The Hudson River’s “Tough Turtles”

During the summer months along the Hudson River south of Troy, New York, it’s easy to notice the tides rising and falling, herons wading in the shallow streams, and the giant cargo ships purposefully streaming up and down the river. Difficult to spot, however, are the river’s many turtles. Several varieties call the Hudson home, but the northern (also called common) map turtle is perhaps the most interesting and understudied.

Northern map turtle “Princess” hanging out while her tracker tag dries before her release. Scientists use these tags to locate the turtles for months after capture.

Northern map turtles (Graptemys geographica) are large river turtles that get their name from the intricate circling pattern on their shells, which resemble the elevation lines on a map. These turtles are relatively secretive. In urban areas they have to work especially hard to find what they need to survive. For one thing, turtles need good basking objects—places where they can come out of the water safely and sun themselves to warm up. Fallen trees or rocks make the best basking habitat, specifically ones that are farther out into the water so they can easily escape from potential predators. Because of the tides, many potentially good basking objects aren’t reachable as they are either too high up the bank or underwater at any given time.

Scientists quietly observe northern map turtle “Luna” nesting from a safe distance. This nest received protection from predators and likely hatched successfully months later.

Another problem is finding places to lay their eggs. All turtles lay eggs and the northern map turtle is no exception. Most turtles prefer loose, sandy soil with plenty of sunlight for the eggs to develop successfully. Temperature determines the gender of the tiny map turtle babies—eggs toward the cooler, bottom of the nest often become males, while those eggs toward the warmer top (that therefore get more sun and heat) will become females. But in this highly urban area, good places to nest are few and far between. Natural areas, like those found in some of the State Parks along the river, help provide habitat for them. These spots seem perfect for northern map turtles, but they do tend to have a couple of drawbacks: 1) road and foot traffic and 2) predators smelling the eggs and destroying the nests soon after they’ve been laid. In addition, well-meaning people who are simply curious about these turtles (and with good reason!) approach nesting females that may “spook” and stop laying. People should give nesting turtles some space and observe quietly from a distance.

Nest — Probable northern map turtle nest destroyed by a predator. Shells that appear twisted indicate some animal has eaten them, whereas more intact shells mean the nest has likely hatched successfully.

Because good turtle habitat is hard to find in an urbanized section of the river, researchers Dr. James Gibbs and Master of Science candidate Julia Vanaman from the State University of New York College of Environmental Science and Forestry are working to identify what habitats are most important to map turtles. Aquatic plants, basking objects, forest along the river banks, and shoreline development all likely play a role in where these turtles choose to spend their time. Once the researchers understand why a turtle likes an area, they can pass along that information to state and local park managers who can protect habitat and take measures to enhance it (e.g., by creating nesting habitat or increasing the number of available basking objects). With these habitat improvements, hopefully these fascinating turtles will stick around for many years to come.

Note: Northern map turtles (Graptemys geographica) occur across much of eastern North America from the Mississippi River, north to the Great Lakes and St. Lawrence River, and east to small portions of the Susquehanna, Delaware and Hudson river systems. In New York State, the map turtle is considered vulnerable to decline and is recognized as a Species of Greatest Conservation Need (SGCN) in the state’s wildlife action plan. For more information, please check out the following links:

New York State Species of Greatest Conservation Need

Rare Animal Status List

Common Map Turtle Distribution Map

Turtles of New York State

NatureServe northern map turtle