Social learning drives handedness in nectar-robbing bumblebees

Bumblebees have been found to observe and copy the behaviour of others with regard to floral choices, particularly when investigating novel flower types. They can also learn to make nectar-robbing holes in flowers as a result of encountering them. Here, we investigate handedness in nectar-robbing bumblebees feeding on Rhinanthus minor, a flower that can be robbed from either the right-hand side or the left-hand side. We studied numerous patches of R. minor spread across an alpine landscape; each patch tended to be robbed on either the right or the left. The intensity of side bias increased through the season and was strongest in the most heavily robbed patches. We suggest that bees within patches learn robbing strategies (including handedness) from one another, either by direct observation or from experience with the location of holes, leading to rapid frequency-dependent selection for a common strategy. Primary robbing was predominantly carried out not only by a specialist robbing species, Bombus wurflenii, but also by Bombus lucorum, a widespread generalist. Both species adopted the same handedness within particular flower patches, providing the first evidence for social learning crossing the species boundary in wild insects.