Some like it hot, but not painted turtles (Image: Lorin Neuman-Lee)

Males don’t stand a chance in a warmer world, if they happen to be painted turtles. A temperature rise of around 1 °C is all it would take for the species to become 100 per cent female and earmarked for extinction.

Painted turtles (Chrysemys picta), found in lakes and streams across North America, are one of many reptile species whose sex is determined by temperature. Eggs in warm nests are likely to hatch as females, while males hatch in cooler nests, although no one is sure why.

In recent years, many researchers have raised concerns that global warming could skew the sex ratios of these reptiles. Rory Telemeco and his colleagues at Iowa State University developed a mathematical model to predict whether the painted turtles might be affected.


For over 25 years, Telemeco’s colleague, Fredric Janzen, documented the nesting times and sex ratio of painted turtle hatchlings on a small island in the Mississippi river in Carroll County, Illinois. He found that females can shift their nesting dates by about 10 days to ensure their eggs develop at temperatures that produce an even mix of males and females.

The team used this finding, along with historical records of soil and air temperatures, to create a mathematical model that predicts the sex ratio of eggs laid at different temperatures. In a preliminary test of the model, the group correctly predicted the sexes of 40 out of 46 hatchlings born in the wild.

Sex ratio

Telemeco’s team then used the same model to predict what might happen to the sex ratio of future hatchlings. Conservative climate models predict that average temperatures in the US Midwest will rise by 4 °C over the next century. The group’s model suggests that this temperature hike would result in nests of all-female hatchlings, even if the turtles nest earlier, when temperatures are cooler. In fact, average temperatures only need to rise by 1.1 °C to have this effect, the team found. “It’s ultimately extinction,” says Telemeco.

Richard Shine, an evolutionary biologist at the University of Sydney in Australia, who was not involved in the study, says the findings are likely to apply to many species where sex is dependent on temperature. “All crocodilians, a smattering of turtles and lizards, plus some fishes”, will be affected, he says. “Just laying your eggs a few weeks earlier won’t be enough to cancel the effects of warming,” he says.

But Shine is optimistic about the turtles’ chances of survival. The animals could adapt by laying their eggs in shadier locations, or evolving the ability to cope with warmer conditions.

Telemeco is less sure. “The problem is that climate change is happening so rapidly that an evolutionary response, especially in long-lived organisms, is not likely,” he says.

Clues to the turtles’ chances of survival may come from a better understanding of the species’ genome, which was sequenced last month. Observing how gene expression changes in response to temperature shifts might provide insight into how the turtles adapted to their current environment and also how they might respond to future changes, says Telemeco.

Journal reference: American Naturalist, DOI: 10.1086/670051; Genome Biology, DOI:10.1186/gb-2013-14-3-r28