Comprehensive molecular and morphological resolution of blubber stratification in a deep-diving, fasting-adapted seal.

Blubber is a modified subcutaneous adipose tissue in marine mammals that provides energy storage, thermoregulation, hydrodynamic locomotion, and buoyancy. Blubber displays vertical stratification by lipid content, fatty acid composition, and vascularization, leading to the assumption that deeper blubber layers are metabolically active, while superficial layers are mainly structural and thermoregulatory. However, few studies have examined functional stratification of marine mammal blubber directly, especially in pinnipeds. We characterized morphological and transcriptional differences across blubber layers in the northern elephant seal, a deep-diving and fasting-adapted phocid. We collected blubber from seals early in their fasting period and divided blubber cores into three similarly sized portions. We hypothesized that the innermost blubber portion would have higher 1) heterogeneity in adipocyte size, 2) microvascular density, and 3) expression of genes associated with metabolism and hormone signaling than outer blubber. We found that adipocyte area and variance increased from outermost (skin-adjacent) to innermost (muscle-adjacent) blubber layers, suggesting that inner blubber has a higher capacity for lipid storage and turnover than outer blubber. Inner blubber had a higher proportion of CD144+ endothelial cells, suggesting higher microvascular density. In contrast, outer blubber had a higher proportion of CD4+ immune cells than inner blubber, suggesting higher capacity for response to tissue injury. Transcriptome analysis identified 61 genes that were differentially expressed between inner and outer blubber layers, many of which have not been studied previously in marine mammals. Based on known functions of these genes in other mammals, we suggest that inner blubber has potentially higher 1) adipogenic capacity, 2) cellular diversity, and 3) metabolic and neuroendocrine signaling activity, while outer blubber may have higher 1) extracellular matrix synthesis activity and 2) responsiveness to pathogens and cell stressors. We further characterized expression of nine genes of interest identified by transcriptomics and two adipokines with higher precision across blubber layers using targeted assays. Our study provides functional insights into stratification of blubber in marine mammals and a molecular key, including CD144, CD4, HMGCS2, GABRG2, HCAR2, and COL1A2, for distinguishing blubber layers for physiological and functional studies in seals.

Influence of season, age, sex, and time of day on the endocrine profile of the common bottlenose dolphin (Tursiops truncatus).

Understanding baseline hormone levels, the magnitude of intra-individual variability, and their variation as a function of life history is difficult in toothed whales (e.g. dolphins and porpoises) because of the effects of capture stress. To determine the endocrine profile of the common bottlenose dolphin (Tursiops truncatus) as a function of season, time of day (TOD), age, sex, and reproductive status, blood corticosteroids, thyroid hormones, and catecholamines were repeatedly measured in a managed-care population exposed to ambient light and water temperatures of San Diego Bay. Additionally, fecal hormone metabolites were assessed for cortisol, aldosterone, and triiodothyronine. Samples were collected at two to four-week intervals over a period of two years, and multiple times within a day at monthly intervals over a year. Samples were collected through the voluntary participation of the dolphins in the blood draws and fecal collections in order to avoid the effects of handling stress. All serum hormones except aldosterone significantly varied with season and all serum hormones except total thyroxine significantly varied as a function of TOD. Fecal glucocorticoid metabolites significantly correlated with circulating cortisol levels, and there was a significant seasonal effect on triiodothyronine fecal metabolites. Strong seasonal effects demonstrated complex interactions with age and sex suggesting that contextual information is critical to interpreting differences in endocrine profiles. Strong circadian patterns further suggest that sampling design is important to the interpretation of blood or fecal collections, particularly since diurnal changes in some serum hormone levels are similar to the magnitude of seasonal differences. Despite potential impacts of feeding schedules on diurnal patterns, managed care populations can provide important insights into seasonal and age-related endocrine changes in toothed whales.

Variation in adrenal and thyroid hormones with life-history stage in juvenile northern elephant seals (Mirounga angustirostris).

