Male Sea Turtles Tracked via Egg-Yolk DNA on Georgia Coast

For nearly two decades, University of Georgia researchers built a detailed pedigree of every female loggerhead sea turtle nesting on the state’s barrier islands. They have almost nothing on the males. A new sperm-DNA technique, published in the journal Ecology and Evolution on March 18, 2026, lets the same team finally count both sides of a population that climate-driven feminization is reshaping faster than fieldwork can follow.

Sampling began this month on Ossabaw Island and will move north along the Georgia coast and into the Carolinas, the first systematic effort in the Southeast to identify breeding male loggerheads without ever needing to spot one in the open ocean.

The Invisible Half of Georgia’s Beach

Georgia listed the loggerhead as a state endangered species in 2006. Since then, the Georgia Sea Turtle Cooperative has cataloged thousands of nests, swabbed eggshells for maternal DNA, and built a working family tree of the females that return to the same stretches of sand year after year. That database is now one of the most complete maternal records of any sea turtle population in the world.

The males have stayed in the water. Adult male loggerheads do not come ashore, and ocean encounters are rare enough that researchers historically had to wait for a fisheries bycatch or a stranding to draw blood. Whole breeding seasons could pass without a single new male added to the genetic record.

“We have a large database of the females, but we just don’t have good baseline data on the males at all,” Brian Shamblin, senior research scientist at the University of Georgia’s Warnell School of Forestry and Natural Resources, told Georgia Public Broadcasting. The phrasing matters. Conservation status is set by population trend, and trend lines drawn from nearly two decades of female-only data assume the other half is keeping pace.

Reading the Father in a Single Egg

The technique Shamblin and his co-authors describe in the Ecology and Evolution paper from UGA’s Warnell School bypasses the ocean problem entirely. When a female lays a clutch, a thin membrane inside each egg traps a residue of sperm from every male that fertilized it. That membrane survives long enough after the nest is laid for a lab to extract usable genetic material.

Co-authors Cheryl Sanchez and Simona Ceriani of the Florida Fish and Wildlife Research Institute and Sean Perry, an assistant professor at Midwestern University, validated the workflow on loggerhead and green turtle nests across the southeastern coast. From a single egg, the team reconstructs paternity for the entire clutch and matches the sperm DNA against any prior sample stored in the regional database.

The method differs from the standard playbook in four ways:

• No adult capture. Researchers never need to intercept a male at sea or fit one with a transmitter.
• No nest disturbance. One egg is removed per nest under permit; the remaining clutch incubates undisturbed.
• Multi-paternity detection. A single clutch routinely shows two or more fathers, all visible in the same yolk membrane.
• Cross-state lineage tracking. Genetic profiles can follow a male from a Georgia nest to a Carolina or Florida nest in the same season.

That last point is the operational shift. Maternal genetics already show that Georgia, South Carolina, and northern Florida share one recovery unit. Paternal genetics could either confirm that or reveal that males circulate across a much smaller or much larger range than the females do.

Why Sand Temperature Decides Sex

Loggerheads, like most turtles, lack sex chromosomes. The pivotal temperature for the species sits near 29 degrees Celsius. Eggs incubating below that line trend male; eggs above it trend female. Field biologists summarize the rule with the unofficial mnemonic “hot chicks, cool dudes.”

Climate warming raises the floor. A multi-site review by marine biologists studying local adaptation in sea turtle populations found incubation temperatures rising at 40 of 64 monitored rookeries worldwide. Some populations carry slightly different pivotal temperatures, evidence that local adaptation buys time, but the rate of warming at most beaches is outpacing the rate at which pivotal temperatures appear to shift.

Georgia’s barrier islands sit at the cooler end of the loggerhead’s nesting range, which is the reason the state’s beaches have been treated as a possible climate refuge. The flip side of that hope is that nobody has measured how heavily the recent run of warm summers has already pushed the Georgia sex ratio. Without a male count, there is no way to back out the answer.

What Raine Island Already Showed

The cautionary case sits in the Coral Sea. Raine Island, the world’s largest green turtle rookery and a roughly 1,500-kilometre boat ride north of Heron Island, has been producing almost no males for two decades.

There basically are no male hatchlings being produced anymore at all.

That is Shamblin, summarizing the 2018 finding by Michael Jensen and colleagues that more than 99% female hatchling production has held on Raine for at least 20 years. A later drone survey commissioned by WWF Australia’s adult turtle aggregation count off the northern Great Barrier Reef found the offshore adult ratio less skewed than the hatchling ratio, a hint that earlier cohorts banked enough males to keep the breeding pool functional for now. Nobody expects that buffer to last if the hatchling ratio stays where it is.

Florida studies have flagged similar pressure on the Atlantic coast loggerhead population. Hidden-behavior monitoring of other marine species has hit the same wall and forced the same kind of methodological reinvention; an underwater camera study of Mediterranean monk seals in bubble caves ran for 141 days before researchers could prove the animals were using the caves at all. Sea turtles got a comparable upgrade with the egg-membrane method. Georgia is the first place it will run at scale on a recovering loggerhead population.

Georgia’s Loggerhead Numbers Year by Year

The recovery story for females is real, and the swings between seasons are the reason scientists want a paternity layer underneath it. Comprehensive surveys on all of the state’s barrier island beaches began in 1989. The Georgia Department of Natural Resources sea turtle recovery tracker publishes the season totals as nests are confirmed.

State biologists model the trend on a three-year cycle, with peak nesting years followed by lower seasons as cohorts rest. A 4,000-nest year is not a population guarantee; it is a snapshot of how many females showed up. Whether their hatchlings carried a viable mix of sexes is the question the new sampling effort is meant to answer.

What a Male Pedigree Would Change

A working male database changes three things for the recovery program. It lets biologists estimate effective breeding population size rather than guess from nest counts. It flags genetic bottlenecks early, before inbreeding shows up in hatch failure or shell deformities. And it offers a way to test, season by season, whether warm summers are quietly producing fewer male contributors to the next generation.

The implication for policy is direct. If Georgia turns out to be the southern Atlantic’s last reliable producer of male loggerheads, the cost of dune shading, beach renourishment, and nest relocation projects on the state’s barrier islands looks small against the alternative. If male production has already slipped below replacement, the funding case for cooler-substrate experiments and managed incubation moves from research curiosity to active conservation triage.

The sampling runs from Ossabaw Island north to the Carolinas through the 2026 nesting season. Each egg pulled from a clutch under permit becomes a row in the database, identifying one or more fathers and time-stamping their presence in the breeding population. By the time the season closes in October, biologists expect to have hundreds of paternity profiles where they previously had a handful.

At Ossabaw this week, the work looks small. A field crew kneels at a marked clutch, lifts a single egg, packs it for the lab. The instrument that will count Georgia’s male loggerheads fits inside that egg, and the count starts now.