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The ontogeny of diving abilities in subantarctic fur seal pups: developmental trade-off in response to extreme fasting?

2011; Wiley; Volume: 25; Issue: 4 Linguagem: Inglês

10.1111/j.1365-2435.2011.01846.x

1365-2435

Delphine Verrier, Christophe Guinet, Matthieu Authier, Yann Tremblay, Scott A. Shaffer, Daniel P. Costa, René Groscolas, John P. Y. Arnould,

Marine animal studies overview

Functional EcologyVolume 25, Issue 4 p. 818-828 Free Access The ontogeny of diving abilities in subantarctic fur seal pups: developmental trade-off in response to extreme fasting? Delphine Verrier, Corresponding Author Delphine Verrier Department of Zoology, University of Melbourne, Parkville, Victoria 3010, Australia Institut Pluridisciplinaire Hubert Curien, Département Ecologie, Physiologie et Ethologie, UMR 7178 CNRS-UdS, 23 rue Becquerel, 67087 Strasbourg Cedex 2, France Correspondence author. Centre de Primatologie, Centre International de Recherches Médicales de Franceville, BP 769, Franceville, Gabon. E-mail: ddlafouine@free.frSearch for more papers by this authorChristophe Guinet, Christophe Guinet Centre d'Etudes Biologiques de Chizé, UPR 1934 CNRS, 79360 Villiers-en-Bois, FranceSearch for more papers by this authorMatthieu Authier, Matthieu Authier Centre d'Etudes Biologiques de Chizé, UPR 1934 CNRS, 79360 Villiers-en-Bois, FranceSearch for more papers by this authorYann Tremblay, Yann Tremblay Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USASearch for more papers by this authorScott Shaffer, Scott Shaffer Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USA Department of Biological Sciences, San Jose State University, One Washington Square, San Jose, CA 95192, USASearch for more papers by this authorDaniel P. Costa, Daniel P. Costa Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USASearch for more papers by this authorRené Groscolas, René Groscolas Institut Pluridisciplinaire Hubert Curien, Département Ecologie, Physiologie et Ethologie, UMR 7178 CNRS-UdS, 23 rue Becquerel, 67087 Strasbourg Cedex 2, FranceSearch for more papers by this authorJohn P.Y. Arnould, John P.Y. Arnould School of Life and Environmental Sciences, Deakin University, Burwood, Victoria 3125, AustraliaSearch for more papers by this author Delphine Verrier, Corresponding Author Delphine Verrier Department of Zoology, University of Melbourne, Parkville, Victoria 3010, Australia Institut Pluridisciplinaire Hubert Curien, Département Ecologie, Physiologie et Ethologie, UMR 7178 CNRS-UdS, 23 rue Becquerel, 67087 Strasbourg Cedex 2, France Correspondence author. Centre de Primatologie, Centre International de Recherches Médicales de Franceville, BP 769, Franceville, Gabon. E-mail: ddlafouine@free.frSearch for more papers by this authorChristophe Guinet, Christophe Guinet Centre d'Etudes Biologiques de Chizé, UPR 1934 CNRS, 79360 Villiers-en-Bois, FranceSearch for more papers by this authorMatthieu Authier, Matthieu Authier Centre d'Etudes Biologiques de Chizé, UPR 1934 CNRS, 79360 Villiers-en-Bois, FranceSearch for more papers by this authorYann Tremblay, Yann Tremblay Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USASearch for more papers by this authorScott Shaffer, Scott Shaffer Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USA Department of Biological Sciences, San Jose State University, One Washington Square, San Jose, CA 95192, USASearch for more papers by this authorDaniel P. Costa, Daniel P. Costa Department of Ecology and Evolutionary Biology, Institute of Marine Sciences, Long Marine Lab, University of California, Santa Cruz, CA 95060, USASearch for more papers by this authorRené Groscolas, René Groscolas Institut Pluridisciplinaire Hubert Curien, Département Ecologie, Physiologie et Ethologie, UMR 7178 CNRS-UdS, 23 rue Becquerel, 67087 Strasbourg Cedex 2, FranceSearch for more papers by this authorJohn P.Y. Arnould, John P.Y. Arnould School of Life and Environmental Sciences, Deakin University, Burwood, Victoria 3125, AustraliaSearch for more papers by this author First published: 14 March 2011 https://doi.org/10.1111/j.1365-2435.2011.01846.xCitations: 25AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat Summary 1. A major hypothesis of life-history theory is that conditions of early development affect future survival and reproductive success. Responses to detrimental environments during early ontogeny may involve trade-offs between current and future fitness. Yet, the functional mechanisms involved in such evolutionary trade-offs remain poorly documented. 2. The physiological and behavioural ontogeny of diving abilities was examined in subantarctic fur seal (Arctocephalus tropicalis Gray) pups to assess whether the repeated extreme fasts they naturally endure throughout the period of maternal dependence impacts on their development. 