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Tift, MS, Huckstadt LA, Ponganis PJ.  2018.  Anterior vena caval oxygen profiles in a deep-diving California sea lion: arteriovenous shunts, a central venous oxygen store and oxygenation during lung collapse. Journal of Experimental Biology. 221   10.1242/jeb.163428   AbstractWebsite

Deep-diving California sea lions (Zalophus californianus) can maintain arterial hemoglobin saturation (S-O2) above 90% despite lung collapse (lack of gas exchange) and extremely low posterior vena caval S-O2 in the middle of the dive. We investigated anterior vena caval P-O2 and S-O2 during dives of an adult female sea lion to investigate two hypotheses: (1) posterior vena caval S-O2 is not representative of the entire venous oxygen store and (2) a well-oxygenated (arterialized) central venous oxygen reservoir might account for maintenance of arterial S-O2 during lung collapse. During deep dives, initial anterior vena caval S-O2 was elevated at 83.6 +/- 8.4% (n = 102), presumably owing to arteriovenous shunting. It remained high until the bottom phase of the dive and then decreased during ascent, whereas previously determined posterior vena caval S-O2 declined during descent and then often increased during ascent. These divergent patterns confirmed that posterior vena caval S-O2 was not representative of the entire venous oxygen store. Prior to and early during descent of deep dives, the high S-O2 values of both the anterior and posterior venae cavae may enhance arterialization of a central venous oxygen store. However, anterior vena caval S-O2 values at depths beyond lung collapse reached levels as low as 40%, making it unlikely that even a completely arterialized central venous oxygen store could account for maintenance of high arterial S-O2. These findings suggest that maintenance of high arterial S-O2 during deep dives is due to persistence of some gas exchange at depths beyond presumed lung collapse.

Tift, MS, Huckstadt LA, McDonald BI, Thorson PH, Ponganis PJ.  2017.  Flipper stroke rate and venous oxygen levels in free-ranging California sea lions. Journal of Experimental Biology. 220:1533-1540.   10.1242/jeb.152314   AbstractWebsite

The depletion rate of the blood oxygen store, development of hypoxemia and dive capacity are dependent on the distribution and rate of blood oxygen delivery to tissues while diving. Although blood oxygen extraction by working muscle would increase the blood oxygen depletion rate in a swimming animal, there is little information on the relationship between muscle workload and blood oxygen depletion during dives. Therefore, we examined flipper stroke rate, a proxy of muscle workload, and posterior vena cava oxygen profiles in four adult female California sea lions (Zalophus californianus) during foraging trips at sea. Flipper stroke rate analysis revealed that sea lions minimized muscle metabolism with a stroke-glide strategy when diving, and exhibited prolonged glides during the descent of deeper dives (>100 m). During the descent phase of these deep dives, 55 +/- 21% of descent was spent gliding, with the longest glides lasting over 160 s and covering a vertical distance of 340 m. Animals also consistently glided to the surface from 15 to 25 m depth during these deeper dives. Venous hemoglobin saturation (SO2) profiles were highly variable throughout dives, with values occasionally increasing during shallow dives. The relationship between SO2 and flipper stroke rate was weak during deeper dives, while this relationship was stronger during shallow dives. We conclude that (1) the depletion of oxygen in the posterior vena cava in deep-diving sea lions is not dependent on stroke effort, and (2) stroke-glide patterns during dives contribute to a reduction of muscle metabolic rate.

McDonald, BI, Ponganis PJ.  2014.  Deep-diving sea lions exhibit extreme bradycardia in long-duration dives. Journal of Experimental Biology. 217:1525-1534.   10.1242/jeb.098558   AbstractWebsite

Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate (f(H)) while diving. We investigated dive f(H) in five adult female California sea lions (Zalophus californianus) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive f(H) (number of beats/duration; 50 +/- 9 beats min(-1)) decreased compared with surface rates (113 +/- 5 beats min(-1)), with all dives exhibiting an instantaneous f(H) below resting (<54 beats min(-1)) at some point during the dive. Both dive f(H) and minimum instantaneous f(H) significantly decreased with increasing dive duration. Typical instantaneous f(H) profiles of deep dives (>100 m) consisted of: (1) an initial rapid decline in f(H) resulting in the lowest instantaneous f(H) of the dive at the end of descent, often below 10 beats min-1 in dives longer than 6 min in duration; (2) a slight increase in f(H) to similar to 10-40 beats min(-1) during the bottom portion of the dive; and (3) a gradual increase in f(H) during ascent with a rapid increase prior to surfacing. Thus, f(H) regulation in deep-diving sea lions is not simply a progressive bradycardia. Extreme bradycardia and the presumed associated reductions in pulmonary and peripheral blood flow during late descent of deep dives should (a) contribute to preservation of the lung oxygen store, (b) increase dependence of muscle on the myoglobin-bound oxygen store, (c) conserve the blood oxygen store and (d) help limit the absorption of nitrogen at depth. This f(H) profile during deep dives of sea lions may be characteristic of deep-diving marine endotherms that dive on inspiration as similar f(H) profiles have been recently documented in the emperor penguin, another deep diver that dives on inspiration.

Meir, JU, Ponganis PJ.  2010.  Blood temperature profiles of diving elephant seals. Physiological and Biochemical Zoology. 83:531-540.   10.1086/651070   AbstractWebsite

Hypothermia-induced reductions in metabolic rate have been proposed to suppress metabolism and prolong the duration of aerobic metabolism during dives of marine mammals and birds. To determine whether core hypothermia might contribute to the repetitive long-duration dives of the northern elephant seal Mirounga angustirostris, blood temperature profiles were obtained in translocated juvenile elephant seals equipped with a thermistor and backpack recorder. Representative temperature (the y-intercept of the mean temperature vs. dive duration relationship) was 37.2 degrees +/- 0.6 degrees C (n=3 seals) in the extradural vein, 38.1 degrees +/- 0.7 degrees C (n=4 seals) in the hepatic sinus, and 38.8 degrees +/- 16 degrees C (n=6 seals) in the aorta. Mean temperature was significantly though weakly negatively related to dive duration in all but one seal. Mean venous temperatures of all dives of individual seals ranged between 36 degrees and 38 degrees C, while mean arterial temperatures ranged between 35 degrees and 39 degrees C. Transient decreases in venous and arterial temperatures to as low as 30 degrees-33 degrees C occurred in some dives >30 min (0.1% of dives in the study). The lack of significant core hypothermia during routine dives (10-30 min) and only a weak negative correlation of mean temperature with dive duration do not support the hypothesis that a cold-induced Q(10) effect contributes to metabolic suppression of central tissues during dives. The wide range of arterial temperatures while diving and the transient declines in temperature during long dives suggest that alterations in blood flow patterns and peripheral heat loss contribute to thermoregulation during diving.

Ponganis, PJ, Meir JU, Williams CL.  2010.  Oxygen store depletion and the aerobic dive limit in emperor penguins. Aquatic Biology. 8:237-245.   10.3354/ab00216   AbstractWebsite

The aerobic dive limit (ADL), dive duration associated with the onset of post-dive blood lactate elevation, has been widely used in the interpretation of diving physiology and diving behavior. However, its physiological basis is incompletely understood, and in most studies, ADLs are simply calculated with an O(2) store/O(2) consumption formula. To better understand the ADL, research has been conducted on emperor penguins diving at an isolated dive hole. This work has revealed that O(2) stores are greater than previously estimated, and that the rate of depletion of those O(2) stores appears to be regulated primarily through a diving bradycardia and the efficiency of swimming. Blood and respiratory O(2) stores are not depleted at the 5.6 min ADL determined by post-dive blood lactate measurements. It is hypothesized that muscle, isolated from the circulation during a dive, is the primary source of lactate accumulation. To predict this 5.6 min ADL for these shallow dives at the isolated dive hole with the classic O(2) store/O(2) consumption formula, an O(2) consumption rate of 2x the predicted metabolic rate of a penguin at rest is required. In contrast, if the formula is used to calculate an ADL that is defined as the time for all consumable O(2) stores to be depleted, then a 23.1 min dive, in which final venous partial pressure of oxygen (P(O2)) was 6 mm Hg (0.8 kPa), represents such a maximum limit and demonstrates that an O(2) consumption rate of about 0.5x the predicted rate of an emperor penguin at rest is required in the formula.

