Lactation costs in southern elephant seals at King George Island, South Shetland Islands

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1 Polar Biol (2004) 27: DOI /s y ORIGINAL PAPER A. R. Carlini Æ M. E. I. Ma rquez Æ H. Panarello S. Ramdohr Æ G. A. Daneri Æ H. Bornemann Æ J. Plo tz Lactation costs in southern elephant seals at King George Island, South Shetland Islands Received: 19 June 2003 / Accepted: 28 November 2003 / Published online: 3 February 2004 Ó Springer-Verlag 2004 Abstract Labelled-water methodology was used to quantify energy costs and energy transfer efficiency in 18 mother-pup pairs of southern elephant seals (Mirounga leonina) during lactation. During the lactation period, mothers lost a mean mass of 227±47 kg. Mass loss included 22% of the protein, 60% of the fat, and 51% of the energy in the mother s body upon arrival. Total body-energy reserves at parturition explained 69% of the variation in the total lactation costs and 50% of the variation in the pup s body energy at weaning. On average, pups retained 48% of the mass, 49% of protein, 53% of fat and 51% of energy lost by their mothers. Greater, fatter females showed a decrease in the efficiency of energy and fat transfer and, at the same time, an increase in the efficiency of protein transfer. This may be due to an increased use of protein as metabolic fuel, as fat demands for milk production increase. There was no evidence that greater total lactation costs influence the ability of mothers to produce a pup in the next breeding season. A. R. Carlini (&) Æ M. E. I. Márquez Depto. de Ciencias Biolo gicas, Instituto Anta rtico Argentino, Cerrito 1248, (1010) Buenos Aires, Argentina acarlini@dna.gov.ar H. Panarello Instituto de Geocronologı a y Geologı a Isoto pica (INGEIS), Pabello n INGEIS, Ciudad Universitaria, (1428) Buenos Aires, Argentina S. Ramdohr Æ H. Bornemann Æ J. Plo tz Alfred-Wegener-Institut fu r Polar-und Meeresforschung, Bremerhaven, Germany G. A. Daneri Depto. de Mamı feros, Museo Argentino de Cs. Naturales B. Rivadavia, Av. Angel Gallardo 470, (1045) Buenos Aires, Argentina Introduction The growth and reproduction of an animal involve energy costs that go beyond those required for maintenance functions. Reproduction is energetically expensive and a current reproductive investment may affect future survival or fecundity (Clutton-Brock 1984). A key aspect of the reproductive biology of all pinnipeds is the spatial and/or temporal separation of feeding at sea and terrestrial parturition, which implies that the bulk of reproductive expenditure occurs in an environment in which animals cannot feed. Reproductive maternal costs include both energy expenditure on gestation (the costs of producing a foetus) and on lactation (the costs of nursing young until weaning), the latter representing a major proportion of the maternal reproductive effort (Costa 1993). Among pinnipeds, phocid seals, which present a relatively short lactation period characterised by maternal fasting and abrupt weaning when mothers depart to the sea, constitute an ideal system to study female reproductive costs. In common with other phocid pinnipeds, the pelagic life of southern elephant seals is intermitted diannually for reproduction and moult. During these periods ashore, the seals metabolism is almost exclusively catabolic. The seaborn energy reserves of a pregnant female coming ashore for parturition must support the energetic costs of both its maintenance metabolism and milk production. Female arrival mass in southern elephant seals varies between 300 and 900 kg (Fedak et al. 1994). Hence, female size will influence the quantity of materials to be transferred to the pup but, at the same time, size will also influence the mother s absolute maintenance costs. Moreover, females with a similar body mass but higher proportions of lean tissue could have different metabolic costs from proportionally fatter females. The effect of maternal mass on the weaning mass of southern elephant seal pups has been reported for several breeding sites (McCann et al. 1989; Campagna et al. 1992; Fedak et al. 1994; Arnbom et al. 1997; Carlini et al.

