The Mystery of Temperature-Dependent Sex Determination in Turtles: How Does Climate Warming Lead to 99% Females?

Nature is a brutal arena where every species strives with all its might to survive and reproduce.

The Survival Wisdom of Turtles

At the “Survival Wisdom” exhibition area of the Shanghai Natural History Museum, the story of turtles is particularly eye-catching. They are not only one of the oldest creatures on Earth but also hide many amazing survival secrets.

Turtle Reproduction: Wisdom Inside the Eggshell

Turtles reproduce by laying eggs, but their eggs differ significantly. Some turtles lay hard-shelled eggs as hard as stone, while others lay leathery eggs as pliable as leather.

Scientists have discovered that sea turtles 120 million years ago also laid hard-shelled eggs, but why some turtle species “abandoned the hard for the soft” remains an unsolved mystery. The shape, size, and quantity of turtle eggs also vary by species. The Leatherback sea turtle (Dermochelys coriacea) can lay 80 oval eggs in a clutch, resembling a pile of white ping-pong balls; while the eggs of the Eastern mud turtle (Kinosternon subrubrum) are less than half the size of Leatherback eggs, and the quantity is also far inferior. Behind these differences lies the turtles’extreme adaptation to the environment. Although mother turtles do not nurture their offspring personally like birds, they spend their energy on “site selection.” The Eastern mud turtle prefers shady places to prevent the eggs from being dried out by the scorching sun; the Diamondback terrapin (Malaclemys terrapin) chooses higher ground to prevent eggs from being submerged during high tides. Scientists have confirmed that these carefully selected locations can indeed significantly improve hatching success rates.

In addition, mother turtles lay eggs at night to avoid the threat of predators. If the environment is not ideal, formed eggs can even pause development inside the mother’s body, waiting for the optimal timing.

Temperature Determines Sex? “It’s Not That Simple!”

The sex determination mechanism of turtles is a miracle of nature. For many turtle species, sex is determined by incubation temperature: low temperatures produce males, high temperatures produce females, or intermediate temperatures produce males while low and high temperatures produce females. For example, the Chinese stripe-necked turtle (Mauremys sinensis) and Red-eared slider (Trachemys scripta elegans) belong to the former, while the Common snapping turtle (Chelydra serpentina) belongs to the latter. However, not all turtles rely on temperature. Scientists have found that species such as the Wood turtle (Glyptemys insculpta) and Narrow-bridged musk turtle (Claudius angustatus) possess a gene-determined sex mechanism, where sex is destined at fertilization. Currently, our understanding of turtle sex determination mechanisms is still very limited; taking the Geoemydidae family as an example, mechanisms for only 8 out of 70 members have been clarified.

The Threat of Global Climate Warming

Global climate warming is posing a huge threat to turtles. Rising temperatures are causing sex imbalance in populations of many turtle species. For example, the Green sea turtle (Chelonia mydas) population in Australia’s Great Barrier Reef has shown a severe trend toward feminization, with the proportion of females on certain beaches reaching as high as 99%. This phenomenon may have lasted for more than 20 years and will intensify in the future. The survival wisdom of turtles is amazing, but their future is full of uncertainty. Protecting these ancient creatures is not only for them but also for maintaining the balance of the Earth’s ecology.