Common Map Turtle
Pictured above: Map turtle head pattern (top right). Note the isolated spots behind each eye.
Description: Common map turtles, like Ouachita and False map turtles, are medium-sized turtles (usually bigger than painted turtles, but smaller than snapping turtles). There is a degree of size difference between males and females, with females having significantly larger carapace, or upper shell, lengths (7 to 10.5 inches) than males (3.5 to 6.5 inches). Their carapace also has blunt vertebral keels (unlike Ouachita and False map turtles, whose keels are prominent) and the back edge of their carapace is serrated. The carapace is usually olive-green to brown with darker blotches (pictured above). The plastron (lower shell) of these turtles is whitish-yellow with a distinct "road map" pattern along the margins (pictured below). The head, legs, and tails of most map turtles are olive or greenish with yellow lines. Common map turtles can also be distinguished from Ouachita and False map turtles by the small isolated oval or triangular spot located behind each eye (see above and below). In Ouachita map turtles, this spot is large and rectangular, while in False map turtles it is crescent shaped and not isolated. In addition to being much larger than males, females have broad jaws for crushing clams and snails (which are favorite food items). The individual pictured above is a female. Common map turtles, like all map turtles in Wisconsin, are members of the family Emydidae.
map turtles are generally found in mid-sized to large rivers with soft bottoms
and many areas for crawling out of the water to bask. They are
occasionally found in backwater sloughs or marshes of large rivers as
well. They have been seen near the La Crosse River in the Myrick Park
Marsh, and frequently are seen along the Mississippi River. As stated
previously, females have broad jaws designed for crushing clams and snails, or
crayfish. Males, on the other hand, are reported to eat other
invertebrates, such as insects and their larvae.
In general, the nesting season is a particularly dangerous time for female turtles of all species. Not only are they more exposed to predators, desiccation, and over-heating; they often traverse dangerous roads and highways to find a suitable site to build their nests. After finding a satisfactory spot, a female turtle digs a flask-shaped burrow with her hind feet and deposits 7 to 13 eggs (map turtle eggs are elliptical, not spherical like snapping turtle eggs; see below). Afterwards, she covers the eggs and returns to the water.
Common map turtles over-winter in the soft sediments of water that remains unfrozen, and well-oxygenated during the winter months. Vogt (1981) reports finding all three species of Map turtle over-wintering behind wing dams in the Mississippi River.
Vogt (1981) makes an interesting point when he explains that female Map Turtles are of particular benefit to humans because they consume many snails, that are often intermediate hosts for trematode parasites that infect, game fish, wildlife, livestock, and humans.
Remarks: If captured, Common map turtles will probably retract into their shells and remain that way until left alone. They are a very shy species that does not appreciate being disturbed by humans. I have encountered them on many occasions along back waters of the Mississippi River during the nesting season, and have seen several in the La Crosse River Marsh during 2001 and 2002. These turtles are relatively common (hence the name) and I believe that they exist throughout the waterways of the La Crosse area. However, I've seen large numbers of them crushed along highways during the nesting season by unobservant motorists. I have also found up to 40 nests destroyed by raccoons and opossums within an area of less than a square mile. Finally, I have found many juveniles killed by predators (mostly crows) as they attempted to reach the water after hatching, and the effects of these sorts of occurrences on map turtle populations has never been quantified.
These are shy, and rather pretty turtles. They are harmless, and defenseless against their only enemy (as adults): humans. To harm a turtle is truly a sign of ignorance an cruelty. Furthermore, the fact that motorists can't be bothered to watch the roads while driving and swerve even the tiniest bit to avoid smashing these turtles during the nesting season continues to stagger me.
Map turtle plastron (left), and head pattern (right).
Above: a map turtle nest which was being raided in the early morning by some sort of mammal. I scared off the critter upon my approach, and found this nest, with all of its' eggs still in place. Because they appeared to be in good shape, I replaced the eggs, and re-covered the nest with dirt. Unfortunately, many more of the nests I find look like the one pictured below: destroyed by raccoons or opossums.