The diving paradox: new insights into the role of the dive response in air-breathing vertebrates

When aquatic reptiles, birds and mammals submerge, they typically exhibit a dive response in which breathing ceases, heart rate slows, and blood flow to peripheral tissues is reduced. The profound dive response that occurs during forced submergence sequesters blood oxygen for the brain and heart while allowing peripheral tissues to become anaerobic, thus protecting the animal from immediate asphyxiation. However, the decrease in peripheral blood flow is in direct conflict with the exercise response necessary for supporting muscle metabolism during submerged swimming. In free diving animals, a dive response still occurs, but it is less intense than during forced submergence, and whole-body metabolism remains aerobic. If blood oxygen is not sequestered for brain and heart metabolism during normal diving, then what is the purpose of the dive response? Here, we show that its primary role may be to regulate the degree of hypoxia in skeletal muscle so that blood and muscle oxygen stores can be efficiently used. Paradoxically, the muscles of diving vertebrates must become hypoxic to maximize aerobic dive duration. At the same time, morphological and enzymatic adaptations enhance intracellular oxygen diffusion at low partial pressures of oxygen. Optimizing the use of blood and muscle oxygen stores allows aquatic, air-breathing vertebrates to exercise for prolonged periods while holding their breath.     

A review of the multi-level adaptations for maximizing aerobic dive duration in marine mammals: from biochemistry to behavior

Marine mammals exhibit multi-level adaptations, from cellular biochemistry to behavior, that maximize aerobic dive duration. A dive response during aerobic dives enables the efficient use of blood and muscle oxygen stores, but it is exercise modulated to maximize the aerobic dive limit at different levels of exertion. Blood volume and concentrations of blood hemoglobin and muscle myoglobin are elevated and serve as a significant oxygen store that increases aerobic dive duration. However, myoglobin is not homogeneously distributed in the locomotory muscles and is highest in areas that produce greater force and consume more oxygen during aerobic swimming. Muscle fibers are primarily fast and slow twitch oxidative with elevated mitochondrial volume densities and enhanced oxidative enzyme activities that are highest in areas that produce more force generation. Most of the muscle mitochondria are interfibriller and homogeneously distributed. This reduces the diffusion distance between mitochondria and helps maintain aerobic metabolism under hypoxic conditions. Mitochondrial volume densities and oxidative enzyme activities are also elevated in certain organs such as liver, kidneys, and stomach. Hepatic and renal function along with digestion and assimilation continue during aerobic dives to maintain physiological homeostasis. Most ATP production comes from aerobic fat metabolism in carnivorous marine mammals. Glucose is derived mostly from gluconeogenesis and is conserved for tissues such as red blood cells and the central nervous system. Marine mammals minimize the energetic cost of swimming and diving through body streamlining, efficient, lift-based propulsive appendages, and cost-efficient modes of locomotion that reduce drag and take advantage of changes in buoyancy with depth. Most dives are within the animal’s aerobic dive limit, which maximizes time underwater and minimizes recovery time at the surface. The result of these adaptations is increased breath-hold duration and enhanced foraging ability that maximizes energy intake and minimizes energy output while making aerobic dives to depth. These adaptations are the long, evolutionary legacy of an aquatic lifestyle that directly affects the fitness of marine mammal species for different diving abilities and environments.     

Ontogeny of total body oxygen stores and aerobic dive potential in Steller sea lions (Eumetopias jubatus)

Two key factors influence the diving and hence foraging ability of marine mammals: increased oxygen stores prolong aerobic metabolism and decreased metabolism slows rate of fuel consumption. In young animals, foraging ability may be physiologically limited due to low total body oxygen stores and high mass specific metabolic rates. To examine the development of dive physiology in Steller sea lions, total body oxygen stores were measured in animals from 1 to 29 months of age and used to estimate aerobic dive limit (ADL). Blood oxygen stores were determined by measuring hematocrit, hemoglobin, and plasma volume, while muscle oxygen stores were determined by measuring myoglobin concentration and total muscle mass. Around 2 years of age, juveniles attained mass specific total body oxygen stores that were similar to those of adult females; however, their estimated ADL remained less than that of adults, most likely due to their smaller size and higher mass specific metabolic rates. These findings indicate that juvenile Steller sea lion oxygen stores remain immature for more than a year, and therefore may constrain dive behavior during the transition to nutritional independence.     

