Pollen grains can be dispersed singly or variously aggregated in groups. Whether the evolu-tion of pollen aggregation is driven by the pollinator remains unexplored. We hypothesize thatan extensive pollen aggregation is favored under a scarcity of pollinators. Variation in pollen aggregation by viscin threads in 13 Rhododendronspecies was mea-sured as it is related to pollen removal in a visit. Visitation rates of functional pollinator groupsthat vary in their grooming behavior were investigated in each species. Pollen deposited onstigmas in the field was also sampled. SevenRhododendronspecies were infrequently pollinated by low-intensity grooming ani-mals, including birds, butterflies and moths. The other six species were more frequently polli-nated by bees with a high intensity of pollen grooming. Bird- and Lepidoptera-pollinatedspecies produced longer pollen-connecting threads that connected more pollen grains. Phylo-genetically independent contrast analysis of the 13 species showed that pollinator visitationfrequency was negatively related to amounts of pollen removal per visit but not to stigmaticpollen loads. The finding of interspecific patterns in pollen removal related to pollinator visitation fre-quency suggests pollinator-mediated selection on pollen packaging strategies, supporting thehypothesis of floral evolution via pollen export.
