Individual differences and variability in the timing of motor activity during walking in insects

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Abstract
The uniformity of the neural physiology of an animal population is a fundamental, rarely tested assumption in most neurophysiological work. In this study, the variability of the timing between the movements of pairs of legs during free walking in cockroaches was assessed. Phases (a measure of timing) of motor bursts in muscles of legs in the American cockroach, Periplaneta americana, were calculated for insects walking straight over a flat, level surface. Student's t, Wallraff, Mann Whitney and Watson U2 two-sample tests were used to compare the phases of motor bursts of the same pairs of legs in different insects. The comparisons showed that in spite of the homogeneity both of the animal population and of the conditions under which the insects walked, most of the inter-leg phases of the animals that were compared were significantly different statistically. Further testing of greater numbers of insects using analysis of variance to test for population uniformity confirmed that the insects we tested were not members of a single statistical population with respect to the timing of motor bursts of the legs during walking. We infer that this unexpectedly large variability in a population thought to be relatively homogeneous reflects subtle but biologically significant differences between animals. The possible sources of these differences and their consequences for the study of behavior and its physiological basis are discussed.