The development of diving bradycardia in bottlenose dolphins (<Emphasis Type="Italic">Tursiops truncatus</Emphasis>)

Bradycardia is an important component of the dive response, yet little is known about this response in immature marine mammals. To determine if diving bradycardia improves with age, cardiac patterns from trained immature and mature bottlenose dolphins (Tursiops truncatus) were recorded during three conditions (stationary respiration, voluntary breath-hold, and shallow diving). Maximum (mean: 117±1 beats·min^–1) and resting (mean: 101±5 beats·min^–1) heart rate (HR) at the water surface were similar regardless of age. All dolphins lowered HR in response to apnea; mean steady state breath-hold HR was not correlated with age. However, the ability to reduce HR while diving improved with age. Minimum and mean steady state HR during diving were highest for calves. For example, 1.5–3.5-year-old calves had significantly higher mean steady state diving HR (51±1 beats·min^–1) than 3.5–5.5-year-old juveniles (44±1 beats·min^–1). As a result, older dolphins demonstrated greater overall reductions in HR during diving. Longitudinal studies concur; the ability to reduce HR improved as individual calves matured. Thus, although newly weaned calves as young as 1.7 years exhibit elements of cardiac control, the capacity to reduce HR while diving improves with maturation up to 3.5 years postpartum. Limited ability for bradycardia may partially explain the short dive durations observed for immature marine mammals.Abbreviations ADL aerobic dive limit - cADL calculated aerobic dive limit - ECG electrocardiogram - HR heart rate - TDR time–depth recorder Communicated by L.C.-H. Wang     

Muscle energy stores and stroke rates of emperor penguins: implications for muscle metabolism and dive performance

In diving birds and mammals, bradycardia and peripheral vasoconstriction potentially isolate muscle from the circulation. During complete ischemia, ATP production is dependent on the size of the myoglobin oxygen (O(2)) store and the concentrations of phosphocreatine (PCr) and glycogen (Gly). Therefore, we measured PCr and Gly concentrations in the primary underwater locomotory muscle of emperor penguin and modeled the depletion of muscle O(2) and those energy stores under conditions of complete ischemia and a previously determined muscle metabolic rate. We also analyzed stroke rate to assess muscle workload variation during dives and evaluate potential limitations on the model. Measured PCr and Gly concentrations, 20.8 and 54.6 mmol kg(-1), respectively, were similar to published values for nondiving animals. The model demonstrated that PCr and Gly provide a large anaerobic energy store, even for dives longer than 20 min. Stroke rate varied throughout the dive profile, indicating muscle workload was not constant during dives as was assumed in the model. The stroke rate during the first 30 s of dives increased with increased dive depth. In extremely long dives, lower overall stroke rates were observed. Although O(2) consumption and energy store depletion may vary during dives, the model demonstrated that PCr and Gly, even at concentrations typical of terrestrial birds and mammals, are a significant anaerobic energy store and can play an important role in the emperor penguin's ability to perform long dives.     

Metabolic limits on dive duration and swimming speed in the southern elephant seal Mirounga leonina

The ability of air-breathing marine predators to forage successfully depends on their ability to remain submerged. This is in turn related to their total O(2) stores and the rate at which these stores are used up while submerged. Body size was positively related to dive duration in a sample of 34 adult female southern elephant seals from Macquarie Island. However, there was no relationship between body size and dive depth. This indicates that smaller seals, with smaller total O(2) stores, make shorter dives than larger individuals but operate at similar depths, resulting in less time being spent at depth. Nine adult female elephant seals were also equipped with velocity time depth recorders. In eight of these seals, a plot of swimming speed against dive duration revealed a cloud of points with a clear upper boundary. This boundary could be described using regression analysis and gave a significant negative relationship in most cases. These results indicate that metabolic rate varies with activity levels, as indicated by swimming speed, and that there are quantifiable limits to the distance that a seal can travel on a dive of a given swimming speed. However, the seals rarely dive to these physiological limits, and the majority of their dives are well within their aerobic capacity. Elephant seals therefore appear to dive in a way that ensures that they have a reserve of O(2) available.     

