Survival costs and reproductive benefits of floral display in a sexually dimorphic dioecious shrub, Leucadendron xanthoconus

The evolutionary causes of sexual dimorphism in plants have not been as wid ely studied as in animals and the importance of sexual selection in causing dimorphism remains controversial. Sexual selection is most obvious when it favours the evolution of a trait which enhances mating success at the expe nse of decreased viability. We studied the relationship between floral disp lay (number of inflorescences), pollinator attraction and plant survival in a dioecious shrub, Leucadendron xanthoconus. Pollinator attraction, measur ed as the number of insect pollinators, increased linearly with floral disp lay in males. However, males with extravagant displays had a higher probabi lity of dying. Our data suggest that male plants are undergoing selection o n floral display for increased mating success counterbalanced by selection against plants with extravagant displays. Seed set in females did not incre ase with floral display, except at very low inflorescence numbers. Nor was female survival correlated with floral display. Because inflorescences are terminal in the species, selection for more inflorescences in males causes increased ramification, thinner terminal branches and smaller leaves. Thus vegetative dimorphism in this species appears to be caused by selection for extravagant floral display in males, but not females. Limits to dimorphism are imposed by survival costs of elaborate display.