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An examination of the adaptive significance of interpopulation variation in the behaviour of the guppy Poecilia reticulata Luyten, Peter Henri


Populations of guppies (Poecilia reticulata) are found in rivers in Trinidad which differ in biotic and abiotic characteristics. Guppies from these rivers differ in a number of behavioural and morphological characteristics. Guppies in a turbid lowland river were found to be more cohesive and inhabit areas closer to shore than fish in clear headstream rivers. Males in the turbid lowland river exhibited a greater frequency of thrusts in their courtship than did males in the headstream rivers. Conversely, headstream males exhibited a greater frequency and duration of sigmoid displays in their courtship than did lowland males. These differences in courtship behaviour between males in headstream and lowland rivers persisted in males bred in the laboratory under identical conditions. This strongly suggests that the differences in behaviour are at least in part genetically determined, and that therefore the behaviours are a product of natural selection. Five experiments were designed in order to test the hypothesis that the identified differences in courtship behaviour exhibited by the populations are adaptations, in part, to differences in the turbidity of the rivers in which the populations naturally occur. If they are adaptations, they will contribute to the mating success of males in their particular environments. Males from turbid and clear rivers were placed in competition for mating with females under both clear and turbid water conditions. Mating success was determined on the basis of number of inseminations and sperm contributed to females by males of the different populations. Sperm where identified using a radioactive labelling technique and standard autoradiographic methods. In clear water, male guppies from a clear headstream river were more successful at mating with their own females and as successful at mating with lowland females as males from a turbid lowland river. In turbid water, males from the turbid lowland river were more successful at mating with their own females and equally successful at mating with headstream females as males from the clear headstream river. These results are consistent with the hypothesis that a high frequency of display and displays of long duration in male courtship are adaptations to clear water conditions. The results are also consistent with the hypothesis that high frequencies of thrusts in male courtship is an adaptation to turbid water conditions. Males exhibiting similar courtship behaviours from two geographically isolated headstream rivers were significantly more successful at mating with females from their own populations. This suggests that other factors, such as colour or pheromones, may also be important in determining mating success.

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