Tracking Bog Turtles in Tennessee

Timothy Calhoun

Masters Student at the University of Tennessee

November 2022

A 3-year-old Bog Turtle being held in a baseball cap – Timothy Calhoun

A major issue impacting wildlife today is loss of habitat as land is degraded or fragmented by human development. This problem is particularly dire for habitat specialists that have few options when a site is destroyed. One such species is the Bog Turtle (Glyptemys muhlenbergii), North America’s smallest freshwater turtle species. These little turtles are mostly found in mountain wetlands through the Appalachians. They like areas with a deep layer of soupy mud with a thin layer of very slow-moving water on top, interspersed with grass or sedge tussocks and sphagnum moss patches. 

A landscape view of a wet meadow – Timothy Calhoun

There’s only one area in the state of Tennessee known to have Bog Turtles, a handful of wetlands in one valley in the northeast corner of the state. In an effort to conserve this species, expansive wetland restoration efforts have been conducted in this area. Over the last few decades, wetland habitats have been expanded from just two acres to an estimated two hundred acres. This should be good for a turtle that specializes in Appalachian wetlands, but bog turtles like to hide in the mud where we cannot see them easily to know for sure. This is where my Masters project at the University of Tennessee comes into the picture.

Grant recipient, Timothy Calhoun in the field with a Bog Turtle – Alex Wyss

To get an idea of how the wetland restoration activities have benefitted the Bog Turtle population on the site, I needed to get at two things. First, what does the Bog Turtle population look like? Are there more turtles on the site than there used to be, and are there young turtles (a sign of successful reproduction) as well as old turtles (a sign of a safe site where turtles are surviving) in the population? Second, where are the turtles spending their time? Are they moving into the restored wetland areas or just sticking to the historic suitable habitat?

A Bog Turtle trap in a wetland, covered by vegetation to protect any trapped turtles – Timothy Calhoun

To answer these questions, I needed to get my hands on some turtles. With help from Zoo Knoxville, I placed live traps throughout the wetland along natural pathways between plant tussocks, so individuals would walk into the trap without bait. I also walked around the wetland with two dowel rods, poking into the mud to find turtles by tapping on their shells. Once I caught a turtle, I weighed each one and measured the dimensions of the shell, then used a file to notch the sides of the shell in a unique placement for each individual. I also drew the pattern of yellow-orange blotches around the head and neck of the turtle to help with future identification, as these are unique among adults.

This unique identification is important to figuring out how many turtles are at the site. By counting how many turtles I catch and mark in one time frame, then counting how many already marked turtles and new unmarked turtles I catch in the next time frame, I can get a rough estimate if what proportion of the total population I’m catching each time. This concept forms the basis of the Spatially Explicit Capture-Recapture (SECR) framework I am using to analyze my capture data from the summer. SECR adds information about where the turtles were when I caught them, as well as the relative spacing of my traps to each other, to refine the population estimate.

A bog turtle trying to escape during data collection – Timothy Calhoun

Looking at the “where” of my captures also helps in figuring out if the turtles are using the expanded wetlands. Similar to using proportions of marked captures to estimate population size, I can use a proportion of captures within the different areas to get an idea of habitat use. Recaptures in different traps can add to this by giving movement information between different areas of the wetland. However, recreating movements just from trap captures offers an incomplete picture of turtle movement at best, especially with a difficult to capture species such as the Bog Turtle. So, to get more complete information on individual movement and home ranges, I attached radio transmitters to six turtles.

A Bog Turtle with a radio transmitter attached to the shell – Timothy Calhoun

To keep track of these radio tagged turtles, I set up 50 passive radio receivers in a grid throughout the site. Whenever these receivers picked up the signal from the radio tags, they recorded the time and the signal strength, which gives an idea of how far from the receiver the turtle is. Each receiver then sends this information back to a central sensor station, which uploads the data for remote access. When multiple receivers pick up the turtle at the same time it is possible to use the signal strength information to triangulate the exact location of the turtle within the wetland. With these receivers constantly listening, this gives very fine scale information about where the turtles are moving and what space they are using.

Unfortunately, due to supply chain issues, I was not able to deploy the radio transmitters until July. Bog Turtles are expected to be most active in May when males are looking for mates, and June when females are looking for nest sites. Getting the tags out so late means I did not get a good idea of home ranges from this last summer. However, the tags have a battery life of about two years, so right now they are collecting data on hibernacula selection and are in place collect information next spring and summer!

The central sensor station collecting the turtle movement data – Terry Cook

I still have a lot of in-depth analyses ahead of me before I am finished with this project, but I can share some of the preliminary results here. I captured several different male and female adults, many of which were old enough that their shells were starting to wear smooth, as well as juveniles ranging from 3 to 9 years old. Thus, there is evidence this population is successfully reproducing, and the diverse age structure and sex ratios bode well for the health of the population. I also caught an individual in the middle of the restored wetland area, then caught that individual again two weeks later approximately 250m away, back in the historic habitat. The turtles here do not seem to be frequently using the new habitat but do appear to be checking it out.

This site contains the only population of Bog Turtles on protected land in Tennessee. As I pull more results from the data collected this summer, this project will provide a more complete understanding of how bog turtles are doing in this state. Getting this important data would have been much more difficult without the small grant funding from The Orianne Society.

A Bog Turtle in hand – Timothy Calhoun