The classical approach to quantifying the impact of stressors on wildlife is through characterization of hormones associated with the generalized stress response. However, interpretation of hormone data can be difficult due to the range of natural variation within a species and potential confounds of individual and life-history variables. Blood adrenal and thyroid hormones were measured in 144 chemically immobilized yearling northern elephant seals (Mirounga angustirostris) to characterize variation between sexes and across semiannual haul-outs. There was no relationship between hormone concentrations and time needed for collecting blood nor evidence of diel patterns, suggesting that collection of samples for baseline values can be accomplished without bias due to handling artifacts or time of day. Serum cortisol concentrations did not vary with gender or across haul-out fasts but increased dramatically during molting. Cortisol was correlated with aldosterone across all measured life-history stages. Thyroid hormone levels were lower in females and decreased with fasting in both sexes during the fall haul-out. Cortisol concentrations were inversely associated with total triiodothyronine (T3) and positively associated with reverse T3 concentrations across all measured life-history stages suggesting an important impact of cortisol on deiodinase enzymes and thyroid function. Epinephrine concentrations increased across fasts and norepinephrine concentrations were higher in males than in females. Significant variation in stress hormone concentrations with gender and life-history stage emphasizes the importance of contextual variables when interpreting serum hormone concentrations.

Blubber transcriptome response to acute stress axis activation involves transient changes in adipogenesis and lipolysis in a fasting-adapted marine mammal.

Stress can compromise an animal's ability to conserve metabolic stores and participate in energy-demanding activities that are critical for fitness. Understanding how wild animals, especially those already experiencing physiological extremes (e.g. fasting), regulate stress responses is critical for evaluating the impacts of anthropogenic disturbance on physiology and fitness, key challenges for conservation. However, studies of stress in wildlife are often limited to baseline endocrine measurements and few have investigated stress effects in fasting-adapted species. We examined downstream molecular consequences of hypothalamic-pituitary-adrenal (HPA) axis activation by exogenous adrenocorticotropic hormone (ACTH) in blubber of northern elephant seals due to the ease of blubber sampling and its key role in metabolic regulation in marine mammals. We report the first phocid blubber transcriptome produced by RNAseq, containing over 140,000 annotated transcripts, including metabolic and adipocytokine genes of interest. The acute response of blubber to stress axis activation, measured 2 hours after ACTH administration, involved highly specific, transient (lasting <24 hours) induction of gene networks that promote lipolysis and adipogenesis in mammalian adipocytes. Differentially expressed genes included key adipogenesis factors which can be used as blubber-specific markers of acute stress in marine mammals of concern for which sampling of other tissues is not possible.

Summing the strokes: energy economy in northern elephant seals during large-scale foraging migrations.

BACKGROUND: The 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. RESULTS: Body 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. CONCLUSIONS: These 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.

Oxidative stress is a potential cost of breeding in male and female northern elephant seals.

The trade-off between current reproductive effort and survival is a key concept of life history theory. A variety of studies support the existence of this trade-off but the underlying physiological mechanisms are not well-understood. Oxidative stress has been proposed as a potential mechanism underlying the observed inverse relationship between reproductive investment and lifespan. Prolonged fasting is associated with oxidative stress including increases in the production of reactive oxygen species, oxidative damage and inflammation.Northern elephant seals (NES) undergo prolonged fasts while maintaining high metabolic rates during breeding. We investigated NES of both sexes to assess oxidative stress associated with extended breeding fasts. We measured changes in the plasma activity or concentrations of markers for oxidative stress in 30 adult male and 33 adult female northern elephant seals across their 1-3 month breeding fasts. Markers assessed included a pro-oxidant enzyme, several antioxidant enzymes, markers for oxidative damage to lipids, proteins and DNA, and markers for systemic inflammation.Plasma xanthine oxidase (XO), a pro-oxidant enzyme that increases production of oxidative radicals, and several protective antioxidant enzymes increased over breeding in both sexes. Males showed increased oxidative damage to lipids and DNA and increased systemic inflammation, while oxidative damage to proteins declined across breeding. In contrast, females showed no oxidative damage to lipids or DNA or changes in inflammation, but showed increases in oxidative damage to proteins. XO activity, antioxidant enzymes, oxidative damage markers, and inflammatory markers were strongly correlated in males but these relationships were weaker or non-existent in females.NES provide evidence for oxidative stress as a physiological cost of reproduction in a capital breeding mammal. Both sexes strongly up-regulated antioxidant defenses during breeding. Despite this response, and in contrast to similar duration non-breeding fasts in previous studies on conspecifics, there was evidence of oxidative damage to tissues. These data demonstrate the utility of using plasma markers to examine oxidative stress but also suggest the necessity of measuring a broad suite of plasma markers to assess systemic oxidative stress.

