MOON, B. R.; Univ. Louisiana, Lafayette: From physiology to fitness: low-impact aerobics in rattlesnakes

Specialized features may incur costs to organisms that may reduce fitness by diverting energy away from reproduction. However, the energetic and fitness costs of having and using specialized features are rarely measured. The specialized shaker muscles of rattlesnakes are an excellent system for studying the effects of specialized features on fitness. Shaker muscle contractions generate rattling, which is one of the fastest sustainable vertebrate movements. Shaker muscle metabolism exceeds the maximum rates reported for most other vertebrate muscles. These features suggest that the high cost of sustaining shaker muscle contractions during rattling may reduce short-term fitness by using energy that could otherwise contribute to reproduction. In this study, I use simple energetic modeling to determine whether the cost of rattling in western diamondback rattlesnakes (Crotalus atrox) is high enough to reduce clutch size, which I use as a simple measure of fitness. The results show that if a 300-g snake rattled for 6.6 min every day of its active season, then the annual cost of rattling would be equal to the annual cost of movement (63 kJ), and would represent only 6% of the total annual energy budget. If this much energy were diverted away from reproduction, then clutch mass could be reduced by up to 6.5 g or 1.7% of an average clutch, which would be unlikely to reduce the number of offspring in the clutch. Less frequent or prolonged rattling would have smaller effects on clutch size and fitness. Many defensive specializations, such as rattling, are used only periodically and probably can be maintained for low cost and with low impact on an animal’s energy budget and fitness.