World / Science & Health

Turtle embryos can partly determine own sex


For some turtle species, the temperature of eggs determines whether the offspring are male or female — a trait that scientists feared could leave the reptiles vulnerable to extinction from climate change.

But a team of researchers in China and Australia have found the embryos can move around inside their eggs to find a “Goldilocks zone” where it is neither too hot nor too cold, thus playing a role in determining their own sex.

Their paper, published on Thursday in Current Biology, described the mechanism as an evolutionary buffer against climate change that may have helped the species survive more severe fluctuations in Earth’s deep past.

“Our research shows that a reptile embryo is not just a passive victim of global warming, but may control their own sex fate to some degree,” said co-author Wei-Guo Du, a professor at the Chinese Academy of Sciences.

The team previously demonstrated that reptile embryos could move around within their egg for thermoregulation, and wanted to learn if this behavior had a big enough impact to determine sex.

They incubated freshwater turtle eggs under various temperatures in lab conditions and in ponds, determining that a single embryo might experience a range of up to 4.7 degrees Celsius (8.5 degrees Fahrenheit) within an egg.

A shift larger than 2 degrees can drastically alter the sex ratio of many turtle species, with higher temperatures leading to a skew toward females.

The researchers applied a chemical that blocks temperature sensors, known as capsazepine, to half the eggs, and left the rest as they were.

They found that those embryos without thermoregulation had developed as either almost all males or all females, depending on their incubation temperature.

Embryos that were able to react to temperature had shifted their positions within their eggs, and the sex ratio was almost even.

“This could explain how reptile species with temperature-dependent sex determination have managed to survive previous periods in Earth history when temperatures were far hotter than at present,” said Richard Shine, a professor at Macquarie University of Australia and one of the co-authors.

The embryos’ control over their sexual destiny does have limits. When mean temperatures in the nests were either very hot or very cold, the embryo’s efforts had no impact on offspring sex.

“(It) may not be enough to protect it from the much more rapid climate change currently being caused by human activities,” Du said.

But he added that the species may have some ways not yet discovered to offset risk, such as laying eggs earlier in a season or in more shaded nests, which would be part of the focus of his future studies.