Mechanical trade-offs explain how performance increases without increasing cost in rattlesnake tailshaker muscle
Brad R. Moon1,*,
J. Johanna Hopp2 and
Kevin E. Conley1,2
1 Department of Radiology, Box 357115 and
2 Department of Physiology and Biophysics, Box 357290, University of Washington Medical Center, Seattle, WA 98195, USA
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Fig. 1. The sinusoidal rattling force of a western diamondback rattlesnake (Crotalus atrox) rattling at 34.5°C and 91 Hz. Snake mass 112 g, tailshaker muscle mass 1.6 g, rattle mass 0.78 g.
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Fig. 2. Peak muscle tension increases dramatically with twitch frequency during rattling in western diamondback rattlesnakes (Crotalus atrox). These tension values are based on the forces recorded with the optical transducer. Each point represents the mean value of five continuous contractions from each of nine snakes. The multivariate regression results for twitch frequency, muscle mass and peak muscle tension are given in Table 1.
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Fig. 3. Individual twitch curves from the tailshaker muscles of a western diamondback rattlesnake (Crotalus atrox) rattling at 10°C (dashed line) and 30°C (solid line). The area under each curve represents the tension–time integral (TTI) for the half-cycle. In this example, the peak tension at 30°C is 3.1 times the value at 10°C, whereas the TTI at 30°C is only 1.5 times the value at 10°C. Mean changes in tension and TTI are shown in Table 2 and Fig. 4. Snake mass 810 g, tailshaker muscle mass 6.2 g.
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Fig. 4. Normalised increases in tailshaker muscle peak tension (open circles) and tension–time integral (TTI, filled circles) per twitch during rattling in western diamondback rattlesnakes (Crotalus atrox). Between 10 and 30°C (26–77 Hz), muscle twitch tension increased an average of 3.5-fold whereas TTI increased by only 1.3-fold. The trend for tension increase is y=0.084x–1.100 (r2=0.79, P<0.001), and that for TTI increase is y=0.016x+0.660 (r2=0.47, P<0.001); N=9 snakes. The slopes are significantly different, as indicated in the text.
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Fig. 5. The trade-off between twitch tension and duration in tailshaker muscle during rattling in western diamondback rattlesnakes (Crotalus atrox). The tension–time integral (TTI) is approximately equal to the product of tension and period, so the dramatic increase in twitch tension trades off with the decrease in twitch period to minimise change in TTI. A TTI value of 1 would indicate an exact trade-off between twitch tension and period. Columns indicate mean values and error bars indicate 1 S.D. in a sample of five snakes.
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Fig. 6. The mechanical work of rattling in western diamondback rattlesnakes (Crotalus atrox). The circles indicate rattlesnake data; the lines indicate the work required to oscillate a pendulum of similar size, shape and mass to a large rattle when pendulum displacement is constant (dashed line) and when it decreases as oscillation frequency increases (solid line). The regression results for mechanical work are given in Table 1.
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Fig. 7. Rattle inertial power in western diamondback rattlesnakes (Crotalus atrox). The circles indicate rattle data; the lines indicate the power required to oscillate a pendulum of similar size, shape and mass to a large rattle when pendulum displacement is constant (dashed line) and when it decreases as oscillation frequency increases (solid line).
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© The Company of Biologists Ltd 2002
