Not many people would think of rattlesnakes as a ‘simple system’. But when Kevin Conley looks at these animals, he doesn’t just see a terrifying viper. To him they are the perfect simplified muscle system. Unlike complicated mammalian muscles, the rattlesnake shaker muscle is built from a single fibre type, which means that looking at the muscle that drives the rattlesnake’s fearsome rattle ‘is like looking at a single muscle cell’ says Conley. These muscles also have a mysterious energy saving property: as the rattle frequency increases, the rattling force also increases, but the energetic cost of each muscular twitch stays constant. The result is that the snake uses the same energy for a fast contraction during high frequency rattling as it does for a slower contraction when rattling less vigorously. Conley had wondered how this could be, until he and Brad Moon analysed the reptile’s furious rattling. They discovered the important mechanical tricks that the snakes use to improve performance without increasing energy use: by contracting more strongly for a shorter period of time, the muscle uses the same amount of energy as it would for a longer, weaker contraction.
Moon explains that the rattlesnakes were fantastically cooperative throughout the experiment ‘because they are so irritable’. He secured each snake in a plastic box with its tail sticking out of a hole, ready to record the rattle. Moon needed to measure the rattle muscle’s force as the rattle’s pitch rose. Fortunately, as the snakes warm up, their rattling frequency increases too. With a snake resting safely in its box, he had plenty of time to gently warm it up while measuring the muscle’s increasing force.
Measuring the twitch tension from individual muscle fibres was complicated. With the help of colleagues in the Zoology department, Moon built a force transducer to measure the net force generated as the muscle contracted. But before he could calculate the muscle’s twitch tension he also measured the fibre’s cross-sectional area by looking at the muscle’s volume and the direction of the muscle’s fibres.
He found that the muscle’s twitch tension increased as the rattle’s frequency rose. But as a stronger twitch lasted for a shorter length of time, the snake used the same energy per contraction, no matter how fast it rattled.
Moon was also surprised to see that the reptiles changed the way their rattles moved as they warmed up. At first they slowly waved the rattle from side to side, making a slow buzzing sound, but as he raised the temperature, the snakes waved the rattle less and began to twist it more to produce a faster buzzing sound. Moon says that the trade-off between the two buzzing styles helps the snake to conserve energy, allowing it to make a louder noise at little extra cost.
So while the rattlesnake’s shaker muscle has helped Conley and Moon answer some fundamental questions about muscle physiology, it also gives a predator enough warning to help a threatened snake ward off its next unwanted encounter, even with an experimental biologist!