3. The ontogeny of pup body oxygen storage capacity was slow, in particular for the muscle compartment, which shows limited increase in myoglobin content (0·23–0·85 g 100 g wet muscle−1). As a consequence, by the time of weaning, mass-specific oxygen stores had only reached 76%, 24% and 61% of adult female capacity for blood, muscle and total, respectively. Concomitantly, in marked contrast to other pinniped infants, they spent decreasing amounts of time in water (16–7%) with age and exhibited very little diving experience and skills. 4. Overall, in addition to experiencing the longest fasting durations throughout the maternal dependence period, subantarctic fur seal pups demonstrate the lowest levels of mass-specific total oxygen stores and maximum dive duration of any otariid near the age of weaning reported to date. 5. Furthermore, dives that exceeded the calculated aerobic dive limit occurred with a very low frequency (0·04%), suggesting that behavioural limitations linked to the necessity to conserve energy to survive repeated fasting, rather than restricted oxygen storage capacity, constrained pup diving behaviour. 6. Hence, these animals appear to trade-off the early development of both their physiological and behavioural diving abilities in favour of body fat accumulation to survive the prolonged fasts they must endure and, potentially, provide a nutritional buffer while they locate appropriate food patches after weaning. While promoting pre-weaning survival, this strategy renders pups more vulnerable to unpredictable changes in environmental conditions and food availability at the transition to independent foraging and, thus, could have negative impact on post-weaning survival. Introduction Because available resources in a particular environment are finite, animals must selectively allocate time, effort and energy expenditure to competing life functions. Life-history theory relies on the concept of 'trade-offs', where a change in one trait or function to improve fitness necessarily has negative consequences for other traits or functions resulting in costs associated with the development or maintenance of a particular phenotype (Stearns 1992). A major hypothesis of life-history theory is that conditions of early development affect future survival and reproductive success. Indeed, responses to detrimental environments during early ontogeny may involve trade-offs between current and future fitness (Lindström 1999). Yet, the functional mechanisms involved in such evolutionary trade-offs remain poorly documented and little is known of the strategies individuals facing strong environmental constraints in early life adopt to optimize both immediate survival and future fitness. The transition to nutritional independence is a critical stage in animals' life history. During the period of parental dependence, infants first depend entirely upon parental provisioning for nutrition. To be able to survive beyond the cessation of maternal care, they must acquire appropriate skills (e.g. behavioural skills, as well as physical and physiological capabilities) allowing them to successfully conduct independent foraging in the surrounding environment (Martin 1984). Due to their physical immaturity and lack of experience, young animals are likely to be less efficient at foraging than adults (Illius & Gordon 1990; Trillmich & Ono 1991; Beauplet et al. 2005). Yet, their ability to ensure adequate foraging success is crucial at both the individual and population scales for it will strongly determine juvenile survival and recruitment, which are major factors driving the dynamics and persistence of animal populations (Caley et al. 1996; Gaillard, Festa-Bianchet & Yoccoz 1998). The transition to nutritional independence can be particularly critical in young predators which, in addition to potentially being exposed to predation themselves, have to be able to locate and capture accessible live prey. In land-born, air-breathing marine vertebrates such as pinnipeds (seals, walruses and sea lions), weanlings must face the additional challenges of shifting from living a terrestrial life being fed ashore on maternal milk (for a nursing period of 3–4 days to 3 years depending on species) to having to forage independently in the ocean for live prey at great depths (Bonner 1984; Gentry & Kooyman 1986). It is essential, therefore, that newly weaned individuals have developed efficient swimming and diving abilities to ensure adequate foraging success (Burns 1999; Jørgensen et al. 2001). While behavioural skills are progressively acquired through swimming practice, learning and experience (Horning & Trillmich 1997b; Jørgensen et al. 2001; Fowler et al. 2006), as air-breathing vertebrates must surface at regular intervals for oxygen supply, physiological diving abilities develop with body oxygen (O2) storage capacity (Burns & Castellini 1996; Kooyman & Ponganis 1998; Fowler et al. 2007; Spence-Bailey, Verrier & Arnould 2007). In vertebrates, blood O2 storage capacity depends upon haemoglobin content (Hb) and blood volume (BV), while muscle O2 storage capacity depends upon muscle myoglobin concentration (Mb) and muscle mass. A hallmark of animals diving to depth is a substantial elevation of Mb. Muscle myoglobin concentrations increase proportionately with diving capacities and are highest in penguins, pinnipeds, and cetaceans. Intermediate concentrations are found in shallow-diving, short-duration divers such as manatees, muskrats, and ducks (Kooyman & Ponganis 1998). Recent studies have shown that, although marine mammals have elevated body O2 stores [through elevated BV, haematocrit (Ht), Hb and Mb] in comparison to terrestrial mammals, their offspring are born physiologically immature and need to actively develop theirs. For instance, although blood typically accounts for ≥50% of the total O2 reserves of adult pinnipeds (Kooyman & Ponganis 1998), neonates and juveniles have significantly lower blood O2 stores than their adult counterparts (Burns 1999; Fowler et al. 2007; Spence-Bailey, Verrier & Arnould 2007; Weise & Costa 2007). In addition, regardless of species, muscles show the longest development time of all O2 storage compartments, in particular with the slow development of Mb (Noren et al. 2001; Fowler et al. 2007; Spence-Bailey, Verrier & Arnould 2007; Weise & Costa 2007; Lestyk et al. 2009), suggesting it is a costly process. Hence, marine mammal infants have to allocate a substantial proportion of the finite nutritional resources provided by their mother (i.e. milk) to both swimming practice and production of O2 reserves throughout their development. Due to the energy-demanding nature of such ontogenetic processes, trade-offs in resource allocation are expected to arise in response to various life-history strategies and developmental pressures (Williams 1966; Stearns 1992). For instance, recent studies of the development of body O2 storage capacity among pinnipeds indicate that the temporal pattern of pup maturation is closely tied to the length of maternal dependency or timing of initiation of independent foraging (Noren et al. 2001; Arnould et al. 2003; Noren, Iverson & Boness 2005; Burns et al. 2007; Fowler et al. 2007). In otariid seals (fur seals and sea lions), adult females adopt a central place foraging strategy throughout lactation (4 months to 3 years depending on species), alternating between short nursing periods at the breeding grounds (1–4 days) and long foraging trips to sea to gather the resources required for milk production during which their pup remains on land (Gentry & Kooyman 1986). Otariid seal pups must, therefore, fast regularly throughout their rearing period, typically from 1–3 days in sea lions to 4–6 days in most fur seal species (Gentry & Kooyman 1986). At Amsterdam Island (Southern Indian Ocean), subantarctic fur seals (Arctocephalus tropicalis, Gray 1872) represent the most extreme example of the otariid life-history pattern. Lactating females undertake the longest maternal foraging trips of any otariid seal due to the great distances they must travel at sea to reach food resources (up to 1800 km away from breeding colonies) (Georges & Guinet 2000a; Beauplet et al. 2004). Pups of this species, therefore, are forced to repeatedly undergo extreme fasting durations from birth to weaning (from an average of 14 days in summer (from birth to 3 months of age) to >30 days in winter (at 7–9 months of age), with regular records up to 2 months), and can spend >85% of the maternal dependence period in repeated fasting episodes (Verrier et al. 2009, 2011). Yet, while almost constantly fasting, they still have to develop and prepare for the transition to nutritional independence. It has been recently demonstrated that, in response to such nutritional constraints, these animals adopt an impressive strategy of energy conservation and protein sparing allowing them to rely on finite body reserves for extended periods of time and making them one of the most advanced evolutionary adaptations of any mammal to conditions of no food and no water during development (Verrier et al. 2009). While promoting pup pre-weaning survival, these adaptations to repeated prolonged fasting appear, however, highly conflicting with the substantial costs of growth, and physiological and behavioural development necessary for independent living at the time of weaning. Whether development is impacted by the extreme nutritional restrictions endured in this species and how individuals cope to optimize their fitness under such energetic constraints is not known. In this context, subantarctic fur seal pups provide an interesting model to address questions that are central to ecological physiology (Costa & Sinervo 