Meir, JU, Champagne CD, Costa DP, Williams CL, Ponganis PJ.  2009.  Extreme hypoxemic tolerance and blood oxygen depletion in diving elephant seals. American Journal of Physiology-Regulatory Integrative and Comparative Physiology. 297:R927-R939.   10.1152/ajpregu.00247.2009   AbstractWebsite

Meir JU, Champagne CD, Costa DP, Williams CL, Ponganis PJ. Extreme hypoxemic tolerance and blood oxygen depletion in diving elephant seals. Am J Physiol Regul Integr Comp Physiol 297: R927-R939, 2009. First published July 29, 2009; doi: 10.1152/ajpregu.00247.2009.-Species that maintain aerobic metabolism when the oxygen (O(2)) supply is limited represent ideal models to examine the mechanisms underlying tolerance to hypoxia. The repetitive, long dives of northern elephant seals (Mirounga angustirostris) have remained a physiological enigma as O(2) stores appear inadequate to maintain aerobic metabolism. We evaluated hypoxemic tolerance and blood O(2) depletion by 1) measuring arterial and venous O(2) partial pressure (PO(2)) during dives with a PO(2)/temperature recorder on elephant seals, 2) characterizing the O(2) hemoglobin (O(2)-Hb) dissociation curve of this species, 3) applying the dissociation curve to PO(2) profiles to obtain %Hb saturation (SO(2)), and 4) calculating blood O(2) store depletion during diving. Optimization of O(2) stores was achieved by high venous O(2) loading and almost complete depletion of blood O(2) stores during dives, with net O(2) content depletion values up to 91% (arterial) and 100% (venous). In routine dives (>10 min) Pv(O2) and Pa(O2) values reached 2-10 and 12-23 mmHg, respectively. This corresponds to SO(2) of 1-26% and O(2) contents of 0.3 (venous) and 2.7 ml O(2)/dl blood (arterial), demonstrating remarkable hypoxemic tolerance as PaO(2) is nearly equivalent to the arterial hypoxemic threshold of seals. The contribution of the blood O(2) store alone to metabolic rate was nearly equivalent to resting metabolic rate, and mean temperature remained near 37 degrees C. These data suggest that elephant seals routinely tolerate extreme hypoxemia during dives to completely utilize the blood O(2) store and maximize aerobic dive duration.

Meir, JU, Ponganis PJ.  2009.  High-affinity hemoglobin and blood oxygen saturation in diving emperor penguins. Journal of Experimental Biology. 212:3330-3338.   10.1242/jeb.033761   AbstractWebsite

The emperor penguin (Aptenodytes forsteri) thrives in the Antarctic underwater environment, diving to depths greater than 500m and for durations longer than 23 min. To examine mechanisms underlying the exceptional diving ability of this species and further describe blood oxygen (O(2)) transport and depletion while diving, we characterized the O(2)-hemoglobin (Hb) dissociation curve of the emperor penguin in whole blood. This allowed us to (1) investigate the biochemical adaptation of Hb in this species, and (2) address blood O(2) depletion during diving, by applying the dissociation curve to previously collected partial pressure of O(2) (P(O2)) profiles to estimate in vivo Hb saturation (S(O2)) changes during dives. This investigation revealed enhanced Hb-O(2) affinity (P(50)=28mmHg, pH7.5) in the emperor penguin, similar to high-altitude birds and other penguin species. This allows for increased O(2) at low blood P(O2) levels during diving and more complete depletion of the respiratory O(2) store. S(O2) profiles during diving demonstrated that arterial S(O2) levels are maintained near 100% throughout much of the dive, not decreasing significantly until the final ascent phase. End-of-dive venous S(O2) values were widely distributed and optimization of the venous blood O(2) store resulted from arterialization and near complete depletion of venous blood O(2) during longer dives. The estimated contribution of the blood O(2) store to diving metabolic rate was low and highly variable. This pattern is due, in part, to the influx of O(2) from the lungs into the blood during diving, and variable rates of tissue O(2) uptake.

