Providing Habitat for Threatened Drymarchon corais couperi Utilizing Gopherus polyphemus Burrows

Part of the Young Naturalist Awards Curriculum Collection.

by David, Grade 12, Floridia - 2008 YNA Winner
Black snake moving along the ground in brown dried twigs and leaves.
Figure 1: Eastern Indigo Snake

Since 1978, the eastern indigo snake, Drymarchan corais couperi(Figure 1), has been a threatened species of snake in the southern part of the United States. In some parts of the indigo snakes' range, its numbers have dwindled so much that seeing one is considered a once-in-a-lifetime event. When I first saw an indigo snake, I was overcome with its beauty and serenity.
This snake is threatened because its significant habitat is the gopher tortoise burrow (Figure 2). The gopher tortoise, Gopherus polyphemus(Figure 3), digs this habitat, and is labeled a "species of special concern." With the drastic decline in gopher tortoise burrows, the indigo snake must compete with humans for living space, thus decreasing their population. The indigo snake was also used in the pet trade since it is a docile and nonvenomous snake, making it perfect for snake lovers.
After hearing of the decline of the indigo snake, I knew I wanted to help out in saving this species. Bok Tower, a local nature conservatory located in Lake Wales, Florida, has about 12 indigo snakes on the property. David Price, director of horticulture and president of Bok Tower, came up with some ideas for my project in 2004. As soon as I heard of the idea of a "snake box," I was hooked. After nearly 200 hours of research and construction, I was ready to assist a species. My six snake boxes might seem farfetched, but over the next four years my project turned out remarkable results.

Left side: Small burrow opening in sandy surface with a few brown leaves and twigs. Right side: a round grayish shell tortoise walking in low grass.
Figure 2: Natural Gopher Tortoise Burrow [left] & Figure 3: Gopher Tortoise [right]

The main area of research was to design a simulated burrow that would sustain life. In 2004, before the construction of the simulated burrows, I visited a serpentarium in Punta Gorda, Florida. This serpentarium's major undertaking is the milking of rattlesnakes to produce antivenin. I saw the wooden snake burrows there and realized that a plastic water-meter box would be more practical because it would hold up for many years. I also visited with gopher tortoise expert Ray Ashton, who explained the shape and sizes of natural gopher tortoise burrows. In addition, Ashton gave me copies of his research, which was a great asset to the project. I hoped this project would serve as a model for the conservation of the eastern indigo snake.

My project is currently in its fourth year. This longitudinal project gave the eastern indigo snake a secondary habitat. Each year, new trials were performed to determine if the simulated gopher tortoise burrows were similar to a natural burrow. By using solid and vented caps, three of each, to cover the observation tubes, I could make a comparison between the natural and the simulated burrows. Each simulated burrow would need to maintain a healthy relative humidity and temperature, similar to a natural burrow, in order to sustain life. In no way were any burrow inhabitants harmed or handled during this research project. The dependent variable, inhabitance of the simulated burrows, directly correlates with the independent variable, the conditions inside the simulated burrow.

Materials and Methods 
The first year of the project centered on the construction and placement of simulated burrows in order to properly sustain life. The initial setup of the simulated burrows consisted of six large water-meter boxes (45.7 cm by 61 cm) that served as the living chamber. A hole (10.2 cm wide) was cut into the side of the box large enough to accept a flexible pipe (10.2 cm wide by 1.8 meters long) that enabled the inhabitant to enter and leave as it wished. To ensure that no water collected in the entrance tube, notches were cut in with a grinder to aid in water dispersion. Another hole was cut on top of the box, where a PVC observation pipe (7.6 cm wide by 20.3 cm long) was attached. Careful planning ensured that no sand or debris could enter the box through the observation pipe. The observation pipe was fitted with either a solid PVC cap or a vented metal roofing cap (Figure 4). The aboveground pipe was painted brown to help it blend in with the surrounding environment.

two designs for observational tube caps to monitor nesting success of the indigo snake
Figure 4: Different Observation Tube Caps (left: solid cap, right: vented cap)

