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Most of Florida's newly-hatched sea turtles are female. Why?

Hatchling loggerhead sea turtles making their way to the ocean on Clam Pass Beach in Florida.
Hatchling loggerhead sea turtles making their way to the ocean on Clam Pass Beach in Florida. (Image credit: Shutterstock)

Almost all sea turtle hatchlings are emerging from their eggs as females on some Florida beaches because of heat waves exacerbated by climate change, Reuters (opens in new tab) reported on Aug. 2.

Five different sea turtle species are found in Florida, including loggerhead turtles (Caretta caretta) and green turtles (Chelonia mydas), according to the Florida Fish and Wildlife Conservation Commission (opens in new tab). Bette Zirkelbach, manager of the Turtle Hospital in the Florida Keys, told Reuters that scientists haven't found any male sea turtles for the past four years. So, what's going on? 

Temperature plays a major role in determining the sex of developing sea turtles. Unlike humans, whose sex determination is largely controlled by the X and Y sex chromosomes, turtle's sex ratios are determined by the temperature at which their eggs are incubated. Higher temperatures at incubation produce more females.

Climate change raises the temperature of nesting sands, causing the sex ratios of turtles to skew toward females, according to the National Oceanic and Atmospheric Administration (opens in new tab) (NOAA). Introducing too many females and not enough males into the turtle populations could reduce the animals' ability to reproduce when the turtles reach adulthood, increasing their risk of local extinction.

Related: Wildfires blaze in London during record heatwave

The strange sex ratio phenomenon isn't just affecting Florida. A 2018 study published in the journal Current Biology (opens in new tab) found that green sea turtles had a 99% female sex bias on warmer, northern Great Barrier Reef nesting beaches and a 65% to 69% female sex bias on cooler, southern beaches Down Under. 

While the skewed sex ratio could be damaging for sea turtles, having more females than males isn't necessarily all that unnatural. Sea turtle nests that are 90% female aren't uncommon and only a few males may be needed in a population to fertilize eggs, Insider (opens in new tab) reported. However, there wouldn't be any fertilization if all males disappeared. 

The temperature threshold for determining the sex of sea turtles is 81.9 degrees Fahrenheit (27.7 degrees Celsius), according to NOAA. Turtles incubated below 81.9 F hatch male and turtles incubated above 81.9 F hatch female, while fluctuating temperatures above and below this threshold produce a mix of males and females.

The process is called temperature-dependent sex determination and it affects a variety of animals, including crocodiles, many fish and some lizards. Scientists aren't certain, but they have theories as to why some animals have their sex determined this way and others don't.

"Our best guess is that temperature-dependent sex determination originated because reptiles do not have parental care and the eggs are in close interaction with the environment," Diego Cortez, a biologist at the National Autonomous University of Mexico in Mexico City, told Live Science in 2021. "We also know that elevated incubation temperatures speed up the development of embryos. So, the sex that is linked to higher incubation temperatures will hatch earlier." 

Temperature-dependent sex determination may also allow mothers to control the sex of their offspring, such as by laying eggs in cooler or warmer spots, if there is a need for more males or more females within that animal's population and species, Live Science previously reported. 

A 2020 study of loggerhead turtles published in the journal Climatic Change (opens in new tab) highlighted other problems that can arise with increased incubation temperatures. In Cabo Verde (also called Cape Verde), a country of islands in the Atlantic Ocean, the researchers found that 33% more embryos perished when incubation temperatures reached 90.1 F (32.3 C) than when incubation temperatures hovered around 85.5 F (29.7 C). The researchers also found that hatchlings incubated at high temperatures were smaller in size and more likely to be killed by crabs on their way to the ocean. 

In other words, hot temperatures can be lethal for developing turtles and reduce their survival chances when they do hatch. 

Originally published on Live Science.

Patrick Pester
Staff Writer

Patrick Pester is a staff writer for Live Science. His background is in wildlife conservation and he has worked with endangered species around the world. Patrick holds a master's degree in international journalism from Cardiff University in the U.K.