The nocturnal sweat bees Megalopta genalis and M. amoena have endogenous bimodal circadian rhythms

Neotropical sweat bees in the genus Megalopta have a bimodal foraging pattern, with activity before dawn and after dusk. We show that in the absence of light cues (Dark:Dark), wild caught M. genalis and M. amoena bees on Barro Colorado Island, Panama, typically exhibit a bimodal circadian activity rhythm, with a 12 and 24 hour peak. However, these peaks do not correspond with local sunrise and sunset times, and are generally much later in the day than their natural foraging times. Some bees show only a unimodal 24 hour peak, and a few are arhythmic. Among newly emerged bees that have never been exposed to light or foraged in nature, some show an endogenous bimodal rhythm, many are arhythmic, and some develop a bimodal rhythm during their first 10 days of adult life. Exposure to natural light (Light:Dark) shifts activity peaks to the times of day (around sunrise and sunset) corresponding to foraging activity in nature. Video recordings of in-nest behavior do not show dramatic peaks, either uni- or bi-modal, outside of foraging times, suggesting that the endogenous peaks are tied to foraging activity. There were not differences in circadian rhythms between males and females, or between social and solitary females, or between the two Megalopta species.