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In Press
Ponganis, PJ, McDonald BI, Tift MS, Gonzalez SC, DaValle B, Gliniecki RA, Stehman CC, Hauff N, Ruddick B, Howard R.  In Press.  Effects of Inhalational Anesthesia on Blood Gases and pH in California Sea Lions. Marine Mammal Science.
Tift, MS, Huckstadt LA, McDonald BI, Thorson PH, Ponganis PJ.  In Press.  Flipper stroke rate and venous oxygen levels in free-ranging California sea lions. The Journal of Experimental Biology.
Huckstadt, LA, Tift MS, Riet-Sapriza FG, Franco-Trecu V, Baylis AMM, Orben RA, Arnould JPY, Sepulveda M, Santos M, Burns JM, Costa DP.  2016.  Regional variability in diving physiology and behavior in a widely distributed air-breathing marine predator, the South American sea lion (Otaria byronia). The Journal of Experimental Biology. 219(15):jeb.138677.
Champagne, CD, Tift MS, Houser DS, Crocker DE.  2015.  Adrenal sensitivity to stress is maintained despite variation in baseline glucocorticoids in moulting seals. Conservation Physiology. 3(1):cov004.
Spraker, TR, Lyons ET, Kuzmina TA, Tift MS, Raverty S, Jaggi N, Crocker DE.  2014.  Causes of death in preweaned northern elephant seal pups (Mirounga angustirostris, Gill, 1866), Año Nuevo State Reserve, California, 2012. Journal of Veterinary Diagnostic Investigation. 26(2):320-326.
Tift, MS, Crocker DE, Ponganis PJ.  2014.  Elevated carboxyhemoglobin in a marine mammal, the northern elephant seal. The Journal of Experimental Biology. 217(10):1752-1757.
Louis, C, Tift MS, Crocker DE, Alexander D, Smith DR, Debier C.  2014.  Isolation of progenitor cells from the blubber of northern elephant seals (Mirounga angustirostris) in order to obtain an in vitro adipocyte model. Marine Mammal Science. 31(2):764-773.
Tift, MS, Ranalli EC, Houser DS, Ortiz RM, Crocker DE.  2013.  Development enhances hypometabolism in northern elephant seal pups (Mirounga angustirostris). Functional ecology. 27(5):1155-1165.   10.1111/1365-2435.12111   AbstractWebsite

Investigation into the development of oxygen storage capacity in air-breathing marine predators has been performed, but little is known about the development of regulatory factors that influence oxygen utilization. Strategies for efficiently using oxygen stores should enable marine predators to optimize time spent foraging underwater.

We describe the developmental patterns of oxygen use during voluntary breath-holds in northern elephant seals (Mirounga angustirostris) at 2 and 7 weeks postweaning. We measured (i) changes in oxygen consumption (VO2) and (ii) changes in venous pH, partial pressure of oxygen (pO2), haemoglobin saturation (sO2), oxygen content (O2ct), partial pressure of carbon dioxide (pCO2), haematocrit (Hct) and total haemoglobin (tHb). To examine the effect of the dive response on the development of oxygen utilization, voluntary breath-hold experiments were conducted in and out of water.

Suppression of VO2 during voluntary breath-holds increased significantly between 2 and 7 weeks postweaning, reaching a maximum suppression of 53% below resting metabolic rate and 56% below Kleiber's standard metabolic rate. From 2 to 7 weeks postweaning, breath-hold VO2 was reduced by 52%. Between the two age classes, this equates to a mean breath-hold VO2 reduction of 16% from resting VO2. Breath-hold VO2 also declined with increasing breath-hold duration, but there was no direct effect of voluntary submergence on reducing VO2.

Age did not influence rates of venous pO2 depletion during breath-holds. However, voluntary submergence did result in slower pO2 depletion rates when compared with voluntary terrestrial apnoeas. The differences in whole-body VO2 during breath-holds (measured at recovery) and venous pO2 (reflective of tissue O2-use measured during breath-holds) likely reflect metabolic suppression in hypoxic, vasoconstricted tissues.

Consistent pCO2 values at the end of all voluntary breath-holds (59·0 ± 0·7 mmHg) suggest the physiological cue for stimulating respiration in northern elephant seal pups is the accumulation of CO2.

Oxygen storage capacity and metabolic suppression directly limit diving capabilities and may influence foraging success in low-weaning weight seals forced to depart to sea prior to achieving full developmental diving capacity.

Viscarra, JA, Rodriguez R, Vazquez‐Medina JP, Lee A, Tift MS, Tavoni SK, Crocker DE, Ortiz RM.  2013.  Insulin and GLP-1 infusions demonstrate the onset of adipose-specific insulin resistance in a large fasting mammal: potential glucogenic role for GLP-1. Physiological reports. 1(2)   10.1002/phy2.23   AbstractWebsite

