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UBC Theses and Dissertations

Behavioural adaptations to stream velocity in the guppy, Poecilia reticulata Crow, Richard Thomas


Populations of guppies living in different streams vary in their "behaviour and morphology. Some populations live in fast-flowing streams while others occupy slow-flowing water. I hypothesized that behavioural differences between the populations may represent adaptations to stream velocity. I tested seven predictions based on this hypothesis in two experiments. In Experiment I, I. examined the behaviour of laboratory reared guppies descendant from three natural populations. These guppies were examined at .03, .07, .10, and .20 m/sec stream velocities. Experiment II used wild-caught guppies from four populations. Their courtship behaviour was examined in still water (.00 m/sec) and at .08 m/sec velocity. As predicted, in Experiment I guppies from slow-flowing water showed l) greater cohesion, 2) less aggressive behaviour, and 3) a greater preference for the surface region of stream tanks than fast water fish. Also as predicted, slow water males performed fewer sigmoid displays "but more thrusts and gonopodial swings than fast water guppies. The persistence of these behavioural differences in laboratory raised descendants of isolated populations demonstrates a genetic contribution to the differences. Therefore the differences are presumably the product of natural selection and represent evolutionary adaptations. Experiment I also showed that stream velocity directly affects the performance of courtship behaviour by males. The frequency of sigmoid displays and gonopodial swings decreased markedly as stream velocity increased. As stream velocity increased, fast water males maintained their frequency and duration of sigmoid displays better than slow water males, thus providing evidence for "behavioural adaptation to stream velocity. The higher ratio of sigmoid displays : thrusts exhibited by fast water fish may also represent an adaptation to stream velocity. Stream velocity had no direct effect on group cohesion and aggression in the guppy. However, it directly affected guppy stream depth preference. Slow water fish could not maintain position at the top of the stream in faster-flowing water, whereas fish bred from an intermediate stream velocity population were able to do so. Fast water fish always preferred the bottom of the stream bed. In Experiment II wild-caught fish from fast and slow-flowing streams were tested together in two stream conditions. Again, as predicted, slow water fish performed fewer sigmoid displays but more thrusts and gonopodial swings than fast water fish. Because of the slow velocity used during the experiment, no effect of a fast stream velocity on courtship behaviour was obtained. During the experiment males generally preferred to court females from their own population. These preferences may represent partial isolating mechanisms. I concluded that differences between guppy populations in courtship behaviour and stream depth preference represent adaptations to different stream velocities. Stream velocity is therefore presumed to have been one of several environmental factors that interacted to shape the guppy's present behaviour.

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