Allometry of kinematics and energetics in carpenter bees (Xylocopa varipuncta) hovering in variable-density gases

We assessed the energetic and aerodynamic limits of hovering flight in the carpenter bee Xylocopa varipuncta. Using normoxic, variable-density mixtures of O₂, N₂ and He, we were able to elicit maximal hovering performance and aerodynamic failure in the majority of bees sampled. Bees were not isometric regarding thorax mass and wing area, both of which were disproportionately lower in heavier individuals. The minimal gas density necessary for hovering (MGD) increased with body mass and decreased with relative thoracic muscle mass. Only the four bees in our sample with the highest body mass-specific thorax masses were able to hover in pure heliox. Wingbeat frequency and stroke amplitude during maximal hovering were significantly greater than in normodense hovering, increased significantly with body mass during normodense hovering but were mass independent during maximal hovering. Reserve capacity for wingbeat frequency and stroke amplitude decreased significantly with increasing body mass, although reserve capacity in stroke amplitude (10-30%) exceeded that of wingbeat frequency (0-8%). Stroke plane angle during normodense hovering was significantly greater than during maximal hovering, whereas body angle was significantly greater during maximal hovering than during normodense hovering. Power production during normodense hovering was significantly less than during maximal hovering. Metabolic rates were significantly greater during maximal hovering than during normodense hovering and were inversely related to body mass during maximal and normodense hovering. Metabolic reserve capacity averaged 34% and was independent of body mass. Muscle efficiencies were slightly higher during normodense hovering. The allometry of power production, power reserve capacity and muscle efficiency were dependent on the assumed coefficient of drag (C_D), with significant allometries most often at lower values of C_D. Larger bees operate near the envelope of maximal performance even in normodense hovering due to smaller body mass-specific flight muscles and limited reserve capacities for kinematics and power production.