Thus we are left with two hypotheses about the evolutionary origin of the rattlesnake rattle—the “tail-vibration as signal precursor hypothesis” and the “caudal luring as signal precursor hypothesis.” The purpose of this study is to use distributions of tail behaviors across modern species of New World pit vipers to better understand the tail behaviors that ancestral rattlesnakes might have had. This will inform the signal precursor debate and will more generally lead to a better understanding of rattle evolution. Such a phylogenetic approach to understanding rattlesnake evolution has not yet been published in the literature. The first step in this project was to map observations of caudal luring and tail-vibration in different New World pit viper species across the entire phylogeny. If one of these behaviors were clustered in close rattlesnake relatives while the other was concentrated in more distant relatives, it could suggest something about the likely behaviors displayed by ancestral rattlesnakes. If ancestral rattlesnakes did not display one of the tail behaviors, then it would of course be exceedingly unlikely that the rattlesnake rattle evolved to enhance that signal. The second step in this project was more closely analyzing the specifics of one of the behaviors and how it changes across the New World pit viper phylogeny. Tail-vibrating was examined because there is wider support in the literature for that behavior being the signal precursor. Different New World pit viper species served as proxies for the evolutionary history of the rattlesnake, depending on how closely related each species is to rattlesnakes. In this way, if the behavior changed in any specific way in closer rattlesnake relatives, it could inform how that behavior changed along the ancestral rattlesnake lineage.