Roads May Skew Turtle Sex Ratios
Anyone who’s seen a turtle cross the road knows that these slow-moving reptiles are no match for the cars whizzing past. But there has been little evidence that roads are a threat to turtle populations. Now new research suggests that cars are picking off the females: painted turtle populations are 73 percent male, and snapping turtle populations are 95 percent male, near roads in upstate New York.
“Our study indicates that females may be taking the brunt of the road kill,” says James Gibbs, who with David Steen presents this work in the August issue of Conservation Biology. Both researchers are at State University New York, Syracuse; Steen will be at the Joseph W. Jones Ecological Research Center in Georgia beginning in August 2004.
The U.S. has about one-fifth (56 out of 257) of the world’s turtle species, and nearly half of them are imperiled. Roads are a likely threat to turtles because the juveniles use them to migrate to find new places to live, and the adults use them to migrate to find mates and nest sites. However, linking roads to declining turtle populations has been difficult because much road expansion has occurred relatively recently and turtles can live for more than 40 years, which means that even disturbed populations could persist for decades.
Steen and Gibbs compared the sex ratios of painted and snapping turtle populations in 35 wetlands in areas with high and low road densities near Syracuse, New York. Most of the high road density study sites were located near the New York State Thruway and Interstate 84, whereas the low road density study sites were in protected areas, including Howland Island Wildlife Management Area and Three Rivers Wildlife Management Area.
The researchers found that “high road density” turtle populations had many more males than “low road density” populations: painted turtles were 73 percent vs. 54 percent male, respectively, and snapping turtles were 95 percent vs. 74 percent male, respectively. This suggests that more females are killed on roads, presumably during their spring-summer nesting migrations. Fewer female turtles could mean fewer baby turtles to replenish the populations.
Turtles can be protected from traffic near wetlands by installing culverts to help them cross roads and short fences to keep them from crossing roads. People can also help turtles by moving them off roads. “When you see a turtle crossing a road in the spring or summer, there’s a good chance it is a female full of eggs, so don’t hit it and don’t take it home or relocate it miles away,” says Gibbs. Instead, turtles should be put on the side of the road they were trying to reach.
Steen, D.A. and J.P. Gibbs. 2004. Effects of roads on the structure of freshwater turtle populations. Conservation Biology 18(4):1143-1148.
Painted turtle (Chrysemys picta) crossing the road in Wright County, Minnesota. Photo by Tony Gamble (2003)