Hummingbird foraging position is altered by the touch-sensitive stigma of bush monkeyflower

The two stigma lobes of bush monkeyflower (Mimulus aurantiacus) close together rapidly in response to touch by a hummingbird pollinator and usually remain closed for the life of the flower, preventing further pollen receipt. Previous work showed that hummingbirds visiting bush monkeyflowers with closed stigmas export more than twice as much pollen to recipient flowers as birds visiting flowers with open stigmas. To investigate how stigma closure increases pollen export, we used videotape to examine the interaction between bird and flower. Hummingbirds altered their method of foraging in response to whether the stigma was open or closed. When stigmas were closed, birds most frequently entered flowers from above, relative to the long axis of the corolla tube, bringing the bill close to the ceiling of the corolla where anthers and stigma are positioned. However, birds were much more likely to enter corollas from below when stigmas were open, keeping the bill farther from the sex organs, and resulting in failure to contact open stigmas 33% of the time. Distance of the bill from the corolla-tube ceiling was associated with frequency of contact with anthers and, when contact was made, explained pollen export in a non-linear fashion. Morphometric analyses showed that smaller corolla openings promoted stigma contact, but corolla openings that were wide relative to their height produced increased contact with anthers. Changes in hummingbird foraging position and reduced pollen export when visiting flowers with open stigmas helps to explain the observation of permanent stigma closure, even when less than full seed set has been achieved.