Optimization of the mammalian respiratory system: symmorphosis versus single species adaptation

Taylor and Weibel's principle of symmorphosis hypothesized optimal design of the mammalian respiratory system, with no excess structure relative to its maximal O2 flux, VO2max. Although they found 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 O2 transport system at normoxic VO2max, the fraction of the total limitation to O2 flux contributed by each of the respiratory transport steps is calculated as either the fractional change (F) in VO2max for a 1% change in each component, or as the fraction of total O2 pressure drop (R(int)) across each component at VO2max. When calculated as F, alveolar ventilation (VA) and pulmonary diffusing capacity (DLO2) are major limiting factors, circulatory convection (Q) is nearly as limiting, and peripheral tissue diffusing capacity (DTO2) is only one-third as important. When calculated as R(int), DLO2 is the major factor, VA and DTO2 contribute significantly, and Q is smallest. These patterns contrast with analogous studies in humans, in which Q is the single major limiting factor. The results suggest that strong selection for aerobic power in horses has maximized the malleable components of their respiratory systems until the least malleable structure, the lungs, has become a major limitation to O2 flux. Symmorphosis cannot determine if such a design is or is not optimized, as every system falls on a continuous distribution of relative optimization among species. However, the concept of symmorphosis is useful for establishing a framework within which a single species can be compared with a quantitatively defined hypothesis of optimal animal design, and compared with other species according to those criteria.