The Dorsal Pigmentation Pattern of Snakes as an Antipredator Strategy: A Multivariate Approach

Each species of the snake fauna of North America north of Mexico was qualitatively scored into one of five dorsal pattern classes and according to 16 behavioral-ecological variables that express the degree of exposure to visually oriented predators and the characteristic response to threatened predation. Multiple discriminant analysis was employed to identify the variables that best distinguish between the pattern types. These most important variables were, in general, behavioral as opposed to habitat variables. Irregularly banded and blotched-spotted patterns are associated with an antipredator strategy of defense rather than flight, and these patterns likely function disruptively to minimize initial detection by predators. Striped and unicolored-speckled patterns are associated with antipredator strategies emphasizing flight more than defense. The advantages of these patterns for a strategy of flight probably lie in their elevation of the threshold velocity for detection of movement and in their reduction of apparent velocity, because these visual effects retard the forward shift of predator visual focus necessary to keep the fleeing snake in sight. On the behavioral-ecological axes, the regularly banded pattern is generally intermediate between the antipredator strategies of defense and flight, and it may combine the advantages of disruptive coloration with those of a uniform pattern through the phenomenon of flicker fusion of the successively identical bands. As predicted, if flicker fusion is an adaptive function of the regularly banded pattern, interspecific variation in band number is negatively correlated with a morphological index of escape speed, positively correlated with body size, and curvilinearly related to (though nonsignificantly) the light-dark ratio of the band cycle. Furthermore, intraindividual variation in band packing is in the direction predicted by speed differentials between middle and rear portions of the body. The multiple discriminant analysis was almost three times as successful in classifying taxa to pattern type as would be expected by chance.