2004), such as the role and impact of physiology on the ecology and behaviour of animals, and the trade-offs made to optimize both immediate survival and future fitness by animals facing strong environmental constraints in early life. Indeed, since early developmental traits affect future performances and survival in many bird (Olsson 1997; Naef-Daenzer, Widmer & Nuber 2001; Blas et al. 2007) and mammal species (Festa-Bianchet et al. 1997; McMahon, Burton & Bester 2000; Hall, McConnell & Barker 2001), nutritional conditions in early life are likely to have important ecological implications at both the individual and population scales (Lindström 1999). Furthermore, the simultaneous development of blood and muscle O2 stores has infrequently been measured in otariids and, in particular, concurrently with the development of behavioural diving abilities (Fowler et al. 2007; Spence-Bailey, Verrier & Arnould 2007). Yet, such information is crucial to decipher growth and resource allocation strategies in animals, and how decision-making processes in early life can affect post-weaning survival, a key factor in animal's population dynamics. The aim of this study, therefore, was to assess whether repeated prolonged fasting throughout the period of maternal dependence affects development in subantarctic fur seal pups, looking at the physiological and behavioural capacities for diving as proxies of development. Specifically, we aimed to determine: (i) whether a developmental trade-off does exist in response to the low maternal provisioning experienced; (ii) whether the predicted trade-off involves pups' physiological or behavioural development, or both; and (iii) whether the pattern of development is different to that reported in other closely related species, which experience much shorter fasting durations. To answer these questions, we investigated the development of physiological (i.e. total O2 stores and their components), and behavioural (i.e. swimming and submergence) capacity for diving in subantarctic fur seal pups at various stages throughout the period of maternal dependence, in the light of the extreme energetic constraints faced. Pup diving capabilities were compared to those of adult counterparts obtained at the same study location. Interspecies comparisons among otariid seals were also conducted and the ecological implications discussed. Materials and methods Study site and animals All procedures involved in this study were approved by the Ethics Committee of the French Polar Institute (IPEV) and the Polar Environment Committee of Terres Australes et Antarctiques Françaises. They complied with the Agreed Measures for the Conservation of Antarctic and sub-Antarctic Fauna and current French laws. The study was carried out on the subantarctic fur seal breeding colony of La Mare aux Elephants, located on the north-east coast of Amsterdam Island, Southern Indian Ocean (37°55′S, 77°30′E). In this colony, adult females give birth to a single pup each year from late November to early January and weaning takes place around mid-October at an age of c. 9–10 months. As part of a long-term population-monitoring programme, c. 150 pups of previously tagged females are sexed and marked each year at birth using temporary codes glued to the fur on the top of their head. At c. 1 month of age, these pups are tagged in the trailing edge of both fore-flippers with an individually numbered plastic tag (Dalton Rototag, Nettlebed, UK) (Georges & Guinet 2000a). Subsample groups were randomly selected among the pup known-age cohorts to estimate pup body O2 stores and investigate pup diving behaviour at six different developmental stages from age 2 weeks to weaning (Tables 1 and 2). Pups were sampled during maternal absence and only once during the study. Additionally, 14 adult females were randomly selected at the study site and sampled for body O2 store estimation. Table 1. Characteristics of the subantarctic fur seals sampled for body oxygen store determination at Amsterdam Island Age class Developmental stage Season Sampling period n Sex ratio (M:F) Body mass (kg) 2 weeks Post-natal Summer December 2003 13 6:7 5·0 ± 0·1 1 month Pre-moult Summer January 2004 14 6:8 6·3 ± 0·2 2 months Pre-moult Summer February 2004 12 7:5 8·1 ± 0·4 3 months Pre-moult Summer March 2004 14 7:7 11·4 ± 0·4 5 months Moult Autumn May 2005 14 7:7 13·5 ± 0·4 9 months Pre-weaning Winter August–September 2005 12 6:6 15·5 ± 0·5 Adult females Lactating Summer February 2001 14 – 43·1 ± 1·3 n represents the number of animals sampled for blood oxygen store determination (M, male pups; F, for female pups). Body mass data are presented as means ± SE. All values are significantly different (anova and Sidak post-hoc: P < 0·05). Table 2. Characteristics of the subantarctic fur seal pups sampled for diving behaviour at Amsterdam Island Age class