EVOLUTIONARY INTERPLAY BETWEEN ECOLOGY, MORPHOLOGY AND REPRODUCTIVE-BEHAVIOR IN THREESPINE STICKLEBACK, GASTEROSTEUS-ACULEATUS

Throughout its range, freshwater populations in the Gasterosteus acule atus species complex display remarkable differentiation of morphology and behavior, much of which reflects differences in ecological conditi ons among habitats. We first describe the ecological conditions that h ave led to morphological and behavioral divergence in two common lake types in British Columbia, Canada. Deep, oligotrophic lakes have favor ed the evolution of slender fish well adapted for feeding on plankton (limnetic, sensu McPhail 1984), whereas shallow, more eutrophic lakes with extensive littoral zones favor fish that are deeper-bodied and we ll adapted for feeding on benthic invertebrates. The latter forage in large groups that attack nests guarded by males and cannibalize the yo ung within. Courtship in these lakes is relatively inconspicuous, a fe ature that apparently enhances nest survivorship. In limnetic populati ons, this form of cannibalism is usually absent and courtship is consp icuous. Because benthic populations tend to have larger bodies and hen ce, larger gapes than do limnetic fish we suggest that cannibalism may be facilitated by large body size or a correlated trait. We test this by comparing the morphology of populations exhibiting both group cann ibalism and a second kind of cannibalism in which solitary females cou rt males, gain access to nests as a consequence, and then cannibalize eggs without spawning. Our results suggest that differences in body si ze cannot explain variation among populations in cannibalistic tendenc ies but that body size may affect the effectiveness of cannibalism by females within populations.