Whole-body acceleration and inertial effects of flippers during swimming in the green sea turtle (Chelonia mydas) ARV 1Florida
1 Rivera ,
2 Rivera ,
G
Atlantic University;
2Iowa
RW
3 Blob ,
J
1 Wyneken
State University;
3Clemson
University
ABSTRACT
RESULTS
CONCLUSIONS
Sea turtles swim using synchronous, dorsoventral movements of elongate flipper-shaped forelimbs to propel themselves through water. These patterns resemble the flapping motions of flight and have been shown to produce thrust during both the upstroke and downstroke phases of the limb cycle, although thrust production during upstroke is less than half of that during downstroke. While thrust has been examined, drag and the cumulative effects of drag and thrust on whole-body acceleration during upstroke and downstroke remains unknown. Furthermore, it is unknown if the lower thrust produced during upstroke is able to overcome the effects of drag. To compare the relative contributions of upstroke and downstroke to forward motion in swimming sea turtles, we analyzed high-speed video of rectilinear swimming by juvenile green sea turtles (Chelonia mydas). Our results show that maximum whole-body acceleration is considerably higher during downstroke than during upstroke. In addition, maximum acceleration during upstroke is near zero, thus indicating that positive acceleration is primarily limited to downstroke. These patterns are likely related to the production of greater average and peak accelerations of the flipper during downstroke, which are facilitated by the hypertrophied pectoralis muscles of sea turtles. Finally, we also calculated the acceleration of the true center of mass and used these data to evaluate the inertial effects of flipper motion.
• Flippers account for 5.8% of total mass; Location of COM and proportions of total mass for segments are provided in Table 1 and Fig. 2
• Turtles decelerate at the start of upstroke, followed by a plateau in acceleration late in upstroke; acceleration increases substantially at the start of downstroke.
• Forward acceleration of the body is negative at the start of upstroke (i.e., velocity decreases), then increases to near zero until the end of upstroke; acceleration then becomes positive (i.e., velocity increases) during downstroke (Fig. 3A) • Maximum forward acceleration of the body is significantly greater during downstroke (P=0.002; Fig. 3A, B) • Inertial effects are greatest at phase transitions (i.e., upstroke-to-downstroke and downstroke-to-upstroke; Fig. 3C)
• Maximum forward acceleration during upstroke is significantly greater than zero for the body (P<0.05), but not for the COM (P=0.09)
Body Proximal Flippers Distal Flippers
METHODS
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N=83 20
10
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-10
Upstroke
Downstroke
-20
30
1. Wyneken J. 1997. Sea turtle locomotion: Mechanisms, behavior, and energetics, In The Biology of Sea Turtles (ed PL Lutz and JA Musick), 165-198. Boca Raton, FL: CRC Press.
Body COM Inertia
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2. Davenport J, Munks SA, Oxford PJ. 1984. A comparison of the swimming of marine and freshwater turtles. Proc R Soc Lond B 220: 447-475.
N=71 10
3. Walker WF, Jr. 1971. Swimming in sea turtles of the family Cheloniidae. Copeia 1971: 229-233.
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Percentage of Swimming Cycle
Downstroke
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Percentage of Swimming Cycle
D 30
25
P=0.002 N=83
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Downstroke
Max Forward Acceleration (BL/sec2)
B
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5. Iriarte-Diaz J, Riskin DK, Willis DJ, Breuer KS, Swartz SM. 2011. Wholebody kinematics of a fruit bat reveal the influence of wing inertia on body accelerations. J Exp Biol 214: 1546-1553. 6. Hedrick TL. 2007. DLTDataViewer2. [www.unc.edu/~thedrick/software1.html]
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P<0.01
N=71 25
7. Walker JA. 1998. Estimating velocities and accelerations of animal locomotion: a simulation experiment comparing numerical differentiation algorithms. J Exp Biol 201: 981-995. 8. Rivera ARV, Wyneken J, Blob RW. 2011. Forelimb kinematics and motor patterns of swimming loggerhead sea turtles (Caretta caretta): are motor patterns conserved in the evolution of new locomotor strategies? J Exp Biol 214: 3314-3323.
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ACKNOWLEDGEMENTS
P=0.1
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0
Body
COM
Upstroke
Figure 1. Images depicting digitized landmarks in lateral (A) and ventral (B) views. Point 5 in lateral view used to define vertical flipper displacement and the switch from upstroke to downstroke. Images shown at different scales.
4. Hedrick TL, Usherwood JR, Biewener AA. 2004. Wing inertia and wholebody acceleration: an analysis of instantaneous aerodynamic force production in cockatiels (Nycphicus hollandicus) flying across a range of speeds. J Exp Biol 207: 1689-1702.
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0
Max Forward Acceleration (BL/sec2)
B
• Maximum inertial forces occur at the beginning of upstroke and downstroke. In contrast, maximum whole-body accelerations occur midphase for both upstroke and downstroke. This temporal shift means that maximum accelerations are less influenced by inertial effects.
