Narrow entrance of short-tubed Aloe flowers facilitates pollen transfer on long sunbird bills

Abstract

Trade-offs are considered a major factor in the evolution of ecological specialization, as organisms can rarely specialize on more than one niche. Here we ask whether flowers can be specialized for more than one pollinator group. We studied an outcrossing plant, Aloe kraussii (Asphodelaceae), whose floral morphology is intermediate between that of exclusively bird- and exclusively insect-pollinated aloes. Previous field experiments showed that bees effectively pollinate A. kraussii; while sunbirds are frequent visitors their effectiveness as pollinators was unknown, especially as their long bills prevent head feathers—the normal location of pollen transfer by birds—from touching A. kraussii's relatively short anthers and stigmas. We tested whether sunbirds can pollinate A. kraussii, and whether this might be facilitated by flowers' narrow corolla opening, which could increase pollen transfer via narrow sunbird bills. Aviary experiments showed that a Malachite sunbird's bill can successfully pick up and deposit A. kraussii pollen, so long-billed sunbirds can be effective pollinators despite the apparent mismatch between bill and corolla length. Flowers with experimentally widened corolla openings experienced lower pollen removal and deposition by sunbirds than unmanipulated flowers with narrow openings. Aloe kraussii's floral morphology therefore enables dual specialization on two divergent pollinator guilds; short corollas facilitate bee pollination by enabling access to nectar, while narrow corolla openings facilitate pollen transfer via sunbird bills.