Pamela Cristina Santana , Jake Mulvaney, Erika Santana, Monika Moir, Bruce Anderson

Many plants have precise pollen placement strategies so that large amounts of pollen can be found over very small and discrete areas located on pollinators. This may lead to male-male competition if pre-existing pollen (1) is smothered or displaced by pollen from subsequent male flowers or (2) prevents subsequent pollen from attaching to pollinators. We investigated these alternative hypotheses using caged sunbirds (Cinnyris chalybeus) and sunbird-pollinated flowers (Tritoniopsis antholyza). We labelled pollen from two different flowers with quantum dots so that their pollen grains could be distinguished. We offered these two male-phase flowers in succession to sunbirds before they were allowed to visit a female-phase flower. In a separate trial, we offered sunbirds a quantum-dot-labelled flower followed by a flower without reproductive structures. This trial established whether discernable amounts of pollen were being lost during the trials due to a “time effect” (over time, pollen falls off, or is groomed from the pollinator). We found that pollen from the second male flower was better represented on the stigmas of the subsequently visited female flowers and that this advantage was not due to a time effect (i.e. less time for the pollen from the last male to fall off the pollinator). Instead, it suggests that pollen from earlier-visited flowers is smothered or displaced by subsequently visited flowers. Because the last male visited may have a reproductive advantage (similar to last-male sperm precedence in animals), plants are likely to evolve strategies to both capitalize on this advantage but also to combat it.