Plant size, sexual selection, and the evolution of protandry in dioecious plants

It is frequently observed that males of dioecious plant species flower earlier in the season than females, although the generality of this pattern has not been quantified. One hypothesis for earlier male flowering is that females require more time for resource acquisition before reproduction; another is that selection for access to unfertilized ovules favors early-flowering males. Here I show that protandry is indeed the usual pattern in dioecious plants--males typically initiate flowering before females--and I propose a new hypothesis to explain this pattern. In many natural plant populations, individuals that begin flowering early are larger and--in the case of females or hermaphrodites--therefore more fecund. When this population-level seasonal decline in size is included in simulations of flowering time evolution in a dioecious plant, males evolve earlier flowering onset than females. Correlations between size (or condition) and reproductive phenology are widespread and likely contribute to the prevalence of protandry in both plants and animals, but their importance seems to have been overlooked by botanists. I suggest that sexual selection (specifically, male-male competition for access to high-quality ovules) may play a more important role in the evolution of flowering phenology than has previously been recognized.