Steller sea lion research>Nutritional Stress>Effects of Diet Changes>Physiological Measures

Physiological Measures and the Health of Steller Sea Lions

Researchers have been using a number of different methods to study the effect of different diets on the physical condition and health of Steller sea lions. One is to measure morphology, such as the length, weight and thickness of the blubber layer. Another is to assess the nutritional status from hormones and other elements contained within blood samples, in the same way that doctors can tell a lot about human health by drawing a sample of blood. Some of these studies are conducted on wild Steller sea lions in the wild and others use a group of trained sea lions in captivity.

Diet Manipulations
In the wild, Steller sea lions eat a number of different types of fish and invertebrates. Recent scientific research suggests that the type of prey available to Steller sea lions in the North Pacific may be changing. Different prey, such as herring or pollock, provide different amounts of energy to the animal. (seeAssessing Prey Quality)

To help determine the possible effects of diet changes, the regular diet of the sea lions at the Vancouver Aquarium was switched from herring to Atka Mackerel pollock and squid, which have less energy. Changes in the food intake, body mass, and metabolism of the sea lions are then carefully monitored. Results indicate that lower quality prey, such as pollock, requires more energy to digest than a higher quality diet of herring. Additionally, the sea lions appear unable to maintain body mass when switched to a lower calorie prey such as squid or pollock, and need to compensate by conserving energy. It appears that the sea lions do not compensate for low energy food sources by increasing their food intake. Instead they seem to use up more of their own energy stores (i.e., they lose weight), and they spend less energy (i.e., they lower their resting metabolism).

Dr. Rosen (UBC) has been assessing the metabolic adaptations employed to combat periods of low food intake. Additional data have been collected on how body composition changes during periods of fasting. Initial results suggest that sea lion's physiological response to low-food conditions may be both seasonal and age-dependent.

Morphology
One way of estimating the health or condition of a pinniped is to measure the thickness of the layer of blubber that lies just below the skin. Calipers measure skinfold thickness and provide a reliable estimate of fat thickness in humans. Researchers Andrew Trites (UBC) and Remko Jonker (UBC) sought to determine whether this simple and inexpensive device could be used to measure the condition of Steller sea lion pups in the wild. They measured the skinfold thickness of dead pups found on the breeding beaches of Alaska. They discovered that heavy pups had higher skinfold thickness than lighter pups. However, this relationship was true for pups with blubber as well as pups that had died of starvation (no blubber). Therefore, skinfold thickness appears to change with body mass rather than condition. These results suggest caliper pinches do not provide a reliable measure of condition in Steller sea lion pups, and alternate methods must be explored.

Blood Chemistry
A co-operative research project with Dr. Lorrie Rea (ADF&G) investigated the effects of fasting and reduced food intake on several physiological parameters. Five juvenile Steller sea lions at the Vancouver Aquarium underwent a 2-week fast and a 4-week period of 50% food intake; the pups fasted for 3 days. The amount of food the animals received was similar to conditions they would experience in the wild. Dr. Rea used blood samples from these sea lions to evaluate the differences in blood chemistry reported in wild Steller sea lions from areas of stable and declining populations. The aim was to determine whether differences in blood biochemistry (e.g., blood urea nitrogen and ketone bodies) reflect nutritional status.

Hormones
Tania Zenteno-Savin (UAF) completed her Ph.D. thesis which focused on three vasoactive hormones - AVP, ANP and ANG II - found in blood plasma. Humans suffering from eating disorders (anorexia, bulimia) have elevated levels of these hormones. It was therefore speculated that Steller sea lions might exhibit similar changes in their blood hormone levels if they are suffering from nutritional stress. To test this hypothesis, blood samples were collected and analyzed from nearly 200 Steller sea lions in Southeast and Northwest Alaska. A concurrent experiment was also run with the captive Stellers at the Vancouver Aquarium to determine how fasting or food restriction affected blood hormone levels.

In the controlled study, ANG II rose in response to food restrictions. AVP was unaffected, while ANP seemed to decline. These feeding experiments seem to indicate that blood hormones do provide some gauge of how well a Steller has been eating . However, in the wild Stellers, ANG II levels were higher in the thriving southeast population compared to the declining Aleutian population. This is not consistent with the hypothesis that Steller sea lions in the western population are nutritionally stressed.

Teeth
The teeth of sea lions have annual growth annuli (rings much like in trees), which scientists have been using to age individuals. However, Dr. David Sampson (OSU) thinks that the chemical composition of the teeth can yield even more information. His idea is that changes in the physical or chemical structure of the teeth can be used to date specific life-history events, such as weaning, the onset of maturity, or age at first lactation. Dr. Roger Nielson (OSU) has been using an electron microprobe to measure the elemental micro-composition of teeth obtained from dead Steller sea lions and from the National Marine Mammal Laboratory (NMFS). This initial study will determine whether archived teeth can be used to reconstruct changes in life history parameters which may provide insight into what happened to Steller sea lions.  

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Fecal triiodothyronine and thyroxine concentrations change in response to thyroid stimulation in Steller sea lions (Eumetopias jubatus). Keech, A.L., Rosen, D.A.S., Nelson Booth, R.K., Trites, A.W., and Wasser, S.K. (in press). General and Comparative Endocrinology abstract Variation in concentrations of thyroid hormones shed in feces may help to identify physiological states of animals, but the efficacy of the technique needs to be validated for each species. We determined whether a known physiological alteration to thyroid hormone production was reflected in hormone concentrations in the feces of Steller sea lions (Eumetopias jubatus). We quantified variation of triiodothyronine (T3) and thyroxine (T4) concentrations in feces following two intramuscular injections of thyrotropin (thyroid-stimulating hormone, TSH) at 24 h intervals in four captive female sea lions. We found fecal T3 concentrations increased 18-57% over concentrations measured in the baseline sample collected closest to the time of the first TSH injection (p=0.03) and 1-75% over the mean baseline concentration (p=0.12) for each animal of all samples collected prior to injections. The peak T3 response occurred 48 h post injection in three animals and 71 h in the fourth. Post-injection T4 concentrations did not differ between the baseline sample collected closest to the time of the first TSH injection (p=0.29) or the mean baseline concentration (p=0.23) for each animal. These results indicate that induced physiological alterations to circulating thyroid hormone concentrations can be adequately detected through analyses of fecal T3 concentrations and that the technique may provide a means of non-invasively detecting metabolic changes in Steller sea lions.
