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While marine top predators can play a critical role in ecosystem structure and dynamics through their effects on prey populations, how the predators function in this role is often not well understood. In the Benguela region of southern Africa, the Cape fur seal (Arctocephalus pusillus pusillus) population constitutes the largest marine top predator biomass, but little is known of its foraging ecology other than its diet and some preliminary dive records. Dive information was obtained from 32 adult females instrumented with dive recorders at the Kleinsee colony (29°34.17' S, 16°59.80' E) in South Africa during 2006-2008. Most dives were in the depth range of epipelagic prey species (less than 50 m deep) and at night, reflecting the reliance of Cape fur seals on small, vertically migrating, schooling prey. However, most females also performed benthic dives, and benthic diving was prevalent in some individuals. Benthic diving was significantly associated with the frequency with which females exceeded their aerobic dive limit. The greater putative costs of benthic diving highlight the potential detrimental effects to Cape fur seals of well-documented changes in the availability of epipelagic prey species in the Benguela.

In capital-breeding marine mammals, prey acquisition during the foraging trip coinciding with gestation must provide energy to meet the immediate needs of the growing fetus and also a store to meet the subsequent demands of lactation. Weddell seals (Leptonychotes weddellii) that give birth following the gestational (winter) foraging period gain similar proportions of mass and lipid as compared to females that fail to give birth. Therefore, any changes in foraging behavior can be attributed to gestational costs. To investigate differences in foraging effort associated with successful reproduction, twenty-three satellite tags were deployed on post-molt female Weddell seals in the Ross Sea. Of the 20 females that returned to the area the following year, 12 females gave birth and eight did not. Females that gave birth the following year began the winter foraging period with significantly longer and deeper dives, as compared to non-reproductive seals. Mid- to late winter, reproductive females spent a significantly greater proportion of the day diving, and either depressed their diving metabolic rates (DMR), or exceeded their calculated aerobic dive limit (cADL) more frequently than females that returned without a pup. Moreover, non-reproductive females organized their dives into 2-3 short bouts per day on average (BOUTshort; 7.06 ± 1.29 hr; mean ± 95% CI), whereas reproductive females made 1-2 BOUTshort per day (10.9 ± 2.84 hr), comprising one long daily foraging bout without rest. The magnitude of the increase in dive activity budgets and depression in calculated DMR closely matched the estimated energetic requirements of supporting a fetus. This study is one of the first to identify increases in foraging effort that are associated with successful reproduction in a top predator and indicates that reproductive females must operate closer to their physiological limits to support gestational costs.

Allometry predicts that the 12–17-g American water shrew (Sorex palustris)—the world’s smallest mammalian diver—will have the highest diving metabolic rate coupled with the lowest total body oxygen storage capacity, skeletal muscle buffering capacity, and glycolytic potential of any endothermic diver. Consistent with expectations, and potentially owing to their low thermal inertia, water shrews had a significantly higher diving metabolic rate in 107C water (8.77 mL O2 g-1 h-1 ) compared with 307C water (6.57 mL O2 g-1 h-1 ). Unlike larger-bodied divers, muscle myoglobin contributed minimally (7.7%–12.4%) to total onboard O2 stores of juvenile and adult water shrews, respectively, but was offset by high blood O2 carrying capacities (26.4%–26.9% v/v). Diving was predominantly aerobic, as only 1.2%–2.3% of dives in 107C and 307C water, respectively, exceeded the calculated aerobic dive limits at these temperatures (10.8–14.4 s). The mean voluntary dive time of water shrews during 20-min trials in 37C–307C water was 5:0 5 0:1 s (N = 25, n = 1, 628), with a mean maximum dive time of 10:1 5 0:4 s. However, the average dive duration (6:9 5 0:2 s, n = 257) of radio-telemetered shrews exclusively foraging in a simulated riparian environment (37C water) for 12–28 h suggests that mean (but not maximum) dive times of water shrews in the wild may be longer. Mean dive duration, duration of the longest dive, and total time in water all decreased significantly as water temperature declined, suggesting that shrews employed behavioral thermoregulation to defend against immersion hypothermia. Additionally, free-diving shrews in the 24-h trials consistently elevated core body temperature by ∼17C immediately before initiating aquatic foraging bouts and ended these bouts when body temperature was still at or above normal resting levels (∼37.87C). We suggest that this observed predive hyperthermia aids to heighten the impressive somatosensory physiology, and hence foraging efficiency, of this diminutive predator while submerged. [ABSTRACT FROM AUTHOR]

BackgroundThe energy requirements of free-ranging marine mammals are challenging to measure due to cryptic and far-ranging feeding habits, but are important to quantify given the potential impacts of high-level predators on ecosystems. Given their large body size and carnivorous lifestyle, we would predict that northern elephant seals (Mirounga angustirostris) have elevated field metabolic rates (FMRs) that require high prey intake rates, especially during pregnancy. Disturbance associated with climate change or human activity is predicted to further elevate energy requirements due to an increase in locomotor costs required to accommodate a reduction in prey or time available to forage. In this study, we determined the FMRs, total energy requirements, and energy budgets of adult, female northern elephant seals. We also examined the impact of increased locomotor costs on foraging success in this species.ResultsBody size, time spent at sea and reproductive status strongly influenced FMR. During the short foraging migration, FMR averaged 90.1 (SE = 1.7) kJ kg(-1)d(-1) - only 36 % greater than predicted basal metabolic rate. During the long migration, when seals were pregnant, FMRs averaged 69.4 (±3.0) kJ kg(-1)d(-1) - values approaching those predicted to be necessary to support basal metabolism in mammals of this size. Low FMRs in pregnant seals were driven by hypometabolism coupled with a positive feedback loop between improving body condition and reduced flipper stroking frequency. In contrast, three additional seals carrying large, non-streamlined instrumentation saw a four-fold increase in energy partitioned toward locomotion, resulting in elevated FMRs and only half the mass gain of normally-swimming study animals.ConclusionsThese results highlight the importance of keeping locomotion costs low for successful foraging in this species. In preparation for lactation and two fasting periods with high demands on energy reserves, migrating elephant seals utilize an economical foraging strategy whereby energy savings from reduced locomotion costs are shuttled towards somatic growth and fetal gestation. Remarkably, the energy requirements of this species, particularly during pregnancy, are 70-80 % lower than expected for mammalian carnivores, approaching or even falling below values predicted to be necessary to support basal metabolism in mammals of this size.

