Mechanisms of Floral Specialization by Pollen-Foraging Bumble Bees

Abstract

A fundamental question in biology is how animals efficiently locate and use diverse resources. Pollinators foraging on flowers are one of our most thoroughly studied examples of generalist foraging behavior and cognition. Individual pollinators typically specialize on a subset of flowering species available to them. Specialization by nectar-foraging pollinators is often the consequence of learned or innate preferences for floral display traits such as color, pattern, and scent. Pollinators must also typically learn to extract nectar from each floral type. By specializing, pollinators reduce costs associated with learning and forgetting nectar extraction routines. Specialization also benefits the plant by enhancing conspecific pollen transfer. Yet nectar is not the only floral reward. The pollen of hundreds of thousands of plant species is collected by pollinators such as bees, beetles, and flies. In fact, solitary and social bees must collect both pollen and nectar to survive. However, much of the vast literature on bee foraging behavior concerns the collection of nectar. This research investigated mechanisms by which generalist bumblebees (Bombus impatiens) specialize on diverse floral resources. Most foragers in a colony were reward generalists over their lifetime, but specialized daily on either pollen or nectar collection. Lifetime patterns of pollen collection were associated with interindividual differences in sensory morphology. Pollen-foraging bumblebees had weak innate preferences, but learned strong preferences for pollen-only plant species, with preferences mediated primarily by anther properties. The anthers provided indirect cues of concealed pollen, and bees learned to prefer properties of the anthers to select potentially rewarding flowers. While learning was involved in the formation of floral preferences by pollen foragers, pollen extraction behavior relied little on learning. Specifically, floral sonication, which is used by bees to extract concealed pollen, was modified only modestly with experience. Furthermore, bees foraged efficiently for pollen from diverse floral resources without relying on instrumental (associative) learning. Efficient foraging involved switching between two distinct motor routines: floral sonication and scrabbling. Switching was regulated by two ubiquitous floral cues: chemical anther cues eliciting sonication and mechanical pollen cues suppressing it (and eliciting scrabbling). I discuss how mechanisms of floral specialization by generalist pollen-foraging bees could drive floral trait evolution.