Foraging by deep-diving birds is not constrained by an aerobic diving limit: a model of avian depth-dependent diving metabolic rate

The theoretical aerobic diving limit (tADL) specifies the duration of a dive after which oxygen reserves available for diving are depleted. The tADL has been calculated by dividing the available oxygen stores by the diving metabolic rate (DMR). Contrary to diving mammals, most diving birds examined to date exceed the tADL by a large margin. This discrepancy between observation and theory has engendered two alternative explanations suggesting that dive duration is extended either anaerobically or by depressing aerobic metabolism. Current formulations of tADL uncritically assume that DMR is independent of depth. However, diving birds differ from other vertebrate divers by having a larger respiratory system volume and by retaining air in their plumage while diving, thereby elevating buoyancy. Because air compresses with depth, diving power requirement decreases with depth. Following this principle, we modeled DMR to depth for Adelie and little penguins and reformulated the tADL accordingly. The model's results suggest that < approximately 5% of natural dives by Adelie penguins exceed the reformulated tADL(d), or maximal aerobic depth, and none in the more buoyant little penguin. These data suggest that, for both small and large species, deep diving birds rarely if ever exceed tADL(d).     

The optimal allocation of time and respiratory metabolism over the dive cycle

Because anaerobic metabolism is much less efficient than aerobicmetabolism in supplying energy, it is widely believed that diversrely predominantly on aerobic metabolism for diving. In thispaper, a time budget model, which assumes that the diver canuse either completely aerobic or partially aerobic metabolismwith additional anaerobic metabolism for diving, is developedand is used to make predictions about patterns in optimal allocationof time and respiratory metabolism during the dive cycle. Theresults derived from the model are (1) a diver that can varythe ratio of energy supplied anaerobically to total energy spentduring dive time is favored by natural selection, but the patternsof time allocation over the dive cycle by the diver do not differ fromthose of a diver that cannot vary the ratio. (2) Even if itis assumed that divers switch their metabolism for diving, anobvious upturn in the surface time with respect to dive timedoes not occur at the aerobic dive limit (ADL) but occurs beyondthe ADL. (3) Use of additional anaerobic metabolism can be favoredfor dives shorter than the ADL. These findings provide a usefulguide to understanding the factors that limit diving behavior.     

High diving metabolism results in a short aerobic dive limit for Steller sea lions (Eumetopias jubatus)

The diving capacity of marine mammals is typically defined by the aerobic dive limit (ADL) which, in lieu of direct measurements, can be calculated (cADL) from total body oxygen stores (TBO) and diving metabolic rate (DMR). To estimate cADL, we measured blood oxygen stores, and combined this with diving oxygen consumption rates (VO₂) recorded from 4 trained Steller sea lions diving in the open ocean to depths of 10 or 40 m. We also examined the effect of diving exercise on O₂ stores by comparing blood O₂ stores of our diving animals to non-diving individuals at an aquarium. Mass-specific blood volume of the non-diving individuals was higher in the winter than in summer, but there was no overall difference in blood O₂ stores between the diving and non-diving groups. Estimated TBO (35.9 ml O₂ kg^?1) was slightly lower than previously reported for Steller sea lions and other Otariids. Calculated ADL was 3.0 min (based on an average DMR of 2.24 L O₂ min^?1) and was significantly shorter than the average 4.4 min dives our study animals performed when making single long dives—but was similar to the times recorded during diving bouts (a series of 4 dives followed by a recovery period on the surface), as well as the dive times of wild animals. Our study is the first to estimate cADL based on direct measures of VO₂ and blood oxygen stores for an Otariid and indicates they have a much shorter ADL than previously thought.     

Effects of the production of perceived exertion during cycle ergometry

Physiological and biomechanical effects of aerobic exercise varying in intensity were studied on the basis of the subjects’ perceived exertion. It was demonstrated that exercise regulated with the use of a 50–100 rating scale was characterized by reliably stable heart-rate and respiratory reactions and biomechanical responses. The relative working heart rate (HR) expressed in percent of the individual HRmax was found to be closely correlated with the values on the 50–100 scale within a wide range during exercise with constant or increasing perceived exertion.     

