Bumblebees forage on flowers of increasingly complex morphologies despite low success

Foraging bees expend considerable time and energy handling flowers that are morphologically complex (with concealed food rewards) while simple flowers (with readily available rewards) bloom simultaneously in their foraging environment. Previous studies have investigated the consequences of floral morphology for both foragers and plants, often treating ‘complex’ and ‘simple’ morphologies as dichotomous traits. How pollinators' foraging choices and success vary along a floral complexity gradient has received less attention. Here we investigated, using real flowers of increasing morphological complexity, how complex flowers are chosen and handled by naïve and experienced bumblebees when presented along with simple ones. Intact flowers of Tecoma x ‘Orange Jubilee’ (Bignoniaceae), Antirrhinum majus (Plantaginaceae) and Lupinus pilosus (Fabaceae) represented a gradient of increasing morphological complexity. We manipulated some flowers of each species to look simple with a readily accessible food reward, while keeping their colour and odour unchanged. Bombus terrestris workers were given four simple and four complex flowers of a single species with equal rewards in choice assays in a flight room. Sixty per cent of naïve foragers chose a complex flower on their first visit to all three flower species arrays. Experienced bees visited complex flower types of all three species but had lower feeding success and longer handling times on the more complex species. Thus, the bees' foraging efficiency on the complex option decreased with increasing complexity of the flowers, while individual variation in feeding success on the complex option was increased. These results suggest that inexperienced foragers and unsuccessful feeding attempts increasingly contribute to floral pollination along the morphological complexity gradient.