Dr. Welch's Research

Fuel Use During Hovering Flight | Metabolic Substrate and Oxygen Consumption | High Performance Muscle Power Output

Fuel Use During Hovering Flight

Research conducted in collaboration with:

Raul K. Suarez, UCSB

Bradley H. Bakken and Carlos Martinez del Rio, U Wyoming

L. Gerardo Herrera M., UNAM (Mexico)

Hovering flight has evolved in only three groups of animals: insects, birds (hummingbirds), and mammals (glossophagine bats). Within each of these groups, those species that possess the ability to hover possess a number of traits in common. All hovering animals are relatively small. Hovering bats and hummingbirds are among the smallest endothermic vertebrates. Hovering flight is characterized by high wing beat frequencies and is enabled by some of the highest mass-
specific metabolic rates observed. Notably, all hummingbirds and those
bats that possess the ability to hover are pollinators and largely
nectarivorous.

Is it simply coincidence that virtually all hovering vertebrates are
nectarivorous? Or, is there a connection between the high metabolic
rates displayed during hovering and the nature of these animals’ diet?

To investigate fuel use during hovering flight, I have employed a
combination of respirometric and stable isotope-tracking techniques.
By tracking relative rates of oxygen (O₂) consumption and carbon
dioxide (CO₂) production, it is possible to determine what metabolic
substrates (fat, carbohydrates, or protein) are being oxidized to support
an animal’s metabolism. A ratio of CO₂ produced to O₂ consumed (the
Respiratory Quotient, RQ) equal to 0.71 is indicative of fatty acid
oxidation, while an RQ equal to 1.0 is indicative of carbon dioxide production. Because, in the case of most exercising vertebrates, the oxidation of protein makes only a negligible contribution to the support of metabolism, it is possible to calculate the relative proportion of metabolism supported by either fat or carbohydrates from the animal’s RQ. RQ values displayed by fasted, hovering

bats and hummingbirds are typically low (~0.71-0.78) indicating that fasted metabolism is largely supported by the oxidation of (stored) fatty acids. However, once allowed to feed freely, RQ values from both bats and hummingbirds rise and approach (or meet) 1.0, indicating a switch to nearly exclusive reliance on carbohydrates to fuel hovering metabolism.

RQ values do not allow determination of the source of oxidized carbohydrates or fat. By feeding bats or hummingbirds sucrose solutions with a distinct ¹³C stable isotopic signature (occurring as a result of differences in the photosynthetic pathway – C3 versus C4 – of the plant from which the sucrose is derived) it is possible to determine the contribution sugar from this exogenous diet towards the fueling of hovering metabolism. This is possible because the ¹³C stable isotopic signature of expired CO₂ reflects the signature of oxidized carbon from which the CO₂ derives. This technique, used in combination with measurement of RQ values, has revealed that hovering hummingbirds can support essentially all (average ~ 95%) of hovering metabolism with sugar ingested

no more than 30 minutes prior. Nectarivorous glossophagine bats can support most of their hovering metabolism with recently ingested sugar (average ~ 75%).

The capacity to fuel most or all of hovering metabolism with exogenous sugar is, based on available studies, an ability restricted to hovering animals. Humans, by comparison, can support only as much as 30-35% of their exercise metabolism with dietary sugars. While this is a concrete example of convergent evolution in the physiology of hovering nectarivores, it remains unclear if such capacities are displayed solely by these animals or are more generally associated with small size, high metabolic rates, nectarivory, or the capacity for hovering flight. More studies are clearly warranted.