Fasting Northern Elephant Seal Research Articles

The demands of lactation promote differential regulation of lipid stores in fasting elephant seals

Fasting animals must ration stored reserves appropriately for metabolic demands. Animals that experience fasting concomitant with other metabolically demanding activities are presented with conflicting demands of energy conservation and expenditure. Our objective was to understand how fasting northern elephant seals regulate the mobilization of lipid reserves and subsequently milk lipid content during lactation. We sampled 36 females early and 39 at the end of lactation. To determine the separate influences of lactation from fasting, we also sampled fasting but non-lactating females early and late (8 and 6 seals, respectively) in their molting fasting period. Mass and adiposity were measured, as well as circulating non-esterified fatty acid (NEFA), triacylglycerol (TAG), cortisol, insulin and growth hormone levels. Milk was collected from lactating females. Milk lipid content increased from 31% in early to 51% in late lactation. In lactating females plasma NEFA was positively related to cortisol and negatively related to insulin, but in molting seals, only variation in cortisol was related to NEFA. Milk lipid content varied with mass, adiposity, NEFA, TAG, cortisol and insulin. Surprisingly, growth hormone concentration was not related to lipid metabolites or milk lipid. Suppression of insulin release appears to be the differential regulator of lipolysis in lactating versus molting seals, facilitating mobilization of stored lipids and maintenance of high NEFA concentrations for milk synthesis. Milk lipid was strongly impacted by the supply of substrate to the mammary gland, indicating regulation at the level of mobilization of lipid reserves.

ACTH administration stimulates both cortisol and aldosterone secretion in fasting northern elephant seals (LB776)

During molting, northern elephant seals exhibit substantial alterations in circulating cortisol concentrations, probably to facilitate pelage synthesis while fasting. How these changes influence the hypothalamic‐pituitary‐adrenal (HPA) axis and the stress response, however, is not known. To investigate the modulation of the HPA axis during molting, we administered ACTH (25 units, intramuscularly) to sixteen juvenile elephant seals at the beginning (n=6), middle (n=5), and end (n=5) of their spring molting period in a cross‐sectional study design. Blood samples were collected immediately before, and then periodically for 2.5 hrs after, ACTH administration. We quantified circulating concentrations of ACTH, and the responses of cortisol and aldosterone. Both cortisol and aldosterone concentrations markedly increased following stimulation by ACTH and remained elevated for the duration of sampling (linear mixed model, F = 153, 25.3, respectively; p < 0.01). There were no detectable differences in the magnitude of hormone responses to ACTH administration among the study groups (p > 0.1). Nonesterified fatty acids increased following ACTH administration, suggesting upregulation of lipolysis; however, blood urea nitrogen concentrations increased only marginally, indicating that protein catabolism was less affected by ACTH administration. ACTH is not a strong aldosterone secretagogue in terrestrial mammals but several studies have measured increased aldosterone after ACTH stimulation in marine mammals, indicating its potential importance in osmoregulation during stress in these species. Cortisol and aldosterone increased in parallel following ACTH stimulation but the slope of this relationship varied among the study groups. This suggests that the release of these hormones is closely coupled in this species and possibly other marine mammals.Grant Funding Source: Supported by the Office of Naval Research

Adiposity and fat metabolism in lactating and fasting northern elephant seals.

Several taxa of animals fast completely from food and water during energy-intensive periods such as lactation, breeding, and development. In elephant seals, these behaviors are sustained by high adiposity, high rates of fat mobilization, and reduced oxidation of carbohydrates and proteins. Adiposity and the regulation of lipolysis directly affect lactation energetics, milk composition, and mating success. Long-term fasting induces changes in regulation of lipolysis and lipid metabolism that influence fatty acid (FA) availability and the onset of insulin resistance. Hypoinsulinemia and elevated circulating FAs are also associated with several unique features of carbohydrate metabolism, including elevated plasma glucose, gluconeogenesis, and Cori cycle activity as well as high rates of pyruvate and tricarboxylic acid cycling. Glucose-lactate pools and triacylglycerol-FA cycles may be linked via glyceroneogenesis and this may be an important pathway influencing both fat and carbohydrate metabolism. Together, these features allow a sustained, high intensity, fat-based metabolism without substantial accumulation of ketoacids.

