — The climate crisis has forced countless species into a simple but brutal choice: adapt or perish. For green sea turtles nesting on the beaches of North Cyprus, adaptation appears to be winning, at least for now. These prehistoric-looking reptiles have been quietly adjusting their reproductive timing in response to warming waters. A 31-year study has revealed these ancient mariners are shifting their nesting schedule earlier by 6.47 days for every degree Celsius increase in sea temperature, a behavioral flexibility that might prove crucial as our planet continues to warm.
Researchers from the University of Exeter and the Society for Protection of Turtles have discovered that individual green turtles (Chelonia mydas) are actively shifting their nesting times in response to rising sea temperatures. This marks the first time scientists have documented individual-level phenological adaptation, changes in the timing of biological events, in a marine ectotherm (cold-blooded animal).
The study, published in Proceedings of the Royal Society B, tracked over 600 individually marked nesting green turtles over a remarkable 31-year period. Researchers found that for every one-degree Celsius increase in sea surface temperature, female turtles advanced their nesting by nearly a week (6.47 days). This ability to adjust nesting schedules in response to environmental cues could prove critical for the species’ survival as ocean temperatures continue to rise.
“To know if the advancement we see now will continue into the future, it is crucial to understand the combined effects of changes in, for example, the age structure of the population, and how individual turtles respond to environmental change,” says lead author Mollie Rickwood from the University of Exeter’s Centre for Ecology and Conservation, in a statement.
What makes this particular study stand out is its focus on individual turtles over time, rather than just observing population-level trends. By monitoring the same turtles across multiple nesting seasons, researchers could separate individual behavioral adjustments from broader demographic or evolutionary changes.
The study site, Alagadi Beach in North Cyprus, has been home to a comprehensive sea turtle monitoring program since 1992. Each night during nesting season (mid-May to mid-August), researchers patrol the beach every 10 minutes between 8:30 PM and 5:30 a.m. to record all turtle activity. Individual turtles are identified through both flipper tags and passive integrated transponder (PIT) tags inserted after they lay their eggs.
This meticulous tracking revealed that about 30% of the observed advancement in nesting dates could be attributed to individual turtles adjusting their behavior in response to warming waters. The remaining change was explained primarily by other factors, including the turtle’s breeding experience (first-time nesters versus veterans), size, and the number of clutches laid per season.
At the population level, the date of the earliest nest each year has advanced by 0.93 days per year since 1993, while the median nest date has advanced by 0.45 days annually. Intriguingly, while nesting is starting earlier, the date of the last nest each year has remained largely unchanged, effectively extending the overall nesting season.
This extended season may provide a buffer against some climate change impacts by spreading reproductive risk across a longer timeframe. It could also allow females to lay more clutches in a single season, potentially helping to offset any climate-related decreases in hatching success.
For green turtles, timing is everything. As ectotherms, their entire reproductive process is highly temperature-dependent, from egg development inside the female to sex determination within the nest. Too hot and nests produce predominantly females or suffer reduced hatching success; too cold and development may be delayed or unsuccessful.
The researchers suggest that rising sea temperatures might accelerate egg development within females, allowing them to begin nesting earlier. However, individual plasticity in response to temperature isn’t the only factor at play. The study found that experienced female turtles nest earlier than first-time mothers, and those that lay more clutches in a season also tend to start earlier.
Temperature plays a particularly crucial role in turtle reproduction beyond just timing. In sea turtles, temperature determines the biological sex of offspring, with warmer temperatures producing more females. Extreme heat can also reduce hatching success or even cause complete nest failure.
What complicates matters for sea turtles is their “natal philopatry,” a strong instinct to return to the very same area where they hatched. This means they can’t easily relocate to cooler regions as temperatures rise. Instead, they must rely on other adaptive strategies, such as shifting their nesting season to take advantage of cooler periods.
The green turtle population at Alagadi Beach has been recovering significantly over the study period, with annual nest numbers increasing from just 55 in 1993 to 402 in 2022. This recovery brings demographic changes that affect nesting timing, including a higher proportion of first-time nesters who tend to arrive later in the season compared to experienced females.
