Meeting Abstract
Few studies have been able to assess maximal performance during flight, a deficiency that hampers our ability to assess animal capacities and adaptations. Graded air density has potential as a method to assess maximal performance during hovering flight but has been poorly explored. We tested the effect of graded air density on flight performance and metabolic rate of honey bees (Apis mellifera) at 23°C and 35°C. At 35°C, metabolic rates during flight increased linearly as air density declined, up to about 30% higher in pure heliox (0.441 kg·m-3; about one third normal air density), and most bees could not sustain hovering, suggesting this was near maximal performance. In contrast, at 23°C, metabolic rate did not increase as air density decreased, and bees could not fly in densities lower than 0.779 kg·m-3. This interactive effect of air density and air temperature resulted from different effects on thorax temperature. At 35°C, thorax temperature was statistically constant as air density declined, while at 23°C, thorax temperature declined linearly with air density, reaching the minimal thorax temperature for flight (32°C) at the lowest air density at which flight could occur. We conclude that at 23°C the high thorax-to-air gradient and the higher convective conductivity of low-density air causes thorax temperatures to decline, inhibiting flight performance in low-density air. In contrast, at 35°C, thorax temperatures are so high, and thorax-to-air temperature gradients relatively low, that graded air density functions nicely as an aerial treadmill for assessing maximal metabolic performance. Supported by USDA 2017-68004-26322.
