New published research: Reproductive ecology of southern Spotted Turtle populations

A female Spotted Turtle with radio transmitter and thread spool attached – Houston Chandler

When I joined The Orianne Society all the way back in 2016, the first project that I spent considerable time working on was a radio telemetry study examining the ecology of Spotted Turtles in Georgia (Chandler et al. 2019).  For such a charismatic species, little research had been conducted at that point on populations at the southern end of their distribution, with the notable exception of Dr. Jackie Litzgus’ work in South Carolina (e.g., Litzgus and Mousseau 2004).  Fast forward six years later and that narrative has started to change.  With ample help from our many partners, we have now completed a variety of projects with Spotted Turtles in the southeast, including the above mentioned telemetry study, a study of their thermal ecology (Chandler et al. 2020), several years of population monitoring in Georgia, and multiple years of surveys attempting to identify new populations in Florida, Georgia, and South Carolina.

In addition to those projects, we recently had a paper published in the journal Ichthyology & Herpetology that describes a multi-year study of the reproductive ecology of southern Spotted Turtle populations (Chandler et al. 2022).  This work was inspired by observations of Spotted Turtles laying multiple clutches of eggs, including one instance of triple clutching, during a single breeding season in South Carolina (Litzgus and Mousseau 2003).  However, since those observations were published no additional work had been completed to assess these and other reproductive behaviors in the southeast.  We set out to change that, mostly focusing on data collected in Georgia but also highlighting some key observations from Florida.

Following Spotted Turtles around can only be described as a dirty job – Ben Stegenga

One of the primary reasons for a lack of data on Spotted Turtle reproduction in the southeast (and really Spotted Turtle ecology in general) is that turtles in these populations are incredibly difficult to study.  They are hard to catch and challenging to track through often large, mucky swamps even with radio transmitters attached.  Not to mention locating nests where almost any bare ground or rotting log could be suitable nesting habitat is a challenging endeavor at best.  Despite these challenges, we initiated our project in 2018 with four main goals: 1) monitoring the timing of the reproductive season, 2) quantify clutch sizes, 3) identify the frequency of multiple clutching events, and 4) to measure nest survival rates.

The first part of this project was reasonably straightforward — catch as many female turtles as possible and attach radio transmitters so that we could reliably locate them throughout the breeding season.  Several years of trapping at two monitoring sites helped with this endeavor, and before long we had a group of female turtles with radio transmitters attached.  Next, we had to figure out how to reliably identify when turtles developed eggs.  It’s easy to count shelled eggs via x-ray imagery (this is the preferred method for small turtles), but how to tell when to take turtles in for x-rays?  We settled on attempting to palpate turtles by sticking a finger into the shell cavity, just above the hind leg and gently feeling for eggs in the body cavity.  I say attempting here because Spotted Turtles in these populations are often so small that they can be difficult to reliably palpate.  With some trial and error, we eventually figured out a mostly reliable technique to at least palpate some of the individuals included in the study.  When we detected shelled eggs via palpation in one or more turtles, we rounded up all individuals and took them in for x-rays to get reliable clutch size estimates (a big thanks to our friends at the Georgia Sea Turtle Center and Oatland Island Wildlife Center for taking the x-rays).

The largest single clutch documented during our study and only observed once.

Now came the hardest part, how do we find Spotted Turtle nests?  Because of the distribution of our turtles and the likelihood that nests would be laid in many dispersed locations at more or less the same time, we did not think we would be able to reliably observe nesting in the field as has been done in other studies.  The next best thing seemed to be attaching a thread spool to each turtle’s shell so that we could follow their movements the following day.  Thus, after determining which turtles were gravid via the x-rays, we began a daily routine of locating, weighing, and palpating turtles to determine whether or not they had laid eggs the previous day.  If we suspected a nest had been laid, we removed the thread spool and began retracing the turtle’s movements, looking for any place where the string passed through potential nesting habitat.  This process was often as tedious as it sounds.  My first time in the field with a turtle that I knew had nested the previous evening, I came up empty.  The string must not have gotten buried in the nest. Despite this first disappointment (and some trepidation that it wasn’t going to work), we eventually found several nests using these techniques.

