Creature Feature: Map Turtles (Graptemys species)

Common (Northern) Map Turtle - Photo D. Gordon/E. Robertson

Common (Northern) Map Turtle - Photo D. Gordon/E. Robertson

Common (Northern) Map Turtle - Photo D. Gordon/E. Robertson

Common (Northern) Map Turtle – Photo D. Gordon/E. Robertson

We were looking at turtles basking along Darby Creek and something wasn’t clicking. The kids were asking about the turtles, and my mind was drawing a blank. I was used to Painted Turtles and Sliders (both common in ponds in the Midwest), but these flat olive turtles didn’t look right. When we tried to edge closer for a better look, they quickly dove into the stream. Definitely unlike the fairly human-tolerant Painted Turtles I had seen in many ponds in Ohio, these turtles also appeared to have a ridge (or keel) along the midline of their shell. Good thing that I didn’t try to shoe-horn them into my concept of common pond turtles. These were Map Turtles, and they were a quite different beast.

Map Turtles take their name from the parallel curving yellow lines that decorate their shells, necks, and legs, said to resemble the elevation lines on topographic maps. These vermiculations are bright yellow in young animals, but fade a bit with age, especially on the dome of the shell, so they don’t make a good field mark unless you are very close to the turtle. The shape of the shell – flattened at the sides and back, with a pronounced ridge running down the center – is often a more helpful field mark from a distance. This flattened shell gives the appearance of streamlining, and that clues us in to the fact that these are very active aquatic turtles.

Map Turtles could just as easily be called Wary River Turtles. These turtles are more commonly found in rivers and streams than Painted Turtles, and they are faster, more adept swimmers. They dive under water to swim and crawl along the stream bottom looking for clams, snails, bugs, and crawfish, which they grab with their jaws. Their “lips” are often thickened and hardened to enable them to bite and crush shells. This more predatory existence fuels a slightly faster metabolism, and these turtles are faster moving and more easily spooked than pond turtles. They also hibernate less, staying active often even after creeks and sloughs freeze over in the fall, and forage under the ice.

As is the case with many other predators, Map Turtles are sexually dimorphic. The males are quite a bit smaller than the females, averaging 4-6 inches in length to the females’ 8-12. This helps to reduce the competition for food in the same stretch of stream. It doesn’t, however, imply that the male helps out at the nest. Like most turtles, it’s all on the female – after mating, she has to leave the water to seek out a site to lay her eggs, burying them in moist sand and leaving them to fend for themselves. Her nest site selection is also a form of sex selection of her offspring. Like many other turtles, Map Turtle gender is determined by the temperature of the nest. Colder temperatures (less than 80 F) incapacitate an enzyme that converts testosterone to estrogen, and most of the offspring become male. Warmer temperatures skew the brood to wards females. Possibly this system evolved to calibrate the sex ratio to food availability: warmer weather indicates more food, which helps the larger females survive and reproduce.

Ouachita Map Turtle - Photo Josh More

Ouachita Map Turtle – Photo Josh More

Map Turtles’ unusual habits have made them diverse – and threatened. Here in Ohio we have two species: the Northern Map Turtle (G. geographica) and the Ouachita Map Turtle (G. pseudogeographica). The former is over much of central and western Ohio, while the latter is mostly in the lower Scioto River drainage. As you move south, their diversity grows, and many different river systems in southern states have their own different Map Turtle species or subspecies. Many of these species are adapted to prey on specific clams and mussels, and the turtles have become scarce as river pollution and channelization has decimated their mollusk food source. Perhaps we had more map turtles before agriculture and industry altered Ohio’s rivers. Our current two species seem to have stable numbers, and you can test your ID skills on them along most of our central Ohio streams. That is, if you can get close to them.