The diving physiology of pinnipeds : an evolutionary enquiry

• Main Author: Mottishaw, Petra Deigh
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/5965

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.