Habitat choice and female preference in a polymorphic stickleback population

Background: A small pond near Bern, Switzerland that is about 90 years old contains a population of threespine stickleback (Gasterosteus aculeatus) with two distinct male phenotypes. Males of one type are large and red, and nest in the shallow littoral zone. Males of the other type are small and orange, and nest offshore at slightly greater depth. The females in this population are phenotypically highly variable but cannot easily be assigned to either male type. Question: Is the existence of two sympatric male morphs maintained by substrate-associated male nest-site choice and facilitated by female mate preferences? Organisms: Male stickleback caught individually at their breeding sites. Female stickleback caught with minnow traps. Methods: In experimental tanks, we simulated the slope and substrate of the two nesting habitats. Males were placed individually in a tank and we observed in which habitat they chose to build their nest. In a simultaneous two-stimulus choice design, we gave females the choice between a large, red male and a small, orange one. We measured female morphology and used linear mixed-effect models to determine whether female preference correlated with female morphology. Results: Both red and orange males preferred nesting in the habitat that simulated the slightly deeper offshore condition. This is the habitat occupied by the small, orange males in the pond. Females showed a broad and bimodal preference distribution, with one group of females choosing the small, orange male and the other females showing a weak tendency to prefer the large, red male. Several aspects of female phenotype correlated with the male type that a female preferred.