Turtle Embryos Move in Eggs to Determine Their Sex, Could Cheat 'Potentially Disastrous' Effects of Climate Change

Scientists believe turtle embryos can move around in their eggs to determine their sex in a way that could help the animals cheat the "potentially disastrous" effects of climate change.

In a process known as temperature-dependent sex determination (TSD), whether some reptiles are male of female partly depends on the conditions in which the embryo develops. A rise in temperature of 2 degrees Celsius can change a turtle's sex, explained the authors of a study published in the journal Current Biology.

Global warming is therefore "potentially disastrous" for the populations of certain animals such as turtles, crocodiles and lizards as expectant mothers might not be able to predict the long-term conditions their eggs will incubate in, warn the authors. This could lead to imbalances in the sexes, and in turn plummeting populations.

While mothers can try to control the temperatures their eggs are exposed to, for instance by laying them in the shade or deep in the soil, they can't control the conditions for the incubation period which can last several weeks.

How, then, the team asked, have turtles previously survived fluctuations to the climate hotter than expected to happen in the next century?

The scientists believe that embryos may be able to influence their sex thanks to a process called "behavioral thermoregulation" to balance out changes to sex ratios caused by global warming.

Co-author Professor Wei-Guo Du of the Chinese Academy of Sciences commented in a statement: "We previously demonstrated that reptile embryos could move around within their egg for thermoregulation, so we were curious about whether this could affect their sex determination.

"We wanted to know if and how this behavior could help buffer the impact of global warming on offspring sex ratios in these species."

The team studied eggs and embryos of freshwater turtles incubated in the laboratory and semi-natural outdoor ponds.

To compare the sex ratios of embryos that could and couldn't thermoregulate, they dosed some eggs with a chemical that blocks temperature sensors. The researchers used the resulting data to predict how climate change might affect the sex ratio of turtles.

Inside an egg, the temperature could shift by 4.7 degrees Celsius, much higher than the 2 degrees required to change sex.

The embryos which couldn't respond to the nest temperature and move in the egg turned out to be almost all male or all female. But nests where embryos could react had a more even split of male and female babies.

"Our analyses suggest that, via behavioral thermoregulation, embryos can ameliorate impacts of climate change on offspring sex ratios and thus on population viability," they authors wrote.

The team concluded: "Embryos of a freshwater turtle with TSD actively thermoregulate, thereby affecting their own sexual destiny."

Du told Newsweek: "The most exciting result is that a tiny embryo can influence its own sex by moving within the egg.

"Our research shows that a reptile embryo is not just a passive victim of global warming. Instead, the embryo can move around inside its egg to find the 'Goldilocks Zone'—not too hot, not too cold— that enables it to buffer extreme thermal conditions imposed by changing temperatures.

"However, the embryos' control over its own sex may not be enough to protect it from the much more rapid climate change currently being caused by human activities, which is predicted to cause severe female-biased populations"

Also, embryonic thermoregulation might not happen if temperature changes inside the eggs are too small for the embryos to detect, Du explained. Early embryos might not have the ability to detect changes and move; while older more developed fetuses might be too big to move.

However, Professor Fredric Janze of Iowa State University who has also studied sex determination in turtles but was not involved in this research, was skeptical about the conclusion, and said it would be unlikely such a phenomenon would play a prominent role in offsetting the sex-ratio effects of climate change.

"We are being asked to believe that embryos preferentially choose/migrate toward cooler temperatures to become males when that is the rarer sex and to preferentially choose/migrate toward warmer temperatures to become females when that is the rarer sex," he told Newsweek.

"All this while they must navigate a massive yolk surrounded by an impenetrable membrane, and do so twice daily because temperatures fluctuate sinusoidally, when a) they do not have fully developed brains or b) a means to 'know' what sex to become," he argued.

"Even buying that embryos migrate actively and adaptively, it seems only to be a couple of millimeters in 34-millimeter eggs, which could hardly make a thermal difference," he argued.

Past studies have suggested that mothers of TSD species adapt to local thermal conditions, for instance by placing nests in shadier locations in hotter locations, said Janze.

"We also know from field studies of numerous species with TSD, however, that seasonal air temperatures even a couple degrees above or below average yield highly biased offspring sex ratios from nests." This often means an entire cohort of offspring in one year is all, or nearly all, one sex.

"Clearly, then, the embryos of those species aren't counteracting even these relatively modest thermal excursions right now, much less to be expected under the more extreme thermal conditions that are being experienced as the planet warms," said Janze.