Hefty or wimpy? A biomechanical hypothesis for plant-pollinator species associations
Though mechanisms underlying pollinator foraging choices have received prolific theoretical and empirical consideration, a general explanation for the frequently observed association between long tongued pollinators and long-tubed flowers is still lacking. Selection for flower handling efficiency (speed) is thought to give rise to this association, despite the fact that proboscis and corolla tube length often covary with other determinants of foraging success. We revisited the tongue-tube length association using a long-tongued bumble bee, Bombus balteatus, and two co-occurring clovers, Trifolium dasyphyllum and T. parryi, which differ in frequency as a B. balteatus host, corolla tube length, and the correlated traits of stem and pedicel strength. We tested two hypotheses explaining B. balteatus preference for the longer-tubed clover, T. parryi: differences in 1) bees’ handling time or 2) handling effort that arise from floral trait variation. We experimentally altered stem strength, pedicel strength, or both for T. dasyphyllum to assess impacts on foraging time and mechanics (Experiment 1) and impacts on bee’s preference and constancy (Experiment 2).
B. balteatus flower handling time did not vary among the clovers; instead, differences in scape and pedicel strength impacted foraging mechanics in ways that contribute to handling effort. Strengthening T. dasyphyllum scapes and pedicels altered bees’ foraging to mimic that while on unmanipulated T. parryi in terms of stem/pedicel deviation, time bees spent supported vs. unsupported, and “errors” while foraging. Strengthening treatments also increased B. balteatus visitation rate on the normally less-preferred clover, T. dasyphyllum, and decreased that for T. parryi. Results suggest that biomechanical aspects of fit may have important consequences for foraging energetics, and should be added to the repertoire of factors driving associations and niche partitioning among floral species for pollinators.