Seasonal influence on the response of the somatotropic axis to nutrient restriction and re-alimentation in captive Steller sea lions (Eumetopias jubatus). Richmond, J.P., T. Jeanniard du Dot, D.A.S. Rosen and S.A. Zinn. (in press). Journal of Experimental Zoology Vol 311A abstract Fluctuations in availability of prey resources can impede acquisition of sufficient energy for maintenance and growth. By investigating the hormonal mechanisms of the somatotropic axis that link nutrition, fat metabolism, and lean tissue accretion, we can assess the physiological impact of decreased nutrient intake on growth. Further, species that undergo seasonal periods of reduced intake as a part of their normal life history may have a differential seasonal response to nutrient restriction. This experiment evaluated the influence of season and age on the response of the somatotropic axis, including growth hormone (GH), insulin-like growth factor (IGF)-I, and IGF-binding proteins (BP), to reduced nutrient intake and re-alimentation in Steller sea lions. Eight captive females (five juveniles, three sub-adults) were subject to 28-day periods of food restriction, controlled re-feeding, and ad libitum recovery in summer (long-day photoperiod) and winter (short-day photoperiod). Hormone concentrations were insensitive to type of fish fed (low fat pollock vs. high fat herring), but sensitive to energy intake. Body mass, fat, and IGF-I declined, whereas GH and IGFBP-2 increased during feed restriction. Reduced IGF-I and IGFBP with increased GH during controlled re-feeding suggest that animals did not reach positive energy balance until fed ad libitum. Increased IGF-I, IGFBP-2, IGFBP-3, and reduced GH observed in summer reflected seasonal differences in energy partitioning. There was a strong season and age effect in the response to restriction and re-alimentation, indicating that older, larger animals are better able to cope with stress associated with energy deficit, regardless of season.
Changes in glucocorticoids, IGF-I and thyroid hormones as indicators of nutritional stress and subsequent refeeding in Steller sea lions (Eumetopias jubatus). Jeanniard du Dot T., Rosen D.A., Richmond, J.P., Kitaysky A.S., Zinn, S.A. and Trites A.W. 2009. Comparative Biochemistry and Physiology, Part A 152:524-534. abstract Physiological responses to changes in energy balance are tightly regulated by the endocrine system through glucocorticoids, IGF-I and thyroid hormones. Changes in these hormones were studied in eight captive female Steller sea lions that experienced changes in food intake, body mass, body composition, and blood metabolites during summer and winter. During a period of energy restriction, one group of sea lions was fed reduced amounts of Pacific herring and another was fed an isocaloric diet of walleye pollock, after which both groups returned to their pre-experimental diets of herring. Cortisol was negatively and IGF-I was positively associated with changes in body mass during periods of energy restriction (mass loss associated with increase in cortisol and decrease in IGF-I) and refeeding (body mass maintenance associated with stable hormone concentrations in summer and compensatory growth linked to decrease in cortisol and increase in IGF-I in winter). Cortisol and IGF-I were also correlated with changes in lipid and lean mass, respectively. Consequently, these two hormones likely make adequate biomarkers for nutritional stress in sea lions, and when combined provide indication of the energetic strategy (lipid vs lean mass catabolism) animals adopt to cope with changes in nutrient intake. Unlike type of diet fed to the sea lions, age of the animals also impacted hormonal responses, with younger animals showing more intense hormonal changes to nutritional stress. Thyroid hormones, however, were not linked to any physiological changes observed in this study.
Energy reallocation during and after periods of nutritional stress in Steller sea lions: low-quality diet reduces capacity for physiological adjustments. Jeanniard du Dot, T., D.A.S Rosen and A.W. Trites. 2009. Physiological and Biochemical Zoology 89:516-530. abstract Two groups of female Steller sea lions (Groups H and P) were subjected to periods of energy restriction and subsequent re-feeding during winter and summer to determine changes in energy partition among principal physiological functions and the potential consequences to their fitness. Both sea lion groups consumed high-quality fish (herring) before and after the energy restrictions. During restrictions, Group H was fed a lower quantity of herring and Group P a caloric equivalent of low-quality fish (pollock). Quantitative estimates of maintenance and production energies and qualitative estimates of thermoregulation, activity and basal metabolic rate were measured. During summer, all animals compensated for the imposed energy deficit by releasing stored energy (production energy). Group H also optimized the energy allocation to seasonal conditions by increasing activity during summer when fish are naturally abundant (foraging effort) and by decreasing thermoregulation capacity when waters are warmer. During winter, both groups decreased the energy allocated to overall maintenance functions (basal metabolic rate, thermoregulation and activity together) in addition to releasing stored energy, but preserved thermoregulatory capacity. Group H also decreased activity levels in winter when foraging in the wild is less efficient, unlike Group P. Overall, sea lions fed pollock did not change energy allocation to suit environmental conditions as readily as those fed herring. This implies that low energy density diet may further reduce fitness of animals in the wild during periods of nutritional stress.
Fecal triiodothyronine assay validation using captive Steller sea lions (Eumetopias jubatus) and subsequent application to free-ranging populations to examine nutritional stress. Keech, A.L. 2009. In Zoology. MSc Thesis, University of British Columbia, Vancouver. 97 pages abstract Reduced availability of high energy-content prey (nutritional stress) is a predominant hypothesis to explain the decline of Steller sea lion (Eumetopias jubatus) populations in western Alaska from the late 1970's to the late 1990's. Animals may respond to eating insufficient prey by increasing stress levels and decreasing metabolic rates. It may thus be possible to identify nutritional stress by measuring concentrations of GC metabolites (stress) and thyroid hormones (metabolism) shed in the feces of Steller sea lions. However, techniques to measure thyroid hormone concentrations from sea lion feces have not been developed. We quantified variation of triiodothyronine (T3) and thyroxine (T4) concentrations in Steller sea lion feces following two injections of thyrotropin (TSH) at 24 h intervals into four captive animals. Glucocorticoid (GC) metabolites were also assayed to examine any relationship to stimulated thyroid hormone secretion. We found that fecal T3 peaked 48 h post-injection and increased 25-57% in three sea lions (all animals, p=0.03). Pre-injection GC increases indicated stress from isolation for baseline fecal collections, but post-injection increases could not be confirmed as a response to TSH injections or as a product of the study design. The results demonstrated that pre- and post-injection changes in fecal GC and T3 concentrations were consistent with predictions of an increased stress response and metabolic rate within the animals. We then measured T3 and GC concentrations in 834 Steller sea lion fecal samples collected in 2005 and 2006 from 15 resting (haulout) and breeding (rookery) sites between British Columbia and the Central Aleutian Islands. Overall, GC concentrations did not differ between haulout populations (western 2006 pre-pupping and eastern 2005 post-pupping). Fecal hard-part analyses revealed a lower energy-content diet in the western population, suggesting that diet quality is a relevant hypothesis to explain slightly higher GC concentrations found in the western population, specifically the Aleutian Islands region. However, the nutritional stress hypothesis could not be substantiated through T3 concentrations. The rookeries possessed the highest energy-content diets, but also exhibited a nutritional stress response with a significantly higher GC and lower T3 concentration than either haulout population (possibly related to lactation or decreased leptin levels), but T3 comparisons performed at scales of site and region were inconclusive.