Parasitological examination for presence of hookworms (Uncinaria spp.) in northern elephant seals (Mirounga angustirostris) at Año Nuevo State Reserve, California (2012).

Northern elephant seals (Mirounga angustirostris Gill, 1866), inhabiting rookeries on the mainland of Año Nuevo State Reserve in central California, were investigated in 2012 for presence of hookworms (Uncinaria spp.). Material collected and examined for hookworms included: blubber (n = 15), stomach and intestines (n = 21) from dead pups; feces from the rectum of weaned pups (n = 23); sand containing apparent feces in areas of weaned pups (n = 28) and sand without apparent feces in areas of weaned pups (n = 54); milk from females (n =23) at 5 days and about 23 to 26 days postpartum; and placenta from one female. Evidence of hookworm presence was not detected in any of the samples examined. Possible reasons why hookworms were not found in northern elephant seals on the mainland of Año Nuevo State Reserve are discussed.

Stable isotope analyses reveal individual variability in the trophic ecology of a top marine predator, the southern elephant seal.

Identifying individuals' foraging strategies is critical to understanding the ecology of a species, and can provide the means to predict possible ecological responses to environmental change. Our study combines stable isotope analysis and satellite telemetry to study the variability in individual foraging strategies of adult female southern elephant seals (Mirounga leonina). Our hypothesis is that female elephant seals from the Western Antarctica Peninsula (WAP) display individual specialization in their diets. We captured adult female elephant seals (n = 56, 2005-2009) at Livingston Island (Antarctica), and instrumented them with SMRU-CTD satellite tags. We collected blood, fur, and vibrissae samples for δ(13)C and δ(15)N analyses. The mean values for all vibrissae were -21.0 ± 0.7‰ for δ(13)C, and 10.4 ± 0.8‰, for δ(15)N. The individual variability of δ(13)C (60%) was more important than the within-individual variability (40%) in explaining the total variance observed in our data. For δ(15)N, the results showed the opposite trend, with the within-individual variability (64%) contributing more to the total variance than the individual variability (36%), likely associated with the effect that the fasting periods have on δ(15)N values. Most individuals were specialists, as inferred from the low intra-individual variability of δ(13)C values with respect to the population variability, with half the individuals utilizing 31% or less of their available niche. We found eight different foraging strategies for these animals. Female elephant seals from the WAP are a diverse group of predators with individuals utilizing only a small portion of the total available niche, with the consequent potential to expand their foraging habits to exploit other resources or environments in the Southern Ocean.

Environment and activity affect skin temperature in breeding adult male elephant seals (Mirounga angustirostris).

The large body size and high rates of metabolic heat production associated with male mating success in polygynous systems creates potential thermoregulatory challenges for species breeding in warm climates. This is especially true for marine predators carrying large blubber reserves intended for thermoregulation in cold water and fuel provision during extended fasts. Thermographic images were used to measure changes in skin temperature (T(S)) in adult male northern elephant seals (Mirounga angustirostris) over the breeding season. Environmental variables, primarily ambient temperature and solar radiation, were the principal determinants of mean and maximum T(S). When controlled for environmental variables, dominance rank significantly impacted mean T(S), being highest in alpha males. Behavioral activity significantly influenced T(S) but in a counter-intuitive way, with inactive males exhibiting the highest T(S). This was likely due to strong impacts of environmental variables on the kinds of behavior exhibited, with males being less active on warm, humid days at peak solar radiation. We classified thermal windows as areas in which T(S) was one standard deviation greater than mean T(S) for the individual seal within a thermograph. Thermal features suggest active physiological thermoregulation during and after combat and significant circulatory adaptations for heat dumping, as evidenced by recurring locations of thermal windows representing widely varying T(S) values. Frequent observations of localized T(S) above 37°C, particularly after combat, suggest the production of thermoregulatory stress during breeding behavior. Our findings demonstrate the importance of environmental drivers in shaping activity patterns during breeding and provide evidence for thermoregulatory costs of successful breeding in large polygynous males.

