Milk intake and energy expenditure of free-ranging northern fur seal, Callorhinus ursinus, pups

Milk ingested by mammalian offspring, coupled with offspring's utilization of this energetic investment, influences survival and growth. A number of studies have examined milk intake in otariids, but few have examined milk intake over the entire lactation period, and none has independently measured energy expenditure concurrent with milk intake. We concurrently examined milk intake, field metabolic rate (FMR), and body composition of 41 pups over the entire lactation interval in 1995 and 1996 on St. Paul Island, Alaska. One hundred two metabolic measurements were obtained with isotope dilution methods. Mean milk intake did not differ annually but increased with age and mass, ranging from 3,400+/-239 to 6,780+/-449 (+/-SE) mL per suckling bout. Milk energy consumption did not vary with age on a mass-specific basis. No differences were detected in milk volume consumed by male and female pups, either absolutely or on a mass-specific basis. Mass-specific FMR peaked during molting, was lowest postmolt, and did not vary by sex. Pups in 1995 had lower FMR than pups in 1996 and were also fatter. Mean milk energy utilized for maintenance metabolism decreased over time from 77% to 43% in 1995 and remained at 71% in 1996. Pup body mass was negatively correlated with the percentage of total body water and positively correlated with the percentage of total body lipid (TBL). Pups increased the percentage of TBL from 16% to 37%. Northern fur seal pups increased energy intake over lactation, while concurrent changes in body composition and pelage condition resulted in mass-specific metabolic savings after the molt.     

Dynamic influence of maternal and pup traits on maternal care during lactation in an income breeder, the antarctic fur seal

Life-history theory predicts that selection will favor optimal levels of parental effort that balance benefits of current reproduction with costs to survival and future reproduction. The optimal level of effort depends on parental traits, offspring traits, and provisioning strategy. Additionally, how these factors influence effort may differ depending on the stage of reproduction. The relative importance of maternal and offspring traits on energy allocation to offspring was investigated in known-age Antarctic fur seals Arctocephalus gazella across four stages of reproduction, using birth mass and milk-consumption measurements. Maternal traits were important during three of the four stages investigated, with larger females giving birth to larger pups and investing more in pups during perinatal and molt stages. Pup mass influenced maternal effort during the premolt stage, and provisioning strategy influenced postnatal maternal effort at all stages. Energy provided to the offspring during an attendance visit was positively related to the duration of the foraging-trip/visit cycle; however, when investment was controlled for trip/visit cycle duration, the overall rate of energy transfer was similar across trip durations. In addition to strong effects of maternal mass, pup traits affected energy allocation, suggesting that pup demand is important in determining maternal care. These findings emphasize the importance of considering state variables in life-history studies and suggest that timing of measurements of effort in species with long provisioning periods may influence conclusions and our ability to make comparisons of reproductive effort among species.     

Factors affecting weaning weight in Antarctic fur seals Arctocephalus gazella at South Georgia

Summary Relationships between size, body condition, age and feeding-attendance patterns during pup rearing of female Antacrtic fur seals Arctocephalus gazella and their effects on the timing of birth and weaning, pup weight, growth and condition were studied at South Georgia in 1981–1982. Twenty-seven (6 male, 21 female) mother-pup pairs were followed from birth to weaning. The analysis of maternal effects was limited to female pups because of the small sample size of male weaners. High weaning weight was associated with those female pups whose mothers spent more time ashore attending their offspring. Weaning weight showed no relationship with perinatal duration, number of feeding trips to sea, days at sea or date of weaning. A further 63 mother-pup pairs were analysed for the effects of maternal body condition (weight/length), age and timing of birth on offspring body weight and condition. Pup weight and condition were weakly correlated with maternal age in female pups. Male pups born earlier in the season were heavier and in better condition. Maternal and offspring body weight and condition were unrelated. For the Antarctic fur seal population at South Georgia where the food supply was apparently not limiting in summer, maternal condition and foraging time were subordinate to maternal care on land (as expressed by attendance duration) in determining offspring weight at weaning.     

