Pollinator-mediated selection on floral traits and size of floral display in Cyclopogon elatus, a sweat bee-pollinated orchid

1. Pollinator-mediated selection on traits associated with mechanical fit and attraction of pollinators were investigated through both sexual functions in Cyclopogon elatus (Sw.) Schlechter (Orchidaceae). 2. Only halictid bees, principally Augochlora nausicaa, were observed as pollinators. The pollinarium becomes attached to the ventral surface of the bee's mouthparts (labrum) when the proboscis, which closely matches the length of floral tube, is projected into the flower to reach nectar. 3. We detected directional selection on nectary depth, with deeper nectaries favoured only through male fitness, because this trait affects pollinaria removal but not deposition. Correlational selection was detected through male function between nectary depth and the number of flowers in an individual's floral display These traits affect pollination in a multiplicative way: flower number is positively related to the number of bee visits, and nectary depth positively affects the effectiveness of pollinaria removal at each visit. 4. We also detected stabilizing selection on display size. For smaller displays there was a strongly positive association between number of flowers and overall reproductive success, which can be attributed to a simple numerical effect. However, the expected performance of individual flowers is impaired in large displays by pollinator limitation, because bees visiting the display pollinate few flowers per visit and each bee carries, at most, one pollinarium.