Animals that hunt for food underwater and return to the surface for air ('divers') are limited by their ability to store oxygen. The aerobic dive limit (ADL) is the dive duration at which all the possible oxygen stores have been used. It has been claimed that models of optimal diving predict that divers will be close to this limit and that the data do not match this prediction. I characterize closeness to the ADL by the fraction ψ of the maximum amount of stored oxygen that is used during a dive. At the ADL this fraction is 1. I show that an optimal diver does not necessarily have a ψ close to 1 and point out that the key feature predicted by the model is that the diver should have no oxygen stores left when it returns to the surface. The data cited against optimal diving do not address this issue. • Animals that dive for food and return to surface for air are limited by oxygen stores. • Models claim that optimal divers should use nearly all their maximum oxygen stores. • I develop a measure of oxygen use for divers and illustrate it with data from whales. • I show that optimal diving does not predict that the maximum amount of oxygen is used. • I argue that the data cited against optimal diving do not address this issue. [ABSTRACT FROM AUTHOR]

Adult Weddell seals (Leptonychotes weddellii) haul-out on the ice in October/November (austral spring) for the breeding season and reduce foraging activities for ~4 months until their molt in the austral fall (January/February). After these periods, animals are at their leanest and resume actively foraging for the austral winter. In mammals, decreased exercise and hypoxia exposure typically lead to decreased production of O2-carrying proteins and muscle wasting, while endurance training increases aerobic potential. To test whether similar effects were present in marine mammals, this study compared the physiology of 53 post-molt female Weddell seals in the austral fall to 47 pre-breeding females during the spring in McMurdo Sound, Antarctica. Once body mass and condition (lipid) were controlled for, there were no seasonal changes in total body oxygen (TBO2) stores. Within each season, hematocrit and hemoglobin values were negatively correlated with animal size, and larger animals had lower mass-specific TBO2 stores. But because larger seals had lower mass-specific metabolic rates, their calculated aerobic dive limit was similar to smaller seals. Indicators of muscular efficiency, myosin heavy chain composition, myoglobin concentrations, and aerobic enzyme activities (citrate synthase and β-hydroxyacyl CoA dehydrogenase) were likewise maintained across the year. The preservation of aerobic capacity is likely critical to foraging capabilities, so that following the molt Weddell seals can rapidly regain body mass at the start of winter foraging. In contrast, muscle lactate dehydrogenase activity, a marker of anaerobic metabolism, exhibited seasonal plasticity in this diving top predator and was lowest after the summer period of reduced activity.

Understanding the diving behavior of semiaquatic mammals, particularly in relation to estimated aerobic dive limits and diet, is important to understand their adaptability and potential vulnerability to changes in prey type and distribution. The diving behavior of African clawless otters, Aonyx capensis , and spotted-necked otters, Hydrictis maculicollis , is poorly known, and no estimates of their dive performance in relation to targeted prey and calculated dive limits have been reported previously for freshwater environments. We investigated the diving behavior of both these otter species in freshwater environments within South Africa through video recordings of direct observations and subsequent video analyses where dive and recovery durations and dive function were recorded. African clawless otters were found to perform longer dives (mean ± SD = 26.9 ± 12.2 s), compared to spotted-necked otters (8.5 ± 7.6 s). African clawless otters showed substantial variation in dive durations, with the shortest dives sometimes lasting < 5 s, and the longest recorded dive being 70 s. The majority of spotted-necked otter dives lasted < 10 s, with the shortest recorded dive lasting 0.66 s and the longest recorded dive lasting 50.9 s. Spotted-necked otters performed different dive types that were evidently dependent on prey targeted, with dives targeting crabs (16.10 ± 1.91 s) being longer than dives targeting rainbow trout, Oncorhynchus mykiss (5.58 ± 0.17 s). The theoretical dive durations of African clawless otters were exceeded during play dives, while spotted-necked otters sometimes exceeded their theoretical dive durations when performing successful foraging dives. The results of this study suggest that spotted-necked otters can vary behavior in relation to prey and exceed theoretical dive duration during successful foraging dives to maximize the net rate of energy gain. Furthermore, when considering known individual-level dietary specialization and plasticity in these species, it may be predicted that dive behaviors are likely to vary substantially among individuals, and in relation to prey availability and localized habitat conditions. [ABSTRACT FROM AUTHOR]

While marine top predators can play a critical role in ecosystem structure and dynamics through their effects on prey populations, how the predators function in this role is often not well understood. In the Benguela region of southern Africa, the Cape fur seal (Arctocephalus pusillus pusillus) population constitutes the largest marine top predator biomass, but little is known of its foraging ecology other than its diet and some preliminary dive records. Dive information was obtained from 32 adult females instrumented with dive recorders at the Kleinsee colony (29°34.17′ S, 16°59.80′ E) in South Africa during 2006–2008. Most dives were in the depth range of epipelagic prey species (less than 50 m deep) and at night, reflecting the reliance of Cape fur seals on small, vertically migrating, schooling prey. However, most females also performed benthic dives, and benthic diving was prevalent in some individuals. Benthic diving was significantly associated with the frequency with which females exceeded their aerobic dive limit. The greater putative costs of benthic diving highlight the potential detrimental effects to Cape fur seals of well-documented changes in the availability of epipelagic prey species in the Benguela.

Diving optimality models predict air breathers to routinely dive within aerobic limits, but predator avoidance dives may be an exception. Lengthening submergence times during a predation threat may enhance survival probability, and we therefore hypothesized that predator avoidance dives in juvenile estuarine crocodiles (Crocodylus porosus) would be partially anaerobically fueled. We also predicted that reliance on anaerobic metabolism would increase at elevated temperatures to offset the faster depletion of body oxygen stores. Crocodiles were maintained at 287 and 347C for 60 d and subsequently underwent simulated predator avoidance dive trials at two test temperatures (287 and 347C). Blood was sampled immediately on surfacing to measure plasma lactate concentrations relative to nondiving (control) values. Aerobic dive limits (cADL; min) were also calculated using known body mass and oxygen storage relationships and rates of diving oxygen consumption and compared with observed dive durations. Postdive plasma lactate levels were elevated beyond resting levels at both test temperatures, indicating that aerobic thresholds were surpassed during simulated predator avoidance dives. Similarly, ≥90% of dive durations exceeded cADLs at both test temperatures. Postdive plasma lactate concentrations were independent of water temperature and thermal acclimation treatment. Together, these findings suggest that reliance on anaerobiosis during simulated predator avoidance dives is important regardless of temperature. [ABSTRACT FROM AUTHOR]

Compared with terrestrial mammals, marine mammals possess increased muscle myoglobin concentrations (Mb concentration, g Mb · 100g−1 muscle), enhancing their onboard oxygen (O2) stores and their aerobic dive limit. Although myoglobin is not homogeneously distributed, cetacean muscle O2 stores have been often determined by measuring Mb concentration from a single muscle sample (longissimus dorsi) and multiplying that value by the animal’s locomotor muscle or total muscle mass. This study serves to determine the accuracy of previous cetacean muscle O2 stores calculations. For that, body muscles from three delphinid species: Delphinus delphis, Stenella coeruleoalba, and Stenella frontalis, were dissected and weighed. Mb concentration was calculated from six muscles/muscle groups (epaxial, hypaxial and rectus abdominis; mastohumeralis; sternohyoideus; and dorsal scalenus), each representative of different functional groups (locomotion powering swimming, pectoral fin movement, feeding and respiration, respectively). Results demonstrated that the Mb concentration was heterogeneously distributed, being significantly higher in locomotor muscles. Locomotor muscles were the major contributors to total muscle O2 stores (mean 92.8%) due to their high Mb concentration and large muscle masses. Compared to this method, previous studies assuming homogenous Mb concentration distribution likely underestimated total muscle O2 stores by 10% when only considering locomotor muscles and overestimated them by 13% when total muscle mass was considered.