Seasonal differences in biochemical adaptation to fasting in juvenile and subadult Steller sea lions (Eumetopias jubatus). Rea, L.D., M. Berman-Kowalewski, D.A.S. Rosen, and A. W.Trites. 2009. Physiological and Biochemical Zoology 82:236-247. abstract Nine Steller sea lions (Eumetopias jubatus) aged 1.756 yr were experimentally fasted for 714 d during the breeding and nonbreeding seasons to identify changes in plasma metabolites that are indicative of fasting and to determine whether the ability of sea lions to fast varies seasonally or with age. Although some animals approached the limit of their protein-sparing ability by the end of our fasting experiments, there was no sign of irreversible starvation biochemistry. Plasma blood urea nitrogen (BUN) concentrations decreased in all animals within the first week of fasting, reflecting a shift to a fasting-adapted state; however, significant increases in plasma BUN concentration at the end of the nonbreeding season fasts suggest that subadult Steller sea lions were not able to maintain a protein-sparing metabolism for a full 14 d during the nonbreeding season. In contrast, juveniles were able to enter protein sparing sooner during the nonbreeding season when they had slightly higher initial percent total body lipid stores than during the breeding season. Subadult and juvenile sea lions had low circulating ketone body concentrations compared with young sea lion pups, suggesting an age-related difference in how body reserves are utilized during fasting or how the resulting metabolites are circulated and catabolized. Our data suggest that metabolite concentrations from a single blood sample cannot be used to accurately predict the duration of fast; however, threshold metabolite concentrations may still be useful for assessing whether periods of fasting in the wild are unusually long compared with those normally experienced.
Steller sea lions Eumetopias jubatus and nutritional stress: evidence from captive studies. Rosen, D.A.S. 2009. Mammal Review 39:284-306. abstract 1. Numbers of Steller sea lions Eumetopias jubatus in the North Pacific have declined. According to the Nutritional Stress Hypothesis, this decline is due to reduced food availability. Data from studies conducted on pinnipeds in the laboratory are used here to test whether the Nutritional Stress Hypothesis can explain the decline of Steller sea lions. 2. Overall, there is strong evidence for biologically meaningful differences in the nutritional quality of major prey species. Steller sea lions can partly compensate for low-quality prey by increasing their food consumption. 3. There appear to be no detrimental effects of low-lipid prey on sea lion growth or body composition when sea lions can consume sufficient quantities of prey. However, the ability to increase consumption is physiologically limited, particularly in young animals. Overall, it is more difficult to maintain energy intake on a diet of low-quality prey than on a normal diet. 4. Under conditions of inadequate food intake (either due to decreased prey availability or quality, or increased energy requirements) the overall impacts of nutritional stress are complex, and are dependent upon season, prey quality, age, and the duration and intensity of the nutritional stress event. 5. Studies on pinnipeds in the laboratory have been instrumental in identifying the conditions under which changes in sea lion prey can result in nutritional stress, and the nature of the physiological impacts of nutritional stress events.
Fasting affects the surface and diving metabolic rates of Steller sea lions (Eumetopias jubatus). Svärd, C., A. Fahlman, D.A.S. Rosen, R. Joy and A.W. and Trites. 2009. Aquatic Biology 8:71-82. abstract Changes in metabolic rates were measured in 3 captive female Steller sea lions (Eumetopias jubatus) that experienced fasts during summer and winter. Metabolic rates were measured (via O2 consumption) before (MRs, surface) and after (DMR, dive + surface interval) the sea lions dove to 10–50 m depths. Measurements were obtained prior to 9-10 day fasts, and following a 14 day recovery period. The sea lions lost significantly more body mass (Mb) during the winter fast (10.6%), compared with the summer (9.5%). Mass-corrected dive metabolic rate (cDMR = DMR • Mb-0.714) was not affected by dive depth or duration, but increased significantly following the winter fasts (13.5 ± 8.1%), unlike the decrease during summer (-1.1 ± 3.2%). However, mass-corrected surface metabolic rate (cMRs) decreased significantly after both the summer (-16.4 ± 4.7%) and winter (-8.0 ± 9.0%) fasts. Consequently, the ratio between cDMR and cMRc was significantly higher in winter, suggestive of an increased thermal challenge and convective heat loss while diving. Increased cDMs following the fast indicated that digestion began during foraging and was not deferred, implying that access to ingested energy was of higher priority than optimizing diving ability. cDMR was elevated throughout the recovery period, independent of season, resulting in a 12% increase in foraging cost in winter and a 3% increase in summer. Our data suggest that Steller sea lions are more sensitive to changes in body condition due to food shortages in the winter compared with the summer.
Steller sea lions show diet-dependent changes in body composition during nutritional stress and recover more easily from mass loss in winter than in summer. Jeanniard du Dot, T., Rosen, D. A. S. , Trites, A. W. 2008. Journal of Experimental Marine Biology and Ecology 367(1):1-10. abstract Controlled feeding experiments were undertaken with captive Steller sea lions (Eumetopias jubatus) to assess seasonal (winter vs. summer) physiological responses of individual animals to reduced quantities and qualities of food that are hypothesised to occur in the wild. Eight animals were randomly divided into two experimental groups fed isocaloric diets: Group H ate Pacific herring (Clupea pallasi) throughout the experiment while Group P was switched to walleye pollock (Theragra chalcogramma) during a 28-day food restriction (after a 28-day baseline) and back to herring during a 28-day controlled re-feeding. Diet type did not impact the rates of body mass lost when food was restricted, but did influence the type of internal energy reserve (protein vs lipids) the sea lions predominantly used. In both summer and winter, Group H lost significantly more lipids and less lean mass than Group P that was fed pollock during the restriction phase. The response of Group H was consistent with the predicted pattern of nutritional stress physiology (i.e. protein sparing and utilization of lipid reserves). Group P lost a surprisingly high proportion of body protein while consuming restricted levels of pollock, which could lead to muscle impairment and vital organ failure on a long-term basis. When given increased amounts of herring during the controlled re-feeding phase, the capacity of both groups to compensate for the previous mass loss was found to depend on season and was independent of previous diet. All of the sea lions increased their rates of mass gain and returned to their pre-experimental weight during winter, but not during summer. Some intrinsic energetic plasticity related to seasonal adaptation to the environment may render winter an easier period than summer to recover from nutritional stress.
