Buzz-pollination leads to size-dependent associations between bumblebees and Pedicularis flowers

An estimated 20,000 angiosperm species conceal pollen inside tubular poricidal anthers or within modified petals. A bee releases pollen by vibrating its thorax and transferring the force through its mandibles while biting the flower. While the floral morphology of buzz-pollinated plants is diverse, the behaviors, dimensions and guilds of buzzing bees are limited. Floral modifications should reflect the relative sizes of their co-adapted pollinator species but we do not know what drives these size associations. We show that the optimal excitation point in the vibration system of bumblebee-pollinated louseworts ( Pedicularis ) occurs precisely where bumblebees bite these flowers. This leads to trait matching between a bumblebee’s individual body length and the beak length of the flowers of each lousewort species. As bumblebees do not visit flowers with beaks (galea) longer than their bodies it guarantees they bite the optimal excitation point for pollen discharge.