Why Do Turtles Cross the Road?

And why do Blanding's turtles do the other things they do? At Weaver Dunes, where these threatened turtles are plentiful, researchers study their every move.

By Jeffrey W. Lang

I waded slowly through frigid water and knee-deep muck. It was early April, 2000. Spring peepers were singing on the fringes of a tiny marsh on the edge of the Mississippi River south of Wabasha. As a flash of white appeared, I lunged forward to grab it. Seconds later, I inspected a Blanding's turtle with a bright white 66 emblazoned on its 10-inch shell. Exposing its bright canary-yellow throat as it extended its long neck and head, the big adult made no attempt to bite. Its legs moved rhythmically, swimming in midair.

This 4-pound male had been taking long trips across the Weaver Bottoms wetlands during the past year. A month earlier, just before the ice disappeared, 66 was located 1.25 miles southwest in an old channel of the Zumbro River, on the edge of the McCarthy Wildlife Management Area. There he'd spent the winter, under snow and ice, buried in the mud bottom. So, why had he moved so early in the spring to this small marsh? With snow still in the air, was he already searching for a mate?

Using radiotelemetry to follow the transmitter glued to the turtle's shell, graduate student Mark Hamernick and I had been tracking 66 since the previous spring when we marked him in this small marsh. From here, he moved 3.5 miles to the mouth of the Whitewater River, where he spent the summer. En route, he stopped for a day or two near breeding females. By mid-May, the male reached the location where he remained through August. Then in September, we located 66 at his overwintering site, more than two miles northwest of his summer haunt.

For nearly a year, we had been following 66 and more than three dozen other Blanding's turtles we'd caught here. A permit from the Department of Natural Resources makes it legal for us to handle this threatened species and conduct research at Weaver Dunes, south of Kellogg. The DNR and The Nature Conservancy maintain the relict sand prairie as a scientific and natural area.

Hundreds of Blanding's turtles nest at Weaver Dunes every year. Some researchers claim that more turtles of this threatened species live in this complex mosaic of river bottoms and sand hills than anywhere else on earth. Our goal has been to find out how individual turtles make a living by using this patchwork of wetlands and uplands on public and private land. Ultimately, we hope to conserve this unique population and to provide management guidelines for Blanding's turtles elsewhere in Minnesota.

Wading ashore, I showed 66 to Mike Pappas, a research collaborator. Mike had been walking the marsh margins, looking for basking turtles. Though the air was chilly and the wind brisk, the sky was clear and sunny. Hidden among sedges, the cold-blooded turtles are able to use the sun to heat themselves up to 40 degrees warmer than the marsh. At this time of year, the turtles might even plod hundreds of feet into the open woods to bask. Mike found and identified number 52, a female that he had seen floating in the marsh a month ago, days after the ice disappeared.

Old Turtles

Mike and I exchanged turtles. The female was smaller, her underside flat, and her tail short and thin. In contrast, the male's underside was distinctly concave in the center, and his tail long and thick.

The Blanding's turtle with its domed shell is easily distinguished from the smaller, common painted turtle with its flattened shell and bright red underside. Compared with other turtles, Blanding's turtles are mild-mannered, docile, and exceptionally long-lived.

The Blanding's longevity record is a 77-year-old female, which probably reproduced for at least 60 years. Interestingly, senescence is unknown in this species. I examined the female's well-worn notches, which Mike carved in the edge of her shell when he first caught her in 1976 in a backwater pond just a mile to the south. At the time, she was already more than 20 years old. Now, nearly a quarter century later, she was still living in the neighborhood and would likely nest again.

Since the mid-1970s, Mike and fellow researcher Bruce Brecke have been studying turtles at Weaver Dunes. Working with landowners, these two naturalists marked more than 1,000 nesting females between 1974 and 1980. They discovered overwintering sites. In the early spring, they watched and photographed turtles emerging from ice-covered ponds and backwaters. Males fought among themselves, then the winners courted and mated with receptive females. Since 1980 Mike and Bruce have been working with scientists and conservationists to protect the turtles and their habitats.

My collaboration with Mike and Bruce grew out of the 1998 Blanding's turtle workshop at the University of Minnesota Bell Museum of Natural History. Organized by herpetologist John Moriarty, this international gathering brought more than 70 turtle biologists and wildlife managers together to "talk turtle." Participants visited important Blanding's turtle locales, including Weaver Dunes and Camp Ripley in central Minnesota.

Blanding's populations differ in size, shape, and coloration from place to place. The species is ancient. Recognizable shell fragments in Nebraska are 5.5 million years old. During the Pleistocene, when glaciers moved back and forth across much of the continent, the species' range was much larger and extended south to what is now the state of Mississippi and west to Oklahoma. Essentially a north temperate species, Blanding's turtles today inhabit marshes and swamps from Nova Scotia westward to central Minnesota and western Nebraska, south to Missouri and Illinois, and eastward to New England. Most populations are small, fragmented, and threatened or endangered. Settlement and agriculture have permanently altered suitable wetlands and adjacent uplands. Large populations still persist in Michigan, Wisconsin, and Minnesota, where the species reaches the northwestern limit of its range.

