THE FLIGHT DIRECTIONALITY OF BUMBLEBEES - DO THEY REMEMBER WHERE THEY CAME FROM

Pollinators usually maintain directionality while moving through a flower patch, presumably to increase foraging success by minimizing revisitation of previously-emptied flowers. Two alternative directionality-generating mechanisms have been proposed: foragers may keep track of their arrival direction at a flower or cluster of flowers and prefer to depart in the same direction (the arrival hypothesis), or they may show little change in orientation while foraging at a flower or flower cluster and prefer to depart in the direction they are last facing (the last-faced hypothesis). Rotation of a flower or inflorescence while the forager is feeding on it will unequivocally distinguish the two hypotheses. We observed worker bumblebees (Bombus melanopygus and B. flavifrons) from captive colonies foraging on a square grid of 25 artificial flowers. Bumblebees were able to keep track of their orientations on a flower relative to their arrival direction, and they choose to leave a flower in about the same direction as their arrival. Whether or not individual flowers or the whole flower patch was rotated 90-degrees clockwise, departure directions were usually the same as arrival directions. In unrotated bees that changed direction between arrival and departure, relatively late changes in orientation during a flower visit tended to compensate for earlier ones. Last-facing directions were better predictors of subsequent movements than were arrival direction, indicating that most compensation for experimentally- or self-imposed rotation while on a flower occurred before a bee left the flower. We conclude that directionality in foraging bumblebees is based on memory of arrival direction.