Comparative physiology has developed a rich understanding of the physiological adaptations of organisms, from microbes to megafauna. Despite extreme differences in size and a diversity of habitats, general patterns are observed in their physiological adaptations. Yet, many organisms deviate from the general patterns, providing an opportunity to understand the importance of ecology in determining the evolution of unusual adaptations. Aquatic air-breathing vertebrates provide unique study systems in which the interplay between ecology, physiology and behavior is most evident. They must perform breath-hold dives to obtain food underwater, which imposes a physiological constraint on their foraging time as they must resurface to breathe. This separation of two critical resources has led researchers to investigate these organisms' physiological adaptations and trade-offs. Addressing such questions on large marine animals is best done in the field, given the difficulty of replicating the environment of these animals in the lab. This Review examines the long history of research on diving physiology and behavior. We show how innovative technology and the careful selection of research animals have provided a holistic understanding of diving mammals' physiology, behavior and ecology. We explore the role of the aerobic diving limit, body size, oxygen stores, prey distribution and metabolism. We then identify gaps in our knowledge and suggest areas for future research, pointing out how this research will help conserve these unique animals., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Survival of air-breathing, diving ectotherms is dependent on their capacity to optimise the time available for obligate underwater activities, such as predator avoidance. Submergence times are thermally sensitive, with dive durations significantly reduced by increases in water temperature, deeming these animals particularly vulnerable to the effects of climate change. The physiological mechanisms underlying this compromised performance are unclear but are hypothesised to be linked to increased oxygen demand and a reduced capacity for metabolic depression at elevated temperatures. Here, we investigated how water temperature (both acute and chronic exposures) affected the physiology of juvenile estuarine crocodiles (Crocodylus porosus) performing predator avoidance dives (i.e. fright-dives). Diving oxygen consumption, 'fright' bradycardia, haematocrit and haemoglobin (indicators of blood oxygen carrying capacity) were assessed at two test temperatures, reflective of different climate change scenarios (i.e. current summer water temperatures, 28°C, and 'high' climate warming, 34°C). Diving oxygen consumption rate increased threefold between 28 and 34°C (Q10=7.4). The capacity to depress oxygen demand was reduced at elevated temperatures, with animals lowering oxygen demand from surface levels by 52.9±27.8% and 27.8±16.5% (means±s.e.m.) at 28°C and 34°C, respectively. Resting and post-fright-dive haematocrit and haemoglobin were thermally insensitive. Together these findings suggest decrements in fright-dive performance at elevated temperatures stem from increased oxygen demand coupled with a reduced capacity for metabolic depression. [ABSTRACT FROM AUTHOR]

Among pinnipeds, southern elephant seals (SESs, Mirounga leonina) are extreme divers that dive deeply and continuously along foraging trips to restore their body stores after fasting on land during breeding or moulting. Their replenishment of body stores influences their energy expenditure during dives and their oxygen (O2) reserves (via muscular mass), yet how they manage their O2 stores during their dives is not fully understood. In this study, 63 female SESs from Kerguelen Island were equipped with accelerometers and time-depth recorders to investigate changes in diving parameters through their foraging trips. Two categories of dive behaviour were identified and related to the body size of individuals, with smaller SESs performing shallower and shorter dives requiring greater mean stroke amplitude compared with larger individuals. In relation to body size, the larger seals had lower estimated oxygen consumption levels for a given buoyancy (i.e. body density) compared with smaller individuals. However, both groups were estimated to have the same oxygen consumption of 0.079±0.001 ml O2 stroke-1 kg-1 for a given dive duration and at neutral buoyancy when the cost of transport was minimal. Based on these relationships, we built two models that estimate changes in oxygen consumption according to dive duration and body density. The study highlights that replenishing body stores improves SES foraging efficiency, as indicated by increased time spent at the bottom of the ocean. Thus, prey-capture attempts increase as SES buoyancy approaches the neutral buoyancy point., Competing Interests: Competing interests The authors declare no competing or financial interests., (© 2023. Published by The Company of Biologists Ltd.)

Our understanding of how air-breathing marine predators cope with environmental variability is limited by our inadequate knowledge of their ecological and physiological parameters. Because of their wide distribution along both coasts of the sub-continent, South American sea lions (Otaria byronia) provide a valuable opportunity to study the behavioral and physiological plasticity of a marine predator in different environments. We measured the oxygen stores and diving behavior of South American sea lions throughout most of its range, allowing us to demonstrate that diving ability and behavior vary across its range. We found no significant differences in mass-specific blood volumes of sea lions among field sites and a negative relationship between mass-specific oxygen storage and size, which suggests that exposure to different habitats and geographical locations better explains oxygen storage capacities and diving capability in South American sea lions than body size alone. The largest animals in our study (individuals from Uruguay) were the shallowest and shortest duration divers, and had the lowest mass-specific total body oxygen stores, while the deepest and longest duration divers (individuals from southern Chile) had significantly larger mass-specific oxygen stores, despite being much smaller animals. Our study suggests that the physiology of air-breathing diving predators is not fixed, but that it can be adjusted, to a certain extent, depending on the ecological setting and or habitat. These adjustments can be thought of as a 'training effect': as the animal continues to push its physiological capacity through greater hypoxic exposure, its breath-holding capacity increases. [ABSTRACT FROM AUTHOR]

Physiological constraints dictate animals' ability to exploit habitats. For marine mammals, it is important to quantify physiological limits that influence diving and their ability to alter foraging behaviors. We characterized age-specific dive limits of walruses by measuring anaerobic (acid-buffering capacity) and aerobic (myoglobin content) capacities of the muscles that power hind (longissimus dorsi) and fore (supraspinatus) flipper propulsion. Mean buffering capacities were similar acrossmuscles and age classes (a fetus, five neonatal calves, a 3 month old and 20 adults), ranging from 41.31 to 54.14 slykes and 42.00 to 46.93 slykes in the longissimus and supraspinatus, respectively. Mean myoglobin in the fetus and neonatal calves fell within a narrow range (longissimus: 0.92-1.68 g 100 g-1 wet muscle mass; supraspinatus: 0.88-1.64 g 100 g-1 wet muscle mass). By 3 monthspost-partum,myoglobin in the longissimus increased by79%, but levels in the supraspinatus remained unaltered. From 3 months post-partum to adulthood, myoglobin increased by an additional 26% in the longissimus and increased by 126% in the supraspinatus; myoglobin remained greater in the longissimus compared with the supraspinatus. Walruses are unique among marine mammals because theyare born withamaturemuscleacid-buffering capacityand attainmaturemyoglobin content early in life.Despite rapid physiological development, small body size limits the diving capacity of immature walruses and extreme sexual dimorphism reduces the diving capacity of adult females compared with adult males. Thus, free-ranging immature walruses likely exhibit the shortest foraging dives while adult males are capable of the longest foraging dives. [ABSTRACT FROM AUTHOR]

