Florida turtle nests are doing great; when eggs hatch, expect mostly girls

Green sea turtles had an exceptional nesting season on Florida's beaches in 2023, with volunteers counting more than 74,300 nests, according to preliminary data. That beats the previous record, from 2017, by a staggering 40%.

“The increase is an explosion” and a welcome surprise, said Simona Ceriani, a research scientist who coordinated the annual survey for the Florida Fish and Wildlife Conservation Commission, the state agency that regulates and manages wildlife. The count will continue through Oct. 31.

Sea turtles don't reach sexual maturity until their 20s or 30s, so what Florida is seeing now is very likely the result of conservation measures put in place after green sea turtles were listed under the Endangered Species Act in 1978, Ceriani said.

But researchers aren't ready to claim a conservation victory just yet. Those impressive nesting numbers are just “half the story,” according to Jeanette Wyneken, a professor at Florida Atlantic University who has studied nesting sea turtles for more than three decades.

That's because, more than most creatures, sea turtles are particularly attuned to a warming climate. In fact, the sex of a baby sea turtle isn't determined by its DNA but by the temperature of the sand in which its egg developed. Cooler temperatures mean males, warmer ones mean more females.

According to Wyneken, who has been monitoring incubation temperatures and sex ratios in the nests of green sea turtles in Palm Beach County since 2005, in recent years the proportion of male green sea turtle hatchlings has dwindled substantially. In the past few seasons, between 87% and 100% of the hatchlings she has tested have been female.

In the short term, such a skewed sex ratio could actually be a boon to green turtles. A breeding female lays between two and nine clutches of about 110 eggs each in a season, and a greater proportion of females in any given generation means more nests in the sand 20 years down the road. That is, Wyneken said, as long as “there's enough boys to service the girls.”

There's some evidence that Florida sea turtles have been producing extremely skewed sex ratios for decades. Limited studies of loggerheads in the late 1980s suggest females already accounted for more than 90% of new hatchlings on some Florida beaches.

Global warming decades ago could be contributing to the boom seen today, though Ceriani and Wyneken agreed that conservation measures deserved most of the credit.

Restrictions on beachfront development and careful monitoring of nests have helped get hatchlings safely to the water, and a gill net ban in 1995 sharply reduced the number of young turtles killed by fishing gear before they hit puberty. A 13-mile stretch of beach in the Archie Carr National Wildlife Refuge, an area south of Orlando set aside in 1991, had about 1,000 green sea turtle nests in 1994, almost 12,000 in 2013, and more than 23,000 this year.

With the potential for accelerating growth as sea turtle populations become increasingly female, it's tempting to think of sea turtles climate change “winners.” But research suggests that climate change will outstrip the adaptive advantage of feminization.

More and more frequently, the nests Wyneken and her colleagues mark in June remain painfully still by August, when they should be teeming with hatchlings. Initial studies by Wyneken and her team indicate that those eggs are not unfertilized. They were most likely killed by a combination of extreme heat and dryness.