C. Leo Ortiz

Water and Energy Flux in Elephant Seal Pups Fasting under Natural Conditions

Total water flux, energy utilization, and mass loss were measured in fasting, newly weaned elephant seal pups, Mirounga angustirostris, in their natural habitat over a period of 32–52 days. Animals lost mass exponentially averaging 5.3 ± 0.9 g·kg−1·day−1. At weaning body water comprised 40% of the total mass while the ratio of adipose to lean body tissue was approximately 1. The mean water turnover rate in all animals was very low, ∼13.0 ± 1.20 ml·liter−1·day−1, and was not affected by entry of the animal into water, supporting the hypothesis that seawater ingestion is not required for water balance. Average metabolic rates calculated from water turnover data were 167.6 ± 19.1 kcal·kg−3/4·day−1. The biological half-time of water, 53.5 ± 5.2 days, is about 2.5 times greater than that of starved harbor seals and significantly exceeds that reported for any other mammal. We conclude that a combination of behavioral and physiological water conservation mechanisms permit these animals to fast on land for up to 3 mo, deriving necessary water and energy from the oxidation of fat stores.

Milk Intake of Elephant Seal Pups: An Index of Parental Investment

Total milk ingested throughout nursing in free-living northern elephant seal pups, derived from the turnover of tritiated water, gives an accurate estimate of the energetic component of parental investment. In 4 wk of nursing, females transferred a mean of 138 kg of milk with a value of 6.04 x 105 kcal, five times the metabolic requirements of pups. The mean rate of energy transfer is more than twice that predicted for the females size from studies of laboratory and domestic animals. In one month, pups receive sufficient energy and essential nutrients from mothers milk to enable them to fast while continuing to develop for an additional 2 1/2 mo before feeding on their own. Measurement of the energetic component of parental investment makes it possible to test predictions from the theory of parental investment.

Hormonal changes associated with the transition between nursing and natural fasting in northern elephant seals (Mirounga angustirostris)

To better interpret previously described hormonal changes observed during the natural postweaning fast (2-3 months) endured by pups of the northern elephant seal (Mirounga angustirostris), we compared plasma cortisol, thyroid hormones, and leptin in pups (n=5) measured during nursing and fasting periods. Blood samples were taken at four times; early (9 days postpartum) and late (18-22 days postpartum) nursing, and early (second week postweaning) and late (eighth week postweaning) fasting. Plasma cortisol increased 39% between early and late nursing and almost 4-fold by late fasting. After the early nursing period, cortisol and body mass were negatively correlated (y=28.3-0.19 x; R=0.569; p=0.027). Total thyroxine (tT(4)), free T(4) (fT(4)), total triiodothyronine (tT3) and reverse T(3) (rT(3)) were greatest at early nursing and reduced by late nursing and remained so throughout the fast, with the exception of tT(4), which increased between late nursing (17.7+/-2.1 ng mL(-1)) and late fasting (30.1+/-2.8 ng mL(-1)) periods. Leptin remained unaltered among the four sampling periods and was not correlated with body mass. Pups appear to exhibit a shift in the relationship between cortisol and body mass suggesting a potential role for cortisol in the regulation of body fat. The higher concentrations of tT(3) and tT(4) during early nursing may reflect enhanced growth and development during this period, however the increase late in fasting is likely physiologically insignificant and an artifact of reduced metabolic clearance of these hormones. Transition of the pups from nursing to fasting states is characterized by a striking lack of change in cortisol, thyroid hormones, and leptin suggesting that any metabolic alterations associated with this transition may occur independent of these hormones.

Urea turnover during prolonged fasting in the northern elephant seal

Abstract 1. 1. Urea turnover was studied in six weaned elephant seal pups fasting under natural conditions. 2. 2. The body urea pool was determined by dilution of [14C]urea and found to have a biological half-time of approx 39 hr. 3. 3. Calculations from these data estimate that protein oxidation contributes no more than 2.2 ± 0.5% to the total metabolism of these fasting animals. 4. 4. The presence of 14CO2 in expired air during the experiments suggests that urea nitrogen may be recycled during starvation.

FabG mediates polyhydroxyalkanoate production from both related and nonrelated carbon sources in recombinant Escherichia coli LS5218.

Polyhydroxyalkanoates (PHAs) composed of a mixture of short‐chain‐length‐medium‐chain‐length (SCL‐MCL) hydroxyacyl monomers are biologically produced polyesters that have properties ranging from thermoplastic to elastomeric, dependent on the molar ratio of SCL to MCL monomers incorporated into the copolymer. Because of the potential wide range of properties and applications for SCL‐MCL PHA copolymers, it is important to develop and characterize novel metabolic pathways for SCL‐MCL PHA production. The current study shows that coexpression of fabG genes from either E. coli or Pseudomonas sp. 61–3 with fabH(F87T) and PHA synthase genes enhances the production of SCL‐MCL PHA copolymer from both related and nonrelated carbon sources in Escherichia coli LS5218, indicating the flexibility of FabG as a monomer‐supplying enzyme for biological PHA production.

Effects of hypoxia and hypercapnia on aerobic metabolic processes in northern elephant seals

An hypoxia-induced metabolic down-regulation has been implicated as an important protective mechanism against tissue deoxygenation in mammals. Whether the same response to hypoxia occurs in northern elephant seals was studied. The effects of hypercapnia were also examined to determine whether the reduced ventilatory response of seals to CO2 is associated with an analogous protective metabolic down-regulation. Thirty three seals (7-300-days-old) were studied using open-flow respirometry with simultaneous monitoring of apnea frequencies and heart rates. Hypoxia (11% O2) and hypercapnia (7% CO2) caused increases in metabolism of up to 38% with corresponding decreases in the percent time spent apneic (%AP) and increases in heart rate. The metabolic, breathing and heart rate responses to altered inspired gases were independent of age. Metabolism was strongly negatively correlated with %AP suggesting that elevated metabolism during hypoxia and hypercapnia exposure is attributable to decreases in %AP. In young elephant seals metabolic down-regulation is not an automatic protective response to experimentally-imposed hypoxia or hypercapnia.

Ontogeny of humoral immunity in northern elephant seal (Mirounga angustirostris) neonates

Northern elephant seal (NES) serum concentrations of total immunoglobulin (Ig) G, an IgG sub-class, and an IgM-like protein were determined by capture immunoassay using three monoclonal antibodies with specificities for Ig of members of the Phocidae pinniped family. These assays were calibrated for use with NES sera using affinity column purified Ig. Concentrations of these Ig populations were estimated in adult female sera sampled at two time points during the lactation period, as well as sera from their pups collected during the first 5 weeks after birth. In pups, concentrations of the IgM-like protein was found to increase rapidly post-partum. In some individuals, values reached mean concentrations within 10-14 days. In addition, rapid increases in pup total IgG and IgG sub-class concentrations were also observed. Collectively, these findings suggest that the majority of post-partum increases in serum Ig can be accounted for by de-novo synthesis.

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

SUMMARY 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.