Myoglobin (Mb) isanoxygenbinding protein found in vertebrate skeletal muscle, where it facilitates intracellular transport and storage of oxygen. This protein has evolved to suit unique physiological needs in the muscle of diving vertebrates that express Mb at much greater concentrations than their terrestrial counterparts. In this study, we characterized Mb oxygen affinity (P50) from 25 species of aquatic and terrestrial birds and mammals. Among diving species, we tested for correlations between Mb P50and routine dive duration. Across all species examined, Mb P50ranged from 2.40 to 4.85 mmHg. Themean P50ofMbfromterrestrial ungulateswas 3.72±0.15 mmHg(range 3.70- 3.74 mmHg). The P50of cetaceans was similar to terrestrial ungulates ranging from 3.54 to 3.82 mmHg, with the exception of the melonheaded whale, which had a significantly higher P50of 4.85 mmHg. Among pinnipeds, the P50ranged from3.23 to 3.81 mmHgand showed a trend for higher oxygen affinity in species with longer dive durations. Among diving birds, the P50ranged from 2.40 to 3.36 mmHg and also showed a trend of higher affinities in species with longer dive durations. In pinnipeds and birds, lowMb P50was associated with species whose muscles are metabolically active under hypoxic conditions associated with aerobic dives. Given the broad range of potential globin oxygen affinities, Mb P50from diverse vertebrate species appears constrained within a relatively narrow range. High Mb oxygen affinity within this rangemay be adaptive for some vertebrates thatmake prolonged dives. [ABSTRACT FROM AUTHOR]

Constraint lines - the boundaries that delimit point clouds in bivariate scattergrams - have been applied in macro-ecology to quantify the effects of limiting factors on response variables, but have not been applied to the behavioral performance and physiological ecology of individual vertebrates. I propose that behavioral scattergrams of air-breathing, diving vertebrates contain informative edges that convey insights into physiological constraints that shape the performance envelopes of divers. In the classic example of repeated cycles of apnea and eupnea in diving, airbreathing vertebrates, the need to balance oxygen consumption and intake should differentially constrain recovery for dives within or exceeding the aerobic dive limit. However, the bulk of variance observed in recovery versus dive duration scattergrams originates from undetermined behavioral variables, and deviations from overall stasis may become increasingly apparent at progressively smaller scales of observation. As shown on dive records from 79 Galápagos fur seals, the selection of appropriate time scales of integration yields two distinct recovery boundaries for dive series within and beyond the estimated aerobic dive limit. An analysis of the corresponding constraint lines is independent of central tendencies in data and avoids violating parametric assumptions for large data sets where variables of interest account for only a small portion of observed variance. I hypothesize that the intercept between these constraint lines represents the effective aerobic dive limit, and present physiological and ecological considerations to support this hypothesis.

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 (VO) 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 O stores by comparing blood O 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 O stores between the diving and non-diving groups. Estimated TBO (35.9 ml O kg) 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 O min) 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 VO and blood oxygen stores for an Otariid and indicates they have a much shorter ADL than previously thought. [ABSTRACT FROM AUTHOR]

Most marine mammals are hypothesized to routinely dive within their aerobic dive limit (ADL). Mammals that regularly perform deep, long-duration dives have locomotor muscles with elevated myoglobin concentrations that are composed of predominantly large, slow-twitch (Type I) fibers with low mitochondrial volume densities (Vmt). These features contribute to extending ADL by increasing oxygen stores and decreasing metabolic rate. Recent tagging studies, however, have challenged the view that two groups of extreme deep-diving cetaceans dive within their ADLs. Beaked whales (including Ziphius cavirostris and Mesoplodon densirostris) routinely perform the deepest and longest average dives of any air-breathing vertebrate, and short-finned pilot whales (Globicephala macrorhynchus) perform high-speed sprints at depth. We investigated the locomotor muscle morphology and estimated total body oxygen stores of several species within these two groups of cetaceans to determine whether they (1) shared muscle design features with other deep divers and (2) performed dives within their calculated ADLs. Muscle of both cetaceans displayed high myoglobin concentrations and large fibers, as predicted, but novel fiber profiles for diving mammals. Beaked whales possessed a sprinter's fiber-type profile, composed of ~80% fast-twitch (Type II) fibers with low VmX. Approximately one-third of the muscle fibers of short-finned pilot whales were slow-twitch, oxidative, glycolytic fibers, a rare fiber type for any mammal. The muscle morphology of beaked whales likely decreases the energetic cost of diving, while that of short-finned pilot whales supports high activity events. Calculated ADLs indicate that, at low metabolic rates, both beaked and short-finned pilot whales carry sufficient onboard oxygen to aerobically support their dives. [ABSTRACT FROM AUTHOR]

Abstract: This paper presents an integrated model of convective O2-transport, aerobic dive limits (ADL), and thermochemical data for oxygen binding to mutant myoglobin (Mb), used to quantify the impact of mutations in Mb on the dive limits of Weddell seals (Leptonychotes weddellii). We find that wild-type Mb traits are only superior under specific behavioral and physiological conditions that critically prolong the ADL, action radius, and fitness of the seals. As an extreme example, the mutations in the conserved His-64 reduce ADL up to 14±2min for routine aerobic dives, whereas many other mutations are nearly neutral in terms of ADL and the inferred fitness. We also find that the cardiac system, the muscle O2-store, animal behavior (i.e. pre-dive ventilation), and the oxygen binding affinity of Mb, K O 2 , have co-evolved to optimize dive duration at routine aerobic diving conditions, suggesting that such conditions are mostly selected upon in seals. The model is capable of roughly quantifying the physiological impact of single-protein mutations and thus bridges an important gap between animal physiology and molecular (protein) evolution. [Copyright &y& Elsevier]

Many optimal foraging models for diving animals examine strategies that maximize time spent in the foraging zone, assuming that prey acquisition increases linearly with search time. Other models have considered the effect of patch quality and predict a net energetic benefit if dives where no prey is encountered early in the dive are abandoned. For deep divers, however, the energetic benefit of giving up is reduced owing to the elevated energy costs associated with descending to physiologically hostile depths, so patch residence time should be invariant. Others consider an asymptotic gain function where the decision to leave a patch is driven by patch-depletion effects - the marginal value theorem. As predator behaviour is increasingly being used as an index of marine resource density and distribution, it is important to understand the nature of this gain function., We investigated the dive behaviour of the world's deepest-diving seal, the southern elephant seal Mirounga leonina, in response to patch quality. Testing these models has largely been limited to controlled experiments on captive animals. By integrating in situ measurements of the seal's relative lipid content obtained from drift rate data (a measure of foraging success) with area-restricted search behaviour identified from first-passage time analysis, we identified regions of high- and low-quality patches., Dive durations and bottom times were not invariant and did not increase in regions of high quality; rather, both were longer when patches were of relatively low quality. This is consistent with the predictions of the marginal value theorem and provides support for a nonlinear relationship between search time and prey acquisition., We also found higher descent and ascent rates in high-quality patches suggesting that seals minimized travel time to the foraging patch when quality was high; however, this was not achieved by increasing speed or dive angle. Relative body lipid content was an important predictor of dive behaviour., Seals did not schedule their diving to maximize time spent in the foraging zone in higher-quality patches, challenging the widely held view that maximizing time in the foraging zone translates to greater foraging success. [ABSTRACT FROM AUTHOR]