Metabolic response to a glucagon challenge varies with adiposity and life-history stage in fasting northern elephant seals

Metabolic adaptations for extended fasting in wildlife prioritize beta-oxidation of lipids and reduced glucose utilization to support energy metabolism. The pancreatic hormone glucagon plays key roles in regulating glycemia and lipid metabolism during fasting in model species but its function in wildlife species adapted for extended fasting is not well understood. Northern elephant seals (NES) undergo natural fasts of 1–3months while under constraints of high nutrient demands including lactation and development. We performed a glucagon challenge on lactating, molting and developing NES, early and late in their natural fasts, to examine the impact of this important regulatory hormone on metabolism. Glucagon caused increases in plasma glucose, insulin, fatty acids, ketones and urea, but the magnitude of these effects varied widely with adiposity and life-history stage. The strong impact of adiposity on glucose and insulin responses suggest a potential role for adipose derived factors in regulating hepatic metabolism and pancreatic sensitivity. Elevations in plasma glucose in response to glucagon were strongly associated with increases in protein catabolism, suggesting negative impacts of elevated glucagon on protein sparing. Glucagon promoted rapid ketone accumulation suggesting that low ketoacid levels in NES reflect low rates of production. These results demonstrate strong metabolic impacts of glucagon and support the idea that glucagon levels are downregulated in the context of metabolic adaptation to extended fasting. These results suggest that the regulation of carbohydrate and lipid metabolism in NES changes with adiposity, fasting duration and under various constraints of nutrient demands.

A profile of carbohydrate metabolites in the fasting northern elephant seal

Northern elephant seals endure prolonged periods of food deprivation at multiple life-history stages and simultaneous with energetically costly activities—including reproduction and development. Most mammals decrease their energy expenditure while fasting, with simultaneous reductions in gluconeogenesis and circulating glucose concentration. Paradoxically, elephant seals maintain high rates of both energy expenditure and gluconeogenesis, and high blood glucose concentrations throughout fasting. We therefore characterized the suite of changes that occur in carbohydrate metabolites during fasting in northern elephant seals. Using a broad-based metabolomics platform we investigated fasting during two states—lactation in adult females and the post-weaning developmental period in pups. A total of 227 metabolites were detected in seal plasma; 31 associated with carbohydrate metabolism were analyzed in the present study. Several compounds showed similar responses during lactation and the post-weaning fast (e.g. glycerol and mesaconate) whereas other compounds displayed quite different abundances between groups (e.g. citrate and pyruvate). This work found that, while the changes that occur with fasting were frequently similar in lactating females and developing pups, the relative abundance of compounds often varied markedly. These differences suggest that the metabolic strategies used to endure prolonged fasts are influenced by life-history or nutrient constraints.

Gluconeogenesis is associated with high rates of tricarboxylic acid and pyruvate cycling in fasting northern elephant seals

Animals that endure prolonged periods of food deprivation preserve vital organ function by sparing protein from catabolism. Much of this protein sparing is achieved by reducing metabolic rate and suppressing gluconeogenesis while fasting. Northern elephant seals (Mirounga angustirostris) endure prolonged fasts of up to 3 mo at multiple life stages. During these fasts, elephant seals maintain high levels of activity and energy expenditure associated with breeding, reproduction, lactation, and development while maintaining rates of glucose production typical of a postabsorptive mammal. Therefore, we investigated how fasting elephant seals meet the requirements of glucose-dependent tissues while suppressing protein catabolism by measuring the contribution of glycogenolysis, glycerol, and phosphoenolpyruvate (PEP) to endogenous glucose production (EGP) during their natural 2-mo postweaning fast. Additionally, pathway flux rates associated with the tricarboxylic acid (TCA) cycle were measured specifically, flux through phosphoenolpyruvate carboxykinase (PEPCK) and pyruvate cycling. The rate of glucose production decreased during the fast (F(1,13) = 5.7, P = 0.04) but remained similar to that of postabsorptive mammals. The fractional contributions of glycogen, glycerol, and PEP did not change with fasting; PEP was the primary gluconeogenic precursor and accounted for ∼95% of EGP. This large contribution of PEP to glucose production occurred without substantial protein loss. Fluxes through the TCA cycle, PEPCK, and pyruvate cycling were higher than reported in other species and were the most energetically costly component of hepatic carbohydrate metabolism. The active pyruvate recycling fluxes detected in elephant seals may serve to rectify gluconeogeneic PEP production during restricted anaplerotic inflow in these fasting-adapted animals.