Biased estimates of fur seal pup mass: origins and implications

The mass of fur seal pups weighed in different years can be used to estimate growth rates or compared with one another to make inferences about the relative condition of a population. However, unless appropriate precautions are taken, many factors can bias estimates of pup mass and lead to incorrect conclusions. Using data collected from tagged and untagged northern fur seal pups ( Callorhinus ursinus ) at the Pribilof Islands, Alaska, I assess how milk consumption, the timing of sampling, and the effects of growth and sample size influence the size of pups captured for weighing. Evidence is presented suggesting that pup mass may increase in a sigmoid fashion, with the most rapid rate of growth occurring when about two months old. This phenomenon can confound efforts to compare the masses of pups weighed on different days in different years, particularly if pups are weighed over the period of rapid growth. Variability in pup mass increases with time because growth rates of individuals vary and because the amount of milk pups consume increases with body size. Thus sample sizes must be increased as the pups grow older in order to detect statistically significant differences in mean body mass. There is also evidence that pups of different ages and sizes are not randomly distributed on the breeding beaches and are not randomly selected for weighing. It appears that the first pups captured for weighing are smaller and younger than subsequent captures, possibly because smaller pups are easier to handle and are segregated to the peripheral rookery regions where sampling begins. These hidden biases, related to sampling error and fur seal biology, must be considered and controlled for when weighing fur seal pups.     

Temporal patterns of milk production in Antarctic fur seals (Arctocephalus gazella)

The timing of milk production in Antarctic fur seals was studied at Bird Island, South Georgia. Like all lactating otariid seals (Pinnipedia: Otariidae), Antarctic fur seal females alternate between short nursing periods ashore and regular foraging trips to sea. Females do not necessarily return to the colony with full mammae, which indicates that mammary volume capacity is unlikely to limit foraging trip duration. Upon arrival at the colony, milk fat (r²= 0.33, P < 0.04) and protein (r²= 0.60, P < 0.002) content were positively correlated to the time spent at sea. A similar trend was observed in the milk produced on land. The rate of milk energy production was much lower at sea (5.02 ± 0.05 MJ day^-1) than on land (23.66 ± 4.4 MJ^-1 day^-1). The rate of milk energy production during the foraging trip was negatively correlated to the time spent at sea (r²= 0.29, P < 0.05), whereas the rate of milk energy production on land was positively correlated (r²= 0.61, P < 0.001) to the duration of the preceding foraging trip. The total amount of milk energy delivered to the pup during each twoday nursing period was positively correlated (r²= 0.60, P < 0.002) to the duration of the previous foraging trip. The overall rate of milk energy delivery, however, was independent of foraging trip duration. This accords with previous observations that the growth rates of Antarctic fur seal pups are unaffected by maternal foraging trip duration patterns.     

Fasting metabolism in Antarctic fur seal (Arctocephalus gazella) pups.

The metabolism of 52-73-day old Antarctic fur seal pups from Bird Island, South Georgia, was investigated during fasting periods of normal duration while their mothers were at sea foraging. Body mass decreased exponentially with pups losing 3.5-3.8% of body mass per day. Resting metabolic rate also decreased exponentially from 172-197 ml (O2) x min(-1) at the beginning of the fast and scaled to M(b)(0.74) at 2.3 times the level predicted for adult terrestrial mammals of similar size. While there was no significant sex difference in RMR, female pups had significantly higher (F(1,18)=6.614, P<0.019) mass-specific RMR than male pups throughout the fasting period. Fasting FMR was also significantly (t(15)=2.37, P<0.035) greater in females (823 kJ x kg(-1) x d(-1)) than males (686 kJ x kg(-1) x d(-1)). Average protein turnover during the study period was 19.3 g x d(-1) and contributed to 5.4% of total energy expenditure, indicating the adoption of a protein-sparing strategy with a reliance on primarily lipid catabolism for metabolic energy. This is supported by observed decreases in plasma BUN, U/C, glucose and triglyceride concentrations, and an increase in beta-HBA concentration, indicating that Antarctic fur seals pups adopt this strategy within 2-3 days of fasting. Mean RQ also decreased from 0.77 to 0.72 within 3 days of fasting, further supporting a rapid commencement of protein-sparing. However, RQ gradually increased thereafter to 0.77, suggesting a resumption of protein catabolism which was not substantiated by changes in plasma metabolites. Female pups had higher TBL (%) than males for any given mass, which is consistent with previous findings in this and other fur seal species, and suggests sex differences in metabolic fuel use. The observed changes in plasma metabolites and protein turnover, however, do not support this.     

Ontogeny of aquatic behaviours in Antarctic fur seal (Arctocephalus gazella) pups in relation to growth performances at Kerguelen Islands

In diving marine predators, such as pinnipeds, the development of diving and foraging skills prior to weaning might be critical to post-weaning survival. Here, we examined the effect of pup mass growth on the amount of time devoted to aquatic activities and the dive performance of Antarctic fur seal, Arctocephalus gazella, pups on Kerguelen Island. Maternal attendance and mass-specific growth rate were assessed for 85 pups. Two types of monitoring were applied: visual observations of behaviours for 60 pups and the deployment of time-depth recorders (TDRs) on 19 female pups. At approximately 2 months of age, pups demonstrated minimal diving behaviour, but displayed considerable aquatic activity. While mothers were foraging at sea, pups fasted on land (6.0 ± 1.3 d). As the mass-specific growth rate was different between sexes, only data on female pups were analysed (n = 31). Mass-specific growth rate was related to maternal attendance patterns and impacted the amount of time allocated by pups to aquatic activities. The time spent in the water by pups was quadratically related to fasting progress. This study shows the importance of growth and fasting progress on the quantity of time pups devoted to aquatic activities. Our results suggest that greater post-weaning survival of heavier pups may be due not only to their greater body reserves, as reported in several studies, but also possibly to from their greater aquatic skills and physiological adaptations developed during the suckling period.     

Growth rates of Antarctic fur seal Arctocephalus gazella pups at South Georgia

Large numbers of pups of the Antarctic fur seal were weighed at regular intervals through their rearing period in six seasons between 1972 and 1981 at South Georgia. Growth rates and birth and weaning weights were estimated from linear regressions of mean weight on mean age. Female pups had significantly lower growth rates and weaning weights than males, but higher variances for all growth parameters. This may reflect the importance to males of polygynous species of attaining high body weight. Except for an inverse relation between growth rate and birth weight there were no systematic relationships between growth parameters. Seasonal differences in growth rates were substantial and there was no indication that the continuing rapid population increase was affecting pup growth rates. The use of these data in detecting seasonal variations in local food availability is discussed. Preliminary estimates of the energy costs of pup rearing indicate that pup existence metabolism consumes c. 80% and mass gain to weaning only c. 15% of the energy provided by the female.     

Field metabolic rate of lactating female Galápagos fur seals (Arctocephalus galapagoensis): the influence of offspring age and environment.

Galápagos fur seals, Arctocephalus galapagoensis, live in a seasonal environment which varies strongly in productivity from year to year. We measured how the field metabolic rates (FMR) of lactating females varied with season, pup age and year. Energy expenditure was measured using doubly labeled water (DLW) during the cold seasons of 1984 and 1985 in 9 mothers of 1-3-month-old pups and 5 mothers of yearlings, and during the 1986 warm season in 8 mothers of 6-month-old pups. Young pups gained 0.84% mass/day during the cold season, but larger pups during the warm season lost 1.25% mass/day. During the warm season, females had lower relative total body water than during the cold season suggesting higher fat content during the warm, less productive season, but the effect was even more marked when comparing different years of the study: fat content was high in 1984 and 1986 and low in 1985. The FMR of mothers varied from 134 to 167 W but did not show significant differences between any of the pup age-groups. Among the years of the study, FMR showed only a trend towards low energy expenditure of mothers of young pups in 1984. The mean FMR was lower than for other otariids. Mothers may limit energy expenditure independent of pup age and season to minimize their own risk of starvation in an environment of comparatively low productivity, varying unpredictably due to frequent El Ni?o events.     

Ecological implications of body composition and thermal capabilities in young antarctic fur seals (Arctocephalus gazella)

In comparison with other homeotherms, young recently weaned marine mammals in high latitudes face exceptional energetic demands when foraging and thermoregulating. Lipids are an important source of energy and a major component of insulation that allows them to meet these demands. To examine the role of lipid stores in a high-latitude pinniped, we measured the body composition and thermoregulatory capabilities of Antarctic fur seal (Arctocephalus gazella) pups and yearlings by using flow-through respirometry and hydrogen isotope dilution. From these data, we constructed a model to examine the importance of postweaning fasting capability in free-ranging young fur seals. Resting metabolic rates were different for pups and yearlings measured in 0.6 degrees C water, 10.3 degrees C water, and ambient air; however, mass and percent lipid as covariates accounted for the different metabolic responses in pups and yearlings for all treatments. The estimated lower critical temperature for combined pups and yearlings was 14.4 degrees C, 10 degrees -15 degrees C above water temperatures normally experienced by Antarctic fur seals. Modeling predicted that a weaned fur seal pup would survive at sea from 9.8 to 36.2 d before succumbing to starvation. The most likely maximum travel distance within this time constraint suggests that food resources close to the natal rookery are important to first-year survival for this species.     

Pup growth in the fur seals Arctocephalus tropicalis and A. gazella on Marion Island

Subantarctic fur seal Arctocephalus tropicalis and Antarctic fur seal A. gazella pups at Marion Island were weighed frequently until weaning, which occurs earlier in A. gazella (112 days) than in A. tropicalis (± 300 days). The mean birth weight of both species was the same (4·2 kg) and males grew faster than females. Arctocephalus gazella growth was linear to weaning and faster than A. tropicalis growth, which was linear to 120 days of age, slowing until 203 days of age, and thereafter losing weight but recovering perceptibly prior to weaning. Tagging had no apparent effect on pup growth. Faster growth in males than in females is part of the differential growth patterns which lead to adult sexual dimorphism. Arctocephalus gazella pup growth at Marion Island is faster than at South Georgia, indicating that conditions at this, their most northerly breeding locality, are not limiting for pup growth. The decrease in A. tropicalis pup body weight in July/August may result from either a scarcity of food in winter or weaning having been initiated. The two groups (polar and non-polar) of fur seals exhibit two strategies, which include differences in pup growth and suckling period; the polar species have a short suckling period and rapid pup growth and are predominantly pelagic over winter, while the more temperate species have a longer suckling period and slower pup growth and are less pelagic over winter.     

How mothers find their pups in a colony of Antarctic fur seals

In Antarctic fur seals, Arctocephalus gazella, mothers must identify their own young among hundreds or even thousands of pups, if they are to invest in their own offspring and avoid misdirecting their parental care. When returning to their breeding colony from a foraging trip of several days at sea, mothers have to find and identify their young before suckling can occur. There appears to be little confusion about which pup belongs to a mother, and adoption is absent or rare. Using behavioral observations, we investigated the means by which female Antarctic fur seals identified their pups in a breeding colony of about 750 mother-pup pairs on Kerguelen Island. We evaluated the importance of vision, scent communication, vocalizations, and rendezvous locations as possible explanations of how mothers find their pups. Every pup that a mother examined, whether her own or not, exchanged naso-nasal inspection with her, suggesting a strong role for olfactory communication in individual recognition. Both mothers and pups called to each other, and mothers that searched for pups over a longer period gave more calls and encountered more pups. Thus, vocalizations may have been used to attract pups that might be offspring. Nursing usually occurred in the same place from the end of one maternal visit to the colony and the arrival at the beginning of the next visit, suggesting that nursing locations may serve as a meeting place, or rendezvous, for mothers and pups. These results suggest that finding pups is a two-stage process for females, in which pups for sampling are attracted by calls or examined at the previous nursing location, and then individual identification is made by olfactory cues.     

GROWTH RATES OF ANTARCTIC FUR SEALS AS INDICES OF ENVIRONMENTAL CONDITIONS

The growth rates of Antarctic fur seal ( Arctocephalus gazella ) pups estimated from weighing cross-sections of the population were compared with measured/ inferred changes in the availability of their main prey species, Antarctic krill ( Euphausia superba ) from 1989 to 2000. There was no relationship between growth rate and mass at weaning and there were counter-intuitive indications of higher growth rates in years of low krill availability. Biases reflecting changes in the component of the population available for sampling appear to invalidate the widely held assumption that interannual differences in growth rate can reliably be derived from differences in the slope of a linear relationship based on cross-sectional population samples. A new index was developed, based on the deviation of pup mass at age in each year compared to the multiyear mean, that was not dependent on assumptions of linearity. The indices of growth deviates produced a more logical relationship with other indices of pup development and related more appropriately to variations in prey availability. The potential impact of methodological biases on the interpretation of growth rate suggests that comparisons of growth rates should not rely on assumptions regarding the underlying growth pattern.     

Fur seals at Macquarie Island: post-sealing colonisation,trends in abundance and hybridisation of three species

Commercial sealers exterminated the original fur seal population at Macquarie Island in the early 1800s. The first breeding record since the sealing era was not reported until March 1955. Three species of fur seal now occur at Macquarie Island, the Antarctic (Arctocephalus gazella), subantarctic (A. tropicalis) and New Zealand (A. forsteri) fur seal. Census data from 54 breeding seasons in the period 1954–2007 were used to estimate population status and growth for each species. Between the 1950s and 1970s, annual increases in pup production for the species aggregate were low. Between 1986 and 2007, pup production of Antarctic fur seals increased by about 8.8% per year and subantarctic fur seals by 6.8% per year. The New Zealand fur seal, although the most numerous fur seal species on Macquarie Island, has yet to establish a breeding population, due to the absence of reproductively mature females. Hybridisation among species is significant, but appears to be declining. The slow establishment and growth of fur seal populations on Macquarie Island appears to have been affected by its distance from major population centres and hence low immigration rates, asynchronous colonisation times of males and females of each species, and extensive hybridisation.     