Measurements of foraging success in a highly pelagic marine predator, the northern elephant seal.

1. Identification of foraging behaviour and the ability to assess foraging success is critical to understanding individual and between-species variation in habitat use and foraging ecology. For pelagic predators, behaviour-dependent foraging metrics are commonly used to identify important foraging areas, yet few of these metrics have been validated. 2. Using the northern elephant seal as a model species, we validated the use of a behaviour-independent measure of foraging success (changes in drift rate) at the scale of the entire foraging migration, and then used this to assess a variety of common foraging metrics that are based on movement patterns and dive behaviour. Transit rate consistently provided the best estimate of daily foraging success, although the addition of other metrics provides insight into different foraging behaviours or strategies. 3. While positive changes in buoyancy occurred throughout most of the migrations, implying successful feeding across much of the north Pacific, the areas of most rapid changes in buoyancy occurred along a latitudinal band (40-50° N) corresponding to a dynamic hydrographic region including Subarctic Gyre and Transition Zone waters. 4. These results support the use of transit rate as an index of foraging success: a metric that is easily derived from tracking measurements on a wide range of marine species.

Condition and mass impact oxygen stores and dive duration in adult female northern elephant seals.

The range of foraging behaviors available to deep-diving, air-breathing marine vertebrates is constrained by their physiological capacity to breath-hold dive. We measured body oxygen stores (blood volume and muscle myoglobin) and diving behavior in adult female northern elephant seals, Mirounga angustirostris, to investigate age-related effects on diving performance. Blood volume averaged 74.4+/-17.0 liters in female elephant seals or 20.2+/-2.0% of body mass. Plasma volume averaged 32.2+/-7.8 liters or 8.7+/-0.7% of body mass. Absolute plasma volume and blood volume increased independently with mass and age. Hematocrit decreased weakly with mass but did not vary with age. Muscle myoglobin concentration, while higher than previously reported (7.4+/-0.7 g%), did not vary with mass or age. Pregnancy status did not influence blood volume. Mean dive duration, a proxy for physiological demand, increased as a function of how long seals had been at sea, followed by mass and hematocrit. Strong effects of female body mass (range, 218-600 kg) on dive duration, which were independent of oxygen stores, suggest that larger females had lower diving metabolic rates. A tendency for dives to exceed calculated aerobic limits occurred more frequently later in the at-sea migration. Our data suggest that individual physiological state variables and condition interact to determine breath-hold ability and that both should be considered in life-history studies of foraging behavior.

Energy reserve utilization in northern elephant seal (Mirounga angustirostris) pups during the postweaning fast: size does matter.

During fasting most mammals preferentially utilize lipid reserves for energy while sparing protein reserves. This presents a potential problem for marine mammals that also depend on lipids as a major component of blubber, the primary thermoregulatory structure. Because of this dual function for lipid, rates of lipid and protein utilization should be closely regulated during the postweaning fast in northern elephant seals (Mirounga angustirostris). To quantify energy expenditure during the fast, we measured body mass and composition of 60 pups at 2.3+/-0.2 days and 55.9+/-0.3 days postweaning in 1999 and in 2000. Body condition differed significantly between years. At weaning, body mass (125.9+/-3.8 kg) and percentage lipid content (39.3+/-0.6% of body mass) in 2000 were significantly greater than body mass (115.2+/-3.1 kg) and percentage lipid content (35.8+/-0.6%) in 1999. In general, percentage lipid content increased with body mass, and fatter pups utilized lipid at relatively higher rates during the fast. Lipid fueled 85-95% and 88-98% of energy expended by pups in 1999 and 2000, respectively. Postweaning fast duration (32-78 days) was positively correlated with body mass and hence lipid content at weaning. This suggests that body composition at weaning influences lipid utilization patterns and ultimately the duration of the postweaning fast in northern elephant seal pups.

Effects of forced diving on the spleen and hepatic sinus in northern elephant seal pups.