2 ), while the influence of maternal body reserves upon arrival on their use during lactation has been informed for South Georgia (Fedak et al. 1996). However, no studies of southern elephant seals provide information on changes in body composition of mother-pup pairs during lactation. The breeding population at King George Island is located in the southernmost distribution range for the species. Recent research at that colony (Burton et al. 1997; Carlini et al. 1997) reports that females coming ashore to breed are heavier than those from other colonies and wean heavier pups. The aims of the present study were to examine total energy lactation costs in relation to mother energy reserves at parturition, and how these influenced pup growth and energy transfer efficiency. We also examined the effects of total maternal energy investment during lactation on the next breeding season. liquid-nitrogen freezing, and the water was separated from the serum by sublimation under vacuum. HDO concentration was measured at the INGEIS laboratories on hydrogen obtained by Zn reduction at 500 C from a subsample (5 ll) of the water obtained, according to the technique described in Coleman et al. (1982). Isotope ratios were determined with a gas-isotope-ratio mass spectrometer (Finningan Delta-s, McKinney). Total body water determination Total body water (TBW) was calculated from the dilution of D 2 O measured in the blood sample after a 4-h equilibration period, according to the equations provided by Schoeller et al. (1980). Measuring TBW by isotope dilution overestimates water mass, because some isotope atoms become incorporated into body tissues and thereby give an erroneously high estimate of TBW (Nagy and Costa 1980). Reilly and Fedak (1990) gave this overestimate as 2.8±0.9% with D 2 O for grey seals, and estimates of TBW were therefore corrected using their regression equation. Materials and methods Study area and sampling protocol The study was conducted at Stranger Point, King George Island (62 14 S, W) during the 1995 and 1996 breeding seasons. In 1995, 18 females were randomly selected on parturition day and immobilised with ketamine hydrochloride using an intramuscular dose of mg/kg body weight. After immobilisation, standard length and girth were measured and females were hot-iron branded. Mothers were weighed in a net stretcher suspended from a load cell (Challenger AZM, 1,000±0.5 kg) attached to an aluminium tripod, and a blood sample (20 ml) was taken to measure background levels of deuterium oxide. Then a preweighed dose (0.043±0.013 g/kg) of deuterium oxide (D 2 O, 99.8% purity) was administered via intramuscular injection. After equilibration of the injected deuterium for 3 4 h (Costa 1987), females were recaptured and a second blood sample was taken. Near the end of lactation, mothers were recaptured, weighed and a blood sample was taken to measure D 2 O levels. Final total body water was then obtained by reinjection of D 2 O (0.042±0.009 g/kg), followed by blood sampling 4 h later. The period measured between the first and last procedure during lactation (19.3±1.0 days, range 17 21) was shorter than the entire suckling period (22.0±2.3 days, range 17 27). We estimated the additional mass loss by females from the last weighing to the end of lactation as in previous work (Carlini et al. 1999). The composition of this mass loss was assumed to be similar to that effectively measured between the two sampling points during lactation, which encompassed % (mean 88±8%) of the lactation period. Pups were weighed, and the procedure to measure total body water was conducted on the same days as for their mothers. The deuterium doses injected were 0.074±0.016 g/kg at the beginning and 0.035±0.07 g/kg near the end of lactation. Additionally, pups were sampled at weaning when the injected deuterium dose was 0.037±0.07 g/kg. During the 1996 breeding season, daily checks were made to search for marked females. Eight of the 18 females marked in 1995 returned to Stranger Point for breeding. On parturition day, animals were immobilised, weighed and total body water was measured as described above. Blood sample analysis All blood samples were left to clot in sealed vials for 8 h, then centrifuged and the serum drawn off by pipette and stored in airtight vials at )14 C. Subsequently, the samples were thawed before Body composition Body composition was calculated from total mass and TBW to give total body fat, total body protein, and total body gross energy (TBGE), using equations provided by Reilly and Fedak (1990) derived from analyses of whole-body chemical composition of grey seals. Calculation of TBGE assumed energy densities of 39.5 kj/g for fat and 23.5 kj/g for protein (Reilly and Fedak 1990). Statistical analyses Values are given as