ANIMAL FLIGHT MECHANICS IN PHYSICALLY VARIABLE GAS-MIXTURES

Empirical studies of animal flight performance have generally been imp lemented within the contemporary Experimental alteration of the physic al of gas mixtures, however, permits construction of novel flight medi a and the non-invasive manipulation of flight biomechanics, For exampl e, replacement of atmospheric nitrogen with various noble gases result s in a tenfold variation in air density at a constant oxygen concentra tion, Such variation in air density correspondingly elicits extraordin ary biomechanical effort from flying animals; hummingbirds and eugloss ine orchid bees hovering in such low-density but normoxic mixtures hav e demonstrated exceptionally high values for the mechanical power outp ut of aerobic flight muscle, As with mechanical power, lift coefficien ts during hovering increase at low air densities in spite of a concomi tant decline in the Reynolds number of the wings, The physical effects of variable gas density may also be manifest in morphological and phy siological adaptations of animals to flight across altitudinal gradien ts. Global variation in atmospheric composition during the late Paleoz oic may also have influenced the initial evolution and subsequent dive rsification of ancestral pterygotes. For the present-day experimenter, the use of physically variable flight media represents a versatile op portunity to explore the range of kinematic and aerodynamic modulation available to flying animals.