© 1999 by The Society for Integrative and Comparative Biology
The Diving Response Mechanism and its Surprising Evolutionary Path in Seals andSea Lions1
Department of Zoology, University of British Columbia Vancouver, B. C, V6T 1Z4, Canada
Correspondence: 2 E-mail: pwh{at}zoology.ubc.ca
During the last half century or more, 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 unravelling 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 basic outlines of the diving response based upon laboratory studies and firmly placing it into proper biological context, underlining its plasticity and species specificities. Now towards the end of the millenium, despite ever more detailed field monitoring of physiology, behaviour and ecology, mechanistic studies are again approaching a point of diminishing returns. To avoid another conceptual stasis, what seems required are new initiatives which we anticipate 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 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 above trait categories in pinniped phytogeny. 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 positive natural selection.