How gravity affects energy production, distribution, maximum mass and metabolic allometric scaling in birds: theory and applied methods

Production of energy, metabolism, is vital for living organisms, on which all their functions
depend. Birds' metabolism has specifics, because their energy expenditures, besides other
functions, have to compensate force of gravity during the flight. However, accounting for energy
expenditures due to gravity is difficult. We advocate a new indirect method, based on the earlier
introduced general concept that metabolic allometric scaling is the result of natural selection and
optimization of distribution of habitat's resources between the species of a food chain. The
proposed method allows finding fraction of metabolic power, depending on mass, required to
compensate force of gravity during steady flights, relative to the power needed to support speed.
It also allows finding metabolic power needed to support acceleration and acquisition of
potential energy during short flights, which represent the most demanding energetic activity of
birds. The discovered increase of fraction of metabolic power required to compensate gravity
with growth of mass is a principal factor, restricting maximum possible mass of flying animals,
while the proportionally decreasing remaining metabolic power has to be redistributed between
the needs to support speed during steady flights, provide acceleration at upward take-offs and
increase of potential energy during ascends. The obtained results show efficiency of the proposed
method for studies of birds' metabolism and related aspects of their flight characteristics and
physiology, and can be also used as a basis for studies of energy distribution in other animals.