REFERENCES Forward Acceleration (BL/sec2)
• In this study we evaluated whole-body acceleration during the two phases of the limb cycle during swimming in green sea turtles (Chelonia mydas). Specifically, we examined how the reported disparity in thrust production2 affects whole-body acceleration, and how inertial forces generated by flipper motion influence this difference. Finally, we tested whether forces produced during upstroke are sufficient to overcome drag and produce positive acceleration.
A
• Inertial effects of flipper motion significantly increase maximum forward acceleration of the body during downstroke, but not upstroke. The reason for this appears to be that maximum acceleration during upstroke coincides temporally with a near-zero plateau of inertial forces.
C Forward Acceleration (BL/sec2)
A
• Studies of flight in birds and bats have demonstrated that inertial effects associated with accelerating wings (i.e., forelimbs) can have significant effects on whole-body acceleration4-5. However, it is unknown if the forelimb flippers of sea turtles may have similar effects.
• Chelonia mydas (5 individuals, 83 total trials; carapace lengths = 66-76mm) • Linear swimming sequences filmed in lateral and ventral views at 200 Hz • Videos digitized using DLTDataViewer26 • 8 landmarks in lateral view (Fig. 1A); 12 landmarks in ventral view (Fig. 1B) • Morphological measurements from 3 deceased turtles used to determine anteroposterior position of COM along the body axis (defined by points 11 and 12) and proximal-distal position of COM for both the proximal (point 2 to 3) and distal (point 3 to 5) flipper segments • These values, along with proportional masses of each segment, used to calculate overall COM in each frame, thus accounting for changes in flipper position throughout the cycle • Calculated flipper kinematics and forward displacement of the body (based on point 12; Fig. 1B) and the calculated COM • Data were smoothed and acceleration profiles generated using quintic splines7-8 • Inertial acceleration calculated as the difference between body and COM accelerations5 • Mixed-model ANOVAs used to compare upstroke vs downstroke, as well as body vs COM • One sample t-tests used to compare body and COM accelerations to zero
Mass Proportion 94.2% 2.3% 3.5%
Figure 2. Circles indicate position of center of mass (COM) for the body (pink) and proximal (blue) and distal (yellow) flipper segments. COM position measured as proportion of length relative to anterior edge of plastron or proximal end of respective flipper segment. Dashed white line represents the horizontal position of the calculated COM for the depicted video frame.
• Sea turtles are distinctive among turtles in having forelimbs that are hypertrophied into long, flattened flippers that are moved up and down in a synchronized motion commonly described as flapping or aquatic flight1-3.
• Although net acceleration is negative during upstroke, small positive whole-body accelerations are generated during upstroke. • Although the flippers account for only a small proportion of the total mass, our results indicate that without the inertial influence on wholebody acceleration, maximum upstroke forces are not capable of producing forward acceleration (accelerations greater than zero).
Table 1. Location of COM and mass proportions of body and flipper segments
INTRODUCTION
• This method of swimming has been demonstrated to produce continuous thrust, though thrust produced during upstroke is less than half of that produced during downstroke2. However, these measures of thrust were from tethered turtles, as such it is unclear how hydrodynamic drag and thrust interact to influence whole-body acceleration during swimming.
• There is a negative net acceleration for upstroke and a positive net acceleration for downstroke (P=0.001).
• Maximum forward acceleration of the body (including inertial effects) and COM (excluding inertial effects) differ significantly for downstroke (P<0.01), but not upstroke (P=0.11) (Fig. 3C, D)
COM 45% 42% 38%
• Maximum acceleration during downstroke is approximately ten-fold greater than maximum acceleration during upstroke.
Body
COM
Downstroke
Figure 3. Comparisons of body, COM, and inertial acceleration for upstroke and downstroke in Chelonia mydas during straight-line swimming. Trials were normalized to the same duration and variables interpolated for 101 equally spaced increments (representing 0-100%) through the swimming cycle. Significance values are listed on plots. A, C: Vertical red line indicates the mean percent at which the switch from upstroke to downstroke occurs. Plots show percentage of the swimming cycle on the xaxis. B, D: Bars indicate means ± S.E.M. for upstroke (maroon) and downstroke (blue). (A) Average forward acceleration of C. mydas throughout the swimming cycle; zero-line indicated for clarity. Symbols represent means ± S.E.M. and are plotted for every 2% of the cycle. Timing and magnitude of maximum acceleration indicated with blue symbols. (B) Maximum forward acceleration during upstroke and downstroke. (C) Forward acceleration of the body (black) and true COM (pink), and inertial effects of flipper motion (green). (D) Maximum forward acceleration during upstroke and downstroke for the body and true COM.
Thanks to Erin Dougherty for assistance with animal care and data collection. We are also grateful to Kim Redmond, Chris Bridges, Casey Gosnell, Libby Thoke, and Nichole Bennett for their help with video analysis. This work was conducted under Florida FWC permit #073 to JW.