Hormone changes indicate that winter is a critical period for food shortages in Steller sea lions. Rosen, D.A.S., Kumagai, S. 2008. Journal of Comparative Physiology B 178:573-583. abstract Given that many marine mammals display seasonal energetic priorities, it is important to investigate whether the impact of unexpected food restriction differs during the year. Steller sea lions (Eumetopias jubatus) fed restricted diets for up to 9 days during spring, summer, fall, and winter lost an average of 10% of their initial body mass. We tracked changes in the levels of three hormones (cortisol, total thyroxine—TT4, total triiodothyronine—TT3) and one blood metabolite (blood urea nitrogen—BUN) following a food restriction in relation to season, body mass, body composition, and metabolism. Degree of changes in cortisol, TT3, and BUN after food restriction was significantly affected by season. The greatest changes in cortisol (+231%), BUN (+11.4%), TT4 (-23.3%), and TT3 (-35.6%) occurred in the winter (November/December) when rates of body mass loss were also greatest. Changes in cortisol levels were positively related to total body mass loss, while changes in TT3 levels were negatively related. While greater increases in BUN were related to greater rates of mass loss, the use of BUN levels as an indicator of metabolic state is complicated by the type and level of food intake. The observed changes in hormone levels support morphological data suggesting Steller sea lions may be more strongly impacted by short-term, reduced energy intake during winter than at other times of the year.
Marine Mammals in the Lab: Tools for Conservation and Science. Rosen, D.A.S. and A.W. Trites. 2008. In North Pacific Universities Marine Mammal Research Consortium. Vancouver, BC. pp. 15 pages abstract Twenty-two participants from a variety of backgrounds and interests discussed how to improve the nature of research with marine mammals in the laboratory and ensure its continuation as a vital scientific resource in the future. There was agreement that captive marine mammals represent a valuable scientific asset. Many of the pressing conservation and scientific research questions pertaining to marine mammals cannot be carried out with their wild counterparts. However, studying marine mammals in the laboratory incurs specific financial, scientific, and logistical challenges. The workshop generated potential solutions to many of these issues. Participants expressed the need for greater cooperation and coordination between scientists to optimize the scientific value of research with captive marine mammals, and to minimize the costs of such research. This could be enhanced through scheduled in-person gatherings and web-based portals for listing active and proposed research. Better use must also be made of scientific resources and expertise, and novel sources of revenue have to be generated. There should also be greater sharing of documents relating to experimental design and research permitting. The effectiveness of research will benefit from greater communication between researchers and husbandry staff at institutions holding animals for research. Such efforts should raise the profile (and acceptance) of captive marine mammals science within the scientific community and for program administrators, leading to greater financial and research opportunities. Nine specific recommendations were forwarded that could be immediately implemented to enhance communication and increase the value of captive marine mammal science: 1. Produce a list of research resources (animals, specialized skills and equipment); 2. Create a list of on-going captive marine mammal studies; 3. Produce a list of publications derived from research with captive marine mammals; 4. Develop a set of guidelines for communication, responsibilities, and intellectual ownership for collaborative projects; 5. Implement means for coordination of future studies (both web-based and scheduled workshop/meetings); 6. Design a means for sharing standard Operating Procedures; 7. Hold a workshop to increase statistical rigor and standards in experimental design; 8. Introduce the use of annual survival rates into institutions holding marine mammals; and 9. Heighten the awareness of the value and prevalence of captive studies to the Us Marine Mammal Commission.
Reductions in oxygen consumption during dives and estimated submergence limitations of Steller sea lions (Eumetopias jubatus). Hastie, G.D., D.A.S. Rosen and A.W. Trites. 2007. Marine Mammal Science 23:272-286. abstract Accurate estimates of diving metabolic rate are central to assessing the energy needs of marine mammals. To circumvent some of the limitations inherent with conducting energy studies in both the wild and captivity, we measured diving oxygen consumption of two trained Steller sea lions (Eumetopias jubatus) in the open ocean. The animals dived to predetermined depths (5–30 m) for controlled periods of time (50–200 s). Rates of oxygen consumption were measured using open-circuit respirometry before and after each dive. Mean resting rates of oxygen consumption prior to the dives were 1.34 (±0.18) and 1.95 (±0.19) liter/min for individual sea lions. Mean rates of oxygen consumption during the dives were 0.71 (±0.24) and 1.10 (±0.39) liter/min, respectively. Overall, rates of oxygen consumption during dives were significantly lower (45% and 41%) than the corresponding rates measured before dives. These results provide the first estimates of diving oxygen consumption rate for Steller sea lions and show that this species can exhibit a marked decrease in oxygen consumption relative to surface rates while submerged. This has important consequences in the evaluation of physiological limitations associated with diving such as dive duration and subsequent interpretations of diving behavior in the wild.
Diet quality and season affect physiology and energetic priorities of captive Steller sea lions during and after periods of nutritional stress. Jeanniard du Dot, T. 2007. MSc Thesis, University of British Columbia, Vancouver. 142 pages abstract The ability of animals to contend with unpredictable seasonal shifts in quality and quantity of prey has implications for the conservation of wildlife. Steller sea lions (Eumetopias jubatus) were subjected to different quantities and qualities of food to determine what physiological and endocrine responses would occur and whether they differed between season (summer and winter) or diet (high-lipid Pacific herring Clupea pallasi vs. low-lipid walleye pollock Theragra chalcogramma). Eight females were divided among two groups. One (Group H) were fed herring for 28 days (baseline), then received a reduced caloric intake for a subsequent 28 days (restriction) to induce a 15% loss of body mass. The second (Group P) were also fed herring during the baseline followed by a reduced isocaloric diet of pollock during the restriction. Both groups subsequently returned to their baseline intake of herring for a 28-day controlled re-feeding. The two groups of sea lions lost identical mass during restrictions independent of species eaten, but did differ in the type of internal energy reserve (protein vs. lipids) they predominantly used. Group H lost significantly more lipids and less lean mass than Group P in both seasons. In summer, Group H also increased activity levels and decreased thermoregulation capacity to optimize energy allocation. No such changes were observed for Group P whose capacity to adjust to the reduced caloric intake seemed to have been blocked by the pollock diet. During winter, the sea lions spared energy allocated to activity (especially Group H) and preserved thermoregulation capacity. Changes in body mass was negatively related to free cortisol and positively related to IGF-1 in winter, but only IGF-1 was related to changes in mass in summer when lean mass regulation seemed more important. Levels of IGF-1 were associated with changes in protein metabolism in both seasons for both groups, but changes in body condition were never explained by the measured metabolites or hormones. The cap! acity to compensate for mass loss was seasonally dependent with sea lions displaying compensatory growth (by restoring lipid stores) in winter but not in summer. Summer appears to be a more difficult season for sea lions to recover from mild nutritional stress. These physiological findings can be used to refine bioenergetic models needed for the conservation of Steller sea lion populations.
