One size fits all:: corolla compression in Aphelandra runcinata (Acanthaceae), an adaptation to short-billed hummingbirds

Co-evolutionary processes have moulded interactions between hummingbirds (Trochilidae) and ornithophilous flowers. Generally, long corollas exclude short-billed trochilids, whereas short corollas allow access for both long- and short-billed species. At La Mucuy, Sierra Nevada National Park, Merida, Venezuela, we studied interactions between Aphelandra runcinata (Acanthaceae) and its avian pollinators. Of 13 trochilid species reported from this site, only two, Coeligena torquata and C. eos, have bills long enough to allow legitimate access to the nectar of A. runcinata flowers. However, we observed that hummingbirds with much shorter bills were also able to extract nectar and pollinate the flowers. This is due to a reversible compression mechanism in A. runcinata, caused by the pressure exerted by the bird in flight and the physical opposition of the inflorescence. The compression mechanism observed in A. runcinata flowers is a morphological characteristic that provides mutualistic advantages for both the plant and its avian visitors. The spectrum of potential pollinators is increased by giving access to nectar even to species with short or medium-long bills. Moreover, nectar-robbing, a foraging strategy that does not result in pollen transfer common among short-billed trochilids, seems to be reduced.