“Behavioral plasticity and the origins of novelty: the evolution of the rattlesnake rattle”

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Bradley C. Allf, Paul A. P. Durst, and David W. Pfennig

The rattlesnake rattle might have evolved via elaboration of a simple behavior: vibrating the tail when threatened

The evolution of the rattlesnake rattle: the snake tail’s tale

Numerous species of snakes, such as this eastern black racer (Coluber constrictor), vibrate their tail when threatened. This simple behavior appears to have paved the way for the evolution of the rattlesnake rattle. Photographed near Hoffman, North Carolina.
(Credit: David W. Pfennig)

How does behavior influence evolution? Can changes in behavior precede—and possibly even facilitate—the evolution of novel morphological features? This topic is contentious, primarily because of a lack of compelling examples from natural populations.

To address this issue, Bradley Allf, Paul Durst, and David Pfennig (three evolutionary biologists at the University of North Carolina at Chapel Hill) focused on one of nature’s most unique morphological features: the rattlesnake rattle. The rattlesnake’s rattle is a truly novel trait, having evolved only once in rattlesnakes; no other snake species (of more than 3000 species) has evolved this feature.

The researchers specifically asked whether rattlesnake rattling behavior—and therefore, the rattle—originated from a simple behavior present in many species of snakes: vibrating the tail when threatened. They tested their idea by simulating attacks and measuring the response of 155 captive snakes from 56 species. These included 38 species of Viperidae (the family to which rattlesnakes belong) and 18 species of Colubridae (the largest snake family, nearly all of which are nonvenomous or have very weak venom). By filming captive (as opposed to wild) snakes, the researchers were able to control for variation in environmental variables (such as temperature) that can affect tail vibration behavior.

Rattlesnakes have evolved one of nature’s most spectacular signals: a vibrating rattle. Here, a western diamondback rattlesnake (Crotalus atrox) vibrates its rattle in response to a perceived threat near Portal, Arizona.
(Credit: David W. Pfennig)

Remarkably, Allf and colleagues found that the more closely related a species was to rattlesnakes, the more similar that species was to rattlesnakes in the duration and rate of its defensive tail vibration behavior. Thus, tail vibration by rattle-less ancestors of rattlesnakes might have served as the evolutionary precursor to rattlesnake rattling behavior. Moreover, this change in behavior—vibrating the tail increasingly more rapidly and longer when threatened—might have favored the evolution of a structure that augmented this behavior: the rattle. Thus, these data emphasize how changes in behavior might precede—and possibly even facilitate—the evolution of morphological novelty. Read the Article