Utilization of stored energy reserves during fasting varies by age and season in Steller sea lions. Rea, L.D., D.A.S. Rosen and A.W Trites. 2007. Canadian Journal of Zoology 85:190-200. abstract Nine captive Steller sea lions (Eumetopias jubatus (Schreber, 1776), 1.75–6 years of age) were fasted for 7–14 d to test the effect of short-term fasting on changes in body mass and body condition. Trials were repeated during both the summer breeding season and the nonbreeding season in seven animals to elucidate whether there was a seasonal component to the ability of Steller sea lions to adapt to limited food resources. Mean percent mass loss per day was higher during the breeding season in juveniles (1.8% ± 0.2%·d–1) than in subadults (1.2% ± 0.1%·d–1), but there were no significant age-related differences during the nonbreeding season (juveniles, 1.5% ± 0.3%·d–1; subadults, 1.7% ± 0.3%·d–1). A decrease in the rate of mass loss occurred after the first 3 d of fasting only in subadults during the breeding season. Percent total body lipid ranged from 11% to 28% of total body mass at the initiation of fasting trials. Animals with lower initial percent total body lipid exhibited higher subsequent rates of mass loss and a lower percentage of tissue catabolism derived from lipid reserves. There was no evidence of metabolic adaptation to fasting in juveniles, which suggests that juvenile sea lions would be more negatively impacted by food limitation during the breeding season than would subadults.
Body mass and composition responses to short-term low energy intake are seasonally dependent in Steller sea lions (Eumetopias jubatus). Kumagai, S., D.A.S Rosen and A.W. Trites. 2006. Comparative Biochemistry and Physiology 179:589-598. abstract Steller sea lions (Eumetopias jubatus) were fed restricted iso-caloric amounts of Pacific herring (Clupea pallasi) or walleye pollock (Theragra chalcogramma) for 8-9 days, four times over the course of a year to investigate effects of season and prey composition on sea lion physiology. At these levels, the sea lions lost body mass at a significantly higher rate during winter (1.6 ± 0.14 kg d-1), and at a lower rate during summer (1.2 ± 0.32 kg d-1). Decreases in body fat mass and standard metabolic rates during the trials were similar throughout the seasons and for both diet types. The majority of the body mass that was lost when eating pollock derived from decreases in lipid mass, while a greater proportion of the mass lost when eating herring derived from decreases in lean tissue, except in the summer when the pattern was reversed. Metabolic depression was not observed during all trials despite the constant loss of body mass. Our study supports the hypothesis that restricted energy intake may be more critical to Steller sea lions in the winter months, and that the type of prey consumed (e.g., herring or pollock) may have seasonally-specific effects on body mass and composition.
Effects of prey composition on the endocrine response to nutrient restriction and re-alimination in Steller sea lions (Eumetopias jubatus). Richmond, J. P., T. Jeanniard du Dot, D. A. S. Rosen and S. A. Zinn. 2006. Symposia of the Comparative Nutrition Society 63:136-141.
Potential effects of short-term prey changes on sea lion physiology. Rosen, D.A., D.J. Tollit, A.J. Winship, and A.W. Trites. 2006. In A.W. Trites, S. Atkinson, D.P. DeMaster, L.W. Fritz, T.S. Gelatt, L.D. Rea and K. Wynne (eds), Sea Lions of the World. Alaska Sea Grant College Program, University of Alaska, Fairbanks. pp. 103-116. abstract hanges in the proximate composition of prey can result in a nutritional imbalance in individual animals, regardless of total energy intake. This mechanism has been hypothesized to have contributed to the decline of Steller sea lions (Eumetopias jubatus). Yet little is known about how otariids react physiologically to short-term changes in prey quality and availability. A series of studies with young captive Steller sea lions tested several potential links between prey quality and sea lion health. Body composition (fat to total mass ratio) of animals fed constant, maintenance-level, isocaloric diets of high- or low-lipid prey changed with season, but overall was not aff ected by prey composition. The sea lions appeared to prioritize maintaining core growth rates even when energy was limited, electing to deplete lipid reserves to fulfi ll energy defi cits, resulting in changes in relative body condition. In contrast, sea lions subject to short- term, sub-maintenance diets of high- or low-lipid prey utilized a greater portion of their lipid reserves when losing body mass on low lipid prey. Experiments with diff erent ad libitum feeding regimes indicated that sea lions are readily able to alter food intake levels to compensate for diff erences in prey energy content and, to a lesser degree, prey availability. However, the results also suggest that decreases in prey quality and/or foraging opportunities can readily combine to require food intake levels that are greater than the digestive capacity of the individual. This is particularly true for young animals that may already be living ?on the edge? energetically.
Interacting physiological constraints to foraging behavior in marine mammals. Rosen, D. A. S., A. J. Winship and L. A. Hoopes. 2006. Symposia of the Comparative Nutrition Society 63:151-156.
Examining the potential for nutritional stress in young Steller sea lions: physiological effects of prey composition. Rosen, D.A.S. and A.W. Trites. 2005. Journal of Comparative Physiology 175:265-273. abstract The effects of high- and low-lipid prey on the body mass, body condition, and metabolic rates of young captive Steller sea lions (Eumetopias jubatus) were examined to better understand how changes in prey composition might impact the physiology and health of wild sea lions and contribute to their population decline. Results of three feeding experiments suggest that prey lipid content did not significantly affect body mass or relative body condition (lipid mass as a percent of total mass) when sea lions could consume sufficient prey to meet their energy needs. However, when energy intake was insufficient to meet daily requirements, sea lions lost more lipid mass (9.16±1.80 kg±SE) consuming low-lipid prey compared with eating high-lipid prey (6.52±1.65 kg). Similarly, the sea lions lost 2.7±0.9 kg of lipid mass while consuming oil-supplemented pollock at maintenance energy levels but gained 5.2±2.7 kg lipid mass while consuming identical energetic levels of herring. Contrary to expectations, there was a 9.7±1.8% increase in metabolism during mass loss on submaintenance diets. Relative body condition decreased only 3.7±3.8% during periods of imposed nutritional stress, despite a 10.4±4.8% decrease in body mass. These findings raise questions regarding the efficacy of measures of relative body condition to detect such changes in nutritional status among wild animals. The results of these three experiments suggest that prey composition can have additional effects on sea lion energy stores beyond the direct effects of insufficient energy intake.
Effects of body condition on resting metabolism in captive and free-ranging Steller sea lions (Eumetopias jubatus). Hoopes, L.A., L.D. Rea, D.A.S. Rosen and G.A.J. Worthy. 2004. Symposia of the Comparative Nutrition Society 2004 5:79-82. abstract The objectives of this study were to compare Resting Metabolic Rate (RMR) from animals in the eastern and western Alaskan populations to discern whether there is any evidence of nutritional stress. Oxygen consumption data were collected from captive Steller sea lions held at the Vancouver Aquarium, Vancouver, BC and from free-ranging Steller sea lions captured from western and eastern Alaskan stocks. In water, RMR ranged from 33.3 to 56.7 MJ/day for sub-adult animals (109-158 kg, 2.9-4.6 times predicted for an adult animal) and from 20.0 to 26.6 MJ/day for pups (57-59 kg, 3.3-4.3 times predicted) at 2°C. RMR, generally decreased with increasing water temperature, but the relationship was not statistically significant. Reduced body condition had a noticeable impact on RMR in juvenile sea lions at colder water temperatures. The results of the present study suggest that young sea lions would be subject to even greater thermoregulatory demands if their body condition were reduced.
