Each year, when spring gives way to summer, female turtles leave the river to lay their eggs. It’s such a predictable event that La Crosse County is known as a hot spot for turtle crossings, turtle species and turtle fatalities.

But cars aren’t the only threats with which turtles must contend. Climate change is skewing turtle embryo development to favor female turtles.

In many turtle species, the temperature dictates whether turtle eggs develop into males or females, said Nicole Valenzuela, professor of ecology, evolution and organismal biology at Iowa State University. Typically, exposure to cooler temperatures favor male turtle development, while warmer temperatures favor female development.

It’s a bit like flipping a switch, Valenzuela said.

At the pivotal temperature, the male to female ratio in a clutch of eggs is one to one. But a temperature deviation as small as 1 degree Celsius above that switch is enough to turn a clutch completely female in some species, Valenzuela said.

In the case of the painted turtle, a North American species found in Wisconsin, extreme temperature fluctuations also increase the likelihood that turtle eggs will skew female, Valenzuela said. In fact, even if the eggs are exposed to daily pulses of both high and low temperatures equidistant from the pivotal temperature, the feminizing effect from the heat would outweigh the masculinizing effect from the same period of cooling.

While there isn’t enough long-term data to quantify the pace at which climate change is feminizing inland turtles, years of data collected on sea turtles show feminization happening at a rapid rate, Valenzuela said.

Turtle populations already are endangered by habitat destruction, overexploitation or road mortality, Valenzuela said. That climate change could thwart turtle reproduction adds insult to injury.

The Wisconsin Department of Natural Resources isn’t sure how climate change affects state turtle species, but a shift toward warmer weather could make the upper Midwest more hospitable to southern turtle species brought here as pets, which could turn into an invasive species, said Andrew Badje, a conservation biologist at the DNR.

Warmer temperatures also could encourage the growth of turtle-infecting bacteria, virus and fungi that previously found Wisconsin too cold, Badje said.

Wisconsin is home to 11 native turtle species, including the ornate box turtle, a state endangered species. There also are state-threatened wood turtles and state protected Blanding’s turtles.

La Crosse County is home to eight or nine species, Badje said.

The DNR asks people to report their turtle sightings as part of its crowdsourced turtle monitoring program.

Knowing where turtles cross allows DNR officials to focus their conservation efforts, especially around key road crossing areas where local turtle populations have taken a hit, Badje said. “We want to keep our common species common.”

In the program’s eight years, the DNR has identified more than 1,900 turtle crossings, Badje said. About 40 sites are considered significant crossings, sites where more than 50 turtles are known to cross or end up as roadkill.

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The area behind the South Side Walmart is one such crossing, Badje said.

Connie Troyanek was getting ready to pick up her granddaughter last week Thursday when she saw a turtle crossing Shelby Road.

“Cars come around here pretty fast,” Troyanek said, from the middle of the road. “I knew if I left it there, it wouldn’t make it to the other side.”

The Coulee Region Humane Society has received half a dozen calls concerning turtles last week, said wildlife rehabilitator Kathy KasaKaitas. They’ve had to euthanize three turtles in two days that were beyond help.

Most of the turtles that get run over are females, the ones sustaining the population, Badje said.

The turtles are looking for open, sandy, loose gravel to dig their nests, Badje said. “People’s lawns are the next best thing.”

The nests can be anywhere from 3 to 6 inches deep for painted turtles, Badje said. Snapping turtles prefer to dig nests closer to 8 inches deep. Some species lay four to six eggs, others lay one to two dozen eggs. Snapping turtles can lay up to 100 eggs.

The eggs typically hatch between August and November. Some hatchlings overwinter in the water. Others stay in the nests until spring.

Badje recommended that people move turtles to whichever side of the street the turtles are headed, lest they end up making the same journey again.

It’s a journey turtles may make more often as climate change extends the turtle nesting season.

Some studies have found that turtles are laying extra clutches later in the year, Valenzuela said. Not only are those eggs more likely to die because they aren’t able to fully develop before it gets too cold, it means pregnant females are crossing the roads more often, increasing their likelihood of getting hit.

While turtles are ancient, long-lived animals, many of which have been able to withstand historic changes in climate, change is now happening at an accelerated pace, Valenzuela said.

For turtles to adapt, they need a large enough population to accumulate and pass on the genes that will allow them to withstand climate change impacts, Valenzuela said. And they need enough time to adapt.

“They can withstand a lot, but not forever,” Valenzuela said. “The longevity will help them, but it also slows down adaptation. It’s a little bit of a two-sided sword.”

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Jennifer Lu is the La Crosse Tribune environmental reporter. You can reach her by phone at 608-791-8217 and by email jennifer.lu@lee.net.


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