We suggest that the capability to retain terminal scale (here renamed sclerophymal) exuviae that were mobile, i.e., origin of the protorattle, was contingent on prior evolution of longitudinal bilobing of the sclerophyma. Two hypotheses for initial functional advantage of such bilobing are tested: the luring effectiveness hypothesis based on putatively greater visual attractiveness during caudal luring, and one version of the dual contact hypothesis based on putatively greater sound intensity and/ or greater audiospectral equitability during tail vibration. Simulated caudal luring that included a bilobed sclerophyma was not more attractive to fence lizards than luring with a conical sclerophyma. Sound intensity produced from mechanically vibrated models of bilobed sclerophymae was lower than sound intensity from other types of sclerophymae, whereas broad-bandedness of the sound was similar to that of other types. In the absence of support for either hypothesis, we discuss possibilities for future research, particularly the need to determine the form of the relationship between sound intensity and the force exerted by the sclerophyma.
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Sisk et al. (1997) studied this question.