Prolonged fasting increases purine recycling in post-weaned northern elephant seals

Northern elephant seals are naturally adapted to prolonged periods (1-2 months) of absolute food and water deprivation (fasting). In terrestrial mammals, food deprivation stimulates ATP degradation and decreases ATP synthesis, resulting in the accumulation of purines (ATP degradation byproducts). Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) salvages ATP by recycling the purine degradation products derived from xanthine oxidase (XO) metabolism, which also promotes oxidant production. The contributions of HGPRT to purine recycling during prolonged food deprivation in marine mammals are not well defined. In the present study we cloned and characterized the complete and partial cDNA sequences that encode for HGPRT and xanthine oxidoreductase (XOR) in northern elephant seals. We also measured XO protein expression and circulating activity, along with xanthine and hypoxanthine plasma content in fasting northern elephant seal pups. Blood, adipose and muscle tissue samples were collected from animals after 1, 3, 5 and 7 weeks of their natural post-weaning fast. The complete HGPRT and partial XOR cDNA sequences are 771 and 345 bp long and encode proteins of 218 and 115 amino acids, respectively, with conserved domains important for their function and regulation. XOR mRNA and XO protein expression increased 3-fold and 1.7-fold with fasting, respectively, whereas HGPRT mRNA (4-fold) and protein (2-fold) expression increased after 7 weeks in adipose tissue and muscle. Plasma xanthine (3-fold) and hypoxanthine (2.5-fold) levels, and XO (1.7- to 20-fold) and HGPRT (1.5- to 1.7-fold) activities increased during the last 2 weeks of fasting. Results suggest that prolonged fasting in elephant seal pups is associated with increased capacity to recycle purines, which may contribute to ameliorating oxidant production and enhancing the supply of ATP, both of which would be beneficial during prolonged food deprivation and appear to be adaptive in this species.

The Effects and function of the Hypothalamic‐pituitary‐thyroid axis on fasting Northern elephant seals

Northern elephant seals sustain long bouts of food deprivation while remaining physically active with relatively high metabolic rates, and unlike hibernating mammals, they also remain normothermic. In terrestrial mammals, food deprivation is associated with reduced thyroid hormone (TH) levels and metabolism; however, elephant seals pups exhibit sustained, if not, elevated TH. The functional relevance of this apparent paradox is not understood. To address the hypothesis that increased TH levels in prolong‐fasted seals are associated with elevated TH receptor expression, we measured the changes in fasting‐induced TH levels and the mRNA expression of the TH receptor, THrβ‐1, in muscle and blubber. To better ascertain the relevance of the changes in THrβ‐1, we also measured the protein content and mRNA expression of products associated with changes in THrβ‐1. Preliminary data shows almost a three‐fold increase in THrβ‐1 gene expression in both muscle and blubber and we observed an elevation in tT3, tT4, and fT4 over the course of the fast decrease in total T3:T4 ratio over the 49 days. Furthermore, both Deiodinase 1 and 2 increased in muscle tissue suggesting an increase in availability of t3 to meet the higher demands of an increase in the receptor. It would be pertinent to attempt to clarify the mechanism of the hypothalamic‐pituitary axis and its overall function and effect on metabolism.

