Oh boy, all girls again: FAU research tracks sea turtle sex ratios

Forty young sea turtles recently reached an important milestone at Florida Atlantic University: They were finally big enough to leave the university’s marine science laboratory and return to the ocean. Their release was a reason to celebrate. But for FAU researchers who have spent years studying loggerhead, leatherback and green sea turtle hatchlings, there is a growing concern about what the next generation of sea turtles may look like: They’re almost all girls.

In the laboratory of Jeanette Wyneken, Ph.D., professor of biological sciences and director of FAU’s Marine Science Laboratory within the Charles E. Schmidt College of Science, she raises hatchlings from local nests to identify the sex ratio. So far, every sea turtle currently being raised and studied is female. While female-biased sex ratios are normal for sea turtles, which develop as males or females depending on the temperature at which their eggs incubate, Wyneken is seeing an increasingly troubling pattern.

For more than two decades of monitoring hatchlings and documenting nest temperatures, she has observed repeated years with no males or only a tiny fraction of males among the turtles examined.

Unlike humans, sea turtles do not have sex chromosomes that determine whether they develop as male or female. Their sex is determined by the temperature inside the nest during incubation. Warmer temperatures produce females, while cooler temperatures produce males. Or as Wyneken puts it, “Cool dudes, warm chicks.”

As Florida’s nesting beaches become warmer, more nests are experiencing temperatures that favor female development, sometimes to the point that no males are produced.

“We expect to see more females than males in sea turtle populations. That’s normal. What is not normal, and what worries me, is how many years we’re seeing either no males at all or only 1% or 2%,” said Wyneken. “We’ve looked at what it takes to sustain a population, and we need approximately 25% males every three to four years. We’re not getting that. We’re not even close.”

The implications may not become apparent for years. Florida’s sea turtles take about 20 to 30 years to reach sexual maturity, meaning the consequences of today’s sex ratios could emerge a few decades from now, when these hatchlings would be returning to Florida’s beaches to reproduce.

“That’s the part that concerns me most,” said Wyneken. “We’re seeing a problem develop now, but we may not see the consequences in the adult population for another 10 or 20 years. By then, we’ve already lost a lot of time to understand what’s happening and how serious it is.”

Wyneken has been monitoring nest temperatures for 24 years, placing data loggers in nests to record the conditions developing embryos experience throughout incubation. The findings show a consistent warming trend. This past summer was particularly hot, with temperatures remaining elevated even during periods when seasonal storms might normally bring cloud cover and cooler conditions.

Excessive nest heat can kill embryos or cause developmental problems in hatchlings, including improperly developed organs and damaged yolk sacs, which provide essential early nourishment.

This year estimated hatching success was approximately 53%, below the expected 65% to 75%. Fewer major storms meant fewer nests were washed out but persistent heat remained a challenge.

Wyneken and her team raise hatchlings in the lab until they are large enough to determine their sex through a minimally invasive procedure. By tracking their development and comparing it with temperatures recorded in their original nests, researchers can better understand how heat affects survival and development. The 40 recently released turtles had grown large enough to return to the ocean.

“Having these turtles in the lab gives us an opportunity to do much more than simply raise them,” said Wyneken. “We know where each one came from, what temperatures its eggs experienced and how it developed. We can verify its sex and understand what those temperatures actually mean for the turtles.”

Cooling nests can produce males, but methods such as shading or cooling the sand have limitations. Meanwhile, female sea turtles typically lay five or six nests across different locations each season. Verified sex-ratio data remain scarce because determining hatchling sex requires specialized expertise and facilities. FAU is among the very few Florida programs worldwide directly verifying hatchling sex.

“We need more information from more locations to understand the full picture,” said Wyneken. “I’m sure males are being produced elsewhere. We know they are. But when we repeatedly see years with little or no male production in the areas we’ve studied, that’s a low or no male pattern one we cannot ignore.”

For now, the 40 hatchlings returning to the ocean represent another generation with a chance to survive the journey to adulthood. But for Wyneken, their release also underscores why decades of research and monitoring matter.

“These little turtles have a remarkable journey ahead of them. Statistically, only about 1 in 1,000 hatchlings survives to adulthood,” she said. “We want to give them every opportunity to make it. But we also have to make sure that, years from now, there are enough males and females to produce the next generation.”

- FAU -

About Florida Atlantic University:

Florida Atlantic University is one of the nation’s fastest-rising public research universities, serving more than 32,000 students in South Florida. Ranked among the Top 100 Public Universities by U.S. News & World Report, Florida Atlantic is also one of only 13 institutions nationwide to hold Carnegie Foundation designations for R1 research, opportunity and community engagement. Guided by its strategic plan, “2031FAU: Where Tomorrow Begins,” the university is focused on delivering career-ready education and experiential learning, driving scholarly inquiry that creates healthier, safer and more prosperous communities, strengthening institutional excellence, and elevating its impact across South Florida and beyond. Florida Atlantic continues to advance its position as Florida’s first quantum university, integrating research, education and industry partnerships around next-generation computing technologies. Through other signature strengths in neuroscience and healthy aging, environmental, ocean and coastal innovation, and national defense and autonomous systems, Florida Atlantic expands knowledge, fuels economic opportunity and fulfills its mission as South Florida’s hometown university.