OXYGEN AND THE UPPER LIMITS TO ANIMAL DESIGN AND PERFORMANCE

Mass-specific rates of aerobic metabolism ((V) over dot(O2)/M-b) scale in inverse proportion to body mass (M-b). Thus, small hummingbirds di splay the highest (V) over dot(O2)/M-b known among vertebrates. Among all animals, higher (V) over dot(O2)/M-b values are known only in flyi ng insects, The high body-mass-specific rates of metabolism seen in hu mmingbirds are made possible by high lung O-2 diffusing capacities, ca rdiac outputs, ratios of capillary surface area to muscle fiber surfac e area, mitochondrial volume densities, cristae surface densities and concentrations of enzymes involved in energy metabolism, Current evide nce from control analyses of O-2 transport through the respiratory and cardiovascular systems and of metabolic fluxes through pathways of en ergy metabolism indicates shared control of maximum flux rates among m ultiple steps (i.e. the absence of single rate-limiting steps). This s upports the suggestion that functional capacities at each step in line ar pathways or processes are matched to each other, and provides an ex planation for why the up-regulation of functional capacities has occur red at virtually all steps in the evolution of the smallest vertebrate homeotherms, Flying insects make use of a tracheal system for Oz tran sport and, like hummingbirds, possess a highly up-regulated biochemica l machinery for substrate oxidation, Studies of hummingbirds and honey bees reveal closer matches between biochemical flux capacities and max imum physiological flux rates than in animals capable of lower maximum (V) over dot(O2)/M-b. It is proposed that the upper limits to functio nal capacities set the upper limit to (V) over dot(O2)/M-b, This upper limit to aerobic metabolic rate may contribute, along with other fact ors, towards establishing the lower limit to vertebrate homeotherm siz e.