Abstract



     The objective of this study is to observe group basking behavior in Western Painted Turtles housed inside of an artificial zoo habitat. As an indicator of security, my hypothesis was to determine whether Western Painted Turtles kept their eyes closed for a longer period of time when group basking-size increased. Continuous focal sampling was used to record the average percentage of time four different group sizes had their eyes closed for. The average number of eye shuts per group size was also recorded. Using an ANOVA, the difference in ‘percent time eyes closed’ and ‘eye shuts’ between the group sizes was tested for significance. A p-value of p < 0.0001 was obtained for the ‘percent time eyes closed’ category suggestive of a significant difference between trait means. However, the significance was indicative of group means decreasing, not increasing like originally hypothesized. A non-significant p-value of p < 0.095 was calculated for average ‘eye shuts’. Based on these p-values, I rejected my hypothesis, but concluded there is a correlation between the amount of time Western Painted Turtles eyes are closed and group basking size. Instead of group security, there might be another factor driving the percentage of time that eyes are shut, such as crowding.



Background



     The Western Painted Turtle (Chrysemys picta bellii) is one of four subspecies of the Painted Turtle. They can be found in streams and lakes from Wisconsin to Oregon. Ideal habitat for the Western Painted Turtle, like other turtles, includes slow moving water with a muddy bottom and submerged logs and vegetation. The Western Painted Turtle is a basking turtle, in that they use the sun to thermoregulate. All ages and both sexes are known to start basking on a log or similar object upon waking from the depths of the lake bottom. After warming to a 17-23 degrees Celsius, they will actively forage in the water (Tran et al. 2006) (Collins 1959). When their body temperature drops they will return to warm up for one or two more cycles of basking and feeding. Not only do Western Painted Turtles bask to contol their body temperature throughout the day, they also use the suns energy to synthesize vitamin D and kill ectoparasites and fungi that might be living on their shell and limbs (Graham 1997).



    Western Painted Turtles are often seen basking in groups with as many as fifty individuals on one log (Painted Turtle, Wikipedia). However, one or two might bask next to each other if they are on a small object. One would speculate that the surface area of the substrate limits the number of turtles basking on the substrate. On the contrary, Western Painted Turtles will bask on top of one another, effectively increasing the amount of turtles that occupy an area and the number of individuals that can detect potential predators (Zipko 1982).

Observations



     The Western Painted Turtles at the Oregon Zoo live in a large indoor aquarium. Although an accurate number of Painted Turtles was not gathered, an estimate puts their number at about seven. Two Pond Turtles cohabitate the tank with the Painted Turtles. The aquarium is divided by a net into a forward space and a backspace. Three of the Painted Turtles lived in the back area during the duration of my study. There was physical contact between turtles on both sides, but neither side could cross into the other side. There was one large rock in the main side (left side) of the tank that could hold four to five basking turtles of various sizes. Two heated lights hung about 40 cm above the rock. There were two protruding logs in the right side of the tank, both with heated lights.



     Upon observation of the Western Painted Turtles, they were either basking in the heat provided by the lamps or they were active in the water. I noticed the turtles on the rocks would open and close their eyes periodically, which appeared not to be affected by my presence. Periodically, other turtles (either species) would climb on the rock, bask on top of other turtles, and sometimes displace the basking turtles. My observations suggest the Western Painted Turtles do not always desire to share the rock with the Pond Turtles. When the Western Painted Turtles occupied the rock and a Pond Turtle attempted to gain access to the rock, the Western Painted Turtles would gape their mouth- a sign of aggression (Zipko 1982). I did not see any antagonistic behavior between two or more Western Painted Turtles.



     This led me to question whether an increasing number of basking Western Painted Turtles would lead to the group feeling less threatened by a competitor, aggressor, or predator compared to a smaller group or a lone turtle. The average amount of time a turtle keeps their eyes open for could be a direct indicator of how secure they feel. As group size increases, I would expect the amount of time each turtle has their eyes closed for increases. Contrastingly, as group size shrinks, turtles will have their eyes open a higher percentage of the time to detect competitors, aggressors, or predators. In this case, the competitor or aggressor would be the Pond Turtles in the tank. There are not any predators in the tank, but I would still expect this conclusion to hold due to the possibility of this action being an innate evolutionary behavior.

 

Scientific Question/Hypothesis



Scientific Question: Is there a correlation between the average amount time Chrysemys picta bellii’s eyes are closed and group basking size?



Hypothesis: Chrysemys picta bellii’s eyes are closed for a longer period of time when there are more turtles basking on the rock?