Water-soluble vitamin homeostasis in fasting northern elephant seals ( Mirounga angustirostris) measured by metabolomics analysis and standard methods

Despite the importance of water-soluble vitamins to metabolism, there is limited knowledge of their serum availability in fasting wildlife. We evaluated changes in water-soluble vitamins in northern elephant seals, a species with an exceptional ability to withstand nutrient deprivation. We used a metabolomics approach to measure vitamins and associated metabolites under extended natural fasts for up to 7 weeks in free-ranging lactating or developing seals. Water-soluble vitamins were not detected with this metabolomics platform, but could be measured with standard assays. Concentrations of measured vitamins varied independently, but all were maintained at detectable levels over extended fasts, suggesting that defense of vitamin levels is a component of fasting adaptation in the seals. Metabolomics was not ideal for generating complete vitamin profiles in this species, but gave novel insights into vitamin metabolism by detecting key related metabolites. For example, niacin level reductions in lactating females were associated with significant reductions in precursors suggesting downregulation of the niacin synthetic pathway. The ability to detect individual vitamins using metabolomics may be impacted by the large number of novel compounds detected. Modifications to the analysis platforms and compound detection algorithms used in this study may be required for improving water-soluble vitamin detection in this and other novel wildlife systems.

Hypoxia inducible factor 1α, hypoxanthine‐guanine phosphoribolsyl transferase and NADPH oxidase 4 from northern elephant seals: cDNA characterization and tissue expression

Northern elephant seals (Mirounga angustirostris) are naturally adapted to prolonged periods (1–3 months) of food deprivation (fasting) and tissue ischemia. Fasting elephant seal pups do not exhibit signs of oxidative damage or inflammation despite increases in the renin‐angiotensin system, NADPH oxidase activity and the superoxide‐generating enzyme, NADPH oxidase 4 (Nox4). Inducible factor 1α (HIF‐1α) and hypoxanthine‐guanine phosphoribosyl transferase (HGPRT) may be activated in response to fasting‐ and hypoxia‐induced increases in ROS. We report the molecular characterization and tissue expression of Nox4, HIF‐1α and HGPRT from fasting northern elephant seal pups. The full‐length sequence for HGPRT is 761 bp long and codes for 218 amino acids, which is similar to other mammalian homologs. The partial sequences of Nox4 and HIF‐1α are 1131 bp and 745 bp long respectively and correspond to approximately 65% and 35% of the full sequence of mammalian homologs, respectively. HGPRT, Nox4 and HIF‐1α sequences are similar to vertebrate homologs and contain conserved functional and regulatory domains. HGPRT, Nox4 and HIF‐1α expression was detected in both muscle and adipose tissue, with higher expression in muscle than in adipose. The present study represents a critical first step towards advancing our understanding of the molecular mechanisms of oxidant regulation in response to prolonged fasting and apnea‐induced ischemia.

A complete profile of carbohydrate metabolism during prolonged fasting in the northern elephant seal

Glucose production is typically suppressed while fasting to facilitate protein sparing by avoiding the commitment of amino acids to gluconeogenesis. Previous work in fasting northern elephant seals, however, has found elevated rates of glucose production. To explore carbohydrate metabolism during prolonged fasting, we used a tracer technique utilizing D2O, [U‐13C]propionate, and [1,6‐13C]glucose. Nuclear magnetic resonance (NMR) was used to measure the abundance of each resulting positional isotope of glucose. We used the relative peak heights from the NMR spectra to quantify the rate of glucose production, the contribution of major gluconeogenic precursors to glucose production, the flux through the tricarboxylic‐acid cycle, and the activity of the phosphoenolpyruvate (PEP) cycle. Rates of glucose production closely agreed with previously measured values in this species. The combined contribution of glycogen and glycerol to gluconeogenesis was low, less than 10% of glucose production. The balance of glucose flux, greater than 90%, derived from PEP. The significant contribution of PEP to glucose production is consistent with high rates of carbon recycling through three‐carbon intermediates such as lactate. This is the first use of this positional isotope technique in a free‐ranging animal and provides the first comprehensive carbohydrate metabolism profile of an animal undertaking a prolonged fast.

Water conservation in fasting northern elephant seals (Mirounga angustirostris).