means±sd except where otherwise indicated, and results were considered to be significant at the P<0.05 level. All percentage data were arcsine-transformed before statistical testing. The Kolmogorov-Smirnov test was used to determine whether data were normally distributed, and the Levene median test was employed to confirm homogeneity of variances. Results Maternal mass and use of body materials Data on mass and body-composition changes for mother-pup pairs are summarised in Tables 1 and 2. There was a large range in maternal mass ( kg), as well as in total-body energy reserves, at the beginning of lactation (5,542 14,623 MJ). Maternal post-partum mass was related to total-body energy reserves (Fig. 1A). In order to analyse whether female body composition upon arrival (fat percentage) changed with post-partum mass, total body fat (kg) upon arrival (TBF) and post-partum mass (Mp) were transformed into logarithms and regressed (log TBF=) log Mp, F 1,16 =320, P<0.001, r 2 =0.95). The slope of this relationship (1.21) was different from 1.0 (t=3.12, P<0.01, df=16), which indicates that larger mothers tended to be fatter than smaller ones at the beginning of lactation (Fig. 1B). We estimated that females lost a mean of 227 kg during lactation, comprising 51% fat, 11% protein and 37% water, with an average energy density of mass loss

3 268 Table 1 Changes in mass and body composition in female southern elephant seals over the suckling period Females bearing male pups (n=9) Females bearing female pups (n=9) t P Data combined (n=18) a Pup mass gain/female mass loss b Pup energy gain/female energy loss c Pup fat gain/female fat loss d Pup protein gain/female protein loss Fat upon arrival (%) 29.2± ±2.6 )0.62 > ±2.4 Arrival mass (kg) 641± ±152 )0.34 > ±131 Mass loss over lactation (kg) 235±50 219± > ±47 Mass loss per day (kg/day) 10.2± ±1.9 )0.001 > ±1.7 Mass loss over lactation (%) 36.6± ± < ±3.1 Fat at parturition (kg) 188±38 200±59 )0.53 > ±49 Fat used over lactation (kg) 124±34 109± > ±36 Fat used over lactation (%) 66.0± ± < ±10.8 Protein at parturition (kg) 109±19 111±23 )0.18 > ±20 Protein used over lactation (kg) 24.1± ±7.7 )0.05 > ±8.1 Protein used over lactation (%) 22.0± ±6.1 )0.09 > ±6.0 Energy reserves at parturition (MJ) 10064± ± > ±2382 Energy used over lactation (MJ) 5527± ± > ±1424 Energy used over lactation (%) 54.8± ± < ±7.3 Energy density of mass loss (MJ/kg) 23.3± ± > ±3.3 Lactation duration (days) 22.9± ± > ±2.2 Efficiency of mass transfer a 0.48± ± > ±0.04 Efficiency of energy transfer b 0.49± ±0.13 )1.00 > ±0.10 Efficiency of fat transfer c 0.49± ±0.17 )1.11 > ±0.14 Efficiency of protein transfer d 0.54± ± > ±0.20 of 22.7 MJ/kg (Table 1). Overall, energy expenditure during lactation represented an average of 51% of the total energy stores available at parturition, comprising 60% stored fat and 22% body protein (Table 1). Figure 2 shows absolute energy, fat and protein lost during lactation in relation to the amounts present in females after giving birth. Body energy reserves at parturition explained much of the variation in the total energy costs (r 2 =0.69, P<0.001; Fig. 2A). There was no difference in the slope or intercept of this relationship between mothers of male or female pups (ANCOVA, P>0.1). The total amount of fat utilised during lactation was related to the amount of fat present in the mother s body at parturition (r 2 =0.67, P<0.001; Fig. 2B). The relationship did not differ in the slope or intercept between mothers of male or female pups (ANCOVA, P>0.1). Although significant, the relation for protein varied considerably (r 2 =0.26, P<0.05; Fig. 2C), and was not significant when data for females bearing males or females were treated separately (P>0.05 both cases). However, higher initial body energy, fat or protein reserves were not related to proportionally greater losses (P>0.5 in all cases), although mothers of male pups lost a higher proportion of their initial body energy reserves and body fat than mothers of female pups (54.8±7.5 vs 42.3±4.1% and 66.0±10.9 vs 54.1±6.8, respectively, t-test P<0.05 both cases). This could be a result of having a slightly, although not significantly, longer lactation period (22.9±2.3 vs 21.4±2.1 days, females bearing males and females, respectively) (Table 1). Loss of body materials in relation to initial body condition Although initial mass or total energy reserves were not related to type of tissue loss, fatter females upon arrival tended to lose more fat per kilogramme lost, and therefore an energetically richer kilogramme than thinner females (r 2 =0.39, P<0.01, Fig. 3A). This did not result in a higher daily percentage of depletion of their initial fat pool (P>0.1). However, fatter females depleted their protein reserves (expressed as a percentage of their initial protein pool) at lower rates than thinner ones (r 2 =0.42, P<0.01, Fig. 3B), using a smaller pro- Table 2 Changes in mass and body composition in pup southern elephant seals over the suckling period Male pups (n=9) Female pups (n=9) t P Data combined (n=18) Birth mass (kg) 49.7± ± > ±7.4 Mass gain (kg) 114.2± ± > ±21.9 Weaning mass (kg) 163.9± ± > ±27.3 Fat gain (kg) 59.7± ± > ±12.8 Protein gain (kg) 11.9± ± > ±2.8 Energy gain (MJ) 2663± ± > ±545 Energy reserves at weaning (MJ) 2997± ± > ±562 Energy density of mass gain 23.3± ±1.9 )1.56 > ±1.7 Fat at weaning (%) 36.8± ±4.8 )1.39 > ±4.1