Ponganis, PJ, Stockard TK, Meir JU, Williams CL, Ponganis KV, Howard R.  2009.  O-2 store management in diving emperor penguins. Journal of Experimental Biology. 212:217-224.   10.1242/jeb.026096   AbstractWebsite

In order to further define O-2 store utilization during dives and understand the physiological basis of the aerobic dive limit (ADL, dive duration associated with the onset of post-dive blood lactate accumulation), emperor penguins (Aptenodytes forsteri) were equipped with either a blood partial pressure of oxygen (P-O2) recorder or a blood sampler while they were diving at an isolated dive hole in the sea ice of McMurdo Sound, Antarctica. Arterial P-O2 profiles (57 dives) revealed that (a) pre-dive P-O2 was greater than that at rest, (b) P-O2 transiently increased during descent and (c) post-dive P-O2 reached that at rest in 1.92 +/- 1.89 min (N=53). Venous P-O2 profiles (130 dives) revealed that (a) pre-dive venous P-O2 was greater than that at rest prior to 61% of dives, (b) in 90% of dives venous P-O2 transiently increased with a mean maximum P-O2 of 53 +/- 18 mmHg and a mean increase in P-O2 of 11 +/- 12 mmHg, (c) in 78% of dives, this peak venous P-O2 occurred within the first 3 min, and (d) post-dive venous P-O2 reached that at rest within 2.23 +/- 2.64 min (N=84). Arterial and venous P-O2 values in blood samples collected 1-3 min into dives were greater than or near to the respective values at rest. Blood lactate concentration was less than 2 mmol l(-1) as far as 10.5 min into dives, well beyond the known ADL of 5.6 min. Mean arterial and venous P-N2 of samples collected at 20-37 m depth were 2.5 times those at the surface, both being 2.1 +/- 0.7 atmospheres absolute (ATA; N=3 each), and were not significantly different. These findings are consistent with the maintenance of gas exchange during dives (elevated arterial and venous P-O2 and P-N2 during dives), muscle ischemia during dives (elevated venous P-O2, lack of lactate washout into blood during dives), and arterio-venous shunting of blood both during the surface period (venous P-O2 greater than that at rest) and during dives (arterialized venous P-O2 values during descent, equivalent arterial and venous P-N2 values during dives). These three physiological processes contribute to the transfer of the large respiratory O-2 store to the blood during the dive, isolation of muscle metabolism from the circulation during the dive, a decreased rate of blood O-2 depletion during dives, and optimized loading of O-2 stores both before and after dives. The lack of blood O-2 depletion and blood lactate elevation during dives beyond the ADL suggests that active locomotory muscle is the site of tissue lactate accumulation that results in post-dive blood lactate elevation in dives beyond the ADL.

Ponganis, PJ, Kreutzer U, Stockard TK, Lin PC, Sailasuta N, Tran TK, Hurd R, Jue T.  2008.  Blood flow and metabolic regulation in seal muscle during apnea. Journal of Experimental Biology. 211:3323-3332.   10.1242/jeb.018887   AbstractWebsite

In order to examine myoglobin (Mb) function and metabolic responses of seal muscle during progressive ischemia and hypoxemia, Mb saturation and high-energy phosphate levels were monitored with NMR spectroscopy during sleep apnea in elephant seals (Mirounga angustirostris). Muscle blood flow (MBF) was measured with laser-Doppler flowmetry (LDF). During six, spontaneous, 8-12 min apneas of an unrestrained juvenile seal, apneic MBF decreased to 46 +/- 10% of the mean eupneic MBF. By the end of apnea, MBF reached 31 +/- 8% of the eupneic value. The t(1/2) for 90% decline in apneic MBF was 1.9 +/- 1.2 min. The initial post-apneic peak in MBF occurred within 0.20 +/- 0.04 min after the start of eupnea. NMR measurements revealed that Mb desaturated rapidly from its eupenic resting level to a lower steady state value within 4 min after the onset of apnea at rates between 1.7 +/- 1.0 and 3.8 +/- 1.5% min(-1), which corresponded to a muscle O(2) depletion rate of 1-2.3 ml O(2)kg(-1) min(-1). High-energy phosphate levels did not change with apnea. During the transition from apnea to eupnea, Mb resaturated to 95% of its resting level within the first minute. Despite the high Mb concentration in seal muscle, experiments detected Mb diffusing with a translational diffusion coefficient of 4.5 x 10(-7) cm(2) s(-1), consistent with the value observed in rat myocardium. Equipoise P(O2) analysis revealed that Mb is the predominant intracellular O(2) transporter in elephant seals during eupnea and apnea.

