Freshwater turtles use the sun to navigate to dark horizons

Researchers knew that Blanding’s turtle (Emydoidea blandingii) hatchlings travel in a straight line toward dark horizons, but they didn’t know how they navigated.

Some turtles, such as loggerhead sea turtles (Caretta caretta), are known to use the earth’s magnetic field to navigate. But biologists weren’t sure what mechanism Blanding’s turtle hatchlings use to guide them toward forests as they emerge from their underground nests, months after their mothers have left.

“They use the sun,” concluded John Krenz, a professor of biology at Minnesota State University and lead author of the study published in Behavioral Ecology and Sociobiology.

In the study, Krenz, along with reptile conservationist Justin Congdon and their colleagues, collected 2- or 3-day-old turtles on a road as they traveled westward toward a forested area. The researchers placed magnets on the shells of half the turtles to disrupt their ability to use geomagnetism. The other half received a piece of metal as a control. The researchers then released the turtles in a cornfield below tall stalks that blocked visual cues but allowed the turtles to see the sky.

After recapturing them, researchers found the turtles continued their westward journey — even those bearing magnets.

In the second half of their experiment, the team kept half the turtles in a laboratory under lighting conditions that mirrored normal day and night hours. The other half underwent a “clock shift,” with the light timing shifted six hours earlier. When placed in the cornfield, the group of turtles that were under normal daylight conditions continued westward. The turtles under the clock shift took a 90-degree turn. A turtle released at 9 a.m., for example, continued as if it were 3 p.m., and following the sun based on its reset internal clock.

“Instead of going west they went south,” Krenz said. “We thought ‘oh my gosh, it worked!’”

The turtles first viewed a dark, forested horizon, Krenz said, then used the sun to navigate toward it. “They shift from that visual cue to their sun compass.”

While the paper focused on the turtles’ navigation, Krenz said, it also raises conservation issues for the turtle, which is considered endangered in much of its range. Because the turtles rely on seeing the forests as an initial visual cue, he said, deforestation can impact their navigation.

“When forests are removed it means the turtles don’t know which way to go,” Krenz said.