4 269 Fig. 1 A Relationship between total body mass (M) and total body energy reserves (TBE) at parturition. TBE=)1, M; F 1,16 =444, P<0.001, r 2 =0.97. B Total body fat (%) upon arrival (F%) plotted as a function of post-partum mass (M) portion of their initial protein pool (r 2 =0.23, P<0.05) by the end of lactation. There was also a positive relationship between body condition (fat percentage) and lactation duration (r 2 =0.34, P<0.05). Pup mass and body composition changes Overall pup mass at birth was 47.8±7.4 kg and did not differ significantly between sexes. During the suckling period, pups gained a mean of 108 kg, constituted on average by 10% protein, 54% fat and 35% water, which represented an increase of 2,580 MJ in their body energy reserves (Table 2). There were no significant differences between male and female pups in total mass gain or in the body constituents (fat and protein) of this mass. Average energy density of mass gain was 23.9±1.7 MJ/ kg, n=18 (Table 2). As expected, an increase in total mass gain (kg) by pups (x) was accompanied by an increase in total fat deposition (y= *x, F 1,16 =62.3, P<0.001, r 2 =0.79), as well as in total protein deposition (y=) *x, F 1,16 =30.1, P<0.001, r 2 =0.65). Overall, the fat to protein ratio in mass gain was variable (5.7±1.6, n=18) and was not Fig. 2 A Total energy used by mothers (EU) plotted as a function of energy reserves upon arrival (TBE). EU= TBE, F 1,16 =37.3, P<0.001, r 2 =0.70. B Total fat used by mothers (FU) plotted as a function of fat reserves upon arrival (FR). FU= FR, F 1,16 =30, P<0.001, r 2 =0.65. C Total protein used by mothers (PU) plotted as a function of protein reserves upon arrival (PR). PU= PR, F 1,16 =5.1, P<0.05, r 2 =0.24 related to pup sex (t-test, P>0.05, df=16), total mass gain (F 1,16 =0.12, P>0.5, r 2 =0.0), length of lactation (F 1,16 =0.01, P>0.5, r 2 =0.0) or total energy reserves at weaning (F 1,16 =1.2, P>0.3, r 2 =0.1). In order to analyse whether pup condition at weaning (fat percentage) changed with weaning mass, total body fat (kg) at weaning (BFW) and weaning mass (Wm) were

5 270 Fig. 3 A Energy density of mass loss (ED) in relation to total body fat (%) upon arrival (F%). ED=) F%, F 1,16 =10.3, P<0.01, r 2 =0.39. B Daily percentage of protein used (DPP) in relation to total body fat (%) upon arrival (F%). DPP= F%, F 1,16 =11.6, P<0.01, r 2 =0.42 transformed into logarithms and regressed (log BFW=) log Wm, F 1,16 =53.1, P<0.001, r 2 =0.79). The slope of this relationship (1.04) was not significantly different from 1.0 (t=0.33, P>0.05, df=16), which indicates that fat percentage was independent of body mass at weaning. Energy and materials transfer efficiency Pups gained a mean of 49% of the protein, 53% of the fat and 51% of the energy lost by their mothers. There was no significant sex difference in the proportion of body constituents gained by pups in relation to maternal loss (Table 1). The differences in female initial totalbody energy reserves explained much of the variation in the total energy reserved by pups during lactation (F 1,16 =12.3, P<0.01, r 2 =0.43), as well as in total pup body energy at weaning (F 1,16 =15.8, P<0.01, r 2 =0.50). These relationships did not differ with pup sex (AN- COVA, P>0.1, both cases). Absolute maternal mass loss was related to pup mass gain over lactation (F 1,16 =87.9, P<0.001, r 2 =0.85). As expected, the Fig. 4 A Relationship between total energy gained by pups (EGP) and total energy used by their mothers (EU) during the lactation period. EGP= EU, F 1,16 =12, P<0.01, r 2 =0.43. B Relationship between total fat gained by pups (FGP) and total fat used by their mothers (FU) during the lactation period. FGP= FU, F 1,16 =5.3, P<0.05, r 2 =0.25. C Relationship between total protein gained by pups and total protein used by their mothers during lactation absolute amount of energy or fat lost by mothers was positively related to total energy or fat gained by their pups (r 2 =0.42, P<0.05; r 2 =0.25, P<0.05, respectively) (Fig. 4A,B). However, greater maternal losses were also related to the lower efficiency (measured as the ratio of pup gain to female loss) of this transfer (Fig. 5A,B).