Six locations known to be frequented by eastern indigo snakes were chosen at random in the nature preserve. The proper placement of the boxes was crucial to sustaining life. The six boxes were buried about half a meter below the soil surface in May 2004 as a part of my project (Figure 5). When burying the boxes, the entrance pipes were placed in a way that ensured that no water would flow into the living chamber. Now my project was underway.
Monitoring the boxes during the first year of this project included regular viewing of the boxes not only for inhabitants but also for signs of debris. To view the inside of the box, an automotive mirror and a flashlight attached to a 1.5-meter-long piece of rope were used. After removing the cap, the flashlight was placed inside of the viewing pipe to illuminate the box, while the mirror was used to check the inside corners of the box. If any inhabitants were found, a digital camera was used to photograph the inside of the box from the viewing pipe.
The second year of this study sought to determine if temperature conditions inside the natural burrows were close to the simulated burrows. In order to record the temperature, a WatchDog data logger was used (Figure 6). This small temperature sensor records the temperature every 10 minutes, 24 hours a day. Sensors were placed inside and outside the natural gopher tortoise burrows and the solid- and vented-cap burrows. This comparison was designed to determine which capped burrow would most closely imitate a natural burrow.

Figure 5: Installation of Simulated Gopher Burrows

In order to accurately determine whether the vented or solid cap best replicated the conditions of a natural burrow, the third year of this study tested not only temperature but also relative humidity. A similar WatchDog data logger was used to record relative humidity and temperature (Figure 7). Sensors were placed outside and inside the natural, vented-cap, and solid-cap burrows. This comparison would help determine which kind of capped burrow, solid or vented, most closely imitates the environmental conditions in the natural burrow.
I worked to keep the simulated burrows free of excessive amounts of dirt and debris in the living chamber by cleaning them out, by digging down to the lid and opening the box. All debris was inspected for feces or other signs of life. The lid was placed back on top and the soil was replaced to restore it to pristine status. All six boxes were inspected and cleaned, no matter what the conditions were inside, to ensure that all variables remained constant. My excitement rose as I found evidence that rats and other small animals were using these burrows.

Figure 6: WatchDog Data Logger used to measure temperature (Year 2)

In the fourth year of this study (2008), I added two additional boxes that were identical to those in Year 1. All eight boxes, the six original and the two new ones, were capped with a solid PVC cap. In addition, a more comprehensive test took place. In the new study, three relative humidity and temperature sensors were placed inside the natural and simulated burrows, and two sensors were placed outside. This study was conducted in July 2007 and again in December 2007. By conducting studies during the summer and the winter, the extremes in relative humidity and temperature inside the burrows were tested for their similarity to natural burrows. The observational viewing of these boxes continued, along with analysis of the data to show the effectiveness of this project not only for the eastern indigo snake but also for other forms of wildlife that inhabit underground burrows.

Data and Results 
Data collection occurred by both observing burrow inhabitants and data analysis. First, throughout this four-year study, each simulated burrow was observed for inhabitants. If any inhabitants were visible, a photo was taken and the box number was noted. Second, during the second, third, and fourth years of the study, data was collected on the conditions inside both the natural and simulated burrows. This data was then analyzed, and the results of each study were determined.

Figure 7: WatchDog Data Logger used to measure temperature and relative humidity (Year 3)

During the first year of the study, only observations took place. Every two weeks, the boxes were inspected, but no animals were found. The snakes or other inhabitants did not find the entrance tubes, as they are not baited in any way. In 2004, central Florida was hit with three major hurricanes, which in turn altered the animals' ecosystem. Even though no animals were found during the first year of study, I continued to remain positive and patient.

The second year of the project included the continued observation and comparison of the temperatures inside the natural gopher burrow and the simulated burrows with vented and solid caps. Year 2 produced breakthrough results. Inhabitants were found living inside the simulated burrows. I was excited when I found an indigo snake in one of the simulated burrows (Figure 8). Along with the indigo snake, I also found an opossum in another simulated burrow. The natural gopher tortoise burrows are used in a symbiotic relationship, as many animals use the same burrow at the same time, including opossums. The discovery of the indigo snake and the opossum encouraged me to continue this project.

