Does Temperature Determine the Sex of a Turtle?
Are there really more females when it is warmer? For many reptile species this really is true. The temperature determines whether they will be genetically male or female. Scientists are learning more about exactly how this works.
In humans, sex is determined by our chromosomes: two identical X‑chromosomes result in female sex, while the combination of a Y‑chromosome and an X‑chromosome results in male sex.
The situation is different for some reptiles though. Environmental temperatures during the earliest stages of their development determine whether they will end up male or female. This process is known as temperature-dependent sex determination.
Temperature
The temperature during incubation determines whether a turtle will be male or female: it turns the development of male or female genitalia on or off. One example is the red-eared slider: if her eggs are in a cool nest at about 26°C, they will all hatch as males. However, if the temperature is 32°C, all the babies are females.
Kdm6b gene
Biologists have been trying to understand this remarkable process for some time. A new study of recently laid turtle eggs has helped them with this. The researchers found that the “Kdm6b” gene becomes much more active at low temperatures (resulting in male turtles) and is virtually deactivated at high temperatures (resulting in females).
When the researchers deactivated the Kdm6b gene in some of the turtle embryos, most of them grew into females – even in a cold nest. This is because activation of the Kdm6b gene (indirectly) leads to the development of the testes. So when the scientists deactivated the Kdm6b gene, that development did not occur.
Germ cells
That was not all: a warm incubation temperature also increased the number of germ cells (the cells that the embryo carries within it from which sperm or eggs will later develop).
These germ cells also appear to have a role in determining sex. Research carried out at a moderate incubation temperature (where the male-female ratio should actually be fifty-fifty) showed that when the mother turtle has more germ cells, there will be more females in the nest.
Getting warmer
Temperature-dependent sex determination is quite risky, though: what happens if global warming continues? The male-female ratio in reptiles could be at risk of serious disruption. A few years ago, researchers found that almost 99% of green sea turtles originating from Australia's warm northern beaches were female.
Further temperature rises could also have a negative impact on embryonic development. At higher temperatures, the turtles that do hatch might be deformed, or the embryos could die. More information about temperature-dependent sex determination will definitely help to ensure that these reptiles can survive into the future.