Prolonged terrestrial fasting is a key element in the life history of elephant seals. While on land seals typically fast without access to fresh water, and thus must maintain positive water balance by reductions in water loss such that they can subsist primarily on metabolic water production (MWP). The terrestrial apnea demonstrated by seals may reduce respiratory evaporative water loss (REWL) to levels that allow seals to make a net gain of water from MWP. We empirically measured REWL in 13 fasting northern elephant seal pups and determined the effects on water conservation of a breathing mode that incorporates a regular pattern of apneas, of > or =1 min in duration, followed by eupneic recovery, compared with a breathing mode with no apneas longer than 20 s and resembling typical breathing patterns in other mammals (normative breathing). Overall REWL fell 41% from 0.075+/-0.013 g min(-1) (mean +/- s.d.) during normative breathing to 0.044+/-0.006 g min(-1) during apneic breathing. The decline in REWL is attributed to a decrease in overall ventilation rate, made possible by a decline in metabolic rate along with an increase in oxygen extraction that would occur during apneic breathing. Data on the range of ambient humidity conditions at the local breeding site were collected and used to bound the range of environmental conditions used in laboratory measurements. Our data showed that the observed variations in ambient humidity had no significant effect on REWL. A combination of apneic breathing and the complex nasal turbinates allows fasting elephant seals to reduce REWL well below the rate of MWP so that they can maintain water balance during the fast.

Angiotensin II and Aldosterone Increase with Fasting in Breeding Adult Male Northern Elephant Seals (Mirounga angustirostris)

The renin-angiotensin-aldosterone system (RAAS) appears to contribute significantly to osmoregulation of fasting northern elephant seal (Mirounga angustirostris) pups; however, RAAS has not been characterized in fasting adult seals. Therefore, this study examined the contribution of RAAS to water turnover rates in fasting adult male northern elephant seals. Blood samples were obtained twice during their breeding fast at an interval of 6.5 wk, and water efflux rate was estimated by isotopic dilution during the same period. Serum electrolytes (Na+, K+, Cl-) and osmolality were unaltered between the two sampling periods, indicating ionic and osmotic homeostasis during the fast. Despite the lack of an increase in vasopressin, serum angiotensin II and aldosterone were increased and were significantly and positively correlated. Changes in aldosterone concentration and water efflux rate were significantly and negatively correlated, suggesting that the greater the increase in aldosterone, the smaller the loss of water. Adult male seals maintain ionic and osmotic homeostasis similar to that of fasting weaned pups, and this homeostasis appears to be mediated, at least in part, by RAAS, which probably contributes to increased water retention as well. The hormonal mechanisms by which northern elephant seals maintain water and electrolyte balance during fasting conditions appear to be similar regardless of age.

Energy reserve allocation in fasting Northern Elephant Seal Pups: inter‐relationships between body condition and fasting duration

1 Organisms are forced to make trade-offs when allocating energy reserves during sustained periods of fasting. 2 For most animals, lipid catabolism is the preferred source of energy to safeguard important protein sources. However, marine mammals also have a compounding pressure to conserve some lipid stores. The main site of lipid storage in these animals is the blubber layer, which is not only an important energy source during fasting, but is also the primary thermal barrier when at sea. 3 To explain how the allocation of protein and lipid reserves during fasting are influenced by body condition (body mass and percentage lipid of total body mass), a dynamic state variable model that takes into account fitness consequences of different allocation strategies was developed. 4 This model was parameterized with respect to conditions faced by weaned Northern Elephant Seal (Mirounga angustirostris Gill) pups. It incorporates the independent effects of body mass and protein mass (the two state variables) on survival and the costs of utilizing either lipid or protein during the postweaning fast. 5 Predictions of lipid and protein allocation by the model were not significantly different from measurements on wild seals. Finally, the model showed that body lipid content and fasting duration both influence allocation of energy reserves.

Acutely elevated vasopressin increases circulating concentrations of cortisol and aldosterone in fasting northern elephant seal (Mirounga angustirostris) pups

