Fig. 1. Hovering kinematics in hypodense air. As ambient air was replaced with normoxic heliox, air density decreased but the partial pressure of oxygen remained constant. The data depicted here were from the experiments performed at 1875 m, although the same trends were evident at 2900 m. (A) Wingbeat frequency increased slightly with decreasing air density. (B) Stroke amplitude increased substantially with decreasing density to a limit near 180°. Values are means ± S.E.M.