Male Malachite Sunbird Probing for Nectar from the Ground-Level Flowers of <i>Babiana ringens</i> (IMAGE)
Caption
For a bird-pollinated plant, Babiana ringens is characterized by two unusual features: a ground-level floral display and a naked inflorescence axis that projects vertically above the flowers. Malachite sunbirds, the primary pollinators of B. ringens, use the naked inflorescence axis as a perch while foraging for nectar. This promotes outcrossing by positioning the birds so that they make contact with the reproductive parts of the flowers. Babiana hirsuta is the sister species of B. ringens and has similar bright red flowers that are also pollinated by malachite sunbirds. However, in this species the inflorescence axis is characterized by numerous side branches, resulting in a much larger floral display. The naked axis and basal flowering of B. ringens appears to have originated through the suppression of apical flowering branches in an ancestor with a similar appearance to B. hirsuta. In a manipulative field experiment, the apical flowers were removed from B. hirsuta inflorescences, simulating the appearance of B. ringens. As a comparative treatment, basal flowers were removed, leaving an apical display on the otherwise naked B. hirsuta inflorescences. Inflorescences with only apical flowers were heavily grazed by antelope; however, no herbivory occurred for inflorescences with only basal flowers. The differences in herbivory affected reproductive success: inflorescences with only basal flowers set significantly more seed than those with only apical flowers. The authors propose that this position-dependent herbivory may have contributed to selection for ground-level flowering and the unique origin of the naked bird perch in Babiana.
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Photo credit: C. E. Smith.
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