Doctoral Dissertations

Date of Award

5-1992

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Life Sciences

Major Professor

Gordon M. Burghardt

Committee Members

Arthur C. Echternacht, John Gittleman, John Malone, Richard Saudargaus

Abstract

Habituation is a relatively simple learning process which has been documented in a number of animal species and which allows an animal to stop responding to irrelevant aspects of its environment. As a basic learning process, habituation has been important in the formulation of theories of learning and memory as well as in the study of the physiological basis of nonassociative learning. How evolutionary considerations affect the expression of habituation is an area with much less emphasis. Habituation of antipredator behaviors and responses to chemical prey extracts was examined in four species of garter snakes (Thamnophis). and four separate experiments were performed. The first experiment compared habituation processes in four species of three-year old laboratory-reared garter snakes. (Thamnophis butleri, T. radix, T. melanogaster and T. sirtalis). In this experiment, all animals were given a standard habituation test to three different threatening stimuli (differing in biological significance) and to two different chemical prey extracts in a within-subjects design. Major questions examined in this experiment were: 1) are the level of response and the rate of habituation related to the biological relevance of the stimulus used, 2) do behaviors show different trajectories over trials, 3) do all species show similar stimulus and behavioral differences, and 4) are the habituation profiles (response curves) similar between two response systems, i.e., antipredator and feeding behaviors. It was found that animals were most responsive in a number of antipredator behaviors to the more biologically relevant stimuli, but differential habituation was shown in only a few behaviors, e.g. orienting. In the extract test, responsiveness and habituation rate were not related to the natural diets of the four species, probably because of conditioning effects due to prior feeding experience. In the antipredator test, behaviors clearly differed in how they changed across trials. Escape and startle behaviors occurred very early in the test and then habituated rapidly. Orienting behavior was very persistent and often increased over trials. Exploratory behavior also increased over trials. There were differences between species in responsiveness and propensity to habituate. Finally, correlations were found between absolute decreases in responses to the weakest threatening stimulus and responses to the two extracts. When examining a relative measure, clustering of curves was observed, especially for the T. melanogaster. The second experiment was similar to the first except that neonates of two species (T. radix and T. melanogaster) were used in a between-subjects experimental design, and only two threatening stimuli were used. Similar questions were examined as in the first experiment. Greater responsiveness in several behaviors was found to the more biologically relevant stimulus (a hawk model) in the T. melanogaster but not in the T radix. The T radix did show differential habituation to the two stimuli in orienting behavior. In the extract test, the T. melanogaster, a specialist species, responded most to its natural diet, whereas T. radix, a generalist species, showed no differentiation between prey extracts. The third experiment dealt with the development of habituation of antipredator behaviors in one species of garter snake, T. melanogaster. Maturational and experiential effects on responsiveness and habituation rate were examined. Striking, fleeing and flips increased between one and 61 days of age, and striking of one day old animals habituated more rapidly than that of 61 day old animals. When tested eight months after the Day 61 test, there was a general decrease in responsiveness and more rapid habituation of striking behavior. Previous test experience suppressed striking behavior but had no effect on orienting. Finally, there was individual consistency between tests for striking propensity and habituation curves for striking. The final experiment examined habituation of antipredator behaviors and responses to chemical prey extracts in wild-caught adult animals (T. sirtalis and T. radix) to assess the generality of findings for laboratory-reared animals. It was found that results for wild-caught animals were generally similar to those of animals raised in the laboratory. There were clearly stimulus, behavioral, individual, and species differences in responsiveness and habituation rate. None of the current theories of habituation can completely explain these results, and a theory of habituation incorporating these differences in an evolutionary framework is needed.

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