Figure 8: Eastern Indigo Snake inside of simulated burrow (Year 2)

The second year of this research project also included a study in which the temperatures inside a natural and a simulated burrow were compared to determine if conditions were similar to a natural burrow. The temperature was measured for a week-long period during the month of December. Every 10 minutes, a WatchDog data logger would record the temperature. The data was downloaded to a laptop computer, where it was averaged and then analyzed. The six data points for each hour were averaged with the data points from the week-long period for each particular hour, and graphed using Microsoft Excel. The temperature inside the vent-cap burrow fluctuated by 3ºC (Graph 1), thus making it imperfect for indigo snake inhabitation. I concluded that solid caps paralleled the natural burrow more closely than vented caps.

Year 3 of this study started with my hopes for another year of evidence. Again, temperature and relative humidity trials were conducted to see which capped burrow most closely represented the conditions inside a natural burrow. The temperature and relative humidity trials, along with the continued observation, sought to prove which capped burrow was the best.
On two separate viewing dates, I observed an indigo snake inhabiting the simulated burrows (Figure 9). Opossums were also found inhabiting the simulated burrows (Figure 10). Their remnants were also found, as I saw pine needles inside many of the burrows. My project demonstrated that simulated burrows can provide an effective habitat not only for the eastern indigo snake, but also for other animals that use gopher tortoise burrows for habitat.

Figure 9: Eastern Indigo Snake inside simulated burrow (Year 3) [left] & Figure 10: Opossum inside simulated burrow (Year 3) [right]

The Year 3 study included a temperature and relative humidity study. This study compared the temperature and relative humidity during the summer, inside and outside a natural burrow and the vented- and solid-cap simulated burrows, to compare with the Year 2 study, which was conducted during the winter. Both conditions were analyzed to see which cap better replicates the conditions in a natural burrow. The temperature study (Graph 2) again indicated that with a solid cap the temperature remains relatively constant, whereas with the vented cap the burrow temperature changes slightly. The comparison between the winter months (Year 2) and the summer months (Year 3) showed a consistency between the two, which proved that simulated burrows with solid caps maintain viable conditions for their inhabitants. Since the burrow temperature with a solid cap is relatively constant, a prediction could be made that the solid cap represented the environmental conditions necessary to sustain life.

Relative humidity was also tested in Year 3. Relative humidity was measured inside a natural burrow, a vented-cap burrow and a solid-cap burrow, and outside the natural burrows. The humidity in the solid-cap burrow remained constant, at about 97 percent in this test (Graph 3). With a successful study in Year 3, I discovered that the simulated burrow with a solid cap most closely replicates the natural gopher burrow.
Ray Ashton's research on temperature and relative humidity in natural burrows showed results similar to mine. Ashton found that the end burrow temperature ranged from 17ºC to 28ºC. Both Year 2 and Year 3's temperature study for the simulated burrows fell directly into this range. Ashton also found that the relative humidity range of more than 2,000 natural burrows was between 78 and 98 percent. As with temperature, the relative humidity of the simulated burrows matched up with the research Ashton had collected on natural burrows. The data set from Ashton was extremely important to have because he tested the end chambers of the gopher burrows, which is the primary model for the simulated burrows. When temperature and relative humidity data was collected for my research, the data loggers were placed about one meter into the gopher burrow, nowhere near the end burrow, thus making the temperatures and relative humidity for my research higher than the data Ashton found. The simulated burrows' conditions were once again reconfirmed.

Figure 11: Opossum inside simulated burrow (Year 4)

Year 4 of this project deals with confirming and expanding upon the results from Year 3. I added two additional simulated burrows in areas where the eastern indigo snake had been seen before. All the simulated burrows were capped with solid caps. The original six were once again cleaned and inspected for any signs of life.

The fourth year's observation produced results quickly. Indigo snakes and opossums were found inhabiting the simulated burrows. The first inhabitant found was an opossum, but most importantly, this opossum was found inside one of the two recently constructed simulated burrows (Figure 11). This helped prove that this project has quick results.
The best visual evidence of inhabitation of the simulated burrows was witnessed in January 2008 (Year 4). An eastern indigo snake was witnessed exiting the simulated burrow (Figures 12 and 13). After waiting nearly an hour, the snake slowly made its way out of the burrow. This observation clearly shows that these burrows are indeed being inhabited by animal species. This moment is conclusive evidence of the success of this project.

