Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Ecology and Evolutionary Biology

Major Professor

Gordon M. Burghardt

Committee Members

Susan E. Riechert, Mary Ann Handel, Arthur C. Echternacht, Arthur W. Jones, Roland M. Bagby


Comparative studies of snake behavior were used to confront three related conceptual issues in ethology: (i) Can behavior evolve? (ii) If so, how can the origins of similarities and differences in behavior among animals be assessed? (iii) What is the significance of this information for evolutionary biology?

Some workers have recently asserted that behavior does not evolve and that behavioral homologies are generally not discernible. A consideration of genetics and developmental biology suggests that both points of view reflect an unrealistic structure-function dualism. In a strict sense, only transcriptional products are genetically determined; all other aspects of the phenotype are dependent on epigenetic effects. At the molecular level, all aspects of the phenotype are variable, dynamic, and have an extended ontogeny subject to environmental influences. There clearly can be a relationship between nucleotide sequences in the genome and behavior patterns. These genes are subject to mutation, drift, selection, and gene flow. Behavior thus can evolve.

Chance, experience, homology, and convergence are best dealt with operationally as alternative hypotheses of resemblance that are potentially falsifiable by comparative and experimental studies. This requires samples of related and unrelated taxa that vary in potential experimental and selective constraints.

Constriction is an action pattern used for prey killing by at least 131 species of snakes in six families. Patterns of variation in four characters were used to describe the constricting behavior of 75 species. Twenty-seven species (13 genera) in the advanced family Colubridae exhibited intergeneric, interspecific, and individual variability in coil application movements. Each character state occurred in more than one taxon, and 19 patterns (based on combinations of character states) were observed. One or two patterns were usually consistent within a genus, but Lampropeltis was highly variable. Boaedon, Elaphe, Pituophis, and Trimorphodon used similar constricting behavior, Arizona, Spalerosophis, and Spilotes were distinct from these genera and from each other. Possible implications of these results for systematic studies are discussed.

Forty-eight species (26 genera) in the primitive families Acrochordidae, Aniliidae, Boidae, and Xenopeltidae usually used a single pattern, despite differences in prior experience, size, shape, habitate, and diet. This implies the shared retention of an action pattern used by their common ancestor. Since these taxa diverged no later than the early Paleocene, constriction must have been used as a prey killing tactic very early in the evolution of snakes. It is suggested that the oldest well-known snake, Dinilysia patagonica of the Cretaceous, was probably a constrictor, and that constricting was an ethological key innovation in the early evolution of snakes.

Defensive displays were compared among 124 species in five families of snakes. Type of defensive behavior was significantly associated with terrestrial or fossorial habits in genera with tail displays and genera with horizontal head displays, and with arboreal habits in genera with vertical head displays. The results of this comparison suggest that convergence has been widespread in the defensive behavior of snakes.

Taken together, these studies point to broader issues: What kinds of motor patterns are stable over long periods of evolutionary time? What kinds change rapidly and why? How are the rates and directions of change constrained by other factors? Rigorous comparative studies might provide answers to these and other questions regarding the evolution of behavior and the role of behavior in evolution.

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