Snapping Turtle Research: Analyses and Conclusions
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This article is still under construction. Expect it to change frequently until this notice is removed. |
Below are the extensive (perhaps more than necessary) details of the methods and results from the 2008-2009 analysis of turtle footage collected by the Snapping turtle research team. Questions should be directed to Colin Carlson or Tobias Landberg.
Contents
Background
Extensive research has been done on the locomotion of other turtles, ranging in size from box turtles to green sea turtles. Limb cycle frequencies in aquatic systems have been reported as reaching up to 2.70 Hz (cycle/s) in Chelonia mydas, 1.7 Hz in Mauremys caspica, 1.0 Hz in Chrysemys scripta and 1.39 Hz in Kinosternon subrubrum in one particular study (Davenport et al., 1984)[1]. More recent studies of terrestrial turtle locomotion have suggested that adult Chrysemys picta may reach speeds up to 1.605 Hz voluntarily, and when forced, can even reach 2.557 Hz (Zani & Clausen, 1994) [2]. There appears to be something of a strong relationship between the typical size of a turtle species and its apparent rate of locomotion, something that will be tested in this research by evaluating where snapping turtles fit in this data set.
Quantified Behaviors
To describe the turtles' behavior, I've classified the turtles' activity into six major behaviors: breathing, horizontal swimming, vertical swimming (up and down), underwater walking, and pausing. Breathing involves surfacing and projecting the tip of the head out of the water to breathe; the turtle does not, however, extend its entire neck out of the water. In Jawless, this behavior is typically prefaced by a bout of horizontal locomotion, in which the turtle simply moves at a constant depth through the water; in contrast, Snippy's breathing generally followed a vertical dive to the bottom of the water, a bout of horizontal locomotion along the bottom (possibly with "underwater walking" interspersed with swimming) and a vertical dive up to the surface of the water. Jawless demonstrated underwater walking relatively rarely, and Snippy only demonstrated two major bouts; while the turtles probably did walk more than it appears on camera, the limbs often aren't visible while the turtle is on the bottom, preventing us from telling whether the turtle is actually walking; in such situations, we assumed that the turtle was in fact swimming.
In order to analyze these behaviors quantitatively, other related factors were brought into play: for instance, the amount of time the turtle spent during one "bout" of a particular behavior (which was often reduced to the amount of time visible on the camera, due to unclear or murky footage) was a useful measurement that helped determine each individual turtle's most common behavior. Another more quantifiable analysis was of limb cycle frequency, which was measured as (number of cycles)/(time), or in short, cycles/s. A cycle was defined as a movement of the front right limb, followed by a movement of the front left limb.
Breath-Dive Analysis
The results of this analysis, which are summarized in Fig. 1 and 2, were different for all three of the turtles, yet there are shared trends in their breathing and diving behavior, and these trends indicate a relationship between these two behaviors.
There was a strong statistical relationship between breath and dive duration. This makes sense, as a longer period of submergence generally creates a larger buildup of carbon dioxide and a greater lack of oxygen, creating a stronger neurological and physiological impulse to breathe (I shall refer to this as the "anoxia response hypothesis", or ARH). Another possible explanation is that a breath could be used to prepare for a longer dive (I will term this the "anoxia preemption hypothesis", or APH). Statistical tests on the correlations found equally strong evidence for the ARH and APH: that is, the order of breaths relative to dives is statistically insignificant.
Another important hypothesis that could be created for this data pertains to the turtles' ecology. For breath duration (Fig. 1), the three turtles were each slightly different, yet Jawless and Lafayette were more similar to each other than to Snippy. This is also true of the distribution of dive durations for the three turtles, summarized in Fig. 2. A potential explanation of this difference is based on the turtles' respective habitats: if deeper water obliges vertical swimming, and vertical swimming takes more energy, then the turtles in deeper water have to breathe more to replenish oxygen levels. The locomotion data has shown that vertical swimming does require faster locomotor speeds than other forms of locomotion; however, because of the small sample size of one turtle in deeper water, the truth of the hypothesis has not been proven one way or another.
Pausing Behavior & Limb Frequency Cycles
The first stage of locomotion analysis in my snapping turtle research was a preliminary analysis of when the turtles paused, and when the turtles were locomoting. This analysis did not include when the turtle was breathing, and did not differentiate between different forms of locomotion. This analysis provided a framework for knowing when the turtles were pausing. The analysis was thus a preparatory step to simply get a sense for the overall behavior trends, before beginning the more specific and quantified analysis.
The major start to analyzing locomotion data came in recording limb frequency cycles for the three turtles. During continuous bouts of locomotion, the number of limb movements on one side (chosen to be the right side, which happens to be more visible in the footage) of the turtle (a logical proxy for the number of limb cycles) is recorded, as is the time duration of the bout. By dividing the movements into duration, a measure of cycles per second for limb cycle frequency is created. This is currently complete for Jawless and Snippy, and is underway for Lafayette.
