TURTLEPOP
Population Structure of Freshwater Turtles along an Urbanization Gradient
An Ecological Research as Education (EREN) Pilot Project
David R Bowne
Assistant Professor
Elizabethtown College
717-361-1317
bowned@etown.edu
Background
Researchers commonly find skewed age and sex ratios in freshwater turtle species (Steen
and Gibbs 2004, Freedberg and Bowne 2006) and these findings are often attributed to
anthropogenic changes in the landscape (Marchand and Litvaitis 2004). A high concentration of
roads near turtle nesting sites may, for example, selectively increase mortality of adult females,
leading to a male-skewed population. Increased predation of nests caused by a high density of
predators that flourish in human-dominated landscapes (e.g. raccoons, opossums) may reduce
juvenile recruitment and cause an adult-skewed population. As of now, each of these statements
has not been rigorously tested due in part to the spatial restrictions of the few studies that have
been conducted. In the TurtlePop project, members of EREN will conduct turtle sampling in
lentic habitats on or near campus in order to determine the population structure of turtles across
an urbanization gradient.
We will test the following primary hypotheses:
1. The secondary sex ratio of turtles will be more male-biased as urbanization increases.
Urbanization will be measured by road density, human population size, and habitat
types within an area scaled to the movement range of the turtle species.
2. The population age distribution will be more biased towards adults as urbanization
increases.
Significance
The turtle species that we study in this project may serve as model species for the
conservation of other freshwater turtles. Worldwide, roughly 45% of freshwater turtle
taxa are listed as threatened in the 2000 IUCN Red List of Threatened Species (Turtle
Conservation Fund 2002). By better understanding how common freshwater turtles respond to
changes along an urbanization gradient, one can devise improved conservation strategies for the
many imperiled freshwater turtle species.
Methods
We will test these hypotheses by conducting a mark-recapture study of freshwater turtles.
The specific species of turtle studied will depend on geographic location and habitat sampled,
but the hypotheses are valid for any species. Because of the added difficulty and danger of
handling, measuring, and marking snapper turtles (Chelydra serpentina) and alligator snapping
turtles (Macroclemys temminckii), we will exclude them from the study, but will still record
when they were caught. We will catch turtles in late summer or early fall (end of August through
September) using nylon hoop traps baited with sardines (Bowne et al. 2006). The exact date of
trapping is not as important as having suitable environmental conditions – minimum water
Bowne, TurtlePop v2.0, 8 February 2016
temperature should be above 15°C (Sexton 1965). Each trap will be set, checked for two
consecutive days, and then removed. See Appendix A for detailed instructions of setting and
checking traps. On each day, every turtle captured will be measured (length of carapace,
plastron, right middle foreclaw, and pre-cloacal tail), sexed, and marked (notches filed into edge
of carapace). See Appendix B for more detailed instructions on handling and marking turtles. If
checking the traps for two consecutive days is not possible, traps could be set, checked and
removed the next day, and then repeated the following week. We need at least two trapping
sessions close in time in order to meet the closed population assumption of population estimation
models (Davis and Winstead 1980). A datasheet for collection of turtle data is given in Appendix
C.
In addition to the direct turtle data, we will also collect the following information
characterizing each pond and surrounding landscape (Marchand and Litvaitis 2004). Please see
Appendix D for additional details.
Pond:
 Latitude and longitude (in decimal degrees) of center of each habitat
sampled (coordinates can be obtained from Google Earth)
 Area of pond
 Dominant habitat substrate type (inorganic or organic)
 Amount of basking sites
 Permanence (Does the pond seasonally dry or does it typically hold water
year-round)
Landscape characteristics:
 Area of land use (forest, wetlands, open water, impervious surface,
agricultural, developed (e.g. lawns), and early successional) within 30 m
of pond perimeters
 Density of road network within 100, 250, 500, 1000, and 2000 m.
 Area of land use (same as above) within 100, 250, 500, 1000, and 2000 m.
 Human population size within surrounding U.S. census block groups
 If one cannot measure these variables with a local Geographic Information
System (GIS), then inform me and I will calculate them.
Pedagogy
The proposed research works well as a stand-alone laboratory exercise, but can be greatly
expanded as more data are collected at more locations. For the past three years, I have used the
turtle trapping lab as a means to teach population estimation techniques through mark-recapture.
Students use a single year’s data to estimate population size using the Lincoln-Petersen Index,
which is explained in most general ecology textbooks. Population estimation requires recaptures,
which does not always happen. I use this occurrence as a “teachable moment” to discuss the
assumptions of the estimation technique. I then supply data from a previous year so that they can
work out the math.