Developmental stage Season Sampling period n Sex ratio (M:F) Deployment duration (d) 1 month Pre-moult Summer January 2004 12 6:6 11·3 ± 0·8 (7–16) 2 months Pre-moult Summer February 2004 13 6:7 12·3 ± 1·7 (4–20) 3 months Pre-moult Summer March 2004 13 7:6 12·5 ± 0·6 (9–16) 5 months Moult Autumn May 2005 20 10:10 23·3 ± 2·3 (9–58) 9 months Pre-weaning Winter August–September 2005 20 10:10 29·8 ± 2·0 (11–46) n represents the number of animals equipped with TDRs for diving behaviour examination (M, male; F, female). Deployment durations are presented as means ± SE with data range between parentheses. Estimation of body O2 stores and aerobic dive limit Subantarctic fur seals' body O2 stores were measured by collecting blood and muscle samples. Pups were manually captured and did not require manual restraint between sampling times. Upon capture, they were weighed in a large Hessian bag using a spring scale (±0·05 kg) and a polyethylene catheter (25 mm long, 0·85 mm OD, 22 G, Surflash I.V. Catheter, Terumo Corporation, Tokyo, Japan) inserted into the brachial vein to ensure blood sampling and intravenous injections. Adult females were captured with a modified hoop net (Fuhrman Diversified, Seabrook, TX, USA) made of a soft mesh with a hole at the end for the animal's nose to facilitate breathing. Upon capture, cows were weighed using a spring scale (±0·5 kg) attached to the net and physically restrained on a wooden board (Georges & Guinet 2000a). In adult females, blood sampling and intravenous injections were made from interdigital rear flipper veins. After collection of an initial background blood sample (4–5 mL), the Evans Blue dilution technique (ICSH 1967) was applied for plasma volume determination. Briefly, each animal received an intravenous injection of a pulse dose (0·6 mg kg−1) of Evans Blue dye (Sigma-Aldrich, St Louis, MO, USA) (El-Sayed, Goodall & Hainsworth 1995). The injection syringe was flushed repeatedly with blood to ensure that all of the dye was administered. Serial blood samples (4–5 mL) were then taken at 10 min intervals for 30 min. Blood was kept on ice until transport to the field laboratory for <3 h prior to treatment and centrifugation. Upon collection of the last serial blood sample, animals were administered an intramuscular dose (0·15 mg kg−1) of the sedative midazolam (Hypnovel®, Roche Products Pty Ltd., Dee Why, NSW, Australia). Once a satisfactory level of sedation and local analgesia (through topical anaesthesia with 2 mL lidocaine, Xylovet®, CEVA, Libourne, France) was obtained (<10 min), a small incision (<1·5 cm) was made through the skin and blubber layer after surgical disinfection and a muscle biopsy of c. 20 mg was collected from the pectoralis complex locomotor muscles using a disposable sterile 6 mm biopsy punch. The biopsy site was then cleansed with 33% hydrogen peroxide and iodine solutions, and the incision stitched. Muscle samples were stored on ice until transported to the field laboratory and stored in air-tight plastic vials at −20 °C until analysis for myoglobin content within 6 months. Upon completion of sampling and recovery from sedation, animals were released at the site of capture in the colony. In the field laboratory, haematocrit (Ht, %) was measured in quadruplicate from the initial background blood sample in capillary tubes following centrifugation for 5 min at 12,000 g. Ten microliter subsamples of whole blood from the initial background sample were also added to 2·5 mL of Drabkin's reagent (Fronine Laboratory Supply Ltd., Riverstone, NSW, Australia) and kept protected from light at room temperature until further analysis for blood haemoglobin concentration within <6 months. All blood samples were then centrifuged and the plasma fraction stored frozen at −20 °C until analysis. Plasma volume (PV, mL) was determined following methods by Foldager & Blomqvist (1991) and El-Sayed, Goodall & Hainsworth (1995). Briefly, upon thawing, serially collected Evans Blue plasma samples were thoroughly mixed using a vortex and centrifuged for 5 min at 7,500 g in order to separate any lipids, fibrinogen or remaining red blood cells. Absorbance of the supernatant was measured at both 624 and 740 nm to account for possible haemolysis and precipitate (Foldager & Blomqvist 1991) and dye concentrations were determined from a serial dilution curve of Evans Blue standards. The instantaneous dilution volume (i.e. PV, mL) was then calculated as the y-intercept of a regression line between log-transformed dye concentrations and the post-injection time of sampling (El-Sayed, Goodall & Hainsworth 1995). Total blood volume (TBV, mL) was determined from PV using Ht value as follows: TBV = 100 × PV/(100−Ht). Haemoglobin concentration (Hb, g dL−1) was determined from whole blood subsamples kept in Drabkin's reagent using the cyan-methhaemoglobin photometric method (ICSH 1967). Absorbance was read at 540 nm and Hb was determined by comparison with standard dilution curves (Haemoglobin Standard Pack SB-0325-006, Fisher Scientific, Suwanee, GA, USA). Mean corpuscular haemoglobin content (MCHC) was calculated using the equation: MCHC = Hb/Ht. Muscle myoglobin content (Mb, g 100 g wet musle−1) was determined using Reynafarje's procedure (1963) modified by Castellini & Somero (1981). Buffer blanks as well as northern elephant seal and California sea lion muscle samples of known Mb (Noren et al. 2001; Weise & Costa 2007) were used as controls on each assay. Total body O2 stores (mL kg−1) were determined from the addition of the various components (e.g. blood, muscle and lung) (Davis & Kanatous 1999; Costa, Gales & Goebel 2001) following the methods previously described (Burns et al. 2007; Fowler et al. 2007). Blood and muscle oxygen stores were obtained in this study and lung oxygen stores were derived by allometric estimates of lung volume for otariids following Costa, Gales & Goebel (2001). Aerobic dive limit (ADL), representing the maximum dive duration without increases in blood lactate can be estimated by the calculated aerobic dive limit (cADL, min) which is used as an index of aerobic and physiological capacity in diving animals (Kooyman & Ponganis 1998; Costa, Gales & Goebel 2001; Butler 2006). Calculated ADL is obtained by dividing total body O2 stores (mL O2 kg−1) by the diving metabolic rate (DMR, mL O2 kg−1 min−1) of the animal (Costa, Gales & Goebel 2001). Because there are no DMR data available for subantarctic fur seals, the approach of Richmond, Burns & Rea (2006) was used by estimating a range of cADL from multiples of resting metabolic rate (RMR): maximum cADL (1 × RMR); mid-range cADL (2 × RMR); and minimum cADL (4 × RMR). Pup RMR previously measured in post-absorptive subantarctic fur seals were used: 13·93 mL O2 min−1 kg−1 at age 0–1 month (as determined in 1–2-week-old pups experiencing their first fast); 12·62 mL O2 min−1 kg−1 at age 2–3 months (as determined in pre-moult pups); 9·93 mL O2 min−1 kg−1 at age 5 months (as determined in moulting pups); and 7·22 mL O2 min−1 kg−1 at age 9 months (as determined in moulted pups throughout the austral winter) (Verrier et al. 2011). For adult females, an average RMR of 9·7 mL O2 min−1 kg−1 recorded in lactating northern fur seals Callorhinus ursinus was used (Ohata, Miller & Kajimura 1977). Diving behaviour and statistical analyses Concurrent with the sampling for O2 storage parameters, separate cohorts of subantarctic fur seal pups were instrumented with electronic data loggers (MK9 TDRs, Wildlife Computers, Redmond, Washington, USA) in order to collect information on their diving behaviour at different stages of the development (Table 2). Confirming previous observations in other fur seal species (Baker & Donohue 2000; Spence-Bailey, Verrier & Arnould 2007), no subantarctic fur seal pups were seen entering the water before the age of 3 weeks. Therefore, diving behaviour sampling was initiated at 1 month of age. Pups were captured the day of the at-sea departure of their mother and TDRs were glued on the short hair of the upper part of a pectoral flipper using quick-set epoxy resin (R.S. Components, Corby, UK). This site was chosen to reduce the possible effect on heat regulation of gluing the TDR on the body fur (Guinet et al. 2005). Devices were retrieved by trimming of the fur underneath upon maternal return to the colony or just after her next departure. Upon TDR retrieval, dive data were downloaded directly to a portable computer, using Wildlife Computers Software (Instrument Helper 0·701, Redmond, WA, USA). All TDRs were equipped with a saltwater switch to determine when the pup was in the water (wet) or on land (dry) and programmed to record pressure every second when wet. The depth resolution of TDRs was ±0·5 m. The proportion of time spent in water (%) was calculated as the total time the animal spent in the water divided by the total time that the device was deployed. Dives were defined as any submergence greater than or equal to 2 m deep (Spence-Bailey, Verrier & Arnould 2007). Diving behaviour variables were analyzed in Matlab (The MathWorks Inc. Natick, MA, USA) using a custom-written dive analysis programme (Y. Tremblay, unpublished data), which calculates a zero offset correction at the surface, corrects the drift of the pressure transducers, and identifies dives on the basis of a minimum depth and duration. Mean dive duration (min), maximum dive duration (min), mean dive depth (m), and maximum dive depth (m) were determined. Data on diving behaviour obtained for subantarctic fur seal pups at different developmental stages were compared to those of adult females obtained at the same study location (Georges, Tremblay & Guinet 2000b). Statistical analyses were performed using spss©