Interspecific aggression and dominance in Gasterosteus

Synopsis

The threespine stickleback, Gasterosteus aculeatus, and the blackspotted stickleback, Gasterosteus wheatlandi, are often found in close association in brackish and marine waters of the Atlantic coast of North America. When the two species are placed together into a large seminatural community tank during the breeding season they interact aggressively. G. aculeatus males prevent G. wheatlandi males from establishing territories in such tanks when individuals of the two species are introduced simultaneously. If several G. wheatlandi males are introduced alone into the tank they establish territories and nests; however, G. aculeatus males subsequently added usually displace resident G. wheatlandi males and establish their own territories. The greater body size of G. aculeatus appears to be the major factor determining the displacement of G. wheatlandi by G. aculeatus. Furthermore, when attack frequencies of the two species are compared in a series of aggression tests, G. aculeatus attains higher levels, suggesting that it is also more aggressive than G. wheatlandi. G. wheatlandi males have a stronger tendency to build nests in close proximity to vegetation than do G. aculeatus males, which is consistent with the generally less conspicuous behavior and mating coloration of G. wheatlandi. The ability of G. aculeatus to interfere with territory establishment and nesting of G. wheatlandi suggest that the slightly later breeding peak of G. wheatlandi might result from interspecific interaction.