6 271 Fig. 5 A Energy transfer efficiency (EE) (pup energy gain/female energy loss) plotted as a function of the mother s energy use during lactation (EU). B Fat transfer efficiency (FE) (pup fat gain/female fat loss) plotted as a function of the mother s fat use (FU) during lactation. C Protein transfer efficiency (PE) (pup protein gain/ female protein loss) plotted as a function of the mother s protein use (PU) during lactation To examine these relationships, total-body energy spent by mothers (TBGEs) (MJ), total body fat spent by mothers (TBFs) (kg), total body energy gained by pups (TBGP) (MJ) and total body fat gained by pups (TBFP) (kg) were transformed into logarithms and regressed (log TBGP= log TBGEs, F 1,16 =16.3, P<0.001, Fig. 6 A Energy transfer efficiency (EE) (pup energy gain/female energy loss) plotted as a function of the mother s total body fat (expressed as percentage) upon arrival (F%). EE= F%, F 1,16 =8.1, P<0.05, r 2 =0.33. B Fat transfer efficiency (FE) (pup fat gain/female fat loss) plotted as a function of the mother s total body fat (expressed as percentage) upon arrival (F%). FE= F%, F 1,16 =7.6, P<0.05, r 2 =0.32. C Protein transfer efficiency (PE) (pup protein gain/female protein loss) plotted as a function of the mother s total body fat (expressed as percentage) upon arrival (F%). PE=) F%, F 1,16 =5.6, P<0.05, r 2 =0.26 r 2 =0.50; TBFP= log TBFs, F 1,16 =7.9, P<0.05, r 2 =0.33, respectively). The slopes of 0.54 and 0.4 were both different from 1.0 (t=3.50, P<0.01, df=16, and t=4.25, P<0.001, df=16), which indicates

7 272 that greater absolute losses were related to lower transfer efficiency (Fig. 5A,B). However, an increase in total protein loss by females was not related to an increase in the protein reserves by their pups. As a consequence, protein transfer efficiency decreased as total protein loss by females increased (Figs. 4C, 5C). Although transfer efficiency (as defined above) was not significantly related to initial mass or absolute body reserves upon arrival, fatter animals upon arrival were able to transfer a greater proportion of their body protein loss to their pups than thinner ones, while at the same time they showed a decreased efficiency to transfer fat and energy (Fig. 6A C). Discussion At King George Island, female arrival mass was positively related to total body energy reserves (Fig. 1A). This is not unexpected because of the wide range in maternal mass after giving birth. However, larger females were also proportionately fatter than smaller ones (Fig. 1B). Thus, in our study, greater arrival mass seems to give females both an absolute and a relative advantage in relation to the quantity of energy available to cover lactation costs. During lactation, females lost an average of 35% of their initial mass. This loss included 22% of the protein, 60% of the fat and 51% of the energy in the mother s body at parturition. The percentage of mass loss, as well as the composition of that loss (Table 1), were similar to those reported by Fedak et al. (1996) for the South Georgia population. However, in absolute terms, and in relation to the greater average arrival mass at King George Island (Burton et al. 1997; Carlini et al. 1997), total body energy reserves upon arrival and pup mass at weaning (and presumably energy reserves) were higher in our study than those reported at South Georgia. The use of body materials as a proportion of those present at the beginning of lactation in female southern elephant seals was studied by Fedak et al. (1996), who did not find a clear overall trend in the proportion of protein, fat or energy used during lactation in relation to the absolute amounts present at parturition. Our data confirm those reported for the South Georgia population. Females showed a wide variation in the proportion of materials employed in relation to those present at the beginning of lactation, although females with higher absolute reserves spent greater absolute quantities of energy and materials (Fig. 2). However, there was a relationship between the type of tissue lost during lactation and that available upon female arrival. Fatter mothers at the beginning of lactation tended to lose more fat per kilogramme lost than thinner ones (Fig. 3A). This did not result in a higher daily percentage of depletion of their initial fat pool (P>0.1). However, the differences in the body materials lost allowed fatter females to spare protein by depleting their protein reserves (expressed as a percentage of their initial protein pool) at lower rates than thinner ones (Fig. 3B), which by the end of lactation resulted in the use of a lower proportion of their initial protein pool. The ability of fatter animals to spare protein, yet fasting longer than thinner animals, suggests that initial body composition is important in determining the ability to maintain fasting. Protein reserves and their use during lactation could act as a threshold level of female departure, although the relation should not be a simple one, since no significant negative relationship