Insect Choice and Floral Size Dimorphism: Sexual Selection or Natural Selection?

Abstract

In considerations of sexual floral size dimorphism, there is a conflict between sexual selection theory, which predicts that larger floral displays attract more pollinators, and optimality theory—particularly the ideal free distribution—which predict that pollinators' visits should match nutritional rewards. As an alternate explanation of this dimorphism, Muller reported that pollinators tend to visit larger male flowers before visiting smaller female flowers, thereby promoting effective pollination. To investigate optimality predictions, I offered pollinators a choice between smaller, less numerous, but more rewarding flowers; and larger, more numerous, but less rewarding flowers. Foragers initially favored the larger and more numerous flowers, but rapidly shifted preferences to conform with the predictions of the ideal free distribution. To test Muller's hypothesis, I offered pollinators choices between larger and smaller corollas of equal caloric reward. Results showed that although pollinators tended to visit larger corollas first, they did not visit them more often. These experiments highlight the need for further investigation into the tradeoff between natural and sexual selection, and their respective influences in pollination ecology.