Bimodal circadian rhythms in the nocturnal sweat bee Megalopta genalis

The Neotropical sweat bee Megalopta genalis exhibits a bimodal foraging pattern. It is typically active before sunrise and after sunset. Here we show that in the absence of light cues (Dark:Dark), M. genalis shows a bimodal circadian activity rhythm, with a 12 and 24 hour peak. However, these peaks do not correspond with local sunrise and sunset times. Exposure to natural light shifts the circadian peaks, but still not to the same times at which bees forage in the field. Video recordings of in-nest behavior do not show dramatic peaks, either uni- or bi-modal. There were not differences in circadian rhythms between males and females, or between social and solitary females. Comparisons with diurnal sweat bees are also discussed.