Volunteer member of Vets for Climate Action, Regina Warne (BSc (Vet Bio)) was recently published in The Canberra Times on 7 May 2023, answering the question for Ask Fuzzy, "What determines a turtle's sex":

There are many things about turtles that are fascinating: they have a shell that acts as a permanent bony shield. They can sleep underwater despite being air-breathers, and some turtle species date back to the age of dinosaurs.

Among these idiosyncrasies is something fundamental to the survival of any species – whether the sex of offspring is male or female.

For humans and other placental mammals, sex is determined when the egg is fertilised by the sperm. But, like alligators and crocodiles, turtles have “temperature-dependent sex determination”.

As that suggests, the environment is more important than the chromosomal arrangement in determining sex.

After the eggs have been laid, if they are incubated at temperatures below 27.7 degrees C, hatchlings will be male. If the temperature is about 31 degrees C, the hatchlings will be female.

At a temperature between 27.7 degrees C and 31 degrees C, the nest will give rise to a mix of female and male hatchlings. This occurs due to regulation of important pathways during embryonic development. While this is an incredibly interesting way for species to evolve, this mechanism may be detrimental to turtles.

With the rise of average temperatures caused by climate change, more hatchlings are being born female.

A recent study of green sea turtles in the Great Barrier Reef demonstrated that 99 percent of turtles born were female. This has the potential to be catastrophic for the survival of this species and is a real threat to all other turtles.

Without males for reproduction, the already endangered green sea turtle could become extinct.

In light of this, the World Wildlife Fund is funding a research project in the Great Barrier Reef.

This includes shading the reefs with native trees or manmade infrastructure, and as irrigating the reef with seawater and rainwater.

These all aim to cool the beach shores to a temperature where the development of male hatchlings is possible.

These studies have demonstrated an average decrease in temperature by 1.3 degrees C which, by no means a ‘fix’, could be a lifeline to turtles on a warming planet.

The article is available here