Constraint lines—the boundaries that delimit point clouds in bivariate scattergrams—have been applied in macro-ecology to quantify the effects of limiting factors on response variables, but have not been applied to the behavioral performance and physiological ecology of individual vertebrates. I propose that behavioral scattergrams of air-breathing, diving vertebrates contain informative edges that convey insights into physiological constraints that shape the performance envelopes of divers. In the classic example of repeated cycles of apnea and eupnea in diving, air-breathing vertebrates, the need to balance oxygen consumption, and intake should differentially constrain recovery for dives within or exceeding the aerobic dive limit (ADL). However, the bulk of variance observed in recovery versus dive duration scattergrams originates from undetermined behavioral variables, and deviations from overall stasis may become increasingly apparent at progressively smaller scales of observation. As shown on dive records from 79 Galápagos fur seals, the selection of appropriate time scales of integration yields two distinct recovery boundaries for dive series within and beyond the estimated ADL. An analysis of the corresponding constraint lines is independent of central tendencies in data and avoids violating parametric assumptions for large data sets where variables of interest account for only a small portion of observed variance. I hypothesize that the intercept between these constraint lines represents the effective ADL, and present physiological and ecological considerations to support this hypothesis. [ABSTRACT FROM AUTHOR]

Northern fur seal ( Callorhinus ursinus; NFS) populations have been declining, perhaps due to limited foraging ability of pups. Because a marine mammal's proficiency at exploiting underwater prey resources is based on the ability to store large amounts of oxygen (O) and to utilize these reserves efficiently, this study was designed to determine if NFS pups had lower blood, muscle, and total body O stores than adults. Pups (<1-month old) had a calculated aerobic dive limit only ~40% of adult females due to lower blood and, to a much greater extent, muscle O stores. Development of the Pectoralis ( Pec) and Longissimus dorsi ( LD) skeletal muscles was further examined by determining their myosin heavy chain (MHC) composition and enzyme activities. In all animals, the slow MHC I and fast-twitch IIA proteins typical of oxidative fiber types were dominant, but adult muscles contained more ( Pec ~50%; LD ~250% higher) fast-twitch MHC IID/X protein characteristic of glycolytic muscle fibers, than pup muscles. This suggests that adults have greater ability to generate muscle power rapidly and/or under anaerobic conditions. Pup muscles also had lower aerobic and anaerobic ATP production potential, as indicated by lower metabolically scaled citrate synthase, β-hydroxyacyl CoA dehydrogenase, and lactate dehydrogenase activities (all P values ≤0.001). In combination, these findings indicate that pups are biochemically and physiologically limited in their diving capabilities relative to adults. This may contribute to lower NFS first year survival. [ABSTRACT FROM AUTHOR]

Odontocetes have an exceptional range in body mass spanning 10 kg across species. Because, size influences oxygen utilization and carbon dioxide production rates in mammals, this lineage likely displays an extraordinary variation in oxygen store management compared to other marine mammal groups. To examine this, we measured changes in the partial pressures of respiratory gases ( $$ P_{{{\text{O}}_{2} }} $$, $$ P_{{{\text{CO}}_{2} }} $$), pH, and lactate in the blood during voluntary, quiescent, submerged breath holds in Pacific white-sided dolphins ( Lagenorhynchus obliquidens), bottlenose dolphins ( Tursiops truncatus), and a killer whale ( Orcinus orca) representing a mass range of 96-3,850 kg. These measurements provided an empirical determination of the effect of body size on the variability in blood biochemistry during breath hold and experimentally determined aerobic dive limits (ADL) within one taxonomic group (odontocetes). For the species in this study, maximum voluntary breath-hold duration was positively correlated with body mass, ranging from 3.5 min in white-sided dolphins to 13.3 min for the killer whale. Variation in breath-hold duration was associated with differences in the rate of change for $$ P_{{{\text{O}}_{2} }} $$ throughout breath hold; $$ P_{{{\text{O}}_{2} }} $$ decreased twice as fast for the two smaller species (−0.6 mmHg O min) compared to the largest species (−0.3 mmHg O min). In contrast, the rate of increase in $$ P_{{{\text{CO}}_{2} }} $$ during breath hold was similar across species. These results demonstrate that large body size in odontocetes facilitates increased aerobic breath-hold capacity as mediated by decreased mass-specific metabolic rates (rates of change in $$ P_{{{\text{O}}_{2} }} $$ served as a proxy for oxygen utilization). Indeed the experimentally determined 5 min ADL for bottlenose dolphins was surpassed by the 13.3 min maximum breath hold of the killer whale, which did not end in a rise in lactate. Rather, breath hold ended voluntarily as respiratory gases and pH fell within a narrow range for both large and small species, likely providing cues for ventilation. [ABSTRACT FROM AUTHOR]

Since the introduction of the aerobic dive limit (ADL) 30 years ago, the concept that most dives of marine mammals and sea birds are aerobic in nature has dominated the interpretation of their diving behavior and foraging ecology. Although there have been many measurements of body oxygen stores, there have been few investigations of the actual depletion of those stores during dives. Yet, it is the pattern, rate and magnitude of depletion of O2 stores that underlie the ADL. Therefore, in order to assess strategies of O2 store management, we review (a) the magnitude of 02 stores, (b) past studies of O2 store depletion and (C) our recent investigations of O2 store utilization during sleep apnea and dives of elephant seals (Mirounga angustirostris) and during dives of emperor penguins (Aptenodytes forsteri). We conclude with the implications of these findings for (a) the physiological responses underlying O2 store utilization, (b) the physiological basis of the ADL and (c) the value of extreme hypoxemic tolerance and the significance of the avoidance of re-perfusion injury in these animals. [ABSTRACT FROM AUTHOR]

Emperor penguins (Aptenodytes forsteri), both at sea and at an experimental dive hole, often have minimal surface periods even after performance of dives far beyond their measured 5.6 min aerobic dive limit (ADL: dive duration associated with the onset of post-dive blood lactate accumulation). Accelerometer-based data loggers were attached to emperor penguins diving in these two different situations to further evaluate the capacity of these birds to perform such dives without any apparent prolonged recovery periods. Minimum surface intervals for dives as long as 10 min were less than 1 min at both sites. Stroke rates for dives at sea were significantly greater than those for dives at the isolated dive hole. Calculated diving air volumes at sea were variable, increased with maximum depth of dive to a depth of 250 m, and decreased for deeper dives. It is hypothesized that lower air volumes for the deepest dives are the result of exhalation of air underwater. Mean maximal air volumes for deep dives at sea were approximately 83% greater than those during shallow (<50 m) dives. We conclude that (a) dives beyond the 5.6 min ADL do not always require prolongation of surface intervals in emperor penguins, (b) stroke rate at sea is greater than at the isolated dive hole and, therefore, a reduction in muscle stroke rate does not extend the duration of aerobic metabolism during dives at sea, and (c) a larger diving air volume facilitates performance of deep dives by increasing the total body O2 store to 68 ml O2 kg-1. Although increased O2 storage and cardiovascular adjustments presumably optimize aerobic metabolism during dives, enhanced anaerobic capacity and hypoxemic tolerance are also essential for longer dives. This was exemplified by a 27.6 min dive, after which the bird required 6 min before it stood up from a prone position, another 20 min before it began to walk, and 8.4 h before it dived again. [ABSTRACT FROM AUTHOR]

