TRACHEAL DEAD SPACE IN THE RESPIRATION OF BIRDS.

Abstract

Some of the most compelling evidence for organic evolution has been provided in cases of evolutionary convergence. The examples of convergence most commonly cited are qualitative morphological descriptions. However, quantitative allometric analysis can direct our attention to the possibility that a given problem may have a very limited number of solutions. If several factors conspire to preclude all but one solution, convergent evolution will result (Gould, 1966). The increasing availability of data on commonly measured physiological and anatomical features permits allometric consideration of convergence, giving rather far-reaching implications and suggestions from such a simple technique. This approach has been particularly valuable in assessing respiratory physiology (e.g., Tenney and Remmers, 1963; Tenney and Bartlett, 1967; Stahl, 1967). Differences between birds and mammals would have interesting bearings upon the relative merits of the two respiratory anatomical designs, while similarities might suggest functional constraints upon the course of respiratory evolution. The spatial separation of cephalic air intake and thoracic lungs confers certain advantages (Ingelstedt, 1956; Jackson and Schmidt-Nielsen, 1964; Schmidt-Nielsen et al., 1970; Getz, 1967). Before reaching the extensive and delicate exchange surfaces of the lungs, inhaled air is partly or completely humidified and equilibrated to body temperature. However, this arrangement also has the inherent disadvantage of tracheal dead air space. At the end of expiration a significant column of