at P=0.05 level was found between daily percentage of protein use and lactation duration (r 2 =0.17, P=0.09). In the northern congener, Crocker et al. (2001) found a relationship between the proportion of lean tissue loss and body condition, with fatter females losing proportionally less lean tissue over lactation. The authors suggested that body composition may serve as a proximate mechanism to initiate weaning by impacting rates of lean-tissue catabolism, allowing the termination of investment prior to levels that carry fitness consequences. Maternal costs related to pup sex In polygynous mammals, mothers are predicted to maximise their own reproductive success by investing more in male than female pups. This could be done by investing more resources throughout lactation or by biasing the sex ratio of offspring depending on the mother s availability of resources (Trivers and Willard 1973; Maynard-Smith 1980). Previous studies on the southern elephant seal, as well as on its northern congener, have shown that, after birth, there were no differences in maternal investment in relation to pup sex (Le Boeuf et al. 1989; McCann et al. 1989; Campagna et al. 1992; Kretzmann et al. 1993; Deutsch et al. 1994; Fedak et al. 1994, 1996; Arnbom et al. 1997; Wilkinson and van Aarde 2001). In our study, mothers giving birth to male or female pups were similar in relation to their initial condition and total body energy reserves. They lost mass at similar rates, with no significant differences in the energy content of the mass lost (Table 1). Moreover, their pups gained energy with no significant sex differences (Table 2). However, mothers giving birth to males made a proportionally greater effort than those giving birth to females, losing a significantly higher proportion of their initial mass (36.7±3.4% vs 33.2±1.5%; t-test, P<0.05) and energy reserves (Table 1). This seems to be a result of having a longer, although not significantly so, lactation period, which is partially supported by the fact that females bearing male or female pups lose mass and energy at not significantly different daily proportional rates (1.6±0.1% vs 1.6±0.2% for mass; 2.4±0.3% vs 2.2±0.3% for energy, t-test, P>0.05, both cases). Since in other studies on southern elephant seals with a much larger number of observations, mothers of male and female pups showed similar lactation periods and proportions of mass loss

8 273 (Fedak et al. 1994; Arnbom et al. 1997; Wilkinson and van Aarde 2001), we interpret our result, considering our limited set of data, as probably due to chance. However, further studies of body-composition changes for both mothers and their offspring are needed before a definite conclusion can be drawn. Energy reserves at parturition and pup growth Southern elephant-seal females fast during lactation, so both pup growth and maintenance costs must be supported from female reserves. In relation to this, the total amount of energy that mothers spend during lactation will be related to that available when they come ashore to breed. Body-energy reserves at parturition accounted for 69% of the variation in total lactation costs (Fig. 2A). Pup growth depended mainly on maternal energy reserves upon arrival, which explained 45% of the variation in pup energy gain during lactation and 52% of the variation in pup energy reserves at weaning. Female size and absolute reserves at parturition were found to be the most important factors in determining pup energy gain in large phocid seals (e.g. Costa et al. 1986; Tedman and Green 1987; Iverson et al. 1993; Fedak et al. 1996) and less important in small phocids like harbour seals, which feed during lactation (Bowen et al. 2001a, 2001b). An increased transfer of resources by mothers during lactation could be of importance in determining subsequent pup survival and, therefore, the future reproductive success of their offspring (Arnbom et al. 1993). Although, in the northern elephant seal, Le Boeuf et al. (1994) showed that mass at weaning was not related to survival, recent studies on southern elephant seals showed that heavier weanlings had higher survival rates during the 1st year of life than lighter ones (McMahon et al. 2000), while body condition at weaning affected the 1st year of survival in grey seals (Hall et al. 2001). The use of female body materials in relation to transfer efficiency Greater sizes will provide mothers, not only with greater absolute reserves, but also with proportionally larger energy stores, which could be used to produce milk because of their lower mass-specific metabolism (Costa 1991, 1993). If, under natural conditions, there was no limit to the conversion of milk into reserves by pups or to the production of milk from body reserves by their mothers, larger females would be expected to pass energy and nutrients more efficiently than smaller ones, since they would have a relatively lower maintenance metabolism (their metabolic overhead, Fedak and Anderson 1982) for equivalent lactation periods. This, however, was not the case at King