The physiological actions of vasopressin (VP) in marine mammals are not well defined. To help elucidate its hormonal and renal effects in this group of mammals, northern elephant seal (Mirounga angustirostris) pups (N=7; 99+/-4 kg) were first infused with 0.9% saline (control; 220 ml), followed 24 h later with VP (as a 20 ng kg(-1) bolus, then 2 ng kg(-1) min(-1) for approximately 35 min in 225+/-16 ml saline). During both control and VP periods, blood samples were collected prior to infusion, and 15, 30, 60, 120 min and 24 h after infusion to examine the hormonal responses of the pups to VP. Renal responses were quantified from 24 h urine samples obtained prior to infusion (control) and 24 h post-infusion. Compared to the control period, infusion of VP increased plasma concentrations of cortisol over a 120 min period and aldosterone over 30 min, while plasma renin activity (PRA) was decreased for a 120 min period. The plasma urea:creatinine ratio was elevated following infusion of VP. Urine output and osmotic clearance were increased by 69+/-18% (mean +/- S.E.M.) and 36+/-10%, respectively, but free water clearance and glomerular filtration rate were not significantly altered 24 h post-infusion of VP. Solute (osmolality, Na(+), K(+) and Cl(-)) excretion and fractional excretion of electrolytes were also increased when compared to control values. The increase in cortisol concentration suggests that VP may possess corticotropin releasing hormone-like activity in elephant seals. If osmotic diuresis and natriuresis are typical consequences of elevated [VP] in fasting pups, then not increasing VP normally during the fast may serve as a protective mechanism to avoid the potential loss of Na(+) induced by elevated [VP]. Therefore, under natural fasting conditions, pups may be highly sensitive to small changes in [VP], resulting in the maintenance of water and electrolyte balance.

Body water handling in response to hypertonic-saline induced diuresis in fasting northern elephant seal pups ( Mirounga angustirostris)

During natural fasting conditions in postweaned northern elephant seal (NES) ( Mirounga angustirostris) pups, urinary water loss is minimized and percent total body water (TBW) is maintained constant. However, following infusion of hypertonic saline, glomerular filtration rate (GFR) and urine output increased in fasting pups. Therefore, we quantified the magnitude of the hypernatremia-induced diuresis relative to the animal's total body water (TBW) pool and the percentage of filtered water reabsorbed. Following a 24 h control period, naturally fasting NES pups ( n=7) were infused (4 ml min −1) with hypertonic saline (16.7%) at a dose of 3 mmol NaCl kg −1 body mass. Total body water was estimated prior to infusion by tritium dilution, GFR was estimated by standard creatinine clearance, and urine output (V) was measured for 24 h during the control and post infusion periods. Percentage of filtered water reabsorbed was calculated as (1−(V/GFR))×100. Twenty-four hours following the infusion, GFR (control: 69±12 ml min −1 and post-infusion: 118±19 ml min −1; mean±S.E.) increased 77±28% above control and the percentage of filtered water reabsorbed was decreased 0.4±0.1%. The increase in urine output (control: 218±47 ml d −1 and post-infusion: 883±92 ml d −1) accounted for 1.7±0.2% of the pups’ TBW. The hypernatremia-induced diuresis was accompanied by the loss of body water indicating the lack of water retention. Although the 77% increase in GFR was only associated with a 0.4% decrease in the percentage of filtered water reabsorbed, this decrease was significant enough to result in a 4-fold increase in urine output. Despite the observed diuresis, fasting NES pups appear to possess an efficient water recycling mechanism requiring only a small percentage of body water to excrete an excess salt load. This water recycling mechanism may allow pups to avoid negative perturbations in body water as they initiate feeding in a marine environment following the fast.

Renal effects of fresh water-induced hypo-osmolality in a marine adapted seal.

With few exceptions, marine mammals are not exposed to fresh water; however quantifying the endocrine and renal responses of a marine-adapted mammal to the infusion of fresh water could provide insight on the evolutionary adaptation of kidney function and on the renal capabilities of these mammals. Therefore, renal function and hormonal changes associated with fresh water-induced diuresis were examined in four, fasting northern elephant seal ( Mirounga angustirostris) (NES) pups. A series of plasma samples and 24-h urine voids were collected prior to (control) and after the infusion of water. Water infusion resulted in an osmotic diuresis associated with an increase in glomerular filtration rate (GFR), but not an increase in free water clearance. The increase in excreted urea accounted for 96% of the increase in osmotic excretion. Following infusion of fresh water, plasma osmolality and renin activity decreased, while plasma aldosterone increased. Although primary regulators of aldosterone release (Na(+), K(+) and angiotensin II) were not significantly altered in the appropriate directions to individually stimulate aldosterone secretion, increased aldosterone may have resulted from multiple, non-significant changes acting in concert. Aldosterone release may also be hypersensitive to slight reductions in plasma Na(+), which may be an adaptive mechanism in a species not known to drink seawater. Excreted aldosterone and urea were correlated suggesting aldosterone may regulate urea excretion during hypo-osmotic conditions in NES pups. Urea excretion appears to be a significant mechanism by which NES pups sustain electrolyte resorption during conditions that can negatively affect ionic homeostasis such as prolonged fasting.

