DILLON, M.E.*; Univ. of Washington, Seattle: Wingbeat kinematics of bumblebees hovering at simulated high altitude

Bumblebees (genus Bombus) thrive in alpine ecosystems, regularly flying to flowers above 4000 m despite the difficulty of producing lift in low-density air. Bumblebees may respond to this challenge by evolving increased wing area, muscle mass or efficiency. Individual bees may compensate with modified wingbeat kinematics. Kinematic mechanisms to increase lift are (1) increasing wing stroke amplitude, (2) increasing wingbeat frequency, or (3) through more detailed changes in the motions of the wings. To explore these kinematic compensatory mechanisms, I collected bumblebees (B. californicus) from sea level. With high speed video, I measured stroke amplitude of bees hovering in a hypobaric chamber at a range of air pressures (101-69 kPa; equivalent to 0-3000 m altitude). A microphone in the chamber recorded wingbeat frequency. Surprisingly, stroke amplitude did not change significantly with declining air pressure. Similarly, wingbeat frequency increased only modestly at the lowest air pressures. The observed minor shifts in kinematics are too small to account for the increased lift required for flight at high altitude. Therefore bumblebees must augment force production during flight in low air pressures through subtle changes in wingbeat kinematics (e.g., increased angle of attack or changes in the timing of wing rotation).