Realized flower constancy in bumble bees: optimal foraging strategy balancing cognitive and travel costs and its possible consequences for floral diversity

Pollinating insects often exhibit flower constancy, i.e., the tendency to make consecutive visits to the same flower species while disregarding others. This behaviour is commonly attributed to the cost of retrieving visual or motor memories from long-term storage while switching between flowers with distinct colours and shapes. Accordingly, researchers often predict co-flowering species to exhibit significantly greater phenotypic diversity than random expectation, thereby minimizing heterospecific pollen transfer. However, field observations have not consistently supported this notion.

The observed inconsistencies may arise from variations in travel costs, which depend on the interaction between the forager’s constancy level and the spatial mixing of plant species. If species are evenly mixed, constant pollinators incur higher levels of travel cost due to frequent skipping of neighbouring flowers. In contrast, if species are patchily distributed, constant pollinators experience lower levels of travel cost, as most neighbours are of the same species. Considering this, ‘realized flower constancy’ may be determined as an optimal strategy for balancing cognitive and travel costs, which dynamically vary across different degrees of spatial species mixing. Here we test this possibility in indoor experiments with bumble bees foraging from two differently coloured artificial flowers (’species’) arranged at three mixing levels.

First, bees dramatically reduced flower constancy as species mixing increased, irrespective of flower spacing. Second, bees were less inclined to switch species after accumulating consecutive visits to one species, suggesting a rapid decay of another species’ information in short-term memory back to long-term storage. This effect may have additionally contributed to the increased flower constancy observed in species with patchy distributions. Third, bees showed minimal constancy for similarly coloured, evenly mixed flower species, suggesting that these flowers were operated with shared short-term memory. The constancy level was hardly affected by colour similarity when species were patchily distributed.

Results support our initial hypothesis that realized flower constancy reflects an optimal foraging strategy rather than a fixed outcome of cognitive limitation. Notably, bees’ constancy increased significantly with greater colour difference only when species were evenly mixed, suggesting a novel perspective: spatial mixing promotes the evolution and maintenance of floral diversity.