OPTIMIZATION OF THE MAMMALIAN RESPIRATORY SYSTEM - SYMMORPHOSIS VERSUS SINGLE-SPECIES ADAPTATION

Taylor and Weibel's principle of symmorphosis hypothesized optimal des ign of the mammalian respiratory system, with no excess structure rela tive to its maximal O-2 flux, (V) over dot (O2max). Although they foun d symmorphosis not to be a general principle of design, it might apply to a highly adapted aerobic athlete, e.g. the Thoroughbred racehorse. Using a mathematical model based on empirical data of the equine O-2 transport system at normoxic (V) over dot (O2max), the fraction of the total limitation to O-2 flux contributed by each of the respiratory t ransport steps is calculated as either the fractional change (F) in (V ) over dot (O2max) for a 1% change in each component, or as the fracti on of total O-2 pressure drop (R-int) across each component at (V) ove r dot (O2max). When calculated as F, alveolar ventilation ((V) over do t A) and pulmonary diffusing capacity (DLO2) are major limiting factor s, circulatory convection ((Q) over dot) is nearly as limiting, and pe ripheral tissue diffusing capacity (DTO2) is only one-third as importa nt. When calculated as R-int, DLO2 is the major factor, (V) over dot A and DTO2 contribute significantly, and (Q) over dot is smallest. Thes e patterns contrast with analogous studies in humans, in which (Q) ove r dot is the single major limiting factor. The results suggest that st rong selection for aerobic power in horses has maximized the malleable components of their respiratory systems until the least malleable str ucture, the lungs, has become a major limitation to O-2 flux. Symmorph osis cannot determine if such a design is or is not optimized, as ever y system falls on a continuous distribution of relative optimization a mong species. However, the concept of symmorphosis is useful for estab lishing: a framework within which a single species can be compared wit h a quantitatively defined hypothesis of optimal animal design, and co mpared with other species according to those criteria. (C) 1998 Elsevi er Science Inc. All rights reserved.