Rapid growth is of crucial importance for Adélie penguin chicks reared during the short Antarctic summer. It partly depends on the rapid ontogenesis of fat stores that are virtually null at hatching but then develop considerably (×40) within a month to constitute both an isolative layer against cold and an energy store to fuel thermogenic and growth processes. The present study was aimed at identifying by RT-PCR the major transcriptional events that chronologically underlie the morphological transformation of adipocyte precursors into mature adipocytes from hatching to 30 days of age. The peak expression of GATA binding protein 3, a marker of preadipocytes, at day 7 posthatch indicates a key proliferation step, possibly in relation to the expression of C/EBPα (C/EBPα). High plasma total 3,5,3′-triiodo-l-thyronine (T3) levels and high levels of growth hormone receptor transcripts at hatching suggested that growth hormone and T3 play early activating roles to favor proliferation of preadipocyte precursors. Differentiation and growth of preadipocytes may occur around day 15 in connection with increased abundance of transcripts encoding IGF-1, proliferator-activated receptor-γ, and C/EBPβ, gradually leading to functional maturation of metabolic features of adipocytes including lipid uptake and storage (lipoprotein lipase, fatty-acid synthase) and late endocrine functions (adiponectin) by day 30. Present results show a close correlation between adipose tissue development and chick biology and a difference in the scheduled expression of regulatory factors controlling adipogenesis compared with in vitro studies using cell lines emphasizing the importance of in vivo approaches.
- gene expression
efficient endothermy, which is restricted to mammals and birds, has played a central role in the conquest of cold environments by vertebrates. This elaborate physiological function requires both an integrative system of sensing and regulating body temperature and a powerful tachymetabolism. It also necessitates an efficient isolative layer to reduce the energetic cost of maintaining body temperature at a high level, independently of ambient temperature that may otherwise represent an excessive part of energy metabolism for endotherms of Polar Regions.
Living below 0°C indeed generates tremendous constraints not only to adult organisms such as penguins but also to newborns that exhibit a high surface-to-volume ratio, a poor insulation, and a low capacity to generate heat (11). Newborn penguins are thus facing an inevitable dilemma between collecting as much energy as possible to rapidly mature in time and allocating energy for thermoregulatory purpose. Solutions can be found in parental thermal protection and rapid building up of efficient insulation and thermoregulatory capacity (11). Adipose tissue may contribute to both last aspects as an isolative layer protecting against cold exposure and as an energy reserve to fuel growing and/or thermogenic tissues. Rapid ontogeny of adipose tissue stores may thus be of major importance for survival of young penguins, but this implies that part of food energy must be stored as adipose tissue to the detriment of the somatic growth of the body.
The Adélie penguin, which can be considered as a long-lived species (predicted maximum lifespan ∼ 24 yr) (23), is of particular interest. Adults weighing ∼4.6 kg (23) breed in Antarctica during the summer season. The nest is made of small-sized stones offering a poor thermal protection against climatic hazards. Eggs are therefore incubated under the brood pouch and maintained warm by the adults. After hatching, the thermal protection offered by adults is limited to the first days of life, until chicks are able to reach the incubation pouch (between 7–15 days). The thermoregulatory ability of newborns is gradually enhanced as chick energy expenditure increases in the nestling phase (age 0 to 11 days) and then stabilizes during the crèche phase (14–40 days) (7). Because the summer period is short, newborns have about 8 wk to reach a critical size and molt before departure to sea to get nutritional emancipation. Because of the high thermal conductance of water and the very low temperature of polar seas, powerful isolation by feathers and subcutaneous fat pads is essential at that stage.
Accumulating fat reserves depends on adipocyte differentiation, proliferation, and lipid synthesis. The biological process of adipogenesis has been extensively studied in vitro using a number of preadipocyte cell lines (12, 20, 25, 30, 39). These studies have identified pro- and anti-adipogenic transcription factors and established that the differentiation process is the result of a subtle equilibrium between these different actors. Two transcription factor families have emerged as the key determinants of terminal adipocyte differentiation: the peroxisome proliferator-