Validation of a fecal glucocorticoid assay for Steller sea lions (Eumetopias jubatus). Hunt, K.E., A.W. Trites, and S.K. Wasser. 2004. Physiology and Behavior 80:595-601. abstract The Steller sea lion (Eumetopias jubatus) is listed as endangered in parts of its range and is suspected of suffering from ecological stressors that may be reflected by fecal glucocorticoid hormones. We validated a fecal glucocorticoid assay for this species with an adrenocorticotropic hormone (ACTH) challenge. Feces were collected from captive Steller sea lions (two males and two females) for 2 days before injection with ACTH, and for 4 or more days postinjection. Feces were freeze-dried, extracted with a methanol vortex method, and assayed for glucocorticoids. The assay demonstrated good parallelism and accuracy. All animals showed the expected peak of fecal glucocorticoid excretion after ACTH injection. However, the two males had higher baselines, higher peaks, and more delayed peaks than the females. Peak glucocorticoid excretion occurred at 5 and 28 h postinjection for the two females, and at 71 and 98 h for the two males. Correction for recoveries by the addition of tritiated hormones produced ACTH profiles that were virtually identical in pattern to uncorrected data, but with higher within-sample coefficients of variation. Based on these results, we conclude that this fecal glucocorticoid assay accurately reflects endogenous adrenal activity of Steller sea lions, and that recovery corrections are not necessary for this species when using the methanol vortex extraction method. More research is needed to address possible sex differences and other possible influences on fecal glucocorticoid concentrations.
Searching for stress: Hematological indicators of nutritional inadequacies in Steller sea lions. Rosen, D.A.S., Hastie, G.D., Trites, A.W. 2004. Symposia of the Comparative Nutrition Society 2004 5:145-149. abstract This experiment examined the response of a suite of hematologic parameters to experimentally induced nutritional stress in a group of captive Steller sea lions. The goal was to identify a suite of parameters that could be used to diagnose comparable conditions among wild Steller sea lions. Previous studies, many with ruminant mammals, have shown that there are significant changes in blood characteristics with nutritional status. However, it is equally clear that there is no overwhelming choice of blood parameter to indicate nutritional stress across different species. Therefore, species-specific empirical tests such as the one carried out in the current study are essential to place results from wild studies in a biologically meaningful context.
Satiation and compensation for short-term changes in food quality and availability in young Steller sea lions (Eumetopias jubatus). Rosen, D.A.S. and Trites, A.W. 2004. Canadian Journal of Zoology pp. 1061-1069. abstract Foraging theory predicts that animals should proportionately increase their food intake to compensate for reduced energy content and/or prey availability. However, the theoretical intake levels will – at some point – exceed the digestive capacity of the predator. We tested the ability of Steller sea lions (Eumetopias jubatus, Schreber, 1776) to compensate for short-term changes in prey energy density and availability, and quantified the maximum amount of food a young sea lion could consume. Five 1-2 year old captive Steller sea lions were alternately offered herring (high-energy) or capelin (low-energy) each day or every second day. When prey were available on a daily basis the sea lions compensated for differences in the energy content of herring and capelin by consuming sufficient quantities of each (8.3 vs. 14.0 kg d-1, respectively) to maintain an equivalent gross energy intake. When herring was available only on alternate days, the sea lions increased their consumption by 52% to 11.5 kg d-1, which was not sufficient to maintain an average gross intake equal to when herring was available every day. When capelin was available only on alternate days, some animals increased their intake for a few days, but average intake (15.2 kg d-1) was far below levels observed during daily feeding. Generally, the sea lions appeared to reach their digestive limit at a level equivalent to 14-16% of their body mass. Our findings suggest that Steller sea lions can alter their food intake in response to short-term changes in prey quality or availability, but that these variables can quickly combine to necessitate food intake levels that exceed the physiological digestive capacities of young animals.
Changes in metabolism in response to fasting and food restriction in the Steller sea lion (Eumetopias jubatus). Rosen, D.A.S. and A.W. Trites. 2002. Comparative Biochemistry and Physiology. 132:389-399. abstract Many animals lower their resting metabolism (metabolic depression) when fasting or consuming inadequate food. We sought to document this response by subjecting five Steller sea lions to periods of: (1) complete fasting; or (2) restricting them to 50% of their normal herring diet. The sea lions lost an average of 1.5% of their initial body mass per day (2.30 kg y d )during the 9 –14-day fast, and their resting metabolic rates decreased 31%, which is typical of a ‘fasting response ’. However, metabolic depression did not occur during the 28-day food restriction trials,despite the loss of 0.30% of body mass per day (0.42 kg y d). This difference in response suggests that undernutrition caused by reduced food intake may stimulate a ‘hunger response ’, which in turn might lead to increased foraging effort. The progressive changes in metabolism we observed during the fasts were related to, but were not directly caused by, changes in body mass from control levels. Combining these results with data collected from experiments when Steller sea lions were losing mass on low energy squid and pollock diets reveals a strong relationship between relative changes in body mass and relative changes in resting metabolism across experimental conditions.While metabolic depression caused by fasting or consuming large amounts of low energy food reduced the direct costs from resting metabolism, it was insufficient to completely overcome the incurred energy deficit.
What is it about food? Examining possible mechanisms with captive Steller sea lions. Rosen, D.A.S. and A.W. Trites. 2002. In D. DeMaster and S. Atkinson (eds), Steller sea lion decline: Is it food II. University of Alaska Sea Grant, AK-SG-02-02, Fairbanks. pp. 45-48. abstract Changes in the quality or quantity of food can have a dramatic effect on the population status of wild animals. Unfortunately, it is difficult to assess (or define) whether nutritional stress is a contributing factor to the decline of any particular species.The “nutritional quality ” of a diet to an animal is a complex matter to assess given the range of components that can influence its value.The effects of different diets on animal health are equally complex, and are particularly difficult to assess in large, wild animals. Research by the North Pacific Universities Marine Mammal Research Consortium with captive Steller sea lions is evaluating the possible mechanisms by which dietary changes might adversely affect the nutritional or health status of individual animals, and ultimately the population as a whole. The research investigates the three potential proximate mechanisms by which changes in diet might impact Steller sea lions:a decrease in energy intake, a decrease in the intake of some essential element, and the over-consumption of an element detrimental to sea lion health.