Protein catabolism in suckling and fasting northern elephant seal pups ( Mirounga angustirostris )

Nursing elephant seal pups are hypothesized to be preadapted to the postweaning fast, yet no comparison of lipid or protein use for meeting metabolic costs has been made between these contrasting nutritional periods. To address this, protein catabolism was estimated in five elephant seal pups from measurements of urea turnover made twice during nursing and twice during the postweaning fast. Changes in body composition were measured in ten separate weaned pups via tritiated water dilution and matched to fasting urea turnover measurements in order to assess errors in protein catabolism derived from urea turnover rates. Estimates of lean mass loss based upon urea turnover and tritiated water dilution were in general agreement, supporting estimates of protein catabolism derived from urea turnover measurements. Protein catabolism was estimated to contribute less than 4% to the average metabolic rate of suckling and fasting pups implying strict protein conservation during both periods and supporting the shypothesis that suckling pups are pre-adapted to fasting. It is proposed that strict protein conservation across suckling and fasting compensates for relative reductions in maternal investment associated with the abbreviated lactation period of the elephant seal.

Plasma carnitine in fasting neonatal and adult northern elephant seals.

Maintenance of adequate body carnitine stores is a requisite for fasting mammals, whose energy is derived mainly from free fatty acid oxidation. The impact of longterm fasting on carnitine status is unclear, and there have been no reports of carnitine during naturally occurring fasts. Total (TC), free (FC), and acylated (AC) plasma carnitine levels were determined in 10 weaned and 11 adult northern elephant seals (Mirounga angustirostris) during natural fasts lasting from 1 to 3 mo. In pups, TC declined little and AC increased only slightly [P greater than 0.05, analysis of variance (ANOVA)] through 11 wk of fasting. Plasma FC dropped by 53 and 26% from week 1 values at 10 and 11 wk fasting, respectively (P = 0.014, ANOVA). The AC/FC ratio did not approach 1.0 until 7 wk of fasting. TC was 38.6 +/- 1.4 microM and 47.6 +/- 4.1 microM in adult females and males, respectively. Adult AC/FC ratios were 0.71 +/- 0.10 (females) and 0.08 +/- 0.04 (males). Plasma TC status is not negatively affected by extended fasting in adult and weaned northern elephant seals. These data support the hypothesis that fasting northern elephant seals defend plasma TC and maintain an attenuated AC/FC ratio well into their prolonged natural fast.

Circulatory water concentration in suckling and fasting northern elephant seal pups

This study examined circulatory water concentrations in the neonatal northern elephant seal (Mirounga angustirostris) in order to determine how suckling and fasting would alter the percentage of water in the whole blood, plasma, and red blood cells (RBC). Plasma water concentration dropped by about 3% during suckling and recovered about 1% during the fast (92.38 +/- 0.48% less than 1 week old, 90.15 +/- 0.36% weaning, 91.02 +/- 0.68% end of fast). RBC water values during this time were more variable than plasma values: RBC water increased about 1% during the first 2 weeks of suckling (from 67.80 +/- 0.28% to 68.68 +/- 0.51%) but dropped to slightly below original neonatal values by weaning (67.15 +/- 0.63%). The first several weeks of fasting were marked by wide variability in RBC water, but by the end of the fast RBC water was comparable to that at weaning. These results indicate: 1) Northern elephant seal pups do not exhibit circulatory dehydration during 10 weeks of fasting; 2) Measurements of plasma or RBC metabolites (such as plasma glucose or RBC hemoglobin) may show variations or trends due not to metabolic regulation but rather to changes in circulatory water concentration.