George Island. The energy reserves of mothers upon arrival showed an opposite (although not significant) tendency to that predicted (r 2 =0.20, P=0.056), with mothers with greater initial body energy reserves being somewhat less efficient. Moreover, energy-transfer efficiency varied, especially in relation to the quantity of material lost by females. Greater absolute energy loss was related to greater absolute energy gain by their pups, which indicated that greater losses were at least partially explained by the quantity of nutrients given to the pups (Fig. 4A). However, the efficiency of this transfer decreased as total female energy loss increased (Fig. 5A). This indicates that only a fraction of the greater effort made by the mothers could be transferred to their pups, causing females to use a greater proportion of their total energy expenditure to supply their own metabolism as their size and total energy reserves after parturition increased. When the use of body materials was analysed separately, similar results were found, with efficiency decreasing as total fat or protein loss of females increased (Fig. 5B,C). In the northern elephant seal, a similar trend was observed by Deutsch et al. (1994), with larger mothers being somewhat less efficient in mass transfer. However, in a more recent publication, Crocker et al. (2001) found that age is the most important factor influencing the efficiency of lactation, showing that metabolic overhead (as a proportion of total energy expenditure) tends to be reduced as maternal age and mass increase. The influence of body condition upon arrival on transfer efficiency As already indicated, fatter females upon arrival tend to lose an energetically richer kilogramme, constituted by a greater proportion of fat versus protein. However, their pups did not gain a richer kilogramme and they were not fatter at weaning either (P>0.1 both cases). This caused a decrease in energy and fat transfer efficiency, as female fat percentage upon arrival increased (Fig. 6A,B). However, fatter females seemed to be more efficient in protein transfer efficiency (Fig. 6C). This suggests that the main differences in the materials used during the lactation period between fatter and thinner animals upon arrival are more related to the type of fuel used by females for metabolic purposes than to the materials given via milk to their pups. Although long-term fasting is characterised by protein sparing, with the bulk of the energy demands being supplied by the oxidation of fatty acids (Worthy and Lavigne 1987; Adams and Costa 1993), it has been shown that the ability to spare protein during fasting depends on the continued availability of lipid fuels (Lowell and Goodman 1987). In birds, fatter individuals at the beginning of a fast are able to achieve and maintain a lower rate of protein catabolism than leaner animals (Cherel et al. 1992). Moreover, for phocid seals, differences in the use of fuel for metabolic purposes were demonstrated in the northern elephant seal by Crocker et al. (1998), who found an increase in protein catabolism in females from mid- to late lactation and a relationship between protein contribution to

9 274 metabolism and body composition. One way to view data from Stranger Point is that mothers below certain fat levels (as a proportion of their initial body-fat reserves) begin to draw in protein reserves, or they do it at faster rates, as metabolic fuels. This would allow fatter females to use protein almost exclusively to give to their pups via milk, allowing them a higher efficiency in protein transfer but, at the same time, they would use relatively more fat to account for their metabolic demands, which would cause a decrease in the energy and fat transfer efficiency when total female losses versus pup gains are taken into account. What could determine such a physiological change? If, to produce a healthy pup, the overall mean energy density of milk must be maintained throughout lactation among different mothers with some independence of female body condition upon arrival, thinner animals could be forced to use part of their body protein as metabolic fuels as lactation progresses. This progress coincides with the production of an energetically richer milk in the southern elephant seal (Carlini et al. 1994; Hindell et al. 1994). Lactating females would act as buffers, giving a roughly similar milk quality, thus changing the rate of protein utilised for metabolism in relation to fat availability for milk production. Since fatter animals were also able to fast longer than thinner ones (r 2 =0.34, P<0.05), probably part of their lower energy-transfer efficiency was also related to the lower pup-growth rates that are characteristic of southern elephant-seal pups during the last part of the lactation period (McCann et al. 1989; Fedak et al. 1994; Carlini et al. 1997). During this period, when milk energy transfer would be lower than in mid-lactation, the mother s maintenance costs would be roughly maintained. Another possible explanation of the differential use of materials observed between fatter and thinner females (Fig. 3) is that they are given different milk qualities. In subantarctic fur seals, Arctocephalus tropicalis, mothers in good condition produce a milk with a higher lipid content, suggesting that individual foraging skills contribute to enhance milk quality (Georges et al. 2001). This possibility cannot be tested by means of a direct approach in our study because milk composition was not measured. However, the lactation pattern for otariids, in which intermittent suckling and feeding at sea is the rule, differs from that of phocid seals, which includes fasting as an integral part (Costa 1991). Milk composition could therefore be less constrained, since lactation costs are spread over several months at sea (Costa et al. 1986). Moreover, in our study, greater females were also fatter. So they would be expected to be potentially able to produce richer milk than thinner ones. If so, they might to produce fatter weanlings. This would be the result of two main occurrences: (1) pups receiving an energetically richer milk, and/or (2) pups receiving more milk. In the latter case, since southern elephant-seal pups are born with very low fat reserves (Hindell et al. 1994; Carlini et al. 2000), it is expected that fat percentage at weaning should increase as mass and energy gain increase, reaching a theoretical maximum point near maximum fat percentage in milk. In making this assumption, we did not consider pup metabolism, which would also be greater for greater animals. However, if smaller and greater pups used a similar fuel for metabolism and this fuel was, as is usually assumed for phocid seals, mainly fat, differences in body condition at weaning must be even greater, because metabolism scales to 0.75 of body mass. The absence of a relation in our study between female condition upon arrival and the energy density of pup mass gain or pup weaning condition, suggests that differences found in transfer efficiency are mainly due to the materials used by females for metabolism, which are in turn related to their initial condition upon arrival, although differences in nutrient assimilation among pups cannot be discarded. The influence of total lactation costs in the next breeding season Since female southern elephant seals make an important energetic effort during lactation, it is possible that they are near a limit, which over-stepped, might compromise their future survival or reduce their subsequent reproductive success (Fedak et al. 1996; Arnbom et al. 1997). At Stranger Point, 8 of the 18 females returned during the next breeding season and were able to breed successfully. Neither of the groups was significantly different in relation to the total energy spent in absolute or in relative terms (Table 3). Moreover, the extreme cases in the quantity of energy and materials lost were found in the group that returned during the next breeding season. If the absence of females at Stranger Point means that they did not survive or breed in 1996, the success during the post-breeding and post-moulting aquatic phase might have, per se, more importance for subsequent successful breeding than the total lactation cost incurred by a particular female during a given breeding season. In conclusion, at King George Island, the variability in pup reserves at weaning was correlated to the energy lost by their mothers, which was in turn accounted for Table 3 Comparison of total lactation costs during the 1995 breeding season, for two female groups: A females that returned to Stranger Point and bred successfully during 1996; B females that were absent from Stranger Point during 1996 Group A (n=8) Group B (n=10) Arrival mass (kg) 657±98 648±158 Final mass (kg) 414±63 433±112 Initial body reserves (MJ) 10560± ±2902 Final energy reserves (MJ) 4928± ±1699 Energy spent (%) a 53.0± ±5.9 Protein used (%) a 23.2± ±5.9 Fat used (%) a 62.5± ±9.7 a Energy, protein or fat used are expressed as a percentage of those present in females upon arrival during the 1995 breeding season

10 275 by initial maternal body reserves, as was seen in other phocid species that fast during lactation. However, greater, fatter females showed a decreased efficiency of energy and fat transfer while, at the same time, they showed an increased efficiency of protein transfer. This may be due to an increased use of protein as metabolic fuel as fat demands for milk production increase. There was no evidence that greater total lactation costs should influence the ability of mothers to produce a pup during the next breeding season. Acknowledgements We wish to thank S. Poljak, R. Conde, M. Alcalde, R. Montiel and G. Moreira for field assistance, and S. Valencio, J.L. Nogueira and A. Corbalán for technical assistance. We thank Nucleoeléctrica Argentina S.A. (Centrales Nucleares Atucha I y II), who provided the D 2 O to perform this study, and Laboratory Holliday-Scott S.A. for providing some of the ketamine used in immobilisation of animals. 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