The effects of food deprivation on serum lipid concentration and content in Steller sea lions (Eumetopias jubatus). Berman, M. and L. Rea. 2000. In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:13-16. abstract The western Alaska population of Steller sea lions has significantly declined over the past thirty-five years. A population estimate of 180,000 individuals in 1965 declined to a current estimate of 50,000. A widely accepted hypothesis for the cause of decline is from indirect competition with the commercial fishing industry. Analysis of Steller sea lion censuses have determined that decline is most evident in the juvenile portion of the population. This could be explained by a decrease in prey availability for juveniles which are physiologically and behaviorally limited in their ability to forage further and deeper for food. Although Steller sea lions naturally fast during their summer breeding season, they are not as biochemically adapted to handle food deprivation at other times of the year (Rea et al. 1999). This study addresses the physiological implications of food deprivation by analyzing the effects of fasting on serum lipid composition and content. Additionally, the breeding and non-breeding seasons were compared to determine if seasonality affects serum lipid composition and content.
The reliability of skinfold-calipers for measuring blubber thickness of Steller sea lion pups (Eumetopias jubatus). Jonker, R.A.H. and A.W. Trites. 2000. Marine Mammal Science 16:757-766. abstract Twelve dead Steller sea lion pups (Eumetopias jubatus) aged 3-14 d were recovered from rookeries in Southeast Alaska. They had a wide range of body sizes and conditions (small to large and fat to no fat). The ability of calipers to estimate the thickness of their blubber layer was assessed with a set of skinfold calipers. Average error of measurement for skin and blubber thickness was an acceptable 5.4%, but the skin and blubber of the pups were highly compressible. Skinfold thickness increased with body mass but did not necessarily reflect the development of blubber, given that pups with no blubber also showed an increase in skinfold thickness with increases in body mass. Skinfold thickness of sea lion pups appears to predict body size better than it predicts blubber thickness, making it difficult if not impossible to develop a simple index of body condition or a calculation of percent body fat for Steller sea lion pups from skinfold caliper measurements.
Changes in serum leptin levels during fasting and food limitation in Steller sea lions (Eumetopias jubatus). Rea, L.D., T.R. Nagy. 2000. In Proceedings of the Comparative Nutrition Society. Asilomar, CA. pp. 171-175. abstract Leptin, also commonly known as the ob protein, is a peptide hormone secreted by adipocytes which has been shown to have a role in energy metabolism and food intake in rodents and man (Campfield et al. 1996); Although the specific molecular and biochemical pathways of action of this hormoneare still the-focus of intensive study, it is thought that leptin acts as a negative feedback signal to satiety centers in the hypothalmus to regulate body energy stores. When adipose reserves are abundant, high levels of leptin are secreted and signal the brain to regulate energy balance (i.e. decrease food intake). The role of leptin in other animal systems has received much less attention to date, thus we chose to investigate how serum leptin concentrations change in response to food deprivation in an animal which is known to undergo periods of voluntary natural fasting in the wild. Female Steller sea lions fast for 1 to 2 weeks during the summer breeding season in order to give birth and nurse their young. Males are also thought to fast while defending territory during the breeding season. By simulating these fasting bouts in a captive environment the effect of complete fasting and body condition (i.e. total fat content) on circulating leptin levels could be addressed. In rodents and humans, food intake has been shown to increase leptin production and fasting consistently decreased leptin secretion by the adipocytes (Saladin et al. 1995, Pratley et al. 1997). To address the related, but individual effects of fasting and decrease in body reserves on leptin production we also held sea lions on a low plane of nutrition (food limitation) for 28 d such that body mass loss was similar to that experienced during 14 d fasting experiments. In several species studied to date, a close correlation between serum leptin concentrations and total body fat mass has been demonstrated. If a close correlation between leptin and body fat content could also be established for Steller sea lions, this hormone could provide an index of body condition that could be more easily monitored in free-ranging animals. Presently the best method for determining body fat content in these animals involves holding captured individuals under an aesthesia during the two hour equilibration period necessary for the dilution of deuterium.
Metabolic response to fasting in 6-week-old Steller sea lion pups (Eumetopias jubatus). Rea, L.D., D.A.S. Rosen and A.W. Trites. 2000. Canadian Journal of Zoology 78:890-894. abstract Four Steller sea lions (Eumetopias jubatus) aged 6 weeks were fasted for 2.5 d to determine how young pups mobilize energy reserves during short periods of fasting similar to those experienced in the wild. At 6 weeks of age, the pups lost 5.1 ± 0.3% of their body mass during 2 d of fasting, with an average daily mass loss of 0.7 ± 0.1 kg·d –1 . Plasma blood urea nitrogen (BUN) concentration increased significantly from 3.0 ± 0.1 mM, after an over-night fast, to 4.8 ± 0.5 mM, after 2.5 d of fasting. It is apparent that BUN levels are quickly depressed, since after only an overnight fast, these pups showed BUN levels 2- to 4-fold lower than those measured after the same pups, when 9 months of age, had recently been fed fish. Plasma ketone body (b-HBA) concentrations of the 6-week-old pups increased significantly from 0.32 ± 0.08 to 0.42 ± 0.08 mM between 0.5 and 1.5 d of fasting. There was no significant change in mean plasma concentration beyond 1.5 d, owing to variable individual responses to extended fasting. Plasma b-HBA levels at 9 months of age ranged from 0.07 to 0.18 mM. Six-week-old Steller sea lion pups showed blood chemistry consistent with metabolic adaptation to fasting within 16 h but were unable to sustain a protein-sparing metabolism for a prolonged period. The pups appeared to revert to protein catabolism after only 2.5 d of fasting. This infers a decrease in lipid catabolism that might be due to the depletion of available lipid resources.
Assessing the role of nutritional stress in the decline of wild populations: a Steller case of scientific sleuthing. Rosen, D.A.S. and A.W. Trites. 2000. In C.L.K. Baer (ed.), Proceedings of the Third Comparative Nutrition Society Symposium. Pacific Grove, California, August 4-9, 2000. 3:182-186. abstract Dry-matter digestibility and energy digestive efficiency were measured in six juvenile Steller sea lions (Eumetopias jubatus) fed three diets each consisting of a single species: herring, pollock, and squid. Two of the animals were also fed pink salmon. Dry-matter digestibility (DMD) and digestive efficiency (DE) were measured using the energy and manganese concentration in fecal and food samples. DE values were high for all prey species (herring: 95.4 ± 0.7% (mean ± SD), pollock: 93.9 ± 1.4%, salmon: 93.4 ± 0.5%, squid: 90.4 ± 1.3%). Steller sea lions appear to digest prey of high energy density more efficiently than prey of low energy density. DMD values were also high for all prey species (herring: 90.1 ± 1.8%, pollock: 86.5 ± 3.4%, salmon: 87.3% ± 2.6, squid: 90.5 ± 1.2%). The low DMD value for pollock compared with herring and squid was due to the high proportion of bony material in pollock. There was a strong linear relationship between DE and DMD for each prey type, but the terms cannot be used interchangeably. DE measures are more meaningful than DMD in conveying the energetic benefits derived by sea lions from different types of prey. Species-specific measures of the digestible energy obtained from an array of prey items are a necessary component in understanding the bioenergetic consequences of consuming different prey species.
