Pinniped diving response mechanism and evolution: a window on the paradigm of comparative biochemistry and physiology

Starting even before the end of World War II, the discipline of comparative physiology and biochemistry experienced a period of unprecedented growth and development that pioneers in this field thought would never end. However, by the mid-1970s many of the major mechanistic problems in the field were pretty well understood in principle, and by the mid-1980s workers in the field widely recognized that the discipline was at the point of diminishing returns. One response to this was disillusionment, which turned out to be premature because the field was already absorbing molecular biology tools which has now caused a kind of renaissance in mechanistic physiology studies. The second major response to the sense of disillusionment led to a search for new approaches, and out of this endeavor the newly rejuvenated field of evolutionary physiology arose, and this research area too is now in a growth phase. These general patterns of growth and development in our discipline as a whole are particularly clearly evident in the field of aquatic mammals and birds. Between the 1930s and the 1970s, studies of diving physiology and biochemistry made great progress in mechanistically explaining the basic diving response of aquatic mammals and birds. Key components of the diving response (apnea, bradycardia, peripheral vasoconstriction, redistribution of cardiac output) were found in essentially all species analyzed and were generally taken to be biological adaptations. By the mid-1970s, this approach to unraveling the diving response had run 'out of steam' and was in conceptual stasis. The breakthrough which gave renewal to the field at this time was the development of microprocessor based monitoring of diving animals in their natural environments, which led to a flurry of studies mostly confirming the essential outlines of the diving response based upon laboratory studies and firmly placing it into a proper biological context, underlining its plasticity and species specificities. Now as we begin a new millenium, despite ever more detailed field monitoring of physiology, behavior and ecology, studies aimed at improving understanding of physiological mechanisms in diving are again approaching a point of diminishing returns. To avoid another conceptual stasis, what seems required are new initiatives which may arise from two differing approaches. The first is purely experimental, relying on magnetic resonance imaging (MRI) and spectroscopy (MRS) to expand the framework of the original 'diving response' concept. The second, evolutionary study of the diving response, is synthetic, linked to both field and laboratory studies. To date the evolution of the diving response has only been analyzed in pinnipeds and from these studies two kinds of patterns have emerged. (1) Some physiological and biochemical characters, required and used in diving animals, are highly conserved not only in pinnipeds but in all vertebrates; these traits are necessarily similar in all pinnipeds and include diving apnea, bradycardia, tissue specific hypoperfusion, and hypometabolism of hypoperfused tissues. (2) Another group of functionally linked characters are more malleable and include (i) spleen mass, (ii) blood volume, and (iii) hemoglobin (Hb) pool size. Increases in any of these traits (or in a morphological character, body size) improve diving capacity. Assuming that conserved physiological function means conserved sequences in specific genes and their products (and that evolving function requires changes in such sequences), it is possible to rationalize both the above trait categories in pinniped phylogeny. However, it is more difficult for molecular evolution theory to explain how complex regulatory systems like those involved in bradycardia and peripheral vasoconstriction remain the same through phylogenetic time than it is to explain physiological change driven by directional natural selection.