Stockard, TK, Levenson DH, Berg L, Fransioli JR, Baranov EA, Ponganis PJ.  2007.  Blood oxygen depletion during rest-associated apneas of northern elephant seals (Mirounga angustirostris). Journal of Experimental Biology. 210:2607-2617.   10.1242/jeb.008078   AbstractWebsite

Blood gases (P-O2, P-CO2, pH), oxygen content, hematocrit and hemoglobin concentration were measured during rest-associated apneas of nine juvenile northern elephant seals. In conjunction with blood volume determinations, these data were used to determine total blood oxygen stores, the rate and magnitude of blood O-2 depletion, the contribution of the blood O-2 store to apneic metabolic rate, and the egree of hypoxemia that occurs during these breath-holds. Mean body mass was 66 +/- 9.7 kg (+/- s.d.); blood volume was 196 +/- 20 ml kg(-1); and hemoglobin concentration was 23.5 +/- 1.5 g dl(-1). Rest apneas ranged in duration from 3.1 to 10.9 min. Arterial P-O2 declined exponentially during apnea, ranging between a maximum of 108 mmHg and a minimum of 18 mmHg after a 9.1 min breath-hold. Venous P-O2 values were indistinguishable from arterial values after the first minute of apnea; the lowest venous P-O2 recorded was 15 mmHg after a 7.8 min apnea. O-2 contents were also similar between the arterial and venous systems, declining linearly at rates of 2.3 and 2.0 ml O-2 dl(-1) min (-1), respectively, from mean initial values of 27.2 and 26.0 ml O-2 dl(-1). These blood O-2 depletion rates are approximately twice the reported values during forced submersion and are consistent with maintenance of previously measured high cardiac outputs during rest-associated breath-holds. During a typical 7-min apnea, seals consumed, on average, 56% of the initial blood O-2 store of 52 ml O-2 kg(-1); this contributed 4.2 ml O-2 kg(-1) min(-1) to total body metabolic rate during the breath-hold. Extreme hypoxemic tolerance in these seals was demonstrated by arterial P-O2 values during late apnea that were less than human thresholds for shallow-water blackout. Despite such low P-O2s, there was no evidence of significant anaerobic metabolism, as changes in blood pH were minimal and attributable to increased P-CO2. These findings and the previously reported lack of lactate accumulation during these breath- holds are consistent with the maintenance of aerobic metabolism even at low oxygen tensions during rest- associated apneas. Such hypoxemic tolerance is necessary in order to allow dissociation of O-2 from hemoglobin and provide effective utilization of the blood O-2 store.

Ponganis, PJ.  2007.  Bio-logging of physiological parameters in higher marine vertebrates. Deep-Sea Research Part Ii-Topical Studies in Oceanography. 54:183-192.   10.1016/j.dsr2.2006.11.009   AbstractWebsite

Bio-logging of physiological parameters in higher marine vertebrates had its origins in the field of bio-telemetry in the 1960s and 1970s. The development of microprocessor technology allowed its first application to bio-logging investigations of Weddell seal diving physiology in the early 1980s. Since that time, with the use of increased memory capacity, new sensor technology, and novel data processing techniques, investigators have examined heart rate, temperature, swim speed, stroke frequency, stomach function (gastric pH and motility), heat flux, muscle oxygenation, respiratory rate, diving air volume, and oxygen partial pressure (PO(2)) during diving. Swim speed, heart rate, and body temperature have been the most commonly studied parameters. Bio-logging investigation of pressure effects has only been conducted with the use of blood samplers and nitrogen analyses on animals diving at isolated dive holes. The advantages/disadvantages and limitations of recording techniques, probe placement, calibration techniques, and study conditions are reviewed. (c) 2007 Elsevier Ltd. All rights reserved.

