Competition for pollen deposition space on pollinators generates last‐male advantage

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Abstract Many plants have precise pollen placement strategies, ensuring that large amounts of pollen are deposited on small and discrete areas of pollinators' bodies. 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 male‐phase flowers with quantum dots, enabling us to distinguish their pollen grains. 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 male‐phase flower followed by a flower without reproductive structures. The last trial established whether pollen loss over time (time effect) influenced our results, such as pollen falling off or being groomed from the pollinator. We found that pollen from the second male‐phase flower was better represented on the stigmas of the subsequently visited female‐phase flowers. This advantage was not attributable to a time effect (i.e. less time for the last male's pollen to fall off the pollinator). Instead, our results suggest 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 exploit or mitigate this effect. Synthesis: Our study demonstrates that pollen grains compete for space on pollinators' bodies. By uncovering how interference competition affects male reproductive success, our findings underscore the importance of pollen‐related traits in sexual selection and open new avenues for investigating floral evolution. Read the free Plain Language Summary for this article on the Journal blog.