Two side-by-side photos of an Eastern Indigo snake exiting a simulated burrow. The  opening of the tube is visible above the ground covered in dry brown leaves.
Figures 12-13: Indigo Snake exiting simulated burrow (Year 4)

Two research studies were completed in Year 4 to confirm the research collected in years 2 and 3. A temperature and relative humidity study was completed for both the summer and winter months. This difference in climate allowed for a comparison of how the burrows sustain needed conditions despite differences in temperature and relative humidity. With the summer research study completed, the results once again showed that temperature (Graph 4) and relative humidity (Graph 5) aligned with the previous years' research.

Error bars helped to prove how consistent the simulated burrows' internal conditions were. The error was calculated through Microsoft Excel's Data Analysis, using ANOVA to calculate the error, using the equation S2 = S(X- )2 / (N-1) . With extremely consistent results between the three sites, the error bars came out to be minimal, thus proving the constancy of the simulated burrows throughout the summer months.

A chart shows the average air temperature and relative humidity inside different parts of a gopher tortoise burrow.
Graph 1: Comparison of Average Temperature (Year 2)

Another research study took place during the month of December to ensure that all conditions were tested again. During the winter study, both temperature (Graph 8) and relative humidity (Graph 7) aligned with previously collected data. The variances were once again low, which shows the consistent results of the study.

The fourth year of this study had groundbreaking results. Witnessing the snake exiting the burrow, and the consistent results in the temperature and relative humidity study, helped to make this project a great success.

After many hours of work, planning, and observing, the simulated gopher tortoise burrows proved to be an alternate habitat for multiple species. A comparison of both types of observation caps during Years 2 and 3, and then repeating the trials in Year 4 of this study, proved that the solid cap is the best cap for this project, and that the simulated gopher burrows were suitable as a substitute habitat.

Chart showing temperatures in gopher burrow over 12 days in July, 2006
Graph 2: Comparison of Average Temperature (Year 3)
A graph shows hourly relative humidity over 13 days in July 2006 in and outside a gopher burrow
Graph 3: Comparison of Average Relative Humidity (Year 3)
Graph 4: Comparison of Average Temperature (Year 4))
A chart showing the relative humidity in natural & simulated gopher burrows over a 14-day period.
Graph 5: Comparison of Average Relative Humidity (Year 4)
Graph 6: Comparison of Average Temperature, Winter Months (Year 4)
Graph 7: Comparison of Average Relative Humidity, Winter Months (Year 4)

To place in perspective the frequency of different species inhabiting the simulated burrows, Ray Ashton's research once again provides a stunning fact. In his book The Gopher Tortoise: A Life History is a chart of the frequency of appearance of other animals as he excavates gopher tortoise burrows. Ashton correlates this frequency to the number of burrows in a specific animal's habitat (Ashton 2004). He rates their frequency in five categories: very common, common, uncommon, rare, and a surprise. Ray Ashton's frequency for sighting the eastern indigo snake is rated as uncommon—about 1 out of 30 burrows in the natural habitat. In his three years of observation, an indigo snake was viewed on 10 different occasions. The frequency of opossums is rated as a surprise, or about 1 in 100 burrows in the correct habitat. In my study, opossums were spotted nearly every time the simulated burrows were observed. In all, 87.5 percent of the simulated burrows were inhabited by an animal. I concluded that the simulated burrows can serve as a good substitute for natural burrows.

The main purpose of this study was to create and maintain substitute housing for the eastern indigo snake, which finds refuge in natural gopher tortoise burrows. I concluded that simulated burrows can be a viable substitute for natural burrows and can provide a home for endangered species. This simple plan for the conservation of wildlife can not only be put into use throughout the eastern indigo snake's home range, but also can be adapted to suit any burrowing animal throughout the world. This simulated burrow can be placed anywhere, as it is both pleasing to the eye and can provide shelter for borrowing animals of any kind. The possibilities are endless to help save endangered wildlife.

Rhea McKinney, my teacher and mentor throughout the project. David Price, advisor from the Historic Bok Sanctuary. My father, Dr. Stephen H. Futch, for a great deal of knowledge and support. My mother and grandmother, Deborah Futch and Betty Reighard, for their endless support. All the faculty and staff at Winter Haven High School, including principals Mike Tucker and Gina Williams, and teachers Karen Girone, Judy Joiner, Jackie Dugger, Nancy Bachman, and Terrence Barber.
All photographs were taken by David Futch.

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