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Origin, evolution, and mechanics of rapid defensive tail vibration in advanced snakes (Serpentes: Caenophidia): rattlesnakes, Agkistrodon, and non-pitviper Colubroids

Reiserer, Randall S.

Chiricahua Desert Museum

Rodeo, New Mexico USA

Smith, Charles F.

Department of Biology

Wofford College

Spartanburg, South Carolina USA

Chiricahua Desert Museum

Rodeo, New Mexico USA

Schuett, Gordon W.

Department of Biology ǀ Neuroscience Institute

Georgia State University

Atlanta Georgia USA

Chiricahua Desert Museum

Rodeo, New Mexico USA

Defensive tail vibration is widespread in advanced (colubroid) snakes and has been assumed to represent the ancestral motor pattern from which rattlesnakes derived their rhythmic tail-shaking behavior. Snakes display a variety of rhythmic tail behaviors, including distally traveling waves displayed during caudal luring. We currently are exploring the motion dynamics of defensive tail motion from all clades of rattlesnakes and their outgroups. Thus far we have examined video footage, including high frame rates (500-1000 fps), of 12 species of Crotalus, several Sistrurus and copperheads (Agkistrodon contortrix) to characterize the movement patterns present in rattlesnakes. Other colubroid species will be investigated. To date we found both similarities and differences in the defensive tail motor patterns of copperheads and rattlesnakes. Our early findings suggest that the rattlesnake tail-shaker bears as much or more resemblance to the rapid undulations seen in caudal luring as to defensive tail vibration in a close pitviper relative, copperheads. Pygmy rattlesnakes (Sistrurus miliarius) display at least two types of defensive tail motor patterns that resemble copperhead tail vibrations more than the tail-shaking of advanced rattlesnakes. In general, rhythmic tail motions become more conservative (reduced amplitudes) in more advanced rattlesnakes (e.g., C. adamanteus, C. atrox, C. scutulatus), with single flexion points at or near the tail terminus, whereas copperheads and pygmy rattlesnakes employ variable displays that can involve multiple flexion points. In rattlesnakes, the rattle resonates via prominent distally traveling waves, a motion that resembles the undulations seen during caudal luring rather than the arching oscillations seen in copperheads and other colubroids. Therefore, we preliminarily suggest that the tail motor pattern dynamics involved in rattlesnake rattling involve elements derived from both ancestral defensive behavior and caudal luring, and that the conservative defensive tail motor patterns of more advanced rattlesnakes resemble caudal luring undulations to a greater extent than the tail vibrations and other motor patterns of non-rattlesnake colubroids.


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