Accurate predictions of predator behavior remain elusive in natural settings. Optimal foraging theory predicts that breath-hold divers should adjust time allocation within their dives to the distance separating prey from the surface. Quantitative tests of these models have been hampered by the difficulty of documenting underwater feeding behavior and the lack of systems, experimental or natural, in which prey depth varies over a large range. We tested these predictions on blue whales (Balaenoptera musculus), which track the diel vertical migration of their prey. A model using simple allometric arguments successfully predicted diving behavior measured with data loggers. Foraging times within each dive increased to compensate longer transit times and optimize resource acquisition. Shallow dives were short and yielded the highest feeding rates, explaining why feeding activity was more intense at night. An optimal framework thus provides powerful tools to predict the behavior of free-ranging marine predators and inform conservation studies. [ABSTRACT FROM PUBLISHER]

The physiological basis of the aerobic dive limit (ADL), the dive duration associated with the onset of post-dive blood lactate elevation, is hypothesized to be depletion of the muscle oxygen (O2) store. A duel wavelength near-infrared spectrophotometer was developed and used to measure myoglobin (Mb) O2 saturation levels in the locomotory muscle during dives of emperor penguins (Aptenodytes forsteri). Two distinct patterns of muscle O2 depletion were observed. Type A dives had a monotonic decline, and, in dives near the ADL, the muscle O2 store was almost completely depleted. This pattern of Mb desaturation was consistent with lack of muscle blood flow and supports the hypothesis that the onset of post-dive blood lactate accumulation is secondary to muscle O2 depletion during dives. The mean type A Mb desaturation rate allowed for calculation of a mean muscle O2 consumption of 12.4 ml O2kg-1 muscle min-1, based on a Mb concentration of 6.4g 100g-1 muscle. Type B desaturation patterns demonstrated a more gradual decline, often reaching a mid-dive plateau in Mb desaturation. This mid-dive plateau suggests maintenance of some muscle perfusion during these dives. At the end of type B dives, Mb desaturation rate increased and, in dives beyond the ADL, Mb saturation often reached near 0%. Thus, although different physiological strategies may be used during emperor penguin diving, both Mb desaturation patterns support the hypothesis that the onset of post-dive lactate accumulation is secondary to muscle O2 store depletion [ABSTRACT FROM AUTHOR]

Rapid changes in sea ice cover associated with global warming are poised to have marked impacts on polar marine mammals. Here we examine skeletal muscle characteristics supporting swimming and diving in one polar species, the narwhal, and use these attributes to further document this cetacean's vulnerability to unpredictable sea ice conditions and changing ecosystems. We found that extreme morphological and physiological adaptations enabling year-round Arctic residency by narwhals limit behavioral flexibility for responding to alternations in sea ice. In contrast to the greyhound-like muscle profile of acrobatic odontocetes, the longissimus dorsi of narwhals is comprised of 86.8%± 7.7% slow twitch oxidative fibers, resembling the endurance morph of human marathoners. Myoglobin content, 7.87 ± 1.72 g/100 g wet muscle, is one of the highest levels measured for marine mammals. Calculated maximum aerobic swimming distance between breathing holes in ice is <1,450 m, which permits routine use of only 2.6%-10.4% of ice-packed foraging grounds in Baffin Bay. These first measurements of narwhal exercise physiology reveal extreme specialization of skeletal muscles for moving in a challenging ecological niche. This study also demonstrates the power of using basic physiological attributes to predict species vulnerabilities to environmental perturbation before critical population disturbance occurs. [ABSTRACT FROM AUTHOR]

Marine turtles spend more than 90% of their life underwater and have been termed surfacers as opposed to divers. Nonetheless turtles have been reported occasionally to float motionless at the surface but the reasons for this behaviour are not clear. We investigated the location, timing and duration of extended surface times (ESTs) in 10 free-ranging loggerhead turtles (Caretta caretta) and the possible relationship to water temperature and diving activity recorded via satellite relay data loggers for 101-450 days. For one turtle that dived only in offshore areas, ESTs contributed 12% of the time whereas for the other turtles ESTs contributed 0.4-1.8% of the time. ESTs lasted on average 90 mm but were mostly infrequent and irregular, excluding the involvement of a fundamental regulatory function. However, 82% of the ESTs occurred during daylight, mostly around noon, suggesting a dependence on solar radiation. For three turtles, there was an appreciable (7°C to 10.5°C) temperature decrease with depth for dives during periods when ESTs occurred frequently, suggesting a re-warming function of EST to compensate for decreased body temperatures, possibly to enhance digestive efficiency. A positive correlation between body mass and EST duration supported this explanation. By contrast, night-active turtles that exceeded their calculated aerobic dive limits in 7.6-16% of the dives engaged in nocturnal ESTs, probably for lactate clearance. This is the first evidence that loggerhead turtles may refrain from diving for at least two reasons, either to absorb solar radiation or to recover from anaerobic activity. [ABSTRACT FROM AUTHOR]

1. Body mass is a key determinant of diving performance in endotherms. In air-breathing ectotherms however, this paradigm occurs with considerably less force. Here, through remote recordings of dive behaviour over a wide size range (5–42 kg body mass, n = 17) of freshwater crocodiles ( Crocodylus johnstoni), we demonstrate why body mass is such a poor determinant of dive duration for ectothermic divers. 2. Crocodiles were released into the wild with a time-depth-recorder attached to their dorsal scutes, and a movement activated radio-tag attached to their tail. Over 15 days, 652·6 ± 58·4 (mean ± SE, n = 17) dives were recorded, with all individuals exhibiting two specific dive-types. These were, a resting-dive (62·7 ± 5·4% of total dive no.), where no activity occurred during the dive, and an active-dive (37·1 ± 6·3% of total no.) associated with swimming. 3. The durations of resting-dives (∼12 min) were similar for all crocodiles. Smaller crocodiles (6·3 ± 0·7 kg, mean ± SE, n = 9) exhibited a significant correlation between dive duration and post-dive surface-interval, whilst larger crocodiles (17·9 ± 3·75 kg, mean ± SE, n = 8) did not. This demonstrated that aerobic dive duration was mass-specific during resting-dives, but other mass specific factors, presumably ecological, determined dive duration. 4. The durations of active-dives were never >1 min, showed no relationship with body mass and no correlation with the post-dive-surface interval. In crocodiles, aerobic metabolic scope is independent of body mass but anaerobic capacity is mass dependent, suggesting that active-dive duration was determined by sustained activity and dives were terminated before anaerobic metabolism became significant. 5. All individuals showed similar diel phase shifts in dive duration, type and depth, illustrating the overwhelming influence of the external environment on dive behaviour. Dive durations which resulted in significant anaerobic debt occurred rarely, but were undertaken in response to a potential threat. 6. Body mass was a poor predictor of diving in C. johnstoni because the external environmental and ecological factors exerted a greater influence on dive duration than oxygen reserves. [ABSTRACT FROM AUTHOR]