Ponganis, PJ, Stockard TK, Levenson DH, Berg L, Baranov EA.  2006.  Cardiac output and muscle blood flow during rest-associated apneas of elephant seals. Comparative Biochemistry and Physiology a-Molecular & Integrative Physiology. 144:105-111.   10.1016/j.cbpa.2006.02.009   AbstractWebsite

In order to evaluate hemodynamics and blood flow during rest-associated apnea in young elephant seals (Mirounga angustirostris), cardiac outputs (CO, thermodilution), heart rates (HR), and muscle blood flow (MBF, laser Doppler flowmetry) were measured.. Mean apneic COs and HRs of three seals were 46% and 39% less than eupneic values, respectively (2.1 +/- 0.3 vs. 4.0 +/- 0.1 mL kg(-1) s(-1), and 54 6 vs. 89 14 beats min(-1)). The mean apneic stroke volume (SV) was not significantly different from the eupneic value (2.3 +/- 0.2 vs. 2.7 +/- 0.5 mL kg(-1)). Mean apneic MBF of three seals was 51% of the eupneic value. The decline in MBF during apnea was gradual, and variable in both rate and magnitude. In contrast to values previously documented in seals during forced submersions (FS), CO and SV during rest-associated apneas were maintained at levels characteristic of previously published values in similarly-sized terrestrial mammals at rest. Apneic COs of such magnitude and incomplete muscle ischemia during the apnea suggest that (1) most organs are not ischemic during rest-associated apneas, (2) the blood O-2 depletion rate is greater during rest-associated apneas than during FS, and (3) the blood O-2 store is not completely isolated from muscle during rest-associated apneas. (c) 2006 Elsevier Inc. All rights reserved.

Ponganis, PJ, Kreutzer U, Sailasuta N, Knower T, Hurd R, Jue T.  2002.  Detection of myoglobin desaturation in Mirounga angustirostris during apnea. American Journal of Physiology-Regulatory Integrative and Comparative Physiology. 282:R267-R272. AbstractWebsite

H-1 NMR solution-state study of elephant seal (Mirounga angustirostris) myoglobin (Mb) and hemoglobin (Hb) establishes the temperature-dependent chemical shifts of the proximal histidyl NdeltaH signal, which reflects the respective intracellular and vascular PO2 in vivo. Both proteins exist predominantly in one major isoform and do not exhibit any conformational heterogeneity. The Mb and Hb signals are detectable in M. angustirostris tissue in vivo. During eupnea M. angustirostris muscle maintains a well-saturated MbO(2). However, during apnea, the deoxymyoglobin proximal histidyl NdeltaH signal becomes visible, reflecting a declining tissue PO2. The study establishes a firm methodological basis for using NMR to investigate the metabolic responses during sleep apnea of the elephant seal and to secure insights into oxygen regulation in diving mammals.

Jobsis, PD, Ponganis PJ, Kooyman GL.  2001.  Effects of training on forced submersion responses in harbor seals. Journal of Experimental Biology. 204:3877-3885. AbstractWebsite

