Theoretical evaluation of pollen transfer by nocturnal and diurnal pollinators: When should a flower open?

Time of flower opening (anthesis) is one of the floral traits comprising a pollination syndrome. The time when pollinators are active has been thought to exert selective pressures on anthesis, but no models have been developed to explain it. Using a simulation model, we estimated the number of pollen grains transferred by both diurnal and nocturnal pollinators in flowers opened at dusk, midnight, morning, and noon. We assumed that pollen removal is proportional to the amount of pollen in the flower and that pollen deposition is an increasing and decelerating function of pollen removal. Pollen removal efficiencies and pollen deposition efficiencies of nocturnal and diurnal pollinators were estimated from field data for Lonicera japonica. Anthesis at dusk was favored, even with a larger number of diurnal pollinators, up to ten, versus two nocturnal pollinators. As long as both the pollen removal efficiency of nocturnal pollinators was lower and their pollen deposition efficiency was higher than those of diurnal pollinators, anthesis at dusk was favored, even when diurnal pollinators transferred more pollen than nocturnal ones. Anthesis at dusk may not be a specialization for nocturnal pollinators.