I usually conduct this lab as the first of the fall semester in a General Ecology course
because it’s an effective team building exercise (students work together in checking traps and
measuring turtles), sets the tone of ecology as a quantitative science, and is usually enjoyable.
One can make the lab more inquiry-based by first conducting a literature search on what turtle
species live in the given geographic region, and then pose a hypothesis as to which species will
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live in the sampled water body. The lab also lends itself to a discussion of conservation biology
given the many species of threatened turtles (Turtle Conservation Fund 2002).
I have created a stand-alone laboratory exercise that uses this project to meet the
following student learning objectives:
1) Perform a basic technique (mark-recapture) to survey a wildlife population
2) Estimate size of a wildlife population from the mark-recapture study using the LincolnPetersen Index.
3) Identify basic external turtle anatomy.
4) Identify turtle species and sex
5) Handle and measure individual turtles.
The exercise can be found on the EREN website: http://erenweb.org/project/turtle-pop-project/.
Logistics
The project requires a substantial time commitment on the part of the faculty since it is a
minimum of three days (set, check, check and pull traps) per season. Working with vertebrates
requires Institutional Animal Care and Use Committee (IACUC) approval. This approval is
usually not difficult to obtain but does require paperwork. I can help with it. Working with wild
vertebrates also requires a scientific collector’s permit from the state. Again, obtaining this
permit is relatively easy but does require time and usually a small fee. A breakdown of costs for
traps and other equipment is given in Appendix E.
Timeframe
Data of these nature have been published after only one field season (Steen and Gibbs
2004), but I would like at least two years worth of data to make sure that results are not artifacts
of environmental conditions of a single year. Having at least two years worth of data will also
increase the accuracy of our population estimate. If participants are able to collect annual data for
many years, we can then ask additional questions about individual turtle growth rates, changes in
age distribution over time, and annual variation in recruitment.
 Academic year of 2011-2012: Obtain IACUC approval and applicable state permits.
Acquire equipment, test protocols in courses
 September 2012 and 2013: Mark-recapture studies of turtles.
 Academic year of 2012-2013: Collect data on landscape characteristics of each site.
 January 2014: Complete data analysis
 April 2014: Rough draft of manuscript
 July2014: Submission of manuscript
Literature Cited
Bowne, D. R., M. A. Bowers, and J. E. Hines. 2006. Connectivity in an agricultural landscape as
reflected by interpond movements of a freshwater turtle. Conservation Biology 20:780791.
David, E.D. and R.L. Winstead. 1980. Estimating the numbers of wildlife populations. Pages
221-245 in S. Schemnitz, editor. Wildlife Management Techniques Manual, 4th edition.
The Wildlife Society. Washington, D.C.
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Freedberg, S. and D. R. Bowne. 2006. Monitoring juveniles across years reveals non-Fisherian
sex ratios in a reptile with environmental sex determination. Evolutionary Ecology
Research 8:1499-1510.
Marchand, M. N., and Litvaitis, J. A. 2004. Effects of habitat feature and landscape composition
on the population structure of common aquatic turtle in a region undergoing rapid
development. Conservation Biology 18:758-767.
Sexton, O.J. 1965. The annual growth and shedding in the Midland painted turtle (Chrysemys
picta marginata). Copeia 1965:314-318.
Steen, D. A., and J. P. Gibbs. 2004. Effects of roads on the structure of freshwater turtle
populations. Conservation Biology 18:1143-1148.
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Appendix A: Site selection, setting and checking hoop traps
Any lentic habitat that supports freshwater turtles (other than snapping turtles) and
contains sufficiently shallow areas to allow the setting of traps (~0.6 m deep, see below) is
suitable for this study. Larger water bodies, however, are more difficult to adequately sample
with a limited number of traps and time available. Therefore if more than one aquatic habitat is
available, I suggest selecting the smaller one. If the water body is less than 2 ha (~5 acres),
distribute the traps equally across the ecosystem. If you have a water body greater than 2 ha, then
cluster your traps in a smaller area so that each is about 20 m apart in order to increase the
probability of recapture. In all cases, one should conduct preliminary trapping to determine what
turtle species are present and if they are present in sufficient numbers – we want more than ten
individuals (see below).
At Elizabethtown College, a large (1 ha) and small (.25 ha) pond are present on campus.
Snapping turtles live almost exclusively in the larger pond and painted turtles live in the smaller
one. My students hypothesize as to why this pattern exists but for the EREN study, I am only
including turtle data from the smaller pond. Direct anthropogenic impacts on turtle species (i.e.,
collecting) may be greater in some ponds than others, but I do not think we can control for this
during site selection. Fortunately, the snapping turtle is the most common species hunted for
food and it will not be part of the study. Please contact me with specific questions about site
selection. I realize that from a statistical design stand-point, this type of site selection is not ideal,
but it may have to suffice – any suggestions are welcome.
Once you have a pond selected, it’s time to start trapping. The turtle trap consists of three
metal hoops (0.9 m diameter) attached by nylon netting (Figure 1). At one end of the trap is an
opening that allows access to the trap. This opening is tapered so that captured turtles have
difficulty escaping from the trap. I recommend reducing the size of this opening in order to
reduce the probability of escape by weaving ¾” braided elastic through the netting encircling the
opening. The opposite end of the trap is knotted with a length of rope. The end of this rope is
used to stake the trap into the pond bottom and prevent the trap from being pulled into deeper
water. The trap is kept rigid by two four foot poles (PVC or wood) extending parallel along the
length of the trap (Figure 1). These poles are secured to the first and third hoop with metal clips.
With the poles secured, the trap will not collapse and can be tossed into the aquatic habitat.
One should set the trap in approximately 0.6 m of water in order for the opening of the
trap to be under water, but no greater than 0.9 m, or else the trap may be completely submerged,
potentially drowning the trapped turtles. One should slightly open a can of sardines and place it
in the rear of the trap, insuring that the can does not float. The rope at the rear of the trap should
be staked into the pond’s bottom.
A minimum of four (4) traps will be set. Each will be placed closed enough to the bank to
meet the 0.6 m depth requirements. In small ponds (< 2 ha), the traps should be equally spaced to
sample the entire habitat. In larger ponds (> 2 ha), the traps should be spaced about 20 m apart
within a subsection of the pond. More intense sampling of a smaller area will increase the
probability of recapture and is preferable to inadequate sampling of a large area. One trap can
capture multiple individuals. We want at least 10 individuals of the same species per pond
(Marchand and Litvaitis 2004).
Approximately 24 hours later, one should check each trap by grabbing hold of the first
and third hoop and lifting the trap out of the water. If turtles are present, remove the stake and
bring the trap onto the shore. With smaller turtles, one can reach into the rigid trap and pull out
each turtle. Place each turtle into a bucket filled with water – each bucket can hold multiple
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turtles. For larger turtles, one might want to remove the support poles and collapse the trap
around the turtle. If a snapping turtle is in the trap, I suggest collapsing the trap and allowing the
snapping turtle to free itself. This may take several minutes but it is always successful. If both a
snapping turtle and another species are in the trap, use one of the trap’s hoops to immobilize the
snapping turtle at one end of the trap and position the trap opening above the other turtle. Keep
an eye on the snapping turtle as you reach in for the other one. If you are unable to immobilize
the snapping turtle, then let the snapping turtle free itself first. The other turtle can wait. Once the
turtles are removed from the trap, the trap should be reset at the original location. The same can
of sardines should be used for both days of trapping.
Figure 1. Hoop trap containing two snapping turtles. The trap opening is located towards the left
and the knotted rope is apparent in the bottom right corner.
Cleaning of traps and other field equipment
In order to minimize risk of unintentionally transmitting organisms or contaminants
between ponds, please take care to clean all field equipment before their use at another study site.
All traps, boots, and other equipment should be scrubbed to remove all mud, algae, and
macroscopic animals. After this initial cleaning, you should spray the equipment with a 70%
ethanol solution to kill microbes (Mitchell 2000). Turtle traps can be difficult to clean. One
method is to soak each trap with a dilute bleach solution in a child’s plastic pool and then rinse
with water.
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Appendix B: Turtle data collection
CAUTION: Turtles are known carriers of Salmonella. Be sure to wash hands before handling
food or drink. One may advise students to wear disposable examination gloves, but this is not
required as long as good hygiene practices are followed.
CAUTION: Turtles may bite and scratch. Turtles, aside from snapping turtles, are unlikely to
bite but it is possible. The best way to avoid being bitten is to keep one’s fingers away from the
turtle’s mouth. It is quite easy to dangle one’s fingers in front of the turtle’s head especially when
measuring the turtle’s length. Pay attention to this and consciously keep the fingers tucked in.
The claws of turtles are quite sharp and one can be easily scratched. Wearing gloves is the easiest
way to protect oneself.
