Graptemys geographica (Lesueur, 1817)

Etymology

Both the common and scientific name of the northern map turtle, Graptemys geographica, come from the markings on its carapace, which resemble contour lines on a map or chart. These markings are typically yellow, tan, or orange, enclosed by dark borders, and the rest of the carapace is olive or greyish brown.

Carapace Marking Ontogeny

The carapace markings fade as the turtle matures; in older individuals, markings are usually only visible when the shell is wet.

Carapace Structure

The carapace is broad, has a hydrodynamic shape, and features a moderately low keel. The rear of the carapace is flared, and the rear marginal scutes form serrations.

Plastron Appearance

The plastron is yellowish, with a central dark blotch that follows the sutures between plastral scutes. This dark marking also fades with age, and many adults have no pattern at all, with an entirely immaculate plastron.

Soft Part Coloration

The head, neck, limbs, and tail are dark green with thin yellow stripes, and there is an oval or triangular spot behind each eye.

Sexual Dimorphism Overview

Like other map turtles, this species shows extreme sexual size dimorphism.

Adult Size Ranges

Males have a carapace length of 10–16 cm (3.9–6.3 in) and weigh 150–400 g (5.3–14.1 oz), while females reach 18–27 cm (7.1–10.6 in) in carapace length and weigh 0.67–2.5 kg (1.5–5.5 lb).

Sexual Head Traits

Females also have much wider heads than males, a trait linked to differences in feeding.

Male Secondary Sex Traits

Males have a narrower carapace with a more distinct keel, a narrower head, and a longer, thicker tail. Unlike females, the male cloacal opening sits beyond the rear edge of the carapace.

Juvenile Carapace Traits

Young northern map turtles have a pronounced dorsal keel.

Hatchling Morphology

Hatchlings have a round, greyish-brown carapace that measures approximately 2.5 cm (0.98 in) long.

Native Eastern Range

The species’ native range stretches from southern Quebec and Ontario to northern Vermont, covering the St. Lawrence River drainage basin.

Native Western & Southern Range

The range extends west through the Great Lakes into southern Wisconsin and eastern Minnesota, west of the Appalachians, and south to Kansas and northwestern Georgia. A population has been recorded in northeastern Mississippi.

Native US River Range

Northern map turtles also live in the Susquehanna River system of Pennsylvania and Maryland, and in the Delaware River.

Introduced Czech Population

A small introduced population exists in the Czech Republic, formed from escaped or released pet turtles and their descendants. In the Czech Republic, this species is considered alien, and it faces the same threats to survival as native Czech turtles, other reptiles, and amphibians from the large invasive population of red-eared sliders (Trachemys scripta elegans).

Habitat Preferences

Northern map turtles live in ponds, rivers, and lakes, preferring large bodies of water with fallen trees and other debris that provide basking sites. They are found more often in rivers than in lakes or ponds.

Range-Specific Habitat Use

In the northern part of their range, they occupy larger rivers and lakes, while in the southern and western parts, they are more commonly found in smaller rocky rivers and streams. Like all turtles, they require sunlight to survive.

Dormancy Period

Northern map turtles are dormant from November through April, with timing dependent on local climate conditions.

Hibernation Behavior

They spend the winter underwater and do not surface to breathe, especially when ice cover makes surfacing impossible. Adults rest on the water bottom or wedge themselves under rocks or logs, and often hibernate in groups with other northern map turtles, remaining somewhat active throughout the winter.

Hibernation Oxygen Requirements

Hibernation sites need to be well oxygenated, because unlike some other turtle species such as painted turtles, map turtles must absorb oxygen from the water to survive the winter.

Activity Patterns

They typically bask in groups and are diurnal, active only during daylight hours.

Anti-Predator Behavior

Northern map turtles are quite shy and difficult to approach; they usually slip into the water and hide at the first sign of danger.

Mating Season

Northern map turtles breed in spring and fall, with most mating occurring in deep water.

Nesting Season & Site Selection

The nesting season runs from May to July. Females strongly prefer unshaded, sandy soil sites, and typically select well-drained areas to deposit their eggs.

Nest Construction

Females dig the nest cavity with their hind feet.

Clutch Traits

Clutch size ranges from six to 20 eggs. The eggs are oval, around 3.2 cm (1.3 in) long, and have flexible shells.

Post-Laying Behavior

After laying, the female fills in the nest cavity.

Incubation & Hatchling Emergence

Incubation takes 50 to 70 days, and most hatchlings emerge between August and September. If a nest hatches late, northern map turtle hatchlings are known to overwinter inside the nest.

Female Clutch Frequency

A female usually lays two or more clutches in a single breeding season.

Temperature-Dependent Sex Determination

Offspring sex is determined by incubation temperature: incubation at 25 °C (77 °F) produces mostly males, while incubation at 30 to 35 °C (86 to 95 °F) produces mostly females.