The overall goal of this experiment is to determine the importance of spatial cognition in over land turtle navigation. This method can help answer key questions in ecology, including how animals respond to changes in their environment or navigate stressful landscapes and give us insight into the underlying cognitive and neurological processes. The main advantages of this technique are that animal movements are studied in a natural system with high spatial precision and behavior is manipulated using well studied pharmacological agents.
Though this method can provide insight into the cognitive basis of animal movements, it can also be applied to other systems, such as conservation and neurobiology. To begin, locate a target body of water that is known to contain turtles. Identify a region within the site that has sufficient water depth to place traps.
Deploy the hoop traps so that four to five inches will remain above water in order to give trapped turtles space to surface and breathe. Expand the traps to their maximum length. Stake the traps into place securely.
Next, bait the traps with dead fish, chicken livers or necks, and a can of cat food or canned vegetables. Check the traps twice daily. Remove the turtles by holding the animal on the sides, avoiding contact with the claws or beak, which can cause injury.
Place the turtle in a holding bag, attached to a spring scale and record the mass of the animal to the nearest gram. To affix the transmitter, first use a spatula and cloth to clear mud, debris, and algae from the attachment area. Swab this area, using 70%isopropanol.
Use a small amount of five minute epoxy to attach the transmitter to the turtle carapace orienting it to ensure the transmitter has maximum contact with the shell. Position the antenna so that it trails behind the animal, parallel to the long axis of the body. Finally, cover the entire transmitter and approximately one centimeter of the surrounding carapace with five minute epoxy.
After the epoxy is cured, usually overnight, return the turtle to the point of capture. To prepare the drug stocks, use an analytical balance in a fume hood to weigh enough scopolamine hydrobromide to make a one milligram per milliliter solution. Add the scopolamine to a conical tube containing the appropriate volume of injection saline.
Vortex until dissolved. Next, weigh enough scopolamine methylbromide to make a one milligram per milliliter working solution. Add this to a conical tube containing the appropriate volume of injection saline.
Vortex the solution until all powder is dissolved. Using a 0.22 micrometer pour syringe filter, transfer the scopolamine hydrobromide solution into a sterile sealed serum vial. Repeat the filtration process using a new syringe to filter the scopolamine methylbromide solution.
Dial in the frequency of the target animal into the receiver. Using a Yagi antenna, and with the receiver gain set to medium, identify the general location of a tagged animal. Move to the general location of the target turtle, remaining at least 25 meters away.
Using a GPS device, record the location. Identify the direction of the strongest signal. Turn the gain down as far as possible, while still receiving a detectable signal.
Move the antenna to the left and record the bearing at which the signal is lost. This is the left knoll. Then repeat this process to determine the right knoll.
These two knolls represent a single set of bearings. Next, move to a different location, ideally 60 to 90 degrees relative to the first bearing, if possible, and take the left and right knoll bearings again. A GPS location needs to be taken each time the tracker changes its position.
Repeat this at different locations for a total of at least three, and up to five, bearing sets to increase the accuracy. Repeat the location collection every 10 to 15 minutes and record the bearing sets for the target animal. This will give movement data for the unmanipulated animals.
Continue until the animal has navigated approximately half of its projected path. Then, proceed to the location of the animal and carefully recapture it. Using the mass collected previously, determine the correct dose of drug to administer for the specific animal collected.
Swab the injection site with a 70%alcohol wipe. Using a one milliliter syringe with a 22 gauge needle, deliver the drug directly into the caudle peritoneal sinus. Release the animal as soon as possible at its site of capture.
Track the movement of the animal using left and right knoll method until it reaches the projected destination. Using each pair of left and right knoll bearings, bisect the angle to find the resultant transmitter bearing. Repeat this for all bearings for a given time point, and triangulate to find the position of the animal at that time.
This map shows the tracked movements of experienced adult turtles between temporary and permanent ponds. Despite maintaining high precision along traditional routes prior to injection, all adults in the test group deviated dramatically after receiving scopolamine treatment. This phenomenon was not seen in individuals treated with the control drug, methylscopolamine.
Conversely, mapping of naive juveniles showed that even with scopolamine treatment, individuals maintained movement within traditional routes. This suggested that juvenile individuals with no previous experience at the site use local cues to navigate, whereas scopolamine treated adults use spatial memory to navigate, as they lost this ability when treated with scopolamine. This plot shows the movement of sample turtles prior to treatment.
All treatment groups are seen to follow the traditional path with a high degree of accuracy. After treatment, adults in the scopolamine treatment group deviated significantly from the traditional path. All other groups continued to navigate with high precision.
Once mastered, each set of bearings can be collected in no more than three minutes if tracking is performed properly. While attempting this procedure, it's important to remember to choose locations that are close enough to collect accurate data, but not too close to disturb the animal. After its development, this technique paved the way for researchers in the field of ecology and animal behavior to explore the neurological basis of cognition under natural conditions.
