Why Are Some Groups of Green Sea Turtles 99% Female?

Green sea turtles Chelonia mydas inhabit the warm waters of tropical and subtropical regions across the globe. With an estimated population of between 85,000 and 90,000 nesting females, these marine reptiles play a vital role in the ecosystems of the Atlantic, Pacific, and Indian Oceans. However, some groups of green sea turtles are producing offspring that are up to 99% female, with alarming implications for the future of their species.

As is the case with many reptiles, the sex development of green sea turtles is influenced by the temperature of their nesting environment. This means that warmer environments produce more females, while cooler environments produce more males. After female turtles reach sexual maturity (which doesn’t happen until at least age 20), they usually nest between three and five times per season, laying around 110 eggs in each nest. This can help their populations bounce back in the face of threats such as climate change.

However, in the long term, as female numbers continue to rise, they are likely to have difficulty finding mates, thus endangering genetic diversity and even the survival of the population.

A study published in November 2023 in Frontiers in Marine Science focused on a group of green sea turtles on Heron Island in Australia’s Great Barrier Reef. The results suggest that pollution, along with a warming climate, could be contributing to the turtles’ female sex development. Researchers found a correlation between levels of heavy metals such as antimony, cadmium, chromium, and lead in the hatchlings’ livers and a greater number of females in the nest. These metals, ingested when the sea turtles consume contaminated algae or sea grass, are thought to mimic estrogen, influencing a rising proportion of female embryos.

Another threat for sea turtles: