Metabolic rate and body composition of harbour seals, Phoca vitulina, during starvation and refeeding

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1 Metabolic rate and body composition of harbour seals, Phoca vitulina, during starvation and refeeding NINA HEDLUND MARKUSSEN AND MORTEN RYG Division of General Physiology, University of Oslo, P. 0. Box 1051, Blindern, 0316 Oslo 3, Norway AND NILS ARE ORITSLAND Norwegian Polar Research, P. 0. Box 158, 1330 Oslo Lufrhavn, Nonvay Received October 1 1, 1990 Accepted September 3, 1991 MARKUSSEN, N. H., RYG, M., and ORITSLAND, N. A Metabolic rate and body composition of harbour seals, Phocu vitulina, during starvation and refeeding. Can. J. Zool. 70: The metabolic rate of normally fed harbour seals (Phocu vitulina) was measured during starvation and the subsequent refeeding period. Resting metabolic rate was calculated from the y-intercept of the metabolic rate versus swimming speed curves and was 20% lower during the starvation period than during normal feeding. During starvation, about 50% of mass loss was fat, i.e., 77% of the energy was obtained from fat. Metabolic rate returned to the initial value about 1 week after the onset of refeeding. MARKUSSEN, N. H., RYG, M., et ORITSLAND, N. A Metabolic rate and body composition of harbour seals, Phoca vitulina, during starvation and refeeding. Can. J. Zool. 70 : Le taux de metabolisme de Phoques communs (Phoca vitulina) nourris normalement a ete mesure au cours d'un jeqne et au cours de la periode d'alimentation qui a suivi le jeqne. Le taux de metabolisme au repos a ete determine par calcul du point d'intersection avec l'ordonnee de la courbe illustrent le taux de metabolisme versus la vitesse de nage et il s'est avere de 20% plus faible chez les phoques soumis au jeqne que chez les phoques nourris normalement. Au cours du jeqne, environ 50% de la perte de masse etait une perte de graisse, c'est-h-dire que 77% de l'energie venait des graisses. Le taux de metabolisme est revenu h sa valeur initiale 1 semaine aprks la reprise de I'alimentation. [Traduit par la redaction] Introduction A large proportion of the body mass of marine mammals is fat, consolidated into a blubber layer that serves both as thermal insulation and energy storage. Fasting therefore presents a problem in that excessive loss of fat might jeopardize thermal stability. True seals undergo natural starvation or fasting periods, pups during the postweaning period and subadults and adults during moulting and lactation. Most studies of changes in body composition of seals during fasting have been carried out on pups or young individuals, with inconsistent results. Reviewing earlier work on seals, Worthy and Lavigne (1 987) suggested that the conflicting reports might be due to previously undetected differences in the energy density of core tissues. They also pointed out the possibility of evolutionary differences between species that usually fast on land (e.g., gray seals, Halichoerus grypus, and elephant seals, Mirounga sp.) and species such as harp seals, Phoca groenlandica, and hooded seals, Cystophora cristata, that may spend much of the postweaning fast in cold water (Worthy and Lavigne 1987). Because death by starvation is linked to depletion of lean body mass, the contribution of fat tissue catabolism to the total energy expenditure also has a direct and profound effect on survival time (Oritsland 1990; Oritsland and Markussen 1990). Basal, or resting, metabolism constitutes a major part of the total energy expenditure of marine mammals and is not significantly different from that of terrestrial mammals of similar size when determinations for both groups are carried out under similar conditions (Oritsland and Ronald 1975; Lavigne et al. 1986a). McNab (1970) and Kleiber (1975) found, in interspecific comparisons, that differences in fat content were not correlated with differences in metabolic rate. Intraspecifically, however, a decrease in body mass with a corresponding change in the body composition due to starvation or undernutrition is followed by a decrease in metabolic rate in terrestrial mammals (Grande et al. 1958; Markussen and Oritsland 1986). Davydov (1984) and Worthy (1987) found such a depressed metabolic rate in fasting harp seal pups. Nordoy et al. (1990) suggested that the development of metabolic depression in grey seal pups is unusual among mammals. The metabolic rate dropped to about half the initial value in about 2 weeks but thereafter remained stable. Discrepancies in the interpretation of changes in body size, composition, and metabolism of pinnipeds during starvation still exist (Nordoy and Blix 1985; Oritsland et al. 1985; Worthy and Lavigne 1987). In the present study, we determined body composition and metabolic rate in harbour seals both during a starvation period that was not connected with natural fasting and also during the following refeeding period. Materials and methods Three subadult harbour seals, Phoca vitulina, (two I -year-olds and one 2-year-old) were held at the University of Oslo with access to two swimming pools, one rectangular (3 x 8 m, 1.5 m deep) and one circular (2.5 m in diameter, 1.5 m deep), connected by dry haul out platforms. Normally the seals were fed herring (Clupea sp.) ad libitum once or twice a day. During two separate starvation periods, food was withheld for 12 and 16 days. In the refeeding period, they received kg herring twice a day for the first week; thereafter they were fed up to 2 kg twice a day (ad libitum). Body mass was determined at 2- or 3-day intervals to the nearest 0.1 kg using a spring balance (Salter). Body composition of the seals was determined three times during the starvation period by means of computed tomography (CT) (Skjervold et al. 1981) (Siemens Somatom 2 scanner). Four cross-sectional images were made, one each in Prinrcd In Canada I Irnprimc' au Canada

2 MARKUSSEN ET AL. DAYS FIG. 1. Changes in body mass (as a percentage of initial body mass) during starvation and refeeding of harbour seals. Solid symbols represent data for the three seals during 12 days' starvation; open symbols represent data from the same three seals during 16 days' starvation. Day 0 represent the onset of refeeding. the pelvic, abdominal, thoracic, and scapular regions. The numbers used to characterize the X-ray absorption in each component of the CT image are referred to as CT numbers. The circumference of each section was outlined with a graphic pen, and the values for the total cross-sectional area were determined. The area of subcutaneous fat in each scan was estimated using CT numbers for fat from to -50. The fat content in each cross section was determined as a percentage of the total area and the mean value of the four sections from each seal was used as a measure of the total fat content. This value was compared and calibrated against fat-content determinations from three dead seals whose fat content was measured. Homogenates of the whole seal (3-5 g) were hydrolyzed by boiling in 175 ml 3 M HCI for 90 min. After filtration the residue was dried under vacuum (20 mmhg172"c) overnight. Fat in the dried residue was extracted by diethyl ether for 3 h, using the Soxhlet extraction technique. The fat extract was dried under vacuum (20 mmhg172"c) overnight, and the fat content of the tissue homogenates was determined. This yielded the equation fat content (ether-extracted fat) = fat content (CT), The energy equivalent of mass loss is designated Q, which may be considered an expression of the proportion of mass taken from lean body and fat tissues during starvation. We estimated the energy equivalent of mass loss from CT data and by combining data on mass loss and maintenance energy requirements (Markussen er al. 1990). Postabsorptive metabolic rates were measured before starvation, during starvation, and during the refeeding period at 2- or 3-day intervals. Oxygen consumption was measured by the "blow-hole" tchnique. Seals were kept in the circular pool (diameter 2.5 m and depth 1.3 m) covered by a net. The animal was trained to breathe into a small Plexiglas hood. Expired gas was collected in Douglas bags and oxygen and carbon dioxide concentrations were determined by means of electrodes and a single-wavelength photometer (ABL300 Acid-Base Laboratory). The volume of the expired gas was measured by means of a Ritter Bochum Langendreer gas meter. Mean swimming speed was estimated by counting the number of laps in the pool during the gas sampling period and dividing by time. An extrapolation of metabolic rate to zero swimming speed represents "resting" metabolic rate and is similar to values for terrestrial mammals standing still (Schmidt-Nielsen 1972). Water temperature during the experiments ranged between 4 and 6 C in the first starvation period and between 8.5 and 11 C in the second. TABLE 1. Body mass (BM) and percent fat before and after starvation, energy equivalent of mass loss (Q), and percentage of energy taken from fat during starvation in harbour seals -- - Seal Before starvation After starvation Q % energy No. BM (kg) Fat (%) BM (kg) Fat (%) (MJIkg) from fat After 12 days' starvation After 16 days' starvation Differences in slope and elevation between the regression lines were tested with analysis of covariance. Results By pooling the data from all seals, relative mass loss during starvation may be described by a linear function: where T is the number of days of starvation and BM% is the body mass as a percentage of the initial body mass (Fig. 1). The total body mass loss during the starvation periods ranged from 5.0 to 7.6 kg. Mass gain during the refeeding period was 0.5 f 0.1 kg daily and initial body mass was reached about 2 weeks after the onset of refeeding (Fig. 1). Fat content ranged from 26.8 to 34.4% before starvation and decreased to between 23.1 and % after starvation (Table 1). The corresponding fat mass loss ranged from 1.6 to 4.0 kg. Assuming energy densities of 9.6 and 39 MJIkg for lean body and fat mass, respectively, the energy equivalent of the mass loss (Q) for the three seals ranged from 19.3 to 26.3 MJIkg during the starvation period (Table 1). This

3 CAN. J. ZOOL. VOL. 70, 1992 SWIMMING SPEED (mls) FIG. 2. Metabolic rate (W/kg0.75) of harbour seals at different swimming speeds (mls). Open sybols represent data from the three seals before starvation; solid symbols represent data from the same three seals during starvation. The regression lines may be described by the following equations: MR = ( ~)BM~.~~ (r2 = 0.82) for normally fed but postabsorptive seals and MR = ( ~.O~V)BM~.~~ (r2 = 0.87) for seals during starvation. implies that 67-87% (mean 77%) of the energy lost during starvation was derived from fat. For velocities up to 0.6 m/s, the metabolic rate (MR, in watts) increased linearly with swimming speed (v). Values for metabolic rate were pooled and may be expressed as and for normally fed and starved seals, respectively (Fig. 2). The slopes were not significantly different (F = 0.104). The elevation of the regression line for the starved animals was significantly lower than of fed animals (F = 32.5). There was no significant difference between the metabolic rate at 4-6 and at "C. The resting metabolic rate increased to prestarvation level after about 1 week of feeding (Fig. 3). Discussion Several questions about the dual and conflicting role of seal blubber, as an energy reservoir to be utilized during starvation and, at the same time, as an insulator against heat loss, remain unresolved. The present study indicates that the contribution of fat tissue catabolism to the total energy expenditure of subadult harbour seals is similar (77%) to that of harp seal pups (72-80 %) (Worthy and Lavigne 1987), but considerably lower than that of grey seal pups (94-97 %) (Nordoy and Blix 1985; Worthy and Lavigne 1987; Nordoy et al. 1990). The difference may be due to age, but may also indicate that the hypothesis of Worthy and Lavigne (1987) should be extended to subadults and adults; this hypothesis states that seal pups which usually spend the postweaning fast on land have a higher rate of blubber utilization than those fasting in cold water, because it is more expendable (Worthy and Lavigne 1987). Harbour seal pups take to the water almost immediately after birth and subadults and adults do not exhibit extended fasting periods ashore (King 1983). It may be worthwhile to note that the present study differs from most other fasting studies of seals in that this was a "forced" fast as opposed to a "normal" fast. Whether the physiology involved in these different kinds of fast differs is unknown at the present time. The result of the simultaneous energy contributions of fat and lean body tissues is conveniently expressed as the energy equivalent of mass loss, Q, which is a function of starvation time for humans (oritsland 1990), a function of the fat content for terrestrial mammals, and perhaps a function of blubber thickness for harp seals (oritsland and Markussen 1990). Q would range between the energy densities of fat and lean body mass. The starvation survival time is related to Q (oritsland and Markussen 1990) and thus it becomes important to obtain more information about the progression of Q during a period of starvation. Alternative calculations based on maintenance values published earlier (Markussen et al. 1990) and the present values for mass loss and metabolic depression during starvation indicated a lower Q value, but we consider the present CT-based values (Table 1) to be more reliable because they are derived from direct measurements. The difference may also be due to changes in the daily activity pattern during starvation. Such changes affect, but are not detected by, determinations of maintenance requirements based on food intake and body growth. Reduction of the basal metabolic rate is one of the most consistent results of undernutrition in humans (Grande et al. 1958), but is rarely expressed quantitatively (Markussen and Qritsland 1986). Most of the contradictory interpretations, and therefore conclusions, reported for marine mammals (Brodie and Pische 1982; Davydov 1984; Nordoy and Blix 1985; oritsland et al. 1985; Worthy and Lavigne 1987; Nordoy et al. 1990) may be explained by assuming that thermoregulatory needs for increased heat production will override and thus potentially mask metabolic depression. Although all true basal metabolic rate measurements are made in the thermoneutral zone, the lower critical temperature will change during starvation (Markussen and oritsland 1986). Metabolic depression might thus be missed if the measurements are made in water, but detected by measurements in air or in warm water. It has been widely accepted that the slope of the metabolic