METHODS



     All trials were preformed on left side of the tank on the large rock to allow consistency. Different group sizes, from a single turtle to four turtles, were observed. Two different data sets were taken- the average amount of time that each group had its eyes closed for and the average number of eye shuts per group. A decreased number of eye shuts might be expected for a smaller group size due to the turtles being more watchful and feeling less secure in a smaller group. Contrastingly, as group size increases, the number of eye shuts should go up as the turtles become less fearful in the presence of other Painted Turtles. Both were used to determine weather a correlation existed between these sets and group size.



     Continuous focal sampling was used; recording the duration (in seconds) that one rock-basking C. P. bellii closes and keeps its eyes closed for in a three-minute period. The number of eye shuts in this period was also recorded. After all basking Western Painted Turtles on the rock had been sampled, this was marked as a trial. The recorded times and eye shuts were averaged for the trial. The turtles were sampled for two more trials. Three trials with one group of turtles marked a completed set. An additional set for each group size was taken at a different time to ensure variability. All times were divided by 180 seconds (3 minutes) to get the percentage of time that the eyes were closed. Eye shut data was taken and grouped in the same manner as above, except were kept as raw averages and not processed as percentages.



     Eyes were considered open if the pupil was seen and closed if the pupil was not seen. Half blinks were not counted. An eye shut shorter than five seconds was not counted in the ‘duration’ trials, but was instead counted as an eye shut. The Painted Turtles had to be basking to be considered in the study. Basking was defined as: under the heat lamp with all limbs and tail extended, head up, and digits fanned. The group size did not change in the middle of data sampling and all trials were considered for analysis.



RESULTS



     The average percentage of time eyes were closed and the average number of eye shuts (Tables 1 and 2, respectively) were graphed and analyzed using an ANOVA. P-values were obtained for both sets of data. A significant correlation (Figure 1: p < 0.0001) was found between group size and the amount of time the Western Painted Turtles eyes were closed. However, this correlation is the opposite of what was hypothesized. As group size increased the percentage of time the turtle’s eyes were closed decreased, that is their eyes were open for longer. A non-significant correlation (Figure 2: p < 0.095) was found between group size and the average number of eye shuts. Observing the two graphs, there appears to be a correlation between the percentage of time eyes were closed and average number of eye blinks. The longer their eyes were closed the more eye shuts occurred. The less time spent with their eyes closed the less eye shuts occurred. However, this correlation was not tested for significance.



     There is higher variance in the smaller group sizes because the sample sizes were smaller. For comparison, observing each trial independently, one turtle made up the whole trial in a group size of one, while four turtles made up the trial in a group size of four.  The wide variation affected the differences between groups in the ‘percent time eyes closed’ category. When each group was compared individually, significant differences result between group size one and four (p < 0.043), group size two and four (p < 0.0001), and group size three and four (p < 0.040). The large variation in group sizes one, two, and three was not enough to produce a significant difference between these groups. However, the larger means in group sizes one, two, and three compared to the narrow variation and smaller mean in group size four had enough power to create a significant difference between these groups.



     Since average number of eye shuts appeared not to be driven by group size, a linear regression was not calculated for this data set. A linear regression, however, was accomplished for the percent time eyes closed (see Figure 3). An R2 value of 0.717 was obtained for the line. This suggests seventy-one per cent of the variation in the amount of time that the Painted Turtles eyes are shut can be explained by group size.

 

DISCUSSION



     The objective of this study was to see if there was a correlation between the amount of time Chrysemys picta belli’s eyes are closed and group basking size. The data suggests there is a correlation. However, my hypothesis was not confirmed. Chrysemys picta belli’s eyes are not closed for a longer period of time when there are more turtles basking on the rock. The average amount of time their eyes are closed decreased with group size (figure 3). It was originally thought Painted Turtles would keep their eyes closed for longer as group size increased due to the advantages of a larger group to lookout for predators.



     There are a few reasons why the data is not indicative of the original hypothesis being true. Considering the study was carried out at a zoo, the turtles are in a semi-artificial environment with no predators and Pond Turtles in close proximity, which most likely affected their behavior. Without any natural predators, there is no chance the turtles will be eaten and little reason for the turtles to keep their eyes open to look out for predators. To say turtles are aware of the future risk of predation based on their environment might be a stretch. This could explain the behavior of the smaller groups, while the behavior of larger group sizes could depend upon the presence of the pond turtles. The Painted Turtles did not seem to care if another Painted Turtles crawled on the rock while they were basking. Yet, they displayed aggressive behavior when the Pond Turtles climbed on the rock while they were basking. As group size increased there was less space on the basking rock and it is possible the larger group sizes of Painted Turtles did not want to share the limited space. Hence, their eyes being open for longer as group sized increased and basking surface decreased. One basking Painted Turtle did not take up much space on the rock so there was sufficient room for more turtles.



