The advantages of being crepuscular for bees: major pollen gain under low competition during the brief twilight period

Abstract

Abstract The shift in flight activity from daylight to twilight in crepuscular bees is assumed to have evolved to escape competitors, but quantitative confirmation of this hypothesis has never been demonstrated clearly. Pseudobombax longiflorum is a chiropterophilous plant, with flowers presenting large amounts of pollen throughout anthesis, thus attracting not only nocturnal visitors, but also crepuscular and diurnal bees. In this dynamic system, the fraction of pollen that flows to different visitors and the putative role of bees in pollination remain unknown. We analysed floral biology, the frequency of visitors in periods with different light intensities and the rate of pollen removal by each visitor group. A pollinator-exclusion experiment showed that bees were not pollinators of Pseudobombax longiflorum, although they collected > 60% of the pollen from their flowers. Crepuscular bees gained the greatest amount of pollen in the few minutes when they foraged without either nocturnal or diurnal competitors, confirming the advantage of foraging under low light. During the short twilight period, these bees foraged alone and removed 26.5 and 15 times more pollen per minute than nocturnal and diurnal visitors, respectively. Therefore, pollen removal by crepuscular bees is particularly efficient when they forage in the brief period while competitors are absent.