A. Turtle Data Collection
Write the date and pond name at the top of the provided datasheet (see Appendix C). For
each capture, record the following on a unique row in the datasheet:
 Last name of the person measuring the turtle (trapper)
 Last name of the person recording the data (recorder)
 Status of the turtle (new capture, recapture, or recapture but identity is uncertain)
 Turtle identification code (usually 3 letters, see below)
 Species (consult field guide or take a picture and send to me)
 Length of carapace, to the nearest 0.1 mm (see directions below)
 Length of plastron, to the nearest 0.1 mm (see directions below)
 Pre-cloacal tail length, to the nearest 0.5 mm (see directions below)
 Right middle foreclaw length, to the nearest 0.1 mm (see directions below)
 Annuli, the number of growth rings on the right abdominal scute (see directions
below)
 Comments on unusual characteristics of the turtle
B. Measuring the turtle
Species
Age/Sex Class: The species, age, and sex of the turtle will be judged from a variety of physical
measurements. Most turtle species are sexually dimorphic but the specific characteristics vary by
species. The most likely species to be caught in campus ponds are the so-called “basking” turtles,
so named for their habit of sunning on logs. These species tend to be size dimorphic, with adult
females being larger than adult males. Adult males, however, tend to have longer foreclaws and a
longer distance from posterior of plastron to the cloacal opening. I recommend using a field
guide such as “A Field Guide to Reptiles & Amphibians of Eastern & Central North America
(Peterson Field Guide Series)” to assist you in identifying species and age/sex class.
Here are ways to determine age/sex class (Freedberg and Bowne 2006) for the most
common species of basking turtle, the painted turtle (Chrysemys picta).
Adult male:
A front middle claw length of ≥10 mm indicates sexual maturity in males only (Frazer et
al., 1993). The carapace and plastron of adult males are shorter than adult females but the precloacal tail length (see below) is much longer, often twice as long.
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Adult female:
Plastron length exceeds 118 mm (see below for measurement). They have shorter claws
and shorter precloacal tail length than adult males.
Subadult:
Both sexes possess at least three annuli (see below). Subadult males have longer precloacal tail
lengths than subadult females. Foreclaw length can be quite variable between the sexes at this
stage and is an unreliable indicator of sex.
Juvenile:
Have two or fewer annuli (see below). One cannot distinguish sex in juvenile turtles.
Carapace (top shell) and plastron (bottom shell): We will measure the straight-line (anterior
to the posterior) distance of each shell using dial calipers (Figure 2).
Figure 2. Measuring with dial calipers the plastron of a painted turtle.
Pre-cloacal tail length: Near the base of the tail is the cloacal opening. The distance from the
base of the tail to this opening is useful in determining the sex of the species – typically, this
length is greater in males. Use the dial caliper to measure this distance.
The pre-cloacal tail length measurement depends on how cooperative the turtle is being
and as such may not be very accurate. It's more a relative measure of long versus short but
measuring it with the calipers gives an objective way of determining that. For example with a
painted turtle, if the true distance is 16.5 mm but you measure 15 mm, that measurement still
demonstrates it's a long precloacal tail and therefore a male. An adult female would be closer to 7
mm.
Right middle foreclaw: The length of the nails is useful for many species in determining both
sex and maturity. Males tend to have longer nails than females and sexually mature males have
longer nails than do juvenile males. For painted turtles, the claw length of ≥10 mm indicates
sexual maturity in males only (Frazer et al., 1993). Use the dial caliper to measure this distance.
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Annuli: The annuli (growth rings) of turtles can be useful in determining age for the first several
years of life. If annuli are visible on the abdominal scute of the plastron, record their number
(Figure 3).
Figure 3. Schematic of annuli on the plastron of a turtle and photograph of the plastron of a
juvenile eastern painted turtle (notice its annuli). The writing on the plastron is this turtle’s
identification code. The writing is not permanent but does ease data collection on recent
recaptures
C. Marking the turtle
The number of marginal scutes (Figure 4) is consistent and therefore marking these scutes is
a useful way to uniquely identify each individual. If the turtles in your study area have already
been marked, then please follow that original marking scheme. If the turtles have not been
marked, then one can use the following methodology. The goal is to have a consistent,
unambiguous plan to uniquely and permanently identify individuals in a manner that does not
harm the animal.
Marking scheme
Holding the turtle with dorsal side up and its head at the 12:00 position, the first scute to the
right of the small nuchal scute is the “A” scute, followed by “B” and “C” (Figure 4). The next
four scutes comprise the bridge connecting the carapace and plastron and are not included in the
naming scheme. After the bridge, the scutes go from “D” to “H” on the right side of the shell.
