(Le Sueur, 1817)
Common map turtle
Recognition
The olive to brown carapace (to 27 cm) has a distinct but low medial keel, a strongly serrated hind margin, and a reticulate pattern of fine yellow lines. The lower edges of the marginals are yellow with circular markings. Vertebrals are broader than long; the 1st is the narrowest, the 5th flared. The bridge is broad, lacks a hinge, and is posteriorly notched. The plastral formula is: abd > an > fem > pect >< gul > hum. The bridge is marked with light bars. The plastron is immaculate yellow to cream colored in adults, but in juveniles it carries a pattern of dark lines bordering the seams. The broad to moderate head has a nonprotruding snout, and the upper jaw lacks either a medial notch or hook. Skin is olive to brown, with yellow stripes. The postorbital mark is somewhat triangular and variable in size. Frequently the anterior end of one neck stripe turns upward across the tympanum, and a few always reach the orbit. The lower jaw is marked with longitudinal yellow stripes, of which the central stripe is the widest.
The diploid chromosome number is 50: 26 macrochromosomes and 24 microchromosomes (Killebrew, 1977a).
Adult males have long, thick tails, with the vent behind the carapacial rim. Females have broad heads, males somewhat narrower heads. In males the carapace is oval, tapering posteriorly; in females it is rounded. Adult males are 10-16 cm in carapace length, adult females 17.5-27 cm.
Distribution
Graptemys geographica ranges from southern Quebec and northwestern Vermont west to southern Wisconsin and, west of the Appalachians, south to Arkansas and Georgia. It also occurs in the Susquehanna River drainage of Pennsylvania and Maryland, and the Delaware River.
Habitat
Graptemys geographica most often frequents large bodies of water, such as rivers or lakes. Mill ponds, oxbows, and the overflow ponds of rivers often contain many individuals. Abundant submerged hiding places, some basking sites, and a rocky or gravel bottom are required. Rapid currents are avoided. Habitat preferences in a Pennsylvania river, as measured by frequency of capture, were as follows: deep, slow-moving areas, 52.9%; shallow areas, 2.3%; and riffles and other areas, 27.0%. Large adults were captured in deep, slow areas more often than expected and small turtles were collected in shallow, slow areas more often that expected. Large adults avoided areas with emergent vegetation, but congregated in areas of fallen limbs (Pluto and Bellis, 1986). In Kansas, the common map turtle is found exclusively in shady, streams with rock and gravel bottoms (Fuselier and Edds, 1994).
Natural History
Newman (1906) reported that none of the females that he found nesting was smaller than 19 cm or less than 14 years old. Females in Wisconsin are still immature at 10-12 years (Vogt, 1980). Courtship and mating occur in both spring and fall and involve active pursuit of the female by the male.
Nesting occurs from late May through mid-July with the peak in June. Most nesting is early in the day, usually before 08:00 hours. The nests are flask shaped, and may contain 9-17 (Gordon and MacCulloch, 1980; White and Moll, 1991) ellipsoidal eggs (32.4-37.4 x 17.9-24.7 mm) with parchmentlike shells. Females may carry as many as 24 enlarged follicles (White and Moll, 1991), and two clutches are laid each season. Eggs incubated at 25°C yield a preponderance of males, while those incubated at 30.5°C yield mostly females (Bull and Vogt, 1979; Ewert and Nelson, 1991). Hatchlings emerge in August-September or overwinter in the nest. A typical incubation period is about 75 days. Hatchlings are nearly round (30 mm) with a dorsal keel. The reticulate pattern of the carapace is bright, and the plastral pattern intense.
Graptemys geographica feeds primarily on freshwater snails and clams, but insects (especially the immature stages), crayfish, and water mites are also taken. Fish are occasionally eaten, usually as carrion, and some plant material is consumed.
These turtles are confirmed baskers, often piling up several deep while basking. They are, however, very shy and difficult to approach.
Remark
The relationship of G. geographica to other map turtles is not well-understood; it is the most divergent species. It probably is most closely related to the broad-headed group (barbouri, ernsti, gibbonsi, and pulchra).
IUCN Red List Status (1996)
Not listed.