If I heard the sound of a rattlesnake's rattle, I'd run away as quickly as possible. Surprisingly, some of my colleagues not only lack this reflex, but go out of their way to elicit and listen closely to snake rattling. In doing so, they are learning some interesting things about the development of this behavior and the underlying tail musculature that drives it.
Within the world of muscles, those that shake the rattlesnake's tail are an impressive bunch, able to contract at very high frequencies for long durations. Recently, Brad Moon and Alexa Tullis observed that the frequency with which a snake shakes its rattle changes over ontogeny, increasing considerably between newborns and adults. This was particularly intriguing given Moon's previous work documenting a small decrease in contractile frequency between medium and large-sized adults. Apparently, contractile performance first increases, then decreases, in relation to body size in rattlesnakes. Not content with just documenting this pattern, Moon and Tullis then set out to determine the underlying causes.
They hypothesized that two disparate mechanisms were at work, one biochemical, and the other mechanical. They believed that shifts in muscle metabolic capacity likely explained the initial increase in contractile performance between newborns and adults. In contrast, they proposed that the subsequent decrease in performance as animals continued to grow was linked to simple mechanics. Bigger snakes have a more ossified, and thus disproportionately heavier, bone in the base of the rattle, and it is harder for the tail muscle to move these larger sound makers.
To test their metabolic hypothesis, the researchers assayed the activity of citrate synthase (CS), an important enzyme for aerobic metabolism, in tail shaker muscle from snakes ranging between 11 and 911 g. CS activity was significantly and positively correlated with contraction frequency, i.e., CS activity changed with body size in a manner similar to contractile frequency,first increasing rapidly then slowly decreasing with body size. To test their biomechanical hypothesis, Moon and Tullis measured how the mass of the rattle,using its first segment as a proxy, changed with growth, and compared that to how the cross-sectional area (CSA) of the tail shaker muscle changed. Because muscle force is proportional to its CSA, rattle mass multiplied by rattle acceleration should be proportional to tail shaker muscle CSA, and to maintain a constant rattle acceleration would require rattle mass to increase proportionately with muscle CSA. In fact, rattle mass increases faster than muscle CSA as snakes grow, implying that rattle acceleration must decrease with size. Because the distance a rattle moves during shaking doesn't decrease with body size, this reduced acceleration necessitates a decrease in rattle frequency.
In summary, the increase in rattle frequency found between newborns and adults can be explained by a concomitant increase in tail shaker muscle aerobic capacity (i.e., CS activity). A subsequent small reduction in this capacity, in addition to a mismatch between the strength of the tail muscle and the mass of the rattle it must shake, underlie the subtle decrease in rattle frequency observed as adult snakes continue to grow. See what you could learn if you were brave enough to stick around and listen carefully to a diamondback's rattle...