Offspring kidnapping with subsequent shared nursing in Antarctic fur seals

Non-offspring maternal care should be common in phocids, but their occurrence would be uncommon among otariids due to the high costs of raising offspring, particularly lactation, and an efficient recognition system that allow for accurate recognition during the frequent mother–pup re-associations. However, non-offspring maternal care has been documented in some otariid species. While the phenomenon in general is not novel among the colonially breeding seals, the exclusive care to a single pup by two lactating females for an extended time is a behavior scarcely documented in natural population. In an extension of this allonursing care, we document the first case of the kidnapping of a pup with subsequent shared nursing in Antarctic fur seal including data on the effect of this interaction on the pup’s growth. While all other lactating females nursed exclusively their own pups, the shared nursing was advantageous for the pup because he grew noticeably larger (in weight and axillary girth) than other of his cohort, particularly after 50 days. This advantage would have been influenced by the asynchrony of the foraging cycle of the biological and foster mother, which resulted in a higher attendance on shore than any other male pups. Although several explanations have been hypothesized for allolactation in mammals, our observations suggest a misguided parental care, associated with recognition errors by the foster mother whose pup was stillborn.     

Observations on the infectivity of parasitic third-stage larvae of Uncinaria lucasi Stiles 1901 (Nematoda: Ancylostomatidae) of Northern fur seals, Callorhinus ursinus Linn., on St. Paul Island, Alaska.

Twelve fur seal pups, which had not nursed their mothers, were used in an infectivity experiment. Pups were exposed to parasitic 3rd-stage larvae of Uncinaria lucasi from belly tissues of fur seal bulls, bachelors, and pregnant cows, to determine maturation capability of the larvae. Hookworms were not recovered from the intestines of 3 pups receiving larvae from belly blubber of bulls, 6 pups receiving larvae from belly blubber of bachelors, and 1 nonexposed pup. Maturation of hookworms did occur in 2 pups exposed to larvae from a mixture of belly blubber, mammary tissue, and milk of pregnant cows. Parasitic 3rd-stage hookworm larvae from belly tissues of pregnant and "non-pregnant" fur seal cows averaged 938.1 and 802.1 micron long, and 34.1 and 31.5 micron wide, respectively; however, larvae from belly tissues of a fur seal bull, bachelors, 2-year-old males, male and female yearlings and pups, and Steller Sea Lion subadults averaged 640.5-732.0 micron long and 20.9-24.9 micron wide.     

Inter-sexual differences in Antarctic fur seal pup growth rates: evidence of environmental regulation?

We investigated the variation of Antarctic fur seal (Arctocephalus gazella) pup growth rates in response to sex, breeding season and duration of both maternal foraging trip and attendance bouts. Data were collected during five consecutive rearing seasons at Cape Shirreff, the most important breeding colony in the South Shetland Islands, Antarctica. Our results showed significant interannual and sexual variations in pup growth rates. Male pups grew significantly faster than female pups during 2000, 2001, 2002 and 2004 seasons, whilst during 2003 no difference was found. The interannual variation in pup growth rates was correlated with the interannual fluctuation in maternal foraging trip and attendance bouts. There was a significant effect of pup sex and maternal foraging trip duration on pup growth rates, which varied between years having foraging trip duration a major effect during 2003, when females spent more time at sea and interestingly on that year there were no sexual differences in pup growth rates. The effect of attendance bout on pup growth rates was not significant. Diet analysis showed that Antarctic krill (Euphausia superba) was the most frequent prey item during the study period. Analysis of krill size distribution showed a significant difference in krill length, during 2003, when A. gazella preyed upon the smallest sizes of krill. In this study, sex was the most important factor on pup growth rates, but when prey availability seemed more limited, there are longer foraging trips and shorter attendance bouts the sex factor became less significant.     

