The functional role of the keel crest in Polygala myrtifolia (Polygalaceae) and its effects on pollinator visitation success

Keel-flowers have evolved structures and mechanisms to protect their pollen and enhance pollen removal by pollinators. Polygala myrtifolia L. (Polygalaceae) has a keel-flower exclusively pollinated by carpenter bees (Hymenoptera: Apidae). The keel also has an elaborate, fluffy appendage (crest) which has been proposed to function as a pollinator attractant, a landing platform, or a lever. This study explored the functional role of the crest. A manipulation experiment determined the effects of crest removal on pollinator visitation and the bees' ability to access the nectar. Foraging times, preference, and ease of access to nectar were recorded for flowers with and without crests. Crest attractiveness through scent was investigated using a neutral red assay as well as potential attraction through colour contrasts using UV and false-colour imaging. Forces required to open the keel were measured at the crest and parts of the keel. Carpenter bees had a stronger initial preference for crested over non-crested flowers and this translated into a higher number of visits for crested flowers. The proportion of visits resulting in successful nectar access was significantly higher for crested flowers. Crested flowers were visited for significantly longer periods than non-crested flowers. Crest colour as perceived by bees strongly contrasted with the rest of the flower suggesting an attractive function, while crests appear not to be involved in scent production. Forces to open the keel were much lower at the crest than at the keel, suggesting a lever function. We suggest that the crest functions both as a lever and as a visual attractant.