The emperor penguin (Aptenodytes forsteri) thrives in the Antarctic underwater environment, diving to depths greater than 500m and for durations longer than 23 mm. To examine mechanisms underlying the exceptional diving ability of this species and further describe blood oxygen (O2) transport and depletion while diving, we characterized the O2-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 O2 depletion during diving, by applying the dissociation curve to previously collected partial pressure of O2 (PO2) profiles to estimate in vivo Hb saturation (SO2) changes during dives. This investigation revealed enhanced Hb-O2 affinity (P50=28 mmHg, pH7.5) in the emperor penguin, similar to high-altitude birds and other penguin species. This allows for increased O2 at low blood PO2 levels during diving and more complete depletion of the respiratory O2 store. SO2 profiles during diving demonstrated that arterial 8O2 levels are maintained near 100% throughout much of the dive, not decreasing significantly until the final ascent phase. End-of-dive venous SO2 values were widely distributed and optimization of the venous blood O2 store resulted from arterialization and near complete depletion of venous blood O2 during longer dives. The estimated contribution of the blood O2 store to diving metabolic rate was low and highly variable. This pattern is due, in part, to the influx of O2 from the lungs into the blood during diving, and variable rates of tissue O2 uptake. [ABSTRACT FROM AUTHOR]

The article characterizes how Marbled Murrelets adjust their foraging effort in response to changes in reproductive demands in an upwelling system in Central California. It states that murrelets spent more time diving during upwelling than oceanographic relaxation, increased their foraging ranges as the duration of relaxation grew longer, and reduced their foraging ranges after transitions to upwelling. Furthermore, to meet reproductive demands during nesting, murrelets adopted a combined strategy or cutting down energy expended commuting to foraging sites and increasing aerobic dive rates.

In order to further define O2 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 (PO2) recorder or a blood sampler while they were diving at an Isolated dive hole in the sea ice of McMurdo Sound, Antarctica. Arterial PO2 profiles (57 dives) revealed that (a) pre-dive PO2 was greater than that at rest, (b) PO2 transiently increased during descent and (c) post-dive PO2 reached that at rest in 1.92±1.89 mm (N=53). Venous PO2 profiles (130 dives) revealed that (a) pre-dive venous PO2 was greater than that at rest prior to 61% of dives, (b) in 90% of dives venous PO2 transiently increased with a mean maximum PO2 of 53±18mmHg and a mean increase in PO2 of 11±12mmHg, (c) in 78% of dives, this peak venous PO2 occurred within the first 3mm, and (d) post-dive venous PO2 reached that at rest within 2.23±2.64mm (N=84). Arterial and venous PO2 values in blood samples collected 1-3mm into dives were greater than or near to the respective values at rest. Blood lactate concentration was less than 2 mmoll-1 as far as 10.5 mm into dives, well beyond the known ADL of 5.6mm. Mean arterial and venous of samples collected at 20-37m 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 PO2 and PN2 during dives), muscle ischemia during dives (elevated venous PO2, lack of lactate washout into blood during dives), and arterio-venous shunting of blood both during the surface period (venous PO2 greater than that at rest) and during dives (arterialized venous PO2 values during descent, equivalent arterial and venous PN2 values during dives). These three physiological processes contribute to the transfer of the large respiratory O2 store to the blood during the dive, isolation of muscle metabolism from the circulation during the dive, a decreased rate of blood O2 depletion during dives, and optimized loading of O2 stores both before and after dives. The lack of blood O2 depletion and blood lactate elevation during dives beyond the AOL suggests that active locomotory muscle is the site of tissue lactate accumulation that results in post-dive blood lactate elevation in dives beyond the AOL. [ABSTRACT FROM AUTHOR]

Optimality models exist for diving endotherms, but are rarely tested with behavioural data or used to estimate oxygen reserves. We used a model for avian divers to study the extreme diving performances of blue-eyed shags. Time–depth recorders were deployed on 15 breeding Kerguelen shags, Phalacrocorax verrucosus. The shags regularly dived deeper than 100m and longer than their behavioural aerobic dive limit (4min). The dive duration to postdive interval ratio peaked for dives lasting 1min, the dive time theoretically necessary to deplete oxygen reserves from the respiratory tract. Most dive parameters of the Kerguelen shag converged with those known for the Crozet shag, Phalacrocorax melanogenis. Yet, whereas the distribution of dive durations matched optimal breathing for the Crozet shag (shallow diving), this was not true for the Kerguelen shag which made mostly deep dives. Thus, regardless of how similar the physiologies of blue-eyed shag species may be, they can adapt their diving behaviour to different environmental conditions, in this case resource distribution. From the model, the volume of body oxygen reserves for blue-eyed shags was calculated as 264ml/kg, which is significantly higher than values found in the literature for avian divers. The volume of the respiratory tract obtained with the model (830ml), however, was realistic. We suggest the model overestimated body oxygen stores because blue-eyed shags have numerous means for reducing their deep-diving metabolism, such as bradycardia, hypothermia or anaerobic metabolism. [Copyright &y& Elsevier]

Blood gas analyses from emperor penguins (Aptenodytes forsteri) at rest, and intravascular PO2 profiles from free- diving birds were obtained in order to examine hypoxemic tolerance and utilization of the blood O2 store during dives. Analysis of blood samples from penguins at rest revealed arterial PO2s and O2 contents of 68±7 mmHg (1 mmHg= 133.3 Pa) and 22.5±1.3 ml O2 dl-1 (N=3) and venous values of 41±10 mmHg and 17.4±2.9 ml O2 dl-1 (N=9). Corresponding arterial and venous Hb saturations for a hemoglobin (Hb) concentration of 18 g dl-1 were >91% and 70%, respectively. Analysis of PO2 profiles obtained from birds equipped with intravascular PO2 electrodes and backpack recorders during dives revealed that (1) the decline of the final blood PO2 of a dive in relation to dive duration was variable, (2) final venous PO2 values spanned a 40-mmHg range at the previously measured aerobic dive limit (ADL; dive duration associated with onset of post-dive blood lactate accumulation), (3) final arterial, venous and previously measured air sac PO2 values were indistinguishable in longer dives, and (4) final venous PO2 values of longer dives were as low as 1-6 mmHg during dives. Although blood O2 is not depleted at the ADL, nearly complete depletion of the blood O2 store occurs in longer dives. This extreme hypoxemic tolerance, which would be catastrophic in many birds and mammals, necessitates biochemical and molecular adaptations, including a shift in the O2-Hb dissociation curve of the emperor penguin in comparison to those of most birds. A relatively higher-affinity Hb is consistent with blood PO2 values and O2 contents of penguins at rest. [ABSTRACT FROM AUTHOR]