     Secondly, apart from the immediate reactions due to group composition and dynamics, there could be an alternate evolutionary reason why Painted turtles had their eyes open for longer in larger groups. This would be an innate response due to group size and not change depending on their surrounding environment. It is possible that smaller groups are less conspicuous and therefore less likely to attract predators where as larger groups are more likely to be seen by predators. Therefore, as group size increases the amount of time that Painted Turtles spend with their eyes open increases, which would heighten their chance of spotting a potential predator.



     Originally, the average number of eye shuts was going to be used as a secondary indicator of security due to group size. The smaller groups were predicted to have less eye-shuts lasting less than five seconds and larger groups have more eye-shuts lasting less than five seconds. Less eye shuts would be indicative of a more watchful turtle(s) and more eye shuts would suggest the opposite. A non-significant opposite trend was found (figure 2) that appeared to match the pattern of the amount of time that the Painted Turtles eyes were closed (figure 1). This makes sense, because the less time turtles spend with their eyes closed the more eye-shuts, and the more time that turtles spend with their eyes closed the less eye-shuts.



     Observing the regression (figure 3), we see how quickly the amount of time that Painted Turtles keep their eyes closed for decreases with group size. If this regression truly explained the pattern then we would expect the graph to level off at a certain value and most likely never approach zero. Painted Turtles are likely to have some eye closure in large groups no matter what the group size. Again, the data was taken in an artificial environment with a small group of turtles and would not accurately reflect data from an observational field study.



     The first two points were possible scenarios that explain the above data. However, the methodologies should be examined to get a better sense of the accuracy of the data. The amount of time that the Painted Turtles eyes are shut might not be a reliable indicator of how secure they feel. A more accurate measure of security and ‘safety in numbers’ might be studied at the endocrine level by measuring stress hormones such as cortisol. Additionally, predation rates can be compared between Painted Turtles in small groups and Painted Turtles in larger groups. Lower predation rates in larger groups might provide evidence of an adaptation to bask in clustered groups. Indirectly, we might infer a “feeling of security” in larger groups.

Though the analysis suggests significance in the data, there was a lot of variation. Only two different groups were measured for each group size and since there are other variables that affect whether their eyes are open are not, a larger sample of groups would have reduced the influence of these outside variables. One of these factors includes the time of day. Though data was taken at different times of the day, most of them- especially the smaller group sizes, were taken in the morning. Another variable that affected the variance within and between groups was the time elapsed since the turtle came out of the water. Turtles that had been on the rock longer were more relaxed and more likely to keep their eyes closed for longer.  Lastly, the movement around the turtle drove some of the variation. I believe this truly explains the higher percentage of time that a group of four turtles had their eyes open for. More turtles on a small rock are more likely to disturb a nearby turtle, especially when they are basking on top of each other. When a basking turtle with its eyes closed was touched by turtle adjusting its position on the rock, it opened up its eyes every time. Boyer (1965) found that emydid turtles probably don’t bask to attain higher elevations to bring themselves closer to a heat source, because turtles pile in shady locations as well. This suggests some other adaptive reason for piling. Hennemann (1979) suggested that piling would help increase a turtle’s internal temperature by moving from a wet and cold substrate to the backs of a warm turtle. The contact between turtles to achieve a higher internal temperature at a faster rate increased as group size went up and probably drove the pattern observed for larger group sizes.



     Figure 1 shows the first turtle with its eyes closed for a shorter period of time than the second turtle. I postulate that this average number is highly flexible based on when and how many times a group size of one is sampled. To make up for the high variance in a group size of one, many more trials should have been completed. If I performed the set of three trials moments before or after I actually did, I would have come out with a completely different number. Taking this into consideration, the group size of one and the regression line should be interpreted carefully.



     I had originally proposed whether an increased number of basking Western Painted Turtles would lead to the group feeling less threatened by a competitor, aggressor, or predator compared to a smaller group or a lone turtle. This may well be the case in nature, but according to the data and analysis we cannot make this conclusion based on this small zoo study. The data suggested the opposite of what was expected and it is possible that the Western Painted Turtles feel less secure and/or more frequently disturbed in a large group as a result of competition for basking space.