Graptemys is a genus of freshwater turtles containing 14 species, commonly known as map turtles. Graptemys are small to medium-sized turtles that are sexually dimorphic, with females attaining as much as twice the length and ten times the mass as males in some species. Depending on the species, adult males range from 7–16 cm (2.75–6.25 in), adult females 10–29.5 cm (4–11.62 in), and hatchlings 2.5–3.8 cm (1–1.5 in),: 202–209 p. although some sources indicate female Barbour's map turtles (Graptemys barbouri) grow to 33 cm (13 in) in length. Most species have a distinctive dark pigmented keel that is often notched or serrated running down the center of the carapace and serrated scutes on the rear margin. The head, neck, and limbs exhibit bold patterns of yellow (occasionally orange or red) lines and spots against darker green, olive, or black base colors. The patterns on the head can be important characters in identifying the various species. The common name "map turtle" is derived from the intricate patterns on their shells that are suggestive of topographical maps, although the patterns are more apparent in some species than others, and often become obscure in older specimens. Some species are occasionally called "sawbacks", in reference to the serrated keels on their shell.
They are endemic to North America, where most species occur in the rivers of the Gulf Coast of the United States, although three species are more wide-ranging, dispersed throughout the eastern two thirds of the greater Mississippi River basin, into the Great Lakes region, and east to New York and southeast Canada. Most Graptemys are loctic, river turtles, having a preference for moving water and larger lakes, and with long legs, broad feet, and long fully webbed digits they are well adapted for swimming in currents. They primarily feed on fresh water mussels, clams, snails, insects (including larva and eggs), bryozoans, sponges, algae, and various vegetation. Fish only appear in the diets of a few species and then only in a relatively small percentage of the diet. Within this spectrum of food items there is significant niche partitioning among the sexually dimorphic males and females, and microcephalic, mesocephalic, and megacephalic species occurring in the same river drainages. Like all turtles, map turtles are oviparous, typically laying eggs from late June to August. Females lay two to 15 eggs per clutch and depending on a number of variables such as species, size, and age among other factors, may skip a year between clutches, or lay as many four clutches a year.
Seven of the 14 species are listed as either near threatened, vulnerable, or endangered by the IUCN Red List as of 2021, and two species, the yellow-blotched map turtle (G. flavimaculata) and the ringed map turtle (G. oculifera) are listed as threatened by the U.S. Federal Government.
Map turtles are endemic to North America. The genus ranges from the Great Lakes region and St. Lawrence River of southern Quebec and Ontario, Canada, south to the Gulf of Mexico, and the Hudson River and Delaware River basins on the Atlantic coast, west to the eastern margins of the Great Plains. Most species occur in rivers, moving waters, or larger lakes. Eleven of the 14 species have relatively limited distributions, restricted to river basins draining into the Gulf of Mexico, in the US states of Texas, Louisiana, Mississippi, Alabama, southwest Georgia, and the Florida panhandle. Three species are more wide ranging, include the Ouachita Map Turtle (G. ouachitensis) and the False Map Turtle (G. pseudogeographica) which range extensively in the Mississippi River drainage and its tributaries including much of the mid west, with the latter also ranging west into east Texas. The Northern Map Turtle (G. geographica) is the most widespread species, occurring in both Atlantic and Gulf of Mexico river drainages, from southern Quebec, Canada, west to Minnesota and eastern Kansas, south to New Jersey and above (north) of the fall line in Georgia, Alabama, and extreme northern Louisiana.: 202–209 pp.
Map turtles are predominantly lotic, living in moving water, such as rivers and larger creeks, streams, and bayous. The northern and wide-ranging Mississippi River species (G. geographica, G. pseudogeographica, G. ouachitensis) tend to inhabit more diverse habitats, including sloughs, oxbow lakes, and backwater areas of river bottoms, as well as lakes and occasionally even ponds and marshlands. Other species use these environments much less frequently, or as juveniles or during seasonal flooding, and a few species have never been reported from areas beyond the main channels of their respective river systems (e.g. G. gibbonsi, G. pearlensis, G. sabinensis). Several species will inhabit manmade reservoirs in their river systems, while other species are very uncommon in such impoundments (e.g. G. oculifera, G. versa), and still others are entirely absent from reservoirs (e. g. G. caglei). With few exceptions (G. barbouri, G. flavimaculata, G. nigrinoda), most species do not occur in tidally influenced waterways, estuaries, or brackish waters, which are occupied by their closest relative the diamondback terrapin (Malaclemys terrapin). The upstream distributional limits of many species are often associated with the level of sunlight penetration through the forest canopy. Larger, wider waterways allow sunlight to reach the surface, providing for the growth of aquatic vegetation and algae (food) and basking sites for Graptemys populations. Smaller streams in the shadow of forest canopies, or that receive brief patchy sunlight, are rarely inhabited by Graptemys.: 84–87, 255–360 pp. : 272–343 pp.
The ecology of many species of map turtles, particularly habitat use and diet, have been well studied. Specific habitat use often differs among males and females of the same species, as well as sympatric species sharing a river system, all inextricably linked to their diets in a somewhat complex example of niche differentiation. Ten of the 14 Graptemys species share a river basin with another Graptemys species, and three species occur in regions of Alabama, and portions of the Mississippi River. Generally the mesocephalic and megacephalic females occupy deeper areas with stronger currents and hard-bottom, limestone sections of rivers, and males inhabit slower, shallower areas of the rivers nearer to the shore, largely correlated with their respective diets and prey distribution. Graptemys rarely walk on land or move between waterways. Most species are reluctant to bask on river banks and are seldom seen even on fallen trees with one end resting on the bank. Basking sites are most often fallen tree and deadwood emerging from water with no direct contact to the banks. Although the wide-ranging, northern species (G. geographica, G. pseudogeographica, G. ouachitensis) tend to be more flexible in their basking habits.: 84–87, 255–360 pp. : 272–343 pp.
The natural meandering of rivers, riparian forest, and fallen trees and deadwood in the water are all key and vital elements to sustain most map turtle populations, particularly the Gulf Coast species. The outer bend in a river is typically the deeper side, with the stronger current of the main channel, eroding and cutting into the outer bank and falling trees of the riparian zone forest as it does. While the inside of the bend is shallower, with a slower current, dropping silt deposits and building beaches and sandbars. The meandering course maintains a diverse spectrum of water depths, temperatures, currents, soft and hardpan bottoms, as well as fallen trees and logs that get lodged in the bends. Beaches and sand bars are essential nesting sites. The presents of partially submerged deadwood and trees are critical, not only for basking, but for the growth of algae, bryozoans, sponges, snails, insects and their eggs larva that live in the growth, which are important food items for turtles.: 84–87, 255–360 pp. : 272–343 pp.