At Camp Ripley, my biology students from the University of North Dakota and I had documented the ecology of Blanding's turtles. We dodged artillery, tanks, and troops, spending long hours and many days studying the small but secure population. We were amazed by the turtles' large body sizes, as well as their exceptionally large clutches, eggs, and hatchlings. Everywhere else, Blanding's turtles are 10 to 15 percent smaller. Why these turtles are so large remains a mystery.

We observed that the turtles at Ripley spent most of their time in and around water. Shrub swamps were their favorite summer and winter hangouts. Individual turtles showed distinct patterns of space use. While some ranged widely in home areas of more than 40 acres, most had ranges of less than 10 acres. Some sedentary turtles never moved from a shrub swamp during our two-year study. We estimated the Camp Ripley population at about 500 turtles--low density compared with other Blanding's turtle populations.

The Weaver Dunes population is much larger, possibly 10 times as large, and much denser. Turtles here travel farther than they do at Ripley. Their home ranges typically parallel the extensive channels and backwaters of the Mississippi bottomlands. While few Weaver Dunes turtles reside within three acres or less, most range from tens to hundreds of acres. Weaver turtles travel back and forth across federal, state, and private boundaries, potentially complicating efforts to conserve and manage them.

Hundreds of Nests

In early June 2000, I stood in our field lab surrounded by 100 white plastic buckets, each with a female Blanding's turtle collected earlier in the day. We quickly measured, weighed, and marked each turtle before releasing it. With the help of assistants and volunteers, we were attempting to count how many turtles nested in Weaver Dunes and how many eggs they laid that season.

Weeks before, hundreds of females had begun migrating into shallow marshes near nesting areas. The exact time of nesting varies with the weather, but it usually begins by the first week in June and lasts two to three weeks.

Each evening, rain or shine, we patrolled roads and checked drift fences to intercept nesting turtles. After several days to a week in the dunes, these females would return to a nearby wetland. Most would follow familiar routes and make a round trip, but a few would take the opportunity to move to another marsh once they finished nesting.

In soft, moist sand, each female dug a nest with her hind feet. Late in the evening, she laid about 10 eggs with translucent shells and covered them immediately, leaving little trace. As with many turtles, egg temperature during incubation determined whether an embryo developed as male (cool temperatures) or a female (warm temperatures). So, most nests contained either brothers or sisters, but usually not both.

Not all eggs hatched. Some failed to develop. Others were destroyed by hungry skunks, raccoons, coyotes, foxes, and crows. We found more than 200 plundered nests. Remarkably, these probably represent less than half of the total lost to predation in a season. Based on our census of reproductive females, more than a thousand nests were made in Weaver Dunes in 2000. So, even though predators raided hundreds of nests, hundreds more produced thousands of hatchlings by late summer.

The hatchlings emerged in late August and early September. Each baby weighed less than half an ounce, with a shell only slightly larger than a quarter. Leftover yolk attached to its shell would fuel the tiny turtle until spring, if need be. Each hatchling, once freed from its egg, must dig itself out of the nest and move across the sand hills to a suitable wetland, where it buries itself in the bottom muck for the winter.

No one knows what cues the little turtles use to find their way to a wetland, a journey of half a mile or more. The dunes are virtually surrounded by marshes and backwaters. So most movements will take a hatchling to water. In late summer and early fall, thousands of hatchlings cross one or more roads--the same paved and gravel roads that several months earlier were covered with nesting females.

Turtles and Roads

In long-lived species, protecting the adults is critical to any conservation strategy. A female turtle may produce as many as 500 eggs during her life. Losing many of these long-lived females, through habitat loss or direct mortality, would seriously jeopardize the ability of a population to maintain itself.

So far, the light traffic on roads that ring Weaver Dunes has killed relatively few adult turtles, but the tiny hatchlings are always at risk. Residents and visitors alike have made way for these seasonal migrants. Rare Turtle Crossing signs have been posted only during nesting season. Elsewhere, busy roads have posed insurmountable barriers for traveling turtles. In the race with fast-moving vehicles, Blanding's turtles rarely win.

How might turtles cross roads safely? Underpasses and culverts beneath roadways are obvious solutions, but how can we design effective crossings that turtles and other wildlife will use? Few studies have tackled this issue. Since we were handling hundreds of turtles as part of our census, some simple experiments provided possible answers.

When we caught traveling females in drift fences, we rerouted some by giving them choices of culverts of different sizes and shapes to crawl through. Turtles readily entered and moved through large culverts, 3 to 4 feet in diameter and long enough to span a county road. Further research will be needed to design landscaping and fencing to lead animals under--rather than across--roads and to decide where to locate underpasses.

We still have a lot to learn from the turtles at Weaver Dunes. With any luck, we'll follow male 66 and female 52 in 2001, our third field season. In future years, we hope to find out how many turtles live in the surrounding wetlands, what areas are critical for hatchlings, and what features make this such a special place for Blanding's turtles. We hope our studies provide information essential to maintaining safe havens for the turtle with the yellow throat elsewhere in Minnesota.

Jeffrey W. Lang is Professor of Biology at the University of North Dakota in Grand Forks.