Morphometric measurements and body condition of healthy and starving Steller sea lion pups (Eumetopias jubatus). Trites, Andrew W. and Remco A.H. Jonker. 2000. Aquatic Mammals 26:151-157. abstract The thickness and weight of skin, blubber, and body core were measured from 12 dead Steller sea lion pups (Eumetopias jubatus). These necropsied pups represented a wide range of body sizes and conditions (small to large, and fat to no-fat), and were chosen to compare the relative body conditions of healthy and starved pups. Seven of the pups lacked blubber and were significantly lighter for a given length compared to the five that had fat at their time of death. Volume exceeded mass by a factor of 1.3% with density averaging 0.987g cm-3. Skin and blubber were not uniformly thick over the body surface. Skin was thinnest on the head and around the flippers (3mm), and became thicker towards the rump (5mm). Skin thickness did not differ between dorsal and ventral sides, unlike blubber, which was thickest on the ventral side, increasing from the snout (1.5mm)to midtrunk (7mm) and decreasing posteriorly (5mm at the tail). Along the back, blubber increased from 1 mm at the snout to about 4.5mm at mid-trunk. The five pups that died of trauma had about 13% skin and 10% blubber (expressed as a proportion of total body mass). Starvelings lost an estimated 43% of their body mass before dying (10% blubber, and 33% body core). Morphometric measurements applied to three proposed indices of body condition suggest that girth is not a good predictor of body condition for Steller sea lion pups. Only the ratio of observed to predicted body mass derived from standardized mass-length relationships could distinguish starvelings from pups with body fat. keywords     morphometric measurements, body condition, Steller sea lions, pups, skin, volume, density, starvation, #2
Seasonal differences in adaptation to prolonged fasting in juvenile Steller sea lions (Eumetopias jubatus). Rea, L.D., D.A.S. Rosen and A.W. Trites. 1999. In The FASEB Journal (Federation of American Societies of Experimental Biology). Washington, D.C., April 17-21, 1999. Vol 13(5) pp. A740 abstract Five juvenile Steller sea lions (Eumetopias jubatus) between the ages of 3 and 4 years were experimentally fasted for 9 to 14 d to assess changes in mass and in key plasma metabolites indicative of biochemical adaptation to fasting. The 5 sea lions lost 20.4 to 35.1 kg each, at a rate of 1 to 2% of their initial body mass per day. Two animals fasted during the natural breeding season (June) exhibited a mean daily loss of 1.6 +/- 0.1kg d-1. This was significantly lower than the mean 2.8 +/- 0.1kg d-1 lost by sea lions fasted outside the normal breeding season in April, October and November (p<0.001). The two sea lion studied in June maintained low BUN concentrations throughout the remainder of the study, while the remaining 3 animals showed significant increases after 7 d of fasting. Only the two juveniles fasted during the breeding season maintained a protein sparing metabolism, typical of the species adapted to long-term fasting. With the exception of the smallest female (after 12 d of fasting), ketone body levels ranged from 0.03 to 0.17 mM. Seasonal differences in how sea lions adapt to fasting suggests that these animals would be more severely impacted by limited food resources during the non-breeding season.
Metabolic effects of low-energy diet on Steller sea lions, Eumetopias jubatus. Rosen, D.A.S. and A.W. Trites. 1999. Physiological Zoology 72:723-731. abstract Diets of six Steller sea lions (Eumetopias jubatus) were switched between a high (herring) and a low (squid) energy density food for 14 d to determine the effects on ingested prey mass, body mass, resting metabolic rate, and the heat increment of feeding. Body mass was measured daily, and resting metabolism was measured weekly by gas respiro-metry. Ingested food mass did not differ significantly be-tween the squid diet and the control or the recovery herring diet periods. As a result of differences in energy density, gross energy intake was significantly lower during the squid diet phase than during either the control or recovery pe-riods. As a result, sea lions lost an average of 1.1 kg/d, totaling 12.2% of their initial body mass by the end of the experimental period. The heat increment of feeding for a 4-kg squid meal was significantly lower than for a similarly sized meal of herring. Decreases in both absolute (24.0 to 18.0 MJ/d, 224%) and mass-corrected (903 to 697 kJ/d/ kg 0.67 , 220%) metabolism were observed by the end of the squid feedings. This study suggests that sea lions can depress their resting metabolism in response to decreases in energy intake or body mass, regardless of satiation level.
Blood Chemistry and Body Mass Changes During Fasting in Juvenile Steller sea lions (Eumetopias jubatus). Rea, Lorrie D., David A.S. Rosen and Andrew W. Trites. 1998. In Proceedings of the Comparative Nutrition Society, Number 2. pp. 174-178. abstract Fasting in bears, penguins and phocid seals is accompanied by predictable changes in plasma metabolite concentrations related to alterations in the body reserves that are catabolized and illustrate a species’ ability to limit protein degradation during long-term fasting (see review in Castellini and Rea 1992, Nordoy et al. 1993, Rea 1995). Steller sea lions (Eumetopias jubatus) also undergo periods of fasting in their natural environment; adult females fast while nursing pups on the rookery, males defend breeding territories and young pups fast on the rookeries while their mothers are at sea foraging. Five juvenile Steller sea lions were fasted ‘in captivity (with free access to fresh water) for 9 to 14 days to test the hypothesis that juvenile Steller sea lions also exhibit changes in key plasma metabolites indicative of biochemical adaptation to fasting. The secondary objective of this study was to determine if blood metabolite concentrations could be used as biochemical indicators of nutritional status in free-ranging juvenile Steller sea lions. keywords     blood chemistry, body mass, fasting, juvenile Steller sea lions, #2
Changes in metabolism in response to varying energy intake in a marine mammal, the Steller sea lion. Rosen, D.A.S. and A.W. Trites. 1998. In Proceedings of the Comparative Nutrition Society, Number 2. pp. 182-187. abstract When faced with decreases in energy intake, an animal has two conditions, hunger stimulates increased foraging activity, a strategy of short-term expenditure off-set by a reasonable-expectation of foraging success. However, when faced wit increased energy h periods of predictable or prolonged shortages of energy intake (although not necessarily energy availability), the animal should limit energy expenditures. The most common response to experimental undernutrition or fasting in homeotherms is metabolic depression. Invoking such physiologic responses that limitenergy expenditures limits tissue loss and delays death by starvation. Some species of marine mammals have exhibited metabolic depression, although its occurrence, scope, and triggers are still unclear. This study was designed to document the extent of metabolic depression in Steller sea lions. It investigated the role of energy and food intake on metabolic depression, and the relationship between changes in body mass and the scope of metabolic depression.