In phocid seals, an increase in hematocrit (Hct) accompanies diving and periods of apnea. The variability of phocid Hct suggests that the total red cell mass is not always in circulation, leading researchers to speculate on the means of blood volume partitioning. The histology and disproportionate size of the phocid spleen implicates it as the likely site for RBC storage. We used magnetic resonance imaging on Northern elephant seals to demonstrate a rapid contraction of the spleen and a simultaneous filling of the hepatic sinus during forced dives (P < 0.0001, R(2) = 0.97). The resulting images are clear evidence demonstrating a functional relationship between the spleen and hepatic sinus. The transfer of blood from the spleen to the sinus provides an explanation for the disparity between the timing of diving-induced splenic contraction ( approximately 1-3 min) and the occurrence of peak Hct (15-25 min). Facial immersion was accompanied by an immediate and profound splenic contraction, with no further significant decrease in splenic volume after min 2 (Tukey-Kramer HSD, P = 0.05). At the conclusion of the dive, the spleen had contracted to 16% of its predive volume (mean resting splenic volume = 3,141 ml +/- 68.01 ml; 3.54% of body mass). In the postdive period, the spleen required 18-22 min to achieve resting volume, indicating that this species may not have sufficient time to refill the spleen when routinely diving at sea, which is virtually continuous with interdive surface intervals between 1 and 3 min.

Renal function in suckling and fasting pups of the northern elephant seal.

Elephant seals fast for prolonged periods without access to water. This is made possible, in part, by reductions in urine production. However, the mechanisms involved in reducing urine production are not understood. In this study, glomerular filtration rate (GFR) was measured in five northern elephant seal pups (Mirounga angustirostris) via the inulin clearance technique. Measurements were made during day 9 and day 18-22 of nursing and the second and eighth week of the postweaning fast. Plasma aldosterone and cortisol concentrations, quantified by radioimmunoassay, were measured in eight other weanlings during the second and eighth week of the fast. Mean GFR was 79.3+/-29.3 ml/min during the early suckling period and 78.2+/-17.1, 89.8+/-52.7, and 80.4+/-12.2 ml/min during the late suckling, early fasting and late fasting periods, respectively. Differences between nursing and fasting were insignificant, possibly because reduced protein oxidation during suckling and rapid recruitment of protein for tissue synthesis obviated the need for postprandial hyperfiltration. Alternatively, maintenance of GFR during fasting may facilitate urea concentration by compensating for reductions in the fractional excretion of urea. It is further hypothesized that aldosterone is primarily responsible for mediating renal water reabsorption in this system.

Respiration and heart rate at the surface between dives in northern elephant seals.

All underwater activities of diving mammals are constrained by the need for surface gas exchange. Our aim was to measure respiratory rate (fb) and heart rate (fh) at the surface between dives in free-ranging northern elephant seals Mirounga angustirostris. We recorded fb and fh acoustically in six translocated juveniles, 1.8-2. 4 years old, and three migrating adult males from the rookery at Año Nuevo, California, USA. To each seal, we attached a diving instrument to record the diving pattern, a satellite tag to track movements and location, a digital audio tape recorder or acoustic datalogger with an external hydrophone to record the sounds of respiration and fh at the surface, and a VHF transmitter to facilitate recovery. During surface intervals averaging 2.2+/-0.4 min, adult males breathed a mean of 32.7+/-5.4 times at a rate of 15. 3+/-1.8 breaths min(-)(1) (means +/- s.d., N=57). Mean fh at the surface was 84+/-3 beats min(-)(1). The fb of juveniles was 26 % faster than that of adult males, averaging 19.2+/-2.2 breaths min(-)(1) for a mean total of 41.2+/-5.0 breaths during surface intervals lasting 2.6+/-0.31 min. Mean fh at the surface was 106+/-3 beats min(-)(1). fb and fh did not change significantly over the course of surface intervals. Surface fb and fh were not clearly associated with levels of exertion, such as rapid horizontal transit or apparent foraging, or with measures of immediately previous or subsequent diving performance, such as diving duration, diving depth or swimming speed. Together, surface respiration rate and the duration of the preceding dive were significant predictors of surface interval duration. This implies that elephant seals minimize surface time spent loading oxygen depending on rates of oxygen uptake and previous depletion of stores.