Abstract: Marine turtle lungs have multiple functions including respiration, oxygen storage and buoyancy regulation, so lung size is an important indicator of dive performance. We determined maximum lung volumes (V L) for 30 individuals from three species (Caretta caretta n =13; Eretmochelys imbricata n =12; Natator depressus n =5) across a range of body masses (M b): 0.9 to 46 kg. V L was 114 ml kg−1 and increased with M b with a scaling factor of 0.92. Based on these values for V L we demonstrated that diving capacities (assessed via aerobic dive limits) of marine turtles were potentially over-estimated when the V L-body mass effect was not considered (by 10 to 20% for 5 to 25 kg turtles and by >20% for turtles ≥25 kg). While aerobic dive limits scale with an exponent of 0.6, an analysis of average dive durations in free-ranging chelonian marine turtles revealed that dive duration increases with a mass exponent of 0.51, although there was considerable scatter around the regression line. While this highlights the need to determine more parameters that affect the duration-body mass relationship, our results provide a reference point for calculating oxygen storage capacities and air volumes available for buoyancy control. [Copyright &y& Elsevier]

A mathematical model was used to explore if elevated levels of N2, and risk of decompression sickness (DCS), could limit dive performance (duration and depth) in king penguins (Aptenodytes patagonicus). The model allowed prediction of blood and tissue (central circulation, muscle, brain and fat) N2 tensions (PN2) based on different cardiac outputs and blood flow distributions. Estimated mixed venous PN2 agreed with values observed during forced dives in a compression chamber used to validate the assumptions of the model. During bouts of foraging dives, estimated mixed venous and tissue PN2 increased as the bout progressed. Estimated mean maximum mixed venous PN2 upon return to the surface after a dive was 4.56±0.18 atmospheres absolute (ATA; range: 4.37–4.78 ATA). This is equivalent to N2 levels causing a 50% DCS incidence in terrestrial animals of similar mass. Bout termination events were not associated with extreme mixed venous N2 levels. Fat PN2 was positively correlated with bout duration and the highest estimated fat PN2 occurred at the end of a dive bout. The model suggested that short and shallow dives occurring between dive bouts help to reduce supersaturation and thereby DCS risk. Furthermore, adipose tissue could also help reduce DCS risk during the first few dives in a bout by functioning as a sink to buffer extreme levels of N2. [ABSTRACT FROM AUTHOR]

The physiological and behavioural development of diving was examined in Australian fur seal ( Arctocephalus pusillus doriferus) pups to assess whether animals at weaning are capable of exploiting the same resources as adult females. Haematocrit, haemoglobin and myoglobin contents all increased throughout pup development though total body oxygen stores reached only 71% of adult female levels just prior to weaning. Oxygen storage components, however, did not develop at the same pace. Whereas blood oxygen stores had reached adult female levels by 9 months of age, muscle oxygen stores were slower to develop, reaching only 23% of adult levels by this age. Increases in diving behaviour corresponded to the physiological changes observed. Pups spent little time (<8%) in the water prior to moulting (age 1–2 months) whereas following the moult, they spent >27% of time in the water and made mid-water dives (maximum depth 35.7 ± 2.9 m) with durations of 0.35 ± 0.03 min. By 9 months (just prior to weaning), 30.5 ± 9.3% of all dives performed were U-shaped benthic dives (maximum depth 65.0 ± 6.0 m) with mean durations of 0.87 ± 0.25 min, significantly shorter than those of adult females. These results suggest that while Australian fur seal pups approaching the age of weaning are able to reach similar depths as adult females, they do not have the physiological capacity to remain at these depths for sufficient durations to exploit them to the same efficiency. [ABSTRACT FROM AUTHOR]

Abstract: [(1)] Interactions between temperature and hypoxia in ectothermic tetrapods (reptiles and amphibians) are reviewed. [(2)] At temperature extremes, mismatches between oxygen demand and oxygen supply can lead to hypoxemia. Thresholds for both physiological and behavioral responses to hypoxia are elevated at high temperature and aerobic scope is reduced at both high and low temperature. [(3)] Environmental hypoxia is uncommon for most species in air, and for those living at altitude or in burrows, low metabolic rates and relatively low temperature probably prevents hypoxia from being a major problem. [(4)] For aquatic species, increasing temperature decreases both the aerobic dive limit and the relative importance of aquatic gas exchange to oxygen supply. [(5)] Tolerance to anoxic submergence is drastically reduced by increased temperature. [Copyright &y& Elsevier]

A defining physiological capability for air-breathing marine vertebrates is the amount of oxygen that can be stored in tissues and made available during dives. To evaluate the influence of oxygen storage capacity on aerobic diving capacity, we examined the extent to which blood and muscle oxygen stores varied as a function of age, body size and sex in the sexually dimorphic California sea lion, Zalophus californianus. We measured total body oxygen stores, including hematocrit, hemoglobin, MCHC, plasma volume, blood volume and muscle myoglobin in pups through adults of both sexes. Blood and muscle oxygen storage capacity was not fully developed by the end of the dependency period, with blood stores not fully developed until animals were larger juveniles (70 kg; 1.5-2.5 years) and muscle stores not until animals were sub-adult size (125 kg; 4-6 years). Differences in aerobic diving capacity among size classes were reflective of these major milestones in the development of oxygen stores. Male sea lions had greater absolute blood volume than females and reflected the larger mass of males, which became apparent when animals were large juveniles. Adult female sea lions had greater muscle myoglobin concentrations compared to males, resulting in greater mass-specific muscle and total oxygen stores. Delayed development of oxygen stores is consistent with the shallow epi-mesopelagic foraging behavior in this species. We hypothesize that the greater mass-specific oxygen stores of female sea lions compared to males is related to differences in foraging behavior between the sexes. [ABSTRACT FROM AUTHOR]

Sound-and-orientation recording tags (DTAGs) were used to study 10 beaked whales of two poorly known species, Ziphius cavirostris (Zc) and Mesoplodon densirostris (Md). Acoustic behaviour in the deep foraging dives performed by both species (Zc: 28 dives by seven individuals; Md: 16 dives by three individuals) shows that they hunt by echolocation in deep water between 222 and 1885 m, attempting to capture about 30 prey/dive. This food source is so deep that the average foraging dives were deeper (Zc: 1070 m; Md: 835 m) and longer (Zc: 58 mm; Md: 47 mm) than reported for any other air-breathing species. A series of shallower dives, containing no indications of foraging, followed most deep foraging dives. The average interval between deep foraging dives was 63 mm for Zc and 92 mm for Md. This long an interval may be required for beaked whales to recover from an oxygen debt accrued in the deep foraging dives, which last about twice the estimated aerobic dive limit. Recent reports of gas emboli in beaked whales stranded during naval sonar exercises have led to the hypothesis that their deep-diving may make them especially vulnerable to decompression. Using current models of breath-hold diving, we infer that their natural diving behaviour is inconsistent with known problems of acute nitrogen supersaturation and embolism. If the assumptions of these models are correct for beaked whales, then possible decompression problems are more likely to result from an abnormal behavioural response to sonar. [ABSTRACT FROM AUTHOR]