The first scute on the left posterior side is “I.” The same naming scheme is used on the left side
with the final scute (bordering the small nuchal scute at the 12:00 position) is “P” (Figure 4).
We will start with a 3-letter code. Simply file a notch about one-half the way through the
appropriate scute. I suggest writing the turtle’s code on its plastron with a permanent marker
(See Figure 3). The written code will disappear when the turtle sheds its scutes, but having it
written does help with data collection on recent recaptures.
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If possible, take two photographs of each turtle, one of the carapace and the other of the
plastron (Figures 3 and 4).
Figure 4. Schematic of turtle carapace with lettering scheme and actual carapace of an adult
eastern painted turtle. This turtle’s 3-letter code is ADN.
D. Releasing the turtle
After the turtle is measured and marked, release the turtle back into its habitat at a location close
to the point of capture.
Literature Cited
Freedberg, S. and D. R. Bowne. 2006. Monitoring juveniles across years reveals non-Fisherian
sex ratios in a reptile with environmental sex determination. Evolutionary Ecology
Research 8:1499-1510.
Frazer, N.B., Greene, J.L. and Gibbons, J.W. 1993. Temporal variation in growth rate and age at
maturity of male painted turtles, Chrysemys picta. Am. Midl. Nat., 130: 314–324.
Mitchell, J.C. 2000. Amphibian Monitoring Methods and Field Guide. Smithsonian National
Zoological Park. Conservation & Research Center. Front Royal, Virginia.
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Appendix C: Datasheet for turtles
Date:
Pond:
Status: New (N), recapture (R), or ID (turtle marked but doesn't match records)
Age-sex: adult male (AM), adult female (AF), Subadult male (SM), subadult female (SF), juvenile (J)
Carapace, plastron, precloacal: straight-line length (to 0.1 mm)
Foreclaw: Length of center nail on right foreclaw (to 0.1 mm)
Annuli: Number of growth rings on right abdominal scute
Trapper
Recorder Status TurtleCode Age-sex
Species
Carapace Plastron PreCloacal Foreclaw Annuli
Comments
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Appendix D: Other data collection
Turtles respond to many local habitat attributes but since our hypotheses focus on
landscape characteristics, we will not measure many pond-level variables. The ones that we will
measure are simple to measure but are still biologically meaningful. They are as follows:
Habitat substrate (organic or inorganic): Simply note its composition as you wade in
the pond to set traps. If it is mostly clay, silt, sand, or rocks, record it as inorganic. If it is mostly
leaf matter or coarse woody debris, record it as organic.
Number of adult turtles that could simultaneously bask (numeric): Basking sites are
items such as logs or large rocks on which a turtle can crawl and sun itself and which are
completely surrounding by open water. A boulder on the bank does not count as a basking site
because of increased risk of predation. A downed tree jutting from the bank into open water is
considered a basking site. To quantify the availability of basking sites, estimate how many adults
of your most common turtle species could simultaneously bask without overlap.
Pond area (numeric): The area of the pond can be determined by use of a hand-held
GPS or can be calculated using a GIS. I can calculate this if you do not have access to either of
these technologies.
Permanence of the habitat (yes or no): Seasonal availability of habitat can strongly
influence population structure (Bowne et al. 2006). Simply record if the pond has retained water
(yes) or if it dried (no) between 2010 and the completion of the study
Landscape characteristics: I will measure landscape characteristics using a geographic
information system. If you want to assist, please let me know and I will give you more detailed
protocols.
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Appendix E: Equipment list and cost
Please note that the following list enumerates the cost for one trap and one set of supporting
equipment. A minimum of 4 traps is required.
Item
hoop trap (3' diameter, 1" mesh)
PVC (1" dia, 10' long)
metal clips
sardines in oil
stakes (plastic, tent)
Quantity
1
2
4
1
1
Unit Price Total price Vendor
$109.00
$109.00 Nylon Net Company
$2.50
$5.00 Any hardware store
$0.30
$1.20 Any hardware store
$0.90
$0.90 Any supermarket
$1.00
$1.00 Any department store
Total for one trap
dial caliper (150 mm); larger caliper
will be required for sliders
chest waders
File (slim taper)
5 gallon plastic bucket
Total for miscellaneous equipment
Catalog #
TN31
$117.10
1
1
1
2
$39.95
$69.95
$7.00
$2.50
$39.95
$69.95
$7.00
$5.00
Forestry Suppliers
Forestry Suppliers
Any hardware store
Any hardware store
59591
93119
$121.90
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