| Summary: | During the last century, 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
generally taken to be biological adaptations, but the evolution of the diving
response has not been seriously examined. This study compares several key
characters of the diving response in the pinnipeds using phylogenetically
independent contrasts. As the traits examined are known to be functionally
important for diving, I expected them to correlate positively with diving ability.
Instead, some physiological/biochemical characters considered adaptive for
diving do not correlate with diving ability. These traits are similar in phocids
(true seals) and otariids (sea lions and fur seals) and include diving apnea and
bradycardia (and probably also include tissue hypoperfusion, and
hypometabolism of hypoperfused tissues). This finding contradicts the generally
accepted theory that these traits are adaptations for extending dive time in
pinnipeds. The key components of the diving response are more appropriately
seen to be ancestral (plesiomorphic) characters. Another group of
physiological/biochemical characters was more variable among the taxa
examined. These included body weight, spleen weight, whole body hemoglobin
content, and blood volume. Increases in these characters correlate with
increased diving capacity (defined as maximum recorded diving duration). This
correlation of characters, along with functional knowledge of current utility, leads
to the conclusion that changes in two traits - spleen size and whole body oxygen
carrying capacities - may have been driven by selection for increased diving
duration in pinnipeds.
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