Temperature effect on the human dive response in relation to cold water near-drowning

To facilitate analysis of mechanisms involved in cold water near-drowning, maximum breath-hold duration (BHD) and diving bradycardia were measured in 160 humans who were submerged in water temperatures from 0 to 35 degrees C at 5 degrees C intervals. For sudden submersion BHD was dependent on water temperature (Tw) according to the equation BHD = 15.01 + 0.92Tw. In cold water (0-15 degrees C), BHD was greatly reduced, being 25-50% of the presubmersion duration. BHD after brief habituation to water temperature and mild, voluntary hyperventilation was more than double that of sudden submersion and was also dependent on water temperature according to the equation BHD = 38.90 + 1.70Tw. Minimum heart rate during both types of submersions (diving bradycardia) was independent of water temperature. The results are pertinent to accidental submersion in cold water and show that decreased breath-holding capacity caused by peripheral cold stimulation reduces the effectiveness of the dive response and facilitates drowning. These findings do not support the postulate that the dive response has an important role in the enhanced resuscitatibility associated with cold water near-drowning, thereby shifting emphasis to hypothermia as the mechanism for this phenomenon.     

Use of the Frank-Starling mechanism during exercise is linked to exercise-induced changes in arterial load

Effective arterial elastance(E(A)) is a measure of the net arterial load imposed on the heart that integrates the effects of heart rate(HR), peripheral vascular resistance(PVR), and total arterial compliance(TAC) and is a modulator of cardiac performance. To what extent the change in E(A) during exercise impacts on cardiac performance and aerobic capacity is unknown. We examined E(A) and its relationship with cardiovascular performance in 352 healthy subjects. Subjects underwent rest and exercise gated scans to measure cardiac volumes and to derive E(A)[end-systolic pressure/stroke volume index(SV)], PVR[MAP/(SV*HR)], and TAC(SV/pulse pressure). E(A) varied with exercise intensity: the ΔE(A) between rest and peak exercise along with its determinants, differed among individuals and ranged from -44% to +149%, and was independent of age and sex. Individuals were separated into 3 groups based on their ΔE(A)I. Individuals with the largest increase in ΔE(A)(group 3;ΔE(A)≥0.98 mmHg.m(2)/ml) had the smallest reduction in PVR, the greatest reduction in TAC and a similar increase in HR vs. group 1(ΔE(A)<0.22 mmHg.m(2)/ml). Furthermore, group 3 had a reduction in end-diastolic volume, and a blunted increase in SV(80%), and cardiac output(27%), during exercise vs. group 1. Despite limitations in the Frank-Starling mechanism and cardiac function, peak aerobic capacity did not differ by group because arterial-venous oxygen difference was greater in group 3 vs. 1. Thus the change in arterial load during exercise has important effects on the Frank-Starling mechanism and cardiac performance but not on exercise capacity. These findings provide interesting insights into the dynamic cardiovascular alterations during exercise.     

EFFECTS OF AEROBIC EXERCISE ON THE CIRCADIAN RHYTHM OF HEART RATE AND BLOOD PRESSURE

Although the effects of aerobic exercise on resting heart rate, heart rate variability, and blood pressure have been investigated, there are scant data on the effects of aerobic exercise on the circadian rhythm of such cardiovascular parameters. In this study, we investigated the effects of aerobic exercise on the 24?h rhythm of heart rate and ambulatory blood pressure in the morning, when cardiovascular events are more common. Thirty-five healthy young subjects were randomized to control and aerobic exercise groups. Subjects in the latter group participated in their respective exercise program for two months, while those in the former group did not exercise. Twenty-four-hour electrocardiogram and ambulatory blood pressure monitoring data were obtained at baseline and at the end of the exercise intervention. The control group showed no changes, while the aerobic exercise group showed a significant decrease in heart rate (73.7?±?6.6?bpm to 69.5?±?5.1?bpm, p?<?0.005) and sympathetic activity such as LF/HF ratio (2.0?±?0.7 to 1.8?±?0.6, p?<?0.05) throughout the 24?h period, particularly in the daytime. The decrease in the heart rate was most prominent in the morning. However, heart rate and LF/HF ratio showed no statistical changes during the night. No significant changes were observed in blood pressure. These findings suggest aerobic exercise exerts beneficial effects on the circadian rhythm of heart rate, especially in the morning. (Author correspondence: hshio@kobe-u.ac.jp)     

