In many species of reptiles such as all sea turtles, some species of freshwater turtles and crocodilians, sex is determined by the temperature of the eggs during a certain period of development. Such type of sex-determination is called temperature-dependent sex determination (TSD). In some species the eggs develop into females under low temperatures and males under high temperatures (F-M pattern). In other species the eggs will develop into males under low temperatures and females under high temperature (M-F pattern). Still, in other species, the eggs develop into females under low temperatures, males under intermediate temperatures, and females under high temperatures (F-M-F pattern).
It means rising temperatures with the climate change has the potential to alter the makeup of populations of these animals. Yes, this is already happening!
In sea turtles, lower temperatures produce more males and higher temperatures produce more females. A very narrow temperature range produces 50% of each sex.
Last year, in a study led by WWF-Australia, a team of researchers found that more than 99% of the green turtles being born on Australia’s Great Barrier Reef are female, with 116 female hatchlings for every male.
“With warming global temperatures and
most sea turtle populations naturally producing offspring above
the pivotal temperature, it is clear that climate change poses
a serious threat to the persistence of these populations”, the study, published in Current Biology reported.
A recent study published in Marine Ecology Progress Series, estimates that 84% of hatchlings currently produced on Cape Verde, home to one of the largest nesting populations of loggerhead turtles in the world, are female with 85% of nests laid on Boa Vista, where incubation temperatures are coolest.
The Intergovernmental Panel on Climate Change (IPCC) temperature projections for 2100 are: low (1.8°C), mid (2.8°C) and high (3.4°C), to which Lucy Hawkers, who led the study said the following.
“Under all three climate change scenarios in our study, by 2100 more than 99 per cent of hatchlings would be female – and under mid and high-emissions scenarios there could be no males at all.”
To keep the nest cool, turtles can benefit from nesting in the shade under trees or they could adapt by nesting earlier in the year when temperatures are lower. But the rate at which climate is changing very likely means they would not be able to evolve fast enough.