Trade-offs in Energy Allocation During Lactation

SYNOPSIS. During lactation, mothers require energy to meet bothmaternal and offspring requirements. If a mother exports toomuch energy to dependent offspring (in milk), her weight lossmay be excessive and maternal risk may increase. Conversely,too little energy allocation to offspring may reduce the growthrate or induce mortality of dependent offspring. This paradigmwas evaluated in cotton rats (Sigmodon hispidus) supportingsmall (3 pup) and large (6 pup) litters from early to late lactation.Several types of evidence indicate that physiological constraintslimit the ability of mothers with large litters to provide resourcesto offspring. Mothers with large litters produced a dilute,energy-poor milk and their rates of food intake, weight lossand energy export per litter appeared to approach physiologicalmaxima. Whereas the energy exported to pups in small littersincreased from early to late lactation, the energy flow perpup in large litters was consistently low; consequently, offspringin large litters had low growth rates. An increase in eithermaternal food intake or weight loss (catabolism of maternaltissue) could have provided additional energy to offset thelow growth rate of pups in large litters. However, mothers withlarge litters did not substantially increase their food intakeor weight loss compared with mothers supporting small litters.These results indicate that the maternal support of offspringin large litters is limited. The pattern of energy allocationshown by cotton rats with large litters likely reflects a compromisebetween meeting maternal and offspring energy requirements (cf.,Parker and Macnair, 1979). The energy flow is greater than optimalfor the parent but less than optimal for the offspring. Lessmaternal-offspring conflict occurs in small than large littersbecause offspring in small litters maintain a high growth rateat a relatively low maternal cost. Yet, under favorable environmentalconditions, the reduction in maternal-offspring conflict hasno apparent fitness benefit.     

Early-life density-dependence effects on growth and survival in subantarctic fur seals

Understanding the regulation of natural populations has been a long-standing research program in ecology. Current knowledge on marine mammals and seabirds is biased toward the adult component of populations and lacking are studies investigating the juvenile component. Our goal was to estimate demographic parameters on the pre-weaning stage of a subantarctic fur seal (Arctocephalus tropicalis) population on Amsterdam Island, suspected to be regulated by density-dependence. The influence of abundance on growth parameters (length and weight) and survival was assessed over a study period spanning 16 years. We evidenced a negative trend in population growth rate when density increased. Density-dependence models were favored for pup body size and mass growth. Abundance had a clear influence on body length at high population-density, pups grew slower and were smaller at weaning than pups born in years with low population density. Abundance partly explained pup body mass variation and a weak effect was detected on pre-weaning survival. The causal mechanisms may be increased competition for food resources between breeding females, leading to a reduction of maternal input to their pups. Our results suggested that pup favored survival over growth and the development of their diving abilities in order to withstand the extreme fasting periods that are characteristic of this fur seal population. This analysis provides significant insight of density-dependent processes on early-life demographic parameters of a long lived and top-predator species, and more specifically on the pre-weaning stage with important consequences for our understanding of individual long-term fitness and population dynamics.     

Fasting metabolism in Antarctic fur seal (Arctocephalus gazella) pups

The metabolism of 52–73-day old Antarctic fur seal pups from Bird Island, South Georgia, was investigated during fasting periods of normal duration while their mothers were at sea foraging. Body mass decreased exponentially with pups losing 3.5–3.8% of body mass per day. Resting metabolic rate also decreased exponentially from 172–197 ml (O₂)·min⁻¹ at the beginning of the fast and scaled to M_b^0.74 at 2.3 times the level predicted for adult terrestrial mammals of similar size. While there was no significant sex difference in RMR, female pups had significantly higher (F_1,18=6.614, P<0.019) mass-specific RMR than male pups throughout the fasting period. Fasting FMR was also significantly (t₁₅=2.37, P<0.035) greater in females (823 kJ·kg⁻¹·d⁻¹) than males (686 kJ·kg⁻¹·d⁻¹). Average protein turnover during the study period was 19.3 g·d⁻¹ and contributed to 5.4% of total energy expenditure, indicating the adoption of a protein-sparing strategy with a reliance on primarily lipid catabolism for metabolic energy. This is supported by observed decreases in plasma BUN, U/C, glucose and triglyceride concentrations, and an increase in β-HBA concentration, indicating that Antarctic fur seals pups adopt this strategy within 2–3 days of fasting. Mean RQ also decreased from 0.77 to 0.72 within 3 days of fasting, further supporting a rapid commencement of protein-sparing. However, RQ gradually increased thereafter to 0.77, suggesting a resumption of protein catabolism which was not substantiated by changes in plasma metabolites. Female pups had higher TBL (%) than males for any given mass, which is consistent with previous findings in this and other fur seal species, and suggests sex differences in metabolic fuel use. The observed changes in plasma metabolites and protein turnover, however, do not support this.