Protein catabolism and renal function in lactating northern elephant seals.

Northern elephant seals, Mirounga angustirostris, fast completely from food and water during lactation. Previous investigations of maternal investment suggested physiological constraints on the level of energy expenditure during lactation. In this study, two components of phocid fasting physiology, protein sparing and reduced glomerular filtration rate, were examined for effects of changing body composition and lactation duration. Protein catabolism was estimated from 14C-urea turnover in five mid- and five late-lactation females. Body composition was determined by using an ultrasound scanner to measure blubber depth coupled with morphometrics. Glomerular filtration rate was measured in five females at mid- and late-lactation using plasma clearance of 3H-inulin. Protein catabolism increased significantly between measurements. The contribution of protein to metabolism varied with body composition and lactation duration. Mass-proportional glomerular filtration rate increased significantly between measurements. These data suggest that conflicting metabolic demands of lactation and fasting might constrain the duration and magnitude of maternal investment in northern elephant seals.

Effects of buoyancy on the diving behavior of northern elephant seals.

Marine mammals experience radical seasonal changes in body composition, which would be expected to affect their buoyancy in the water. The aim of this study was to examine the relationship between such changes in buoyancy and diving behavior in northern elephant seals Mirounga angustirostris. This was achieved by modifying the buoyancy of 13 juvenile elephant seals translocated from Año Nuevo State Reserve, CA, USA, and released at various sites in Monterey Bay, CA, USA. The buoyancy of each seal was calculated and was increased or decreased using syntactic foam or lead weights, and their diving behavior was recorded as they returned to Año Nuevo. The seals were divided into three groups: increased buoyancy (B+), reduced buoyancy (B-) and control seals (Bc). Mean descent rates were 0.77+/-0.3 ms-1 for the B+ seals, 0.82+/-0.2 ms-1 for the control seals and 0.87+/-0.3 ms-1 for the B- seals, and were significantly different. Mean ascent rates for the three treatments were 0.82+/-0.3 ms-1 for the B+ seals, 0.86+/-0.3 ms-1 for the control seals and 0.82+/-0.3 ms-1 for the B- seals. All the B+ seals ascended faster than they descended, while four of the five B- seals descended faster than they ascended. There was a significant negative correlation between buoyancy and descent rate, with less buoyant seals descending faster than more buoyant seals. There was, however, no correlation between ascent rate and buoyancy. This suggests that seals may use negative buoyancy to drift passively during descent, but that all seals may swim continuously during ascent. There was a significant correlation between buoyancy and the drift descent rate of C-type drift dives, including upwards drift in the most buoyant seal. Buoyancy was not correlated with diving depth, trip duration, dive duration or surface-interval duration. This study demonstrates that buoyancy plays a significant role in shaping diving behavior in northern elephant seals and that elephant seals may adjust their behavior to suit their buoyancy, rather than adjusting their buoyancy to suit a dive. This study also validated the truncated cones method of calculating body composition in this species by comparing it with body composition determined using tritium dilution.

Blood volume and diving ability of the New Zealand sea lion, Phocarctos hookeri.

We test the hypothesis that the New Zealand sea lion is physiologically better equipped for prolonged, continuous diving than other otariids (fur seals and sea lions) by measuring its blood volume, an important component of its oxygen storage. Mass, hematocrit, and plasma volume were measured and blood volume calculations were completed on 14 adult females and five juvenile females. Plasma volume was determined using the Evans blue dye dilution technique. Mean plasma volume for all subjects was 74 mL kg(-1). Mass-specific plasma volume was significantly higher in adult females (15.3%) than in juveniles (14.6%). Blood volume (150 mL kg(-1)) and hematocrit (51%) were not significantly different between adults and juveniles. The aerobic dive limit can be estimated by dividing the animal's oxygen stores by its metabolic rate. The estimated aerobic dive limit for adult animals was between 5.5 and 7.8 min, depending on the assumed metabolic rate. New Zealand sea lions have the highest blood volume yet reported for an otariid, which supports the hypothesis that they have a physiological capability suited to their unique diving behavior.