Prolonged food deprivation increases lipid oxidation and utilization, which may contribute to the onset of the insulin resistance associated with fasting. Because insulin resistance promotes the preservation of glucose and oxidation of fat, it has been suggested to be an adaptive response to food deprivation. However, fasting mammals exhibit hypoinsulinemia, suggesting that the insulin resistance-like conditions they experience may actually result from reduced pancreatic sensitivity to glucose/capacity to secrete insulin. To determine whether fasting results in insulin resistance or in pancreatic dysfunction, we infused early- and late-fasted seals (naturally adapted to prolonged fasting) with insulin (0.065 U/kg), and a separate group of late-fasted seals with low (10 pmol/L per kg) or high (100 pmol/L per kg) dosages of glucagon-like peptide-1 (GLP-1) immediately following a glucose bolus (0.5 g/kg), and measured the systemic and cellular responses. Because GLP-1 facilitates glucose-stimulated insulin secretion, these infusions provide a method to assess pancreatic insulin-secreting capacity. Insulin infusions increased the phosphorylation of insulin receptor and Akt in adipose and muscle of early- and late-fasted seals; however, the timing of the signaling response was blunted in adipose of late-fasted seals. Despite the dose-dependent increases in insulin and increased glucose clearance (high dose), both GLP-1 dosages produced increases in plasma cortisol and glucagon, which may have contributed to the glucogenic role of GLP-1. Results suggest that fasting induces adipose-specific insulin resistance in elephant seal pups, while maintaining skeletal muscle insulin sensitivity, and therefore suggests that the onset of insulin resistance in fasting mammals is an evolved response to cope with prolonged food deprivation.

Currylow, AF, Tift MS, Meyer JL, Crocker DE, Williams RN.  2013.  Seasonal variations in plasma vitellogenin and sex steroids in male and female Eastern Box Turtles, Terrapene carolina carolina. General and comparative endocrinology. 180:48-55. AbstractWebsite

The Eastern Box Turtle (Terrapene carolina carolina) is a widespread species that has recently experienced precipitous declines throughout its range. Although many studies have documented aspects of reproduction in box turtles, reproductive physiology of free-ranging animals is unknown and can be crucial in this species’ recovery. Over a two-year period, we measured reproductive parameters, (vitellogenin [Vtg], estradiol-17β [E2], and total testosterone [TT]), in plasma of 116 free-ranging Eastern Box Turtles across their active season. We found similar seasonal variations of Vtg and E2 within females. Mid-season, females showed a sharp peak in E2 that correlates with the putative beginning of the ovarian cycle. Individual females lacking these expected peaks of both Vtg and E2 suggest that some female T. c. carolina may not reproduce annually. Females typically expressed undetectable levels of TT, yet there was a small peak in TT early in the active season. Male Eastern Box Turtles exhibited a dual peak in TT. Elevated TT in males was significantly associated with observed sexual behaviors and smaller home ranges. Body condition had no effect on the concentration of TT or E2 in either sex. This is the first study to (1) document Vtg and sex steroid hormones in free-ranging animals of this genus, and (2) relate those metrics to individuals, the population, the purported annual cycle, and to other chelonian species.

Houser, DS, Crocker DE, Tift MS, Champagne CD.  2012.  Glucose oxidation and nonoxidative glucose disposal during prolonged fasts of the northern elephant seal pup (Mirounga angustirostris). American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 303(5):R562-R570.
Lyons, ET, Kuzmina TA, Spraker TR, Jaggi N, Costa DP, Crocker DE, Tolliver SC, Tift MS.  2012.  Parasitological examination for presence of hookworms (Uncinaria spp.) in northern elephant seals (Mirounga angustirostris) at Año Nuevo State Reserve, California (2012). Parasitology research. 111(4):1847-1850.
Kelso, EJ, Champagne CD, Tift MS, Houser DS, Crocker DE.  2012.  Sex differences in fuel use and metabolism during development in fasting juvenile northern elephant seals. The Journal of Experimental Biology. 215(15):2637-2645.
Vázquez-Medina, JP, Zenteno-Savín T, Tift MS, Forman HJ, Crocker DE, Ortiz RM.  2011.  Apnea stimulates the adaptive response to oxidative stress in elephant seal pups.. The Journal of experimental biology. 214(24):4193-4200.
Tift, MS.  2011.  Development enhances hypometabolism and the dive response in northern elephant seal pups (Mirounga angustirostris). ( Crocker DE, Ed.)., Rohnert Park, CA, USA: Sonoma State University
Tift, MS, Houser DS, Crocker DE.  2011.  High-density lipoprotein remains elevated despite reductions in total cholesterol in fasting adult male elephant seals ( Mirounga angustirostris). Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology. 159(4):214-219. AbstractWebsite

We examined changes in lipid profiles of 40 adult northern elephant seal bulls over the 3-month breeding fast and the 1-month molting fast to investigate impacts of fasting on serum total cholesterol (TC), triglycerides (TG) and lipoproteins. Total cholesterol and low-density lipoprotein (LDL) levels were initially high (3930 ± 190 mg L− 1and 1610 ± 170 mg L− 1, respectively) and decreased significantly over the breeding season. Total cholesterol and LDL declined significantly with adipose tissue reserves (p < 0.001), and LDL levels as low as 43 mg L− 1 were measured in seals late in the breeding fast. Less dramatic but similar changes in lipid metabolism were observed across the molting fast. High-density lipoproteins (HDL) remained consistently elevated (> 1750 mg L− 1) suggesting that elephant seals defend HDL concentrations, despite significant depletion of TC and LDL across the breeding fast. Triglyceride levels were significantly higher during the molt, consistent with lower rates of lipid oxidation needed to meet metabolic energy demands during this period. The maintenance of HDL during breeding is consistent with its role in delivering cholesterol from adipose tissue for steroidogenesis and spermatogenesis and potentially mitigates oxidative stress associated with fasting.