DISCRIMINATION BY POLLEN-COLLECTING BUMBLEBEES AMONG DIFFERENTIALLY REWARDING FLOWERS OF AN ALPINE WILDFLOWER, CAMPANULA-ROTUNDIFOLIA (CAMPANULACEAE)

We studied bumblebees (Bombus wurflenii and B. sichelii) that were collecting pollen from the flowers of Campanula rotundifolia in an alpine meadow in Switzerland. Flower size varied greatly among individual plants. while pollen and nectar availability varied among the flowers in association with their sexual phase. Compared to random samples, bumblebees significantly favoured female phase flowers on one day. but favoured male phase flowers on the following day and this change coincided with a decline in overall pollen availability. The bumblebees' behaviour is consistent with a simple economic interpretation: initially. bumblebees favoured female phase flowers, many of which offered both pollen and nectar, but as pollen became less abundant, bumblebees increasingly favoured the relatively pollen-rich. nectarless male phase flowers. which implies that pollen was the more valued resource. Like previous studies, we showed that bumblebees are capable of responding to pollen availability, apparently by using visual assessment of pollen before landing on a flower as the basis for discriminatory foraging. Unlike previous studies, we found no evidence for discrimination among flowers on the basis of size, which was uncorrelated with pollen availability.