Sea turtles are beautiful, peaceful animals, but unfortunately, they are very vulnerable to climate change in ways we may not initially expect. In their 2018 study, “Environmental Warming and Feminization of One of the Largest Sea Turtle Populations in the World”, researchers Michael Jensen and Camryn Allen studied an unexpected effect of climate change—almost all sea turtles born today are female.
Temperature-dependent sex determination (TSD) is when the incubation temperature during embryonic development determines the sex of an organism. Species with TSD, including sea turtles as well as crocodiles and some freshwater turtles, are especially impacted by climate change. In sea turtles, higher incubation temperatures increase the proportion of hatchlings that are female, and lower temperatures produce more males. The pivotal temperature is the incubation temperature that produces 50% of each sex, and is inherited, varying across species and populations. The temperatures that create 100% males or 100% females are separated only by a few degrees, so climate change has a very significant impact. High incubation temperatures resulting from climate change produce female-only clutches of eggs and increase clutch mortality.
The northern Great Barrier Reef (nGBR) in Australia is home to one of the largest green turtle populations in the world, with over 200,000 nesting females. The researchers faced a challenge in estimating the population’s sex ratio at rookeries, where turtles are born: the most accurate way to determine sex in juveniles is to look at the gonads (the ovary or testis) but the hatchings must be dead to do this. The ethical dilemma prevented researchers from sacrificing hatchlings to determine their sex, and using only the dead hatchlings found in nests could result in sampling bias. The researchers instead developed a new way of determining sex ratios by using genetic analysis to link mature individuals of known sex in a foraging ground (FG), where the turtles eat, to the beach where they were born.
The researchers found that nGBR turtle populations were extremely female-biased, with 99.1% of juveniles, 99.8% of subadults, and 86.8% of adult turtles being female. Turtle populations from the south Great Barrier Reef (sGBR), where temperatures are cooler, were also female-biased, but less so, with about 60-70% in each size class being female. Since female bias is greater in younger age classes, they believe that feminization has increased in recent decades, and that nests have consistently been incubated above the pivotal temperature since the 1990s. Since climate change is causing increased sand temperatures on beaches, almost no male turtles are being born in the nGBR green turtle populations.
The low number of males will limit the reproduction of sea turtle populations. Sea turtles return to the beach where they were born to reproduce, so the nGBR population is isolated, and they have a long lifespan, so natural selection takes a long time to act. Both of these suggest many sea turtle populations like the nGBR will have little opportunity to adapt to the rapid global warming predicted. This poses a serious threat to the survival of sea turtles as a species and should serve as a wake-up call to us humans to do our part to protect this species and the planet as a whole.