Hot Chicks, Cool Dudes
How warming temperatures are affecting the male/female ratio in nests and what this may mean
By Corri Osborne
It’s sea turtle nesting season again on Florida’s beaches, where 90 percent of sea turtles laying eggs in the United States come ashore. Sea turtles exhibit nest site fidelity, and after reaching sexual maturity at 20 to 30 years after hatching, female sea turtles return to the same beach they were born on to lay their own eggs year after year. Sea turtles’ natural lifespan is estimated to be 50-100 years, with females continuing to nest throughout their lifetime.
The summer heat that often has Florida’s human residents complaining plays a critical role in sea turtle egg development. Many reptiles, including all seven species of sea turtles, use temperature-dependent sex determination (TSD) to assign the makeup of their nest’s sex ratio. This means that the embryos are not male or female because of sex chromosome combinations (genotypic sex determination or GSD) but instead are influenced by the temperature of the environment during a critical period of their development.
In the case of sea turtles, research indicates that incubation temperatures below 81.86 degrees Fahrenheit will produce male turtle hatchlings. Temperatures above this will generate female hatchlings. It is typical for there to be a range of temperatures on the same beach rookery and also within the same nest’s clutch. A recent analysis of 75 sea turtle rookeries worldwide showed the average ratio of females to males was roughly 3 to 1. In fact, some turtle populations produced fewer males than females even a century ago. However, as temperatures rise globally, studies indicate trends of increased female to male hatchlings.
Alarmingly, in a survey of the Pacific’s largest green sea turtle rookery, located along the northern stretch of the Great Barrier Reef, females were found to outnumber males by at least 116:1. Turtles that hatched there around the 1970s and 1980s were also mostly female, but only by a ratio of 6:1. This area of the reef has suffered from increased temperatures and significant coral bleaching. In the southern end of the reef, where researchers found healthier coral and lower temperatures, the female to male ratio in green sea turtles today is 2:1.
In precise terms, even a modest increase in temperature (3.6 degrees F) changes the sex ratio in sea turtle nests to almost entirely female. Animals such as sea turtles that use TSD will be in a race to evolve and adapt to the global temperature increase and its impact on their reproductive strategy. It may only take an increase of 7.2 degrees Fahrenheit to effectively eliminate production of male offspring. Given that hatchlings survive to adulthood at a rate of between 1:1000 and 1:10,000, how long will it take for older males to become nonviable breeding partners and the lack of new, younger males replacing them to mean the end of entire species?
In the case of more radical temperature increases, human intervention has begun. In nest sites where it is now too hot for eggs to hatch at all, scientists have begun moving eggs to nurseries – providing holes dug to a certain depth on cooler areas of the beach. Eventually, hatchlings are escorted from nest to water, providing protection from land predators. Danger still awaits them in the water and as in most human interference, this doesn’t address the bigger picture or the challenge these species face on a global scale.
Websites and scholarly articles for further reading:
https://conserveturtles.org/information-sea-turtles-threats-climate-change/
https://www.scientificamerican.com/article/experts-temperature-sex-determination-reptiles/
https://oceanservice.noaa.gov/news/june15/sea-turtles.html
https://www.cell.com/current-biology/fulltext/S0960-9822(17)31539-7 – Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World
https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1523-1739.2010.01531.x – Breeding Periodicity for Male Sea Turtles, Operational Sex Ratios, and Implications in the Face of Climate Change
https://onlinelibrary.wiley.com/doi/abs/10.1046/j.1365-2486.2003.00606.x – Climate change and sea turtles: a 150‐year reconstruction of incubation temperatures at a major marine turtle rookery
https://www.pnas.org/content/91/16/7487.short – Climate change and temperature-dependent sex determination in reptiles
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