A Spotted Turtle nest with the string from a thread spool passing through it – Ben Stegenga

We implemented these general methodologies at two sites in Georgia during 2018 and again at a single site in 2020, although Covid-19 complications ended up impacting data collection during 2020.  We supplemented these two field seasons with some observations from 2016 in Georgia and other observations from Florida.  So what did we find?  Well to answer the most exciting question first: turtles frequently developed and laid more than one clutch of eggs during a single breeding season.  Across the two breeding seasons, 80% of females produced multiple clutches per year, with six individuals triple-clutching during 2018!  Clutch sizes ranged from 1–4 eggs (mean = 2.1), and clutch number appeared to have a negative effect on clutch size (i.e., 3rd clutches were smaller than 1st or 2nd clutches).  Furthermore, this was the first time that single egg clutches have been documented in wild Spotted Turtles.  The timing of these reproduction events occurred over an approximately 3-month period from May to July.  Interestingly, gravid female turtles were detected in Florida in late March, which is almost a month earlier than the earliest detections in Georgia.

Timing of Spotted Turtle nesting events during 2018 (above line) and 2020 (below line). Black dots represent times when turtles were first identified as gravid and gray points represent nesting dates. – Houston Chandler

We were able to locate a total of 24 Spotted Turtle nests across the two breeding seasons (approximately 50% of the total nests laid).  Nine of these nests either hatched or partially hatched (37.5%), producing 17 hatchling Spotted Turtles.  Other nest fates included predated (41.7%), unhatched due to other factors (8.3%), and unknown (12.5%).  We observed both armadillos and raccoons predating turtle nests.  Nests were laid in a variety of substrates, including loose soil and leaf litter, sphagnum moss clumps, rotting logs, and raised hummocks containing thick grass clumps.  On multiple occasions, turtles failed to completely cover eggs.  Female turtles often made relatively long movements (100s of meters) to lay eggs before returning to their normal activity areas.

Spotted Turtle egg left mostly exposed after the female turtle was finished nesting – Houston Chandler

Overall, the results of our study indicate that multiple clutching is a common reproductive strategy employed by Spotted Turtles at the southern end of their range.  Producing multiple clutches increases overall reproductive output, which is especially important when considering how small individual clutches are in these populations (restricted by small body size in adults).  This is also a form of bet-hedging where the risks of predation are spread across multiple nesting events.  A longer activity season and multiple clutching allows turtles in these population to produce more eggs per year than larger individuals laying larger clutches in northern populations.

Our work with Spotted Turtles is likely winding down in many respects, with most of our effort now focused on annual population monitoring.  The reproductive data described here, along with our mark-recapture data, will eventually be used to better understand the population dynamics of southern Spotted Turtle populations.  This project was full of challenges but was incredibly rewarding to complete and generate a snapshot of rarely observed behaviors.

You can find the full publication on our Publications page or at the links below.

 

Literature Cited

Chandler, H.C., B.S. Stegenga, and D.J. Stevenson. 2019. Movement and space use in southern populations of Spotted Turtles (Clemmys guttata). Southeastern Naturalist 18:602–618.

Chandler, H.C., B.S. Stegenga, and D.J. Stevenson. 2020. Thermal ecology of Spotted Turtles (Clemmys guttata) in two southern populations. Copeia 108:737–745.

Chandler, H.C., B.S. Stegenga, and J.D. Mays. 2022. Compensating for small body size: The reproductive ecology of southern Spotted Turtle (Clemmys guttata) populations. Ichthyology & Herpetology 110:268–277.

Litzgus, J.D., and T.A. Mousseau. 2003. Multiple Clutching in Southern Spotted Turtles, Clemmys guttata. Journal of Herpetology 37:17–23.

Litzgus, J.D., and T.A. Mousseau. 2004. Home range and seasonal activity of southern Spotted Turtles (Clemmys guttata): Implications for management. Copeia 2004:804–817.

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