Researchers from Iowa State University say there's a danger climate change will warp the sex ratio of painted turtles, leading to dramatic reductions in reproduction.
For many reptiles, including the painted turtle, gender is determined by the environment during nesting, says Iowa State biologist Rory Telemeco — in contrast to mammals, whose chromosomes determine gender.
"The temperature during a fairly short window of about a month in the middle third of development determines whether or not the offspring will be male or female, with cool nests producing males and warm nests producing females," Telemeco explains. Potentially, then, warmer temperatures could mean many more females are born than males.
Like many other of Earth's organisms, shifts in the painted turtle's onset of reproduction are rapidly occurring, Telemeco says. "We see flowers blooming earlier, leaves bursting earlier on trees. Birds and butterflies are migrating earlier. Frogs are singing earlier, and things like turtles and lizards are nesting earlier in the year. It's a really common response," he explains. "This leads to the question: Are these organisms [effectively] buffering themselves from climate change by nesting and doing their things earlier in the year when it's a little cooler? We really wanted to know whether or not that was going to work. So, we looked at these turtles."
Telemeco and his colleagues examined 25 years of research on a single population of painted turtles in the Mississippi River and plugged their data into a mathematical model. They found that nesting earlier, by itself, will do little good.
"If that's the only thing that the turtles had at their disposal to try to protect themselves from climate change, it would only buffer them through about a one degree Celsius increase in temperature," he explains. "Any more than that, it would have no buffering effect. We'd expect to see 100 percent female sex ratios in these populations, and potentially even really high mortality in the nests of these populations, as well."
The Midwest is predicted to see a temperature increase of four or more degrees Celsius increases over the next century, according to current climate models.
Typically, painted turtles lay their eggs in late spring, around June 1, with the expectation that temperatures will hold relatively steady in the middle third of development, roughly in mid-July. If, as a result of warming temperatures, the turtles move their nesting date a couple of weeks earlier, their new expectation would be that by mid-June temperatures are holding steady. But climate change may thwart this expectation.
While the nest temperature on May 15 may be what it once was on June 1, Telemeco says, the nest temperature will actually warm much more rapidly throughout development than before, which will lead to many more female births.
Turtles have been on Earth for about 200 million years, so they have certainly learned to evolve and adapt to changing conditions, Telemeco says. The difference now is that climate change is occurring so quickly, with temperature increases or four to eight degrees Celsius within the next 100 years.
Turtles can reproduce starting around age five, after which they can reproduce every year for another 20 to 25 years. "That rate of generation time really slows the ability of populations to evolve," Telemeco explains. "It means that you would need dramatic evolutionary changes within just a few generations for them to be able to cope with the types of environmental change we're predicting over the next century."
While humanity can't completely stop all climate change, "we can pull it back and keep the climate from changing so much," Telemeco believes. "Short of that, the things we can do to help support these organisms, or keep them from going extinct, at least, are the same sorts of things we would do under any other circumstances. But it's going to be really difficult and, sadly, I really don't think we're going to be able to save all of them."