Northern Map Turtle
Graptemys geographica
Graptemys geographica
If you were to gaze upon the shell of a Northern Map Turtle, you may be forgiven for thinking you were staring at a map and wondering where it leads. The unique yellow pattern on the carapace (upper shell) resembles the contour lines found on a topographic map, hence their name. Map Turtles are medium-sized, with the carapace of adult males reaching up to 16cm in length and adult females reaching up to 27cm. They are sexually dimorphic, meaning males and females look different – females can be more than twice the size of males.
The carapace (upper shell) of Map Turtles ranges in colour from brown to olive and can be distinguished from Ontario’s other turtles by the presence of a ridge down the midline of its shell, known as a keel, along with a serrated rear margin. The plastron (lower shell) is yellow to cream and the skin of the neck, head and limbs are dark olive to brown-black and covered with yellow stripes. Map Turtles also have a characteristic yellow spot behind each eye.
A specialized carnivore, Northern Map Turtles feed primarily on molluscs such as mussels and snails but their diet also includes crayfish and aquatic insect larvae. Map Turtles, particularly the females, have a broad head and strong jaw that make quick work of the protective shells much of their prey use as a defence.
The diet of females consists more of molluscs whereas the smaller males have a diet that is more similar to juveniles who consume more aquatic insects.
Northern Map Turtles can be found in rivers and marshy or nearshore areas of lakes, they have a preference for waters that are shallow, have moderate flows, a soft-bottom, and good basking habitat such as logs, rock, and exposed banks. They lead a highly aquatic lifestyle and require areas with good water quality and are intolerant of waters with low oxygen levels. Terrestrial habitat is rarely used except during the nesting season and for basking. Described as a social species, Map Turtles will gather and jostle at prime basking sites to absorb the sun’s warmth. To survive the harsh Ontario winters, Map Turtles overwinter for 5 to 6 months at the soft-bottom of oxygen-rich lakes and rivers, 0.5-11m below the surface, hiding under exposed ledges, boulders, and tree trunks.
The Northern Map Turtle is found in eastern North America with approximately 10% of its global range in Canada occurring in central and southern Ontario and southwestern Quebec in the Great Lakes-St. Lawrence basin.
In southwestern Ontario, Map Turtles live in areas of Lake St. Clair, Lake Erie, and Lake Ontario with undeveloped shoreline as well as major tributaries in the region including the Sydenham, Ausable, Thames, Detroit, and Grand rivers.
Major threats to Northern Map Turtles include mortality from boat collisions and death caused through commercial fisheries bycatch. Shoreline development and activities that modify natural flow regimes and sediment transport reduce the amount of habitat needed for nesting, basking, foraging, and hibernation. Unnaturally high predation rates by mammalian egg predators, especially raccoons, decreases the number of young turtles being added to the population, threatening the persistence of their populations. Since Map Turtles have a specialized diet, threats like siltation and pollution that impact molluscs and other benthos also threaten Map Turtles.
© 2024 · St. Clair Region Conservation Authority - The Sydenham River Watershed