Natural Notes: Turtles of Cook County

This blog post consists of my notes from a lecture on the natural history of turtles in Cook County, Illinois, delivered by Chris Anchor, wildlife biologist with the Forest Preserves of Cook County.  The lecture and following staff training was conducted at Thatcher Woods in River Forest, IL during the morning of August 29, 2018 to an audience of mostly Forest Preserves staff.  

Native Cook County Turtle Species:

• Common Snapping Turtle (Chelydra serpentina)
• Painted Turtle (Chrysemys picta)
• Spiny Softshell (Apalone spinifera)
• Stinkpot or Musk (Sternotherus odoratus)
• Box Turtle (Terrapene carolina)
• Map Turtle (Graptemys geographica)
• False Map Turtle (G. pseudogeographica)
• Red-eared Slider (Trachemys scripta)
• Spotted Turtle (Clemmys guttata)

Snapping Turtle

Figure 1:  Common Snapping Turtle displaying its jaws

Snapping Turtles have jaws that crush prey rather than cut them (figure 1).  Males frequently wander to look for females to mate with.  They eat mostly live prey, but can also scavenge.  They often can eat enough at once to survive a month or more without food, as their metabolisms are quite slow.  Snapping turtle sexes can be distinguished by carefully picking up a mature turtle and rocking it side-to-side, which mimics their copulation movements.  If there is a protrusion from the cloaca, it is a male.  

Painted Turtle
Painted turtles feed mostly on aquatic invertebrates.  They are also a vector for various parasites. 
The sex of painted turtles can be best distinguished by behavioral observation.  Males will try to impress females by rubbing their front feet against each other or wiggling them. 

Red-eared Sliders
Red-eared sliders are mostly likely native to southern Illinois but have advanced northward over the last 100 years, likely due to people dumping them into local waterways.  They occupy the same habitat as painted turtles, often out-competing them.  Sliders and snapping turtles are also more tolerant of water contamination than painting turtles, and as such are often used as biomonitors since they absorb many types of toxins.

Spiny Softshell

Figure 2:  Spiny softshell turtles have spine-like
protrusions from the side of their carapace.  Photo credits:  LA Dawson

Spiny Softshells are common in rivers and slow moving streams and are very aggressive.  Softshell turtles have very sharp, scissor-like jaws, and their head can reach almost all the way back, close to their tail.  These animals should be handled with extreme caution, and must be held as far back on the shell as possible.  Their name is based on the small "spines" visible near the front end of their carapace (figure 2). 


Softshell turtles are sexually dimorphic, with males a much smaller size than females.  This size difference allows the males to easily mount the females for copulation.  The feeding habits of males and females consequently are also quite different, with males feeding primarily on small invertebrates while larger females feed on fish, shellfish, and mollusks.  Both males and females will also consume algae. 

Stinkpots (Musk Turtles)
Musk turtles are extremely common in muddy and murky pond and river bottoms, and generally are submerged underwater except to come up for air.  Musk turtles can often release an unpleasant odor. 

Spotted Turtles
Spotted Turtles populations as well as many other species of turtles in the genus Clemmys are in a steep decline across their entire range.  They are extremely rare in Cook County, and the source of their decline has been primarily habitat destruction, predation by meso-predators, and illegal collection by humans. 

Box Turtles
Box Turtles are native to southern Cook County and the Lake Michigan Shoreline, but are now found throughout Cook County.  Box turtles do best in areas with sandy soils where they can easily dig below the frost line to overwinter.  Box turtles cannot overwinter in lowland areas with shallow soils since they will freeze to death during periods of frosts and hard freezes. 

Map Turtles and False map Turtles
These two closely related species frequently hybridize.  Both are native to cook county but are extremely rare.  They are species of river turtle. 

Alligator Snapping Turtle (Macrochelys temminckii) & Non-native Turtles
Alligator snapping turtles are falsely observed by residents in Cook County, and other than pets that are illegally dropped into local waterways, are only found in extreme-Southern Illinois.  Alligator snapping turtles are a subtropical turtle that cannot survive the harsh winters of Northern Illinois.  There are currently state-level efforts to restore their depleted populations in southern Illinois, but predation by River Otters (Lontra canadensis) has made recovery efforts challenging.  Forest Preserve field biologists have observed over 20 species of non-native turtles and other reptiles over the past 30 years in Cook County that have been illegally deposited, most being tropical or subtropical species that do not survive through winter. 

Determining Turtle Age

Figure 3:  Turtle scutes with growth rings.  Photo credits:  Jonathan Zander 

After about 6-8 years, growth rings are discernible underneath some of the scutes (figure 3).  Using a microscope, the number of rings can be used to estimate age.   As the turtle ages the rings become more difficult to discern.  At this point measuring the length of the turtle can be used to estimate age, and the growth rate of turtles living in warmer climate zones with longer growing seasons is higher than those of cooler climates.  Turtle growth ceases during their overwintering period, and slows considerably with age on most species. 

Improving Turtle Habitat in Cook County
Turtles have been on a decline in Cook County.  Cook County has some of the highest populations of meso-predators (mid-level predators such as raccoons) in the state, which prey upon turtles and turtle eggs.  Turtles require areas with abundant sunshine, and invasive woody species like Buckthorn have restricted available sunlight to trails and right-of-ways, where eggs from egg-laying females often succumb to predation.  Lack of prescribed burns has reduced the duff layer that turtles need in order to dig to lay eggs.  Removal of invasive species as well as top-soil restoration through prescribed burns has helped to improve turtle offspring success in Cook County. 

Turtles as Biomonitors
Compared with more recently evolved mammals, turtles have very active immune systems that fight off most diseases, and turtles are not affected by mammalian pathogens, though they often are a vector.  Turtles absorb many toxins as well, and drawing blood samples can often reveal environmental problems.  Turtles also have closely-associated parasites such as certain species of leeches, and parasite associates are a topic of research of their own.