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UBC Theses and Dissertations

Size-assortative mating in threespine sticklebacks from two sites on the Salmon River, British Columbia Borland, Marjory


Size-assortative mating was studied in freshwater Gasterosteus aculeatus from two sites on the Salmon River, British Columbia. In the upper river females tend to be larger than males but in the lower river females are generally smaller than males. Male size is similar in both areas. The hypothesis tested was that female size influences male mate choice. Freshwater males from the two areas showed a pronounced difference in mate preference with respect to female size. In choice tests, males from the upper river favour females larger than themselves, whereas lower river males favour females smaller than themselves. Differences in courtship behaviour also exist between males from the two sites. After an initial assessment period, the relative frequencies and occurrence of some male behaviour patterns are strongly associated with either the relative size or genetic type of the female. Other behaviours are independent of the size or type of female courted. Lower Salmon males appear to be constrained from choosing larger, and therefore more fecund, females by the presence during the breeding season of the marine form of threespine stickleback. The marine form of G. Aculeatus is anadromous and as adults are adapted to a pelagic, marine existence. They represent a discrete and different gene-pool from the freshwater form. Females of the marine form are consistently the largest stickleback females encountered in the lower river. Lower Salmon males clearly distinguish between marine and freshwater females and to some extent this discrimination is related to female size. The absence of this constraint in the upper river allows upper river males to increase their reproductive potential by preferentially mating with the largest and most fecund females in their area.

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