Conservationists worldwide have been lauding recent surges in sea‑turtle nesting activity, yet a new study warns that a booming count of nests may conceal a looming demographic crisis. Researchers from Queen Mary University of London, using drone imagery and fifteen years of nesting records, have uncovered a striking female‑biased sex ratio among loggerheads (Caretta caretta) that could undermine future breeding success.
The team’s pre‑print, posted to bioRxiv on 20 January, suggests that rising sand temperatures—driven by global warming—are skewing hatchling sex towards females. Without a sufficient number of males, the apparent “boom” in nest numbers could mask an impending collapse of the population.
How temperature shapes turtle sex
Sea turtles, like many reptiles, exhibit temperature‑dependent sex determination (TSD). Eggs incubated in warmer sand (>≈30 °C) tend to develop into females, while cooler conditions (<≈27 °C) favor males. The pivotal temperature for an even sex split in loggerheads is roughly 29 °C, a value reported by the U.S. National Oceanic and Atmospheric Administration (NOAA) and echoed in the IUCN Red List assessment for the species (IUCN).
“The way we describe it in my lab is ‘hot chicks and cool dudes,’” explains Dr Jeanette Wyneken, a sea‑turtle biologist at Florida Atlantic University, who was not involved in the study. As sand temperatures climb, a larger proportion of hatchlings become female, intensifying the gender imbalance.
Drone surveys reveal a 9‑to‑1 female‑to‑male ratio
By flying drones over nesting beaches, the researchers could differentiate adult males from females based on tail morphology—males possess longer, thicker tails. Their analysis of the Cape Verde loggerhead population showed an estimated 9 females for every 1 male (unconfirmed). “We can differentiate between females and males by the tails,” notes Dr Fitra Arya Dwi Nugraha, a co‑author of the study.
Wyneken cautions that identifying sub‑adult turtles from aerial images is challenging; immature males may be mis‑counted as females, potentially inflating the skew (unconfirmed). She adds that definitive sex determination of hatchlings usually requires laparoscopy, a more invasive but accurate technique.
Why a female‑heavy population could be problematic
Turtles have evolved strategies to mitigate extreme sex ratios. Males can mate more frequently than females, females store sperm to fertilize multiple clutches and both sexes typically have several partners. Nevertheless, persistent female dominance can threaten long‑term viability. “You expect more females, but seasons where we get 100 percent female, again and again, or 98 percent female? That’s not sustainable,” Wyneken warns.
Such skewed ratios are already reported in warmer regions, including the northern Great Barrier Reef where climate‑induced temperature rises have produced predominantly female hatchlings (Nature Scientific Reports, 2022).
Implications for conservation policy
The authors stress that the apparent increase in nest numbers should not lead to complacency. “We think that there’s a bit of a mirage,” says Dr Christophe Eizaguirre, a conservation geneticist at Queen Mary University. “What we really don’t want is the effort to stop.” Ongoing measures—such as marine protected areas, regulated fisheries, and the protection of egg clutches—remain vital, but must adapt to the changing thermal landscape.
Conservation programs that relocate eggs to hatcheries aim to buffer against predation and environmental threats. However, Wyneken notes that “you’re putting all your eggs in one basket” if hatcheries become the sole strategy, especially if temperature control is not precise.
Going forward, researchers advocate for integrated monitoring that combines drone surveys, in‑situ temperature logging, and periodic hatchling sex assessments. Such data will help calibrate management actions, including shading of nesting beaches or artificial cooling, to preserve a more balanced sex ratio.
Readers are invited to share their thoughts on how best to safeguard sea‑turtle populations in a warming world. Please comment below and spread the word.
