- The sex of some turtle species is influenced not by genes but by the temperatures they experience in the nests. Embryos of the Chinese pond turtle, however, can move inside the eggs toward cooler or hotter spots and influence their own sex, at least to some extent, a new study has found.
- This is good news because it means that, at least in theory, the turtles might be able to buffer some of the predicted shifts in the sex ratio because of climate change.
- But while the embryos seem to be influencing their sex under ideal conditions, researchers say that it may not be enough to counter the rapidly changing climate brought about by human activities.
The sex of some turtle species is influenced not by genes but by the temperatures they experience in the nest. Eggs incubated at cooler temperatures develop into males, while those that face warmer temperatures turn out to be females. When temperatures fluctuate between cool and warm, the eggs produce a mix of male and female babies.
The Chinese three-keeled pond turtle (also called the Chinese pond turtle) is one such species. But its embryos seem to have some control over their own sexual fate, according to a new study.
The embryos can move inside the eggs toward cooler or hotter spots, researchers have found, influencing their own sex to some extent. This is good news because it means that, at least in theory, the turtles might be able to buffer some of the predicted shifts in sex ratio because of climate change. Since hotter temperatures produce only female babies, rising temperatures due to climate change could end up creating populations of mostly female turtles, scientists say, leading to population declines.
“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,” Du Wei-Guo, a professor at the Chinese Academy of Sciences and corresponding author of the study, told Mongabay.
In previous research, Du and his colleagues had shown that embryos of the freshwater Chinese pond turtle (Mauremys reevesii), an endangered species, move inside eggs in response to temperatures. The significance of this behavior, though, remained unclear.
To find out more, the researchers conducted experiments on Chinese pond turtle eggs both in the laboratory — using eggs collected from a private commercial turtle farm in China’s Zhejiang province — and in an outdoor pond where farm turtles had laid some eggs.
When incubation temperatures are cooler than 26 degrees Celsius (79 degrees Fahrenheit), the turtle’s eggs all hatch male babies. When the temperature rises above 30 degrees Celsius (86 degrees Fahrenheit), every embryo is a female. At 29 degrees Celsius (84 degrees Fahrenheit), or the pivotal temperature, the eggs are known to produce a 50:50 sex ratio.
The researchers used capsazepine, a chemical that blocks the eggs from sensing temperature, on half of both the laboratory and outdoor eggs, and monitored the embryos throughout their development. When the eggs hatched, the team found that the embryos inside the eggs treated with capsazepine did not move as much compared to those in eggs that hadn’t been treated. The treated eggs also produced all male babies when the incubation temperature was low, and all females when the temperature was high. Embryos in the untreated eggs, meanwhile, had moved around inside the eggs and hatched into a 50:50 mix of male and female turtles.
“Until a few years ago, we thought that even given the choice, turtles would not be able to choose among temperatures in the egg,” Rory Telemeco, an assistant professor at California State University, Fresno, who was not involved in the study, told Mongabay in an email. “Then, thanks to earlier work by this laboratory, as well as myself and other colleagues, we thought that [embryos] could choose among temperatures, but may never be given the opportunity in nature. This study confirms that, at least in this species of turtle, both the choice of thermal environment and ability to choose among them can be available for embryos. Moreover, when available, embryos appear able to make the ‘good’ choice and choose the environment that will result in a more 50:50 sex ratio.”
But a turtle embryo likely has very limited control over its own sex in the wild, researchers say. “The sexes of the baby turtles are most sensitive to conditions available in the environment and the mother’s nesting choices,” Telemeco said.
The extent to which the embryos can counteract the effects of climate change also remains unclear.
Telemeco said that while the embryos seem to be influencing their sex under ideal conditions, those conditions “might not be available much of the time, especially given climate change predictions.”
“For embryos to meaningfully alter their temperatures within the egg, eggs must be large, near the surface, and average temperature during a 1-month window must be very close to the pivotal temperature for sex determination,” Telemeco said. “This study confirmed that this behavior only works under those conditions.
“Most reptiles produce eggs that are too small, or buried too deep, or exposed to too extreme of average conditions for this behavioral response to have any effect. Therefore, we cannot consider embryo behavioral thermoregulation to be a panacea allowing this species or others to respond to climate change,” he added.
Ideal conditions aside, Du agreed that the embryos’ power over their own sex may not be enough to counter the rapidly changing climate brought about by human activities.
“However, the discovery of this surprising level of control in such a tiny organism suggests that in at least some cases, evolution has conferred an ability to deal with such challenges,” Du said.
Ye, Y., Ma, L., Sun, B., Li, T., Wang, Y., Shine, R., and Du, W. (2019) The embryos of turtles can influence their own sexual destinies. Current Biology. doi:10.1016/j.cub.2019.06.038