The Costs of Foraging in Three Dimensions for Bumble Bees

Foraging decisions that optimize returns should be favoured by natural selection. Nectarivorous animals face problems associated with foraging in three-dimensional space, including difficulty remembering foraging paths and expending energy to work against gravity. Captive individual bumble bees ( Bombus impatiens) were video-taped visiting artificial inflorescences to test the effects of three-dimensional (3D) movements and gravity on their behaviour. Bumble bees responded to increased dimensionality by probing fewer flowers for longer durations and responding to self-imposed variation, as a consequence of depleting flowers, by visiting fewer flowers per floral inspection on three-dimensional inflorescences than on two-dimensional inflorescences. When confronted with inflorescences inclined at angles from 0° to 90°, bees demonstrated that flying upward is less time-consuming than flying downward or horizontally, suggesting that their flight capacity is optimized for upward movement. The results provide new insights on the role of cognition and upward movements in bumble-bee foraging.