4 MARKUSSEN ET AL. DAYS FIG. 3. Mean resting metabolic rate (W/kg0.75) in harbour seals during starvation and refeeding. Food was withdrawn on day - 15 and refeeding started on day 0. Resting values were determined by extrapolating the metabolism - swimming speed relationship to zero swimming speed. regression line for mammals is 0.75 (Kleiber 1975; Schmidt- Nielsen 1984) and metabolic rates of marine mammals scale in relation to body mass in a similar manner to those of terrestrial mammals (Lavigne et al ). We found that metabolic rate was about 20% lower in starving seals than in normal fed seals, and appeared to remain at the same level throughout the 2-week starvation period (Fig. 3). Although starvation caused a decrease in metabolic rate in the present study, we have chosen to relate metabolic rate to metabolic body size, i.e., body mass to the power Even if we express metabolic rate per unit of total body mass, the metabolic depression follows the same course. The present pattern of metabolic depression (Fig. 3) is different from patterns described in terrestrial mammals, where the metabolic rate declines throughout the starvation period and the metabolic depression factor is a function of mass loss (Grande et al. 1958; Markussen and oritsland 1986). Nordoy et al. (1990) recently suggested a pattern almost similar to ours for fasting grey seal pups. They found a rapid decrease in metabolic rate the first 2 weeks of fasting, while in our experiment the decrease occurred on the first day. After the initial drop, the metabolic rate remained stable throughout the fasting period. Reanalysis of their results, taking metabolic body size (kg0.75) into account, indicates a trend of further metabolic depression, i.e., a "terrestrial" trend. The differences in time until a metabolic rate reached a more or less stable level may be due to differences between normal fasting versus force-fasting seals. harbour seals), feeding level, or a natural versus a forced-fast situation. Davis et nl. (1985) measured the energy cost of swimming at velocities between 0.5 and 1.4 m/s and determined that metabolism increased curvilinearly with speed. Castellini et al. (1985) reported the metabolic rate of grey seals swimming at 1.25 m/s to be twice the resting rate. In the present experiment there was a linear relationship between swimming speed and metabolic rate for speeds up to 0.6 m/s. A higher energy cost of swimming is suggested by the present results than by the results from Davis et al. (1985). This may be due to the additional effect of the need to turn constantly in the circular pool. In conclusion, the present work suggests that starvation causes a depression of the basal metabolic rate in harbour seals, which is still evident during activity. The fact that the regression lines for swimming speed versus metabolic rate of both normal fed and starving seals are parallel supports our contention that the basal metabolic rate is depressed during starvation. The major part of this depression appears to be established during the first days of starvation, while recovery may progress at a slower rate. Both fat and lean body tissues play an important role as energy stores utilized during starvation in harbour seals. Acknowledgements - Fat content determinationsowere carried out at the Agricultural University of N ~ A~, N ~ ~ and ~ the ~ gas ~ analyses ~ ~ ~, Humans show an increase in metabolic rate with the onset at the ~~~~i~~~~ of ~ ~ ~ di i oslo, ~ ~ N i ~ ~ ~ to, whom i ~ ~ of feeding after a fast (Grande et al. 1958). The metabolic rate we are grateful. ~ h are ~ extended ~ to k D ~. D ~ M. ~~~i~~~ increases gradually during the recovery period, reaching the and an anonymous reviewer for helpful comments on the initial value after about 1 week of food intake (Grande ct al. manuscript. The work was financially supported by the N ~ ~ ). he metabolic rate increase in harp and grey seal PUPS wegian Fisheries Research Council (ProMare and Sea Mamreported by Worthy (1987) was seen immediately after the mal program). onset of feeding. This increase was not due to the heat increment of feeding3 because measurements were performed Brodie, P. F., and pische, A. J Density-dependent condition with animals in a postabsorptive state. The results presented and energetics of marine mammal population in multispecies fishhere suggest that the metabolic rate is still depressed 1 week eries management. ln ~ ~ l approaches ~ to i fisheries ~ man- ~ ~ ~ after the onset of feeding, and could be a way of saving energy agement advice. Edited by M. C. Mercer. Can. Spec. Publ. Fish. during the recovery period. These contradictory results may Aquat. Sci. No. 59. pp be due to species, age (harp and grey seal pups versus subadult Castellini, M. A., Murphy, B. J., Fedak, M., Ronald, K., Gofton, N.,

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