Plotting the LCF variable over time (using locomotion bout number as a chronological proxy for time), there is clearly a negative trend in Jawless's limb cycle frequency (See Fig. 3). This suggests an escape response, with high initial locomotion rates that decline as the turtle "relaxes," or in more scientific (and accurate) terms, decreases its overall stress level in response to a lack of human intervention and adjustment to the context. The Snippy data does not suggest an escape response, as Snippy's behavior is relatively constant throughout the video. Lafayette, who ranks in inconsistency somewhere between Snippy and Jawless in this footage, may demonstrate an escape response, though the data analyzed so far is inconclusive.
Methods of Locomotion
For these turtles, there are different methods of locomotion that can be differentiated into four behaviors: moving horizontally, swimming up, swimming down, and walking. To monitor limb frequency cycles differing between these behaviors, every bout of locomotion was recorded with the kind of behavior as well as limb cycle frequency.
From an ANOVA it appears swimming horizontally for Snippy was significantly slower than swimming vertically (in either direction) and walking (Fig. 5). However, because Jawless is in shallower water most of his behaviors can only be classified as horizontal swimming, as he has less of an "opportunity" to engage in vertical swimming. Moreover, Jawless and Snippy both demonstrate little *visible* walking, but whereas Jawless is likely not engaging at all in this behavior, Snippy is likely walking with his limbs not visible, making it mostly impossible to document this behavior. Consequently, this allows the analysis to be possible for Snippy, but as most of Jawless' behavior is horizontal locomotion, an ANOVA-like test would not be possible or particularly relevant.
This analysis will soon be complete for Lafayette as well; however, I expect to see results that are very similar overall to Jawless' behavior, though perhaps with more walking behavior than for the other two turtles. However, it should be noted that predictions such as these are not particularly reliable, as the three turtles all demonstrate individual differences.
Behavioral and Morphological Novelties
There are many unusual features of behavioral and morphological traits demonstrated by the turtles. The most striking example comes from Jawless, the 39-pound snapper without a lower jaw. Certainly a well fed turtle! But how is this possible without a lower jaw? Do typical suction feeding mechanisms still work without half the feeding mechanism? We really can't answer this question, but it's certainly an interesting one to think about.
The second key novelty demonstrated by the turtles was "the hitchhiking sunfish." During part of the footage collected from Snippy's Crittercam, a sunfish is visible, swimming along the turtle's neck as the turtle continues to swim, possibly unaware of the fish. While symbiosis between sunfish and snapping turtles would seem to be as probable as symbiosis between former President Clinton and a quarter-pounder Big-Mac, the sunfish certainly isn't phased by the turtle, and at times appears to be eating dead skin off the turtle's neck. Is this an example of some sort of symbiosis or commensalism? Unlike for Jawless' morphology, there is in fact a way to further examine the relationship between the sunfish and the turtle: more Crittercam! As the project is expected to get longer footage from more turtles during Summer 2009, future research will be able to examine this for evidence of a mutualism.
The third key behavioral/morphological novelty from the footage was fish activity surrounding the turtle. One could hypothesize that the one sunfish was just a particularly foolish (though clearly not appetizing enough for the turtle to notice) fish, and that most fish would be more wary of the turtles. The data suggests otherwise: all three turtles' footage includes resting periods during which large schools of fish aggregate in the vegetation surrounding the turtle and take no notice of the large carnivorous powerful-jawed predator watching them from its resting spot. Former research suggests that while juvenile snappers are active predators, adult snappers are in fact sit-and-wait predators, and while the behavior of these turtles in waiting for large schools of fish to slowly aggregate seems similar to such a feeding behavior, there is no predation documented in the footage. Why? Are the turtles simply not hungry? Is there some sort of hope for a skin-cleaning behavior like that the sunfish may have exemplified? As with the sunfish, more data will help answer this question.
Current Analyses
Right now, I've finished analyzing breathing rates for all three turtles, a simple analysis of diving vs. pausing, and the data on limb locomotion rates for Jawless and Snippy. Although an analysis for Lafayette was attempted, unfortunately, Lafayette's footage does not show her limbs as much as would be necessary to complete an analysis, and so the locomotion data is restricted to the two turtles for which the limbs are visible. Consequently, analysis of the actual data is no longer "in progress." At this stage in the research, I am:
- Searching for explanations for breathing behavior based on the turtles' morphology.
- Comparing the two turtles' limb and breath cycles and the factors that may be relevant to each.
- Drawing conclusions about morphology's influence on behavior in these turtles (See section "Conclusions")
- Reviewing literature concerning the locomotion rates of other turtles. (See section "Background")
Conclusions
Coming soon to a turtle near you!
References
- ↑ Davenport J, Munks SA, Oxford PJ. A comparison of the swimming of marine and freshwater turtles. Proceedings of the Royal Society of London.Series B, Biological Sciences 1984 Feb. 22;220(1221):447-75.
- ↑ Zani PA, Claussen DL. Voluntary and forced terrestrial locomotion in juvenile and adult painted turtles, chrysemys picta. Copeia 1994 May 16;1994(2):466-71.