THE RELEVANT USE OF THE TRADITIONAL TUNISIAN GAME ‘‘RAQASSA’’ FOR CARDIOVASCULAR STIMULATION IN SCHOOLCHILDREN

The aim of this study was to investigate the heart rate (HR) responses, the rate of perceived exertion (RPE), and the feeling during physical education schooling while performing traditional games activities compared to intermittent exercise. Nineteen pre-pubertal children randomly performed on different days two types of lessons (intermittent running mode vs. traditional Tunisian “Raqassa” game) lasting 12-min each. HR was continuously recorded during both lessons, while ratings of perceived exertion and Feeling values were recorded after the sessions. The mean HR value during the traditional game was significantly higher than during intermittent exercise (p<0.05). Conversely, the perceived exertion score was significantly higher after intermittent exercise than the traditional exercise game (p<0.05), showing that the higher cardiovascular strain of the game was perceived as “lighter” than the run. Simultaneously, the children''s Feeling was significantly higher after the traditional game than intermittent exercise (p<0.001), showing a higher satisfaction from playing with respect to running. Exercise based on the “Raqassa” traditional game could be used in pre-pubertal children as an alternative or as an additional method for suitable cardiovascular stimulation during physical education lessons with lower perceived exertion and better feeling compared to intermittent running.     

Role of hypoxemia for the cardiovascular responses to apnea during exercise

We sought to define the role of hypoxemia in eliciting the cardiovascular responses to apnea during exercise. Eleven men performed repeated apneas during 100-W steady-state exercise, either with normoxic gas (air) or 95% oxygen (oxygen). Beat-by-beat arterial blood pressure, arterial oxygen saturation, and heart rate (HR) were determined, and stroke volume (SV) was estimated from impedance cardiography calibrated with soluble gas rebreathing. There were large interindividual variabilities of HR, mean arterial pressure (MAP), and total peripheral resistance (TPR) at end-apnea (ea). However, for each individual, HR(ea), MAP(ea), and TPR(ea) were highly correlated between air and oxygen (R = 0.94, 0.78, and 0.93). HR decreased and MAP increased faster during apnea with air than with oxygen (ANOVA, P < 0.05), but MAP(ea) was not different between conditions. Cardiac output was reduced by 33% with air and by 11% with oxygen (P < 0.001 for air vs. oxygen). We conclude that the hypoxemia component cannot account for the wide interindividual differences of HR and TPR responses to apnea. However, hypoxemia augments the HR and TPR responses and may limit the MAP response to apnea by preventing a bradycardia-associated increase of SV.     

The maximum oxygen consumption and aerobic scope of birds and mammals: getting to the heart of the matter

Resting or basal metabolic rates, compared across a wide range of organisms, scale with respect to body mass as approximately the 0.75 power. This relationship has recently been linked to the fractal geometry of the appropriate transport system or, in the case of birds and mammals, the blood vascular system. However, the structural features of the blood vascular system should more closely reflect maximal aerobic metabolic rates rather than submaximal function. Thus, the maximal aerobic metabolic rates of birds and mammals should also scale as approximately the 0.75 power. A review of the literature on maximal oxygen consumption and factorial aerobic scope (maximum oxygen consumption divided by basal metabolic rate) suggests that body mass influences the capacity of the cardiovascular system to raise metabolic rates above those at rest. The results show that the maximum sustainable metabolic rates of both birds and mammals are similar and scale as approximately the 0.88 +/- 0.02 power of body mass (and aerobic scope as approximately the 0.15 +/- 0.05 power), when the measurements are standardized with respect to the differences in relative heart mass and haemoglobin concentration between species. The maximum heart beat frequency of birds and mammals is predicted to scale as the -0.12 +/- 0.02 power of body mass, while that at rest should scale as -0.27 +/- 0.04.     

To what extent is the foraging behaviour of aquatic birds constrained by their physiology?

Aquatic birds have access to limited amounts of usable oxygen when they forage (dive) underwater, so the major physiological constraint to their behaviour is the need to periodically visit the water surface to replenish these stores and remove accumulated carbon dioxide. The size of the oxygen stores and the rate at which they are used (V dot o2) or carbon dioxide accumulates are the ultimate determinants of the duration that aquatic birds can remain feeding underwater. However, the assumption that the decision to terminate a dive is governed solely by the level of the respiratory stores is not always valid. Quantification of an optimal diving model for tufted ducks (Aythya fuligula) shows that while they dive efficiently by spending a minimum amount of time on the surface to replenish the oxygen used during a dive, they dive with nearly full oxygen stores and surface well before these stores are exhausted. The rates of carbon dioxide production during dives and removal during surface intervals are likely to be at least as important a constraint as oxygen; thus, further developments of optimal diving models should account for their effects. In the field, diving birds will adapt to changing environmental conditions and often maximise the time spent submerged during diving bouts. However, other factors influence the diving depths and durations of aquatic birds, and in some circumstances they are unable to forage sufficiently well to provide food for their offspring. The latest developments in telemetry have demonstrated how diving birds can make physiological decisions based on complex environmental factors. Diving penguins can control their inhaled air volume to match the expected depth, likely prey encounter rate, and buoyancy challenges of the following dive.     