In several pinniped species, the heart rates observed during unrestrained dives are frequently higher than the severe bradycardias recorded during forced submersions. To examine other physiological components of the classic 'dive response' during such moderate bradycardias, a training protocol was developed to habituate harbor seals (Phoca vitulina) to short forced submersions. Significant changes were observed between physiological measurements made during naive and trained submersions (3-3.5min). Differences were found in measurements of heart rate during submersion (naive 18 +/-4.3 beats min(-1) versus trained 35 +/-3.4 beats min(-1)), muscle blood flow measured using laser-Doppler flowmetry (naive 1.8 +/-0.8 ml min(-1) 100 g(-1) versus trained 5.8 +/-3.9 ml min(-1) 100 g(-1)), change in venous P-O 2 (naive -0.44 +/-1.25 kPa versus trained -1.48 +/-0.76 kPa) and muscle deoxygenation rate (naive -0.67 +/-0.27 mvd s(-1) versus trained -0.51 +/-0.18 mvd s(-1), a relative measure of muscle oxygenation provided by the Vander Niroscope, where mvd are milli-vander units). In contrast to the naive situation, the post-submersion increase in plasma lactate levels was only rarely significant in trained seals. Resting eupneic (while breathing) heart rate and total oxygen consumption rates (measured in two seals) were not significantly different between the naive and trained states. This training protocol revealed that the higher heart rate and greater muscle blood flow in the trained seals were associated with a lower muscle deoxygenation rate, presumably secondary to greater extraction of blood O-2 during trained submersions. Supplementation of muscle oxygenation by blood O-2 delivery during diving would increase the rate of blood O-2 depletion but could prolong the duration of aerobic muscle metabolism during diving. This alteration of the dive response may increase the metabolic efficiency of diving.

Ponganis, PJ, Kooyman GL.  2000.  Diving physiology of birds: a history of studies on polar species. Comparative Biochemistry and Physiology a-Molecular and Integrative Physiology. 126:143-151.   10.1016/s1095-6433(00)00208-7   AbstractWebsite

Our knowledge of avian diving physiology has been based primarily on research with polar species. Since Scholander's 1940 monograph, research has expanded from examination of the 'diving reflex' to studies of free-diving birds, and has included laboratory investigations of oxygen stores, muscle adaptations, pressure effects, and cardiovascular/metabolic responses to swimming exercise. Behavioral and energetic studies at sea have shown that common diving durations of many avian species exceed the calculated aerobic diving limits (ADL). Current physiological research is focused on factors, such as heart rate and temperature, which potentially affect the diving metabolic rate and duration of aerobic diving. (C) 2000 Elsevier Science Inc. All rights reserved.

Ancel, A, Starke LN, Ponganis PJ, Van Dam R, Kooyman GL.  2000.  Energetics of surface swimming in Brandt's cormorants (Phalacrocorax penicillatus Brandt). Journal of Experimental Biology. 203:3727-3731. AbstractWebsite

The energy requirements of Brandt's cormorants (Phalacrocorax penicillatus) during surface swimming were measured in birds swimming under a metabolic chamber in a water flume. From the oxygen consumption recordings, we extrapolated the metabolic rate and cost of transport at water speeds ranging from 0 to 1.3 ms(-1). In still water, the birds' mean mass-specific rate of oxygen consumption ((V)over dot(O2),) while floating at the surface was 20.2ml O-2 min(-1) kg(-1), 2.1 times the predicted resting metabolic rate. During steady-state voluntary swimming against a how, their Po, increased with water speed, reaching 74 mi O-2 min(-1) kg(-1) at 1.3 ms(-1), which corresponded to an increase in metabolic rate from 11 to 25 W kg(-1). The cost of transport decreased,vith swimming velocity, approaching a minimum of 19 J kg(-1) m(-1) for a swimming speed of 1.3 m s(-1) Surface swimming in the cormorant costs approximately 18% less than sub-surface swimming. This confirms similar findings in tufted ducks (Aythya fuligula) and supports the hypothesis that increased energy requirements are necessary in these bird diving to overcome buoyancy and heat submergence.

Ponganis, PJ, Kooyman GL, Zornow MH, Castellini MA, Croll DA.  1990.  Cardiac output and stroke volume in swimming harbor seals. Journal of Comparative Physiology B-Biochemical Systemic and Environmental Physiology. 160:473-482.   10.1007/BF00258974   AbstractWebsite

Cardiac output was measured by the thermodilution method in three young harbor seals, at rest and while swimming up to the maximum effort for which they could be trained. Stroke volume was determined by counting heart rate simultaneously with determination of cardiac output. Cardiac outputs varied widely between surface breathing (7.8 and breath-holding while swimming under water (1.8 Stroke volume while at the surface was almost twice the volume while submerged. Surface cardiac output was always near maximal despite work effort, whereas submerged cardiac output gradually increased at higher work efforts. The cardiovascular performance of seals at the maximum MO2 we could induce from them is equivalent to that of the domestic goat.