Long-and short-billed hummingbirds as pollinators of Palicourea demissa, a distylous treelet of Neotropical cloud forests

Abstract

Distyly has been interpreted as a mechanism that promotes cross-pollination between floral morphs. According to this hypothesis, pollen from anthers positioned at different heights could adhere to different body parts of the pollinator that would correspond to those points where stigmas of compatible morph contact the animal. In this regard, hummingbird species with different bill sizes may play different roles as pollinators of each morph. If pollinators mobilize more legitimate pollen towards one of the two morphs, gender specialization may occur. This work aimed to assess experimentally the role of long- and short-billed hummingbirds as pollinators of short-style (SS) and long-style (LS) flowers of Palicourea demissa, a distylous, hummingbird-pollinated treelet in Venezuelan cloud forests. Flowers were emasculated and exposed to a single visit of the hummingbird Coeligena torquata (long-billed), Heliangelus spencei (short-billed) or Adelomyia melanogenys (short-billed). Later, stigmas were removed, and pollen load counted under a microscope to calculate the probability of legitimate- and illegitimate-pollen transfer by hummingbirds. The probability analyses of pollen transference showed that short-billed hummingbirds have higher pollination probabilities from SS-anthers to LS- and SS-stigmas, and from LS-anther to LS-stigmas than from LS-anther to SS-stigmas. In contrast, long-billed hummingbirds have higher probabilities of pollen transference from LS-anthers to SS-stigmas than in other directions. A deeper view of the sexual expression of each morph in P. demissa will depend on future studies that determine possible morpho-differences in the biological function of male and female floral structures, and the role played by less frequent floral visitors as mediators of legitimate pollination between floral morphs.