WISE-2005: adrenergic responses of women following 56-days, 6 degrees head-down bed rest with or without exercise countermeasures

We tested the hypotheses that women completing 56 days, 6 degrees head-down bed-rest (HDBR) would have changes in sensitivity of cardiovascular responses to adrenergic receptor stimulation and that frequent aerobic and resistive exercise would prevent these changes. Twenty-four women, eight controls, eight exercisers (lower body negative pressure treadmill and flywheel resistance exercise), and eight receiving nutritional supplement but no exercise were studied in baseline and during administration of the beta-agonist isoproterenol (ISO) and the alpha- and beta-agonist norepinephrine (NOR). In the control and nutrition groups, HDBR increased heart rate (HR) and reduced stroke volume (SV), and there was a significantly greater increase in HR with ISO after HDBR. In contrast, the HR and SV of the exercise group were unchanged from pre-HDBR. After HDBR, leg vascular resistance (LVR) was greater than pre-HDBR in the exercise group but reduced in control and nutrition. LVR was reduced with ISO and increased with NOR. Changes in total peripheral resistance were similar to those of LVR but of smaller magnitude, perhaps because changes in cerebrovascular resistance index were directionally opposite to those of LVR. There were no changes in sensitivity of the vascular resistance responses to adrenergic stimulation. The HR response might reflect a change in sensitivity or a necessary response to the reduction in SV after HDBR in control and nutrition groups. The reduced peripheral vascular resistance after HDBR might help to explain orthostatic intolerance in women. Exercise was an effective countermeasure to the HDBR effects.     

Body size and skeletal muscle myoglobin of cetaceans: adaptations for maximizing dive duration

Cetaceans exhibit an exceptionally wide range of body mass that influence both the capacities for oxygen storage and utilization; the balance of these factors is important for defining dive limits. Furthermore, myoglobin content is a key oxygen store in the muscle as it is many times higher in marine mammals than terrestrial mammals. Yet little consideration has been given to the effects of myoglobin content or body mass on cetacean dive capacity. To determine the importance of myoglobin content and body mass on cetacean diving performance, we measured myoglobin content of the longissimus dorsi for ten odontocete (toothed whales) and one mysticete (baleen whales) species ranging in body mass from 70 to 80000 kg. The results showed that myoglobin content in cetaceans ranged from 1.81 to 5.78 g (100 g wet muscle)(-1). Myoglobin content and body mass were both positively and significantly correlated to maximum dive duration in odontocetes; this differed from the relationship for mysticetes. Overall, the combined effects of body mass and myoglobin content accounts for 50% of the variation in cetacean diving performance. While independent analysis of the odontocetes showed that body mass and myoglobin content accounts for 83% of the variation in odontocete dive capacity.     

BUFFERING CAPACITY OF THE LOCOMOTOR MUSCLE IN CETACEANS: CORRELATES WITH POSTPARTUM DEVELOPMENT, DIVE DURATION, AND SWIM PERFORMANCE

Skeletal muscles of marine mammals must support the metabolic demands of exercise during periods of reduced blood flow associated with the dive response. Enhanced muscle buffering could support anaerobic metabolic processes during apnea, yet this has not been fully investigated in cetaceans. To assess the importance of this adaptation in the diving and swimming performance of cetaceans, muscle buffering capacity due to non-bicarbonate buffers was measured in the longissimus dorsi of ten species of odontocete and one mysticete. Immature specimens from a subset of these species were studied to assess developmental trends. Fetal and neonatal cetaceans have low buffering capacities (range: 34.8–53.9 slykes) that are within the range measured for terrestrial mammals. A lengthy developmental period, independent of muscle myoglobin postnatal development, is required before adult levels are attained. Adult cetacean buffering capacities (range: 63.7–94.5 slykes) are among the highest values recorded for mammals. Cetacean species that demonstrate extremely long dive durations or high burst speed swimming tend to have greater buffering capacities. However, the wide range of body size across cetaceans may complicate these trends. Enhanced muscle buffering capacity may enable small-bodied species to extend breath-hold beyond short aerobic dive limits for foraging or predator evasion when necessary.