“This is a bit of good news, as we’ve shown that these turtles are responding to the elevated temperatures brought about by climate change by shifting to cooler months to nest,” says study co-author Annette Broderick. “There is no guarantee that they carry on doing this though – it’s very much dependent on how much the temperatures rise, and also what they are eating. If the timing of production in terms of where their food’s coming from shifts, then they could start to be disconnected ecologically between where they forage and where they breed.”
This ecological disconnect represents a significant concern. Sea turtles typically time their breeding migrations based on environmental cues at their foraging grounds, which may be hundreds or thousands of miles from their nesting beaches. If climate change affects these different areas at different rates, turtles could find themselves migrating at suboptimal times, potentially disrupting their reproductive success.
In a related study published in Endangered Species Research, the same research team placed temperature loggers into turtle nests to monitor conditions and estimate future requirements. They calculated that turtles would need to nest 0.5 days per year earlier to maintain current sex ratios and 0.7 days per year earlier to prevent egg-hatching failures. Data showed that loggerhead turtles (a related species that shares the same beaches) are already advancing their nesting by 0.78 days per year since 1993, potentially enough to ensure continued hatching success.
Looking ahead, green turtles’ ability to adjust their nesting timing could provide some resilience against climate change. However, the researchers caution that this plasticity alone may not be sufficient to keep pace with rapidly warming temperatures, particularly for populations nearing their thermal limits at lower latitudes.
“Although our turtles appear to be coping with current rising temperatures, it is unclear how long they may be able to do this before conditions in Cyprus are no longer suitable, but cooler locations in the Mediterranean may become available for them to nest,” notes Dr. Damla Beton from the Society for Protection of Turtles (SPOT).
The study also revealed some unexpected findings regarding migration patterns. Until 2010, most green turtles nesting in Cyprus were known to migrate from three main foraging sites in Turkey and Libya. However, in recent years, an increasing proportion of turtles have been coming from Lake Bardawil in Egypt, requiring a shorter migration distance. This shift in foraging locations might be contributing to population-level changes in nesting timing beyond what can be explained by temperature alone.
While many animal species appear to be struggling to adapt to our rapidly changing world, the green turtles of North Cyprus demonstrate a remarkable flexibility that offers a glimmer of hope. Their individual ability to respond to rising sea temperatures suggests that at least some long-lived marine species may have more adaptation tools in their arsenal than previously thought.
For a species that has navigated the world’s oceans for more than 100 million years, green sea turtles have witnessed climate shifts before. But the current pace of warming presents unprecedented challenges. As climate change accelerates, the question remains whether these ancient mariners can continue to adjust their timing quickly enough. Their struggle to maintain suitable nesting conditions represents a smaller version of the larger challenge facing countless species: finding ways to adapt to a rapidly changing world before time runs out.
Paper Summary
Methodology
Researchers monitored Alagadi Beach in North Cyprus for 31 years (1992-2022), creating an extensive dataset tracking individual green turtles. During nesting season (mid-May to mid-August), teams patrolled the beach every 10 minutes through the night to record all turtle activity. Each nesting female was tagged with both external flipper tags and internal PIT (passive integrated transponder) tags, similar to microchips used in pets. These tags allowed researchers to identify individual turtles each time they returned to nest, sometimes years apart. For each turtle, they recorded when they laid their first nest of the season, their size (curved carapace length), and how many clutches they laid. The team also collected sea surface temperature data for the waters adjacent to the nesting beach and known foraging grounds. In a complementary study, they placed temperature loggers directly into nests to monitor conditions throughout incubation. Using statistical models, they separated the observed changes in nesting dates into individual-level effects (how the same turtle’s behavior changed over time) versus population-level effects (how the overall population’s timing changed). They also investigated how much of these changes could be attributed to temperature versus other factors like breeding experience, clutch frequency, and turtle size.
Results
The analysis revealed several key findings. First, the population’s first nesting date has advanced by 0.93 days per year since 1993, while the median nest date has advanced by 0.45 days annually. At the individual level, turtles advanced their nesting by 6.47 days for every 1°C increase in sea surface temperature, accounting for about 30% of the observed individual-level change in nesting timing. Experienced breeders (those that had nested before) laid their first nest about 3.75 days earlier than first-time nesters. Turtles that laid more clutches in a season also started nesting earlier, with each additional clutch corresponding to starting 4.52 days earlier. Interestingly, as individual turtles grew larger over time, they nested earlier, but populations composed of larger turtles actually nested later than those with smaller individuals. The researchers found that sea surface temperatures increased by about 0.05°C per year during the study period. The overall nesting season has grown longer because while nesting starts earlier each year, the end date of the nesting season has remained relatively stable. Their calculations indicate that turtles would need to nest 0.5 days earlier per year to maintain current sex ratios, and 0.7 days earlier to prevent hatching failures—rates that appear to be currently achieved.
Limitations
The researchers acknowledge several limitations to their study. Survey efforts were reduced in certain years (1992, 2000, 2020, and 2021), which may have affected data completeness. The early years of the study (1993-1994) were excluded from individual-level analyses because all females were unmarked at the beginning, making it impossible to differentiate between new recruits and returning breeders. The team also had to estimate the number of clutches some females laid when internesting intervals were unusually long, assuming missed nesting events. While the study found correlations between sea temperature and nesting timing, the exact physiological mechanisms behind this plasticity remain speculative. Additionally, the study cannot fully separate the effects of plasticity from potential genetic adaptation over time, though the researchers suggest the contribution of genetic change is likely negligible given the long generation time of green turtles (estimated at over 30 years to reach sexual maturity). The researchers also note that while turtles are currently adapting successfully, future temperature increases may exceed their capacity for adjustment.
Discussion and Takeaways
This study provides the first evidence of individual-level phenological plasticity in a marine ectotherm, contributing significantly to our understanding of how such species might respond to climate change. The researchers emphasize that while plasticity plays an important role in the observed nesting advancement, other factors like breeding experience and clutch frequency contribute equally to individual-level changes in timing. At the population level, however, sea surface temperature emerged as the dominant driver of nesting advancement. The study also highlights how demographic shifts in a recovering population (with more first-time nesters) can partially offset the advancement in nesting dates caused by warming temperatures. The researchers suggest that this phenological plasticity indicates a “comparatively positive outlook” for this green turtle population but caution that it remains unclear whether these adjustments will be sufficient to preserve optimal thermal conditions for egg development as climate change accelerates. They note that similar adaptive mechanisms in other species have proven insufficient to avoid negative climate impacts. A key concern raised is the potential ecological disconnection between foraging areas and breeding grounds if climate change affects these habitats at different rates. Future research should examine whether the observed phenological shifts help maintain appropriate thermal conditions for embryo development and hatching success over the long term.
Funding and Disclosures
The study was funded by various organizations including the British Associate of Tortoise Keepers, British Chelonia Group, British High Commission, British Residents Society, Carnegie Trust for the Universities of Scotland, Erwin Warth Foundation, Karşıyaka Turtle Watch, Glasgow University Court, Kuzey Kıbrıs Turkcell, the MAVA Foundation, MEDASSET UK, and Roger de Freitas. The research was conducted under permits from the North Cyprus Department for Environmental Protection to the Society for the Protection of Turtles, with ethical approval granted by the University of Exeter Research Ethics Committee.
Publication Information
The paper, titled “Individual plasticity in response to rising sea temperatures contributes to an advancement in green turtle nesting phenology,” was published in Proceedings of the Royal Society B (Volume 292) in 2025. The lead author is Mollie L. Rickwood, accompanied by co-authors Eve Tucker, Damla Beton, Sophie Davey, Brendan J. Godley, Robin T. E. Snape, Erik Postma, and Annette C. Broderick from the Centre for Ecology and Conservation at the University of Exeter and the Society for Protection of Turtles in North Cyprus. A companion study, “Phenological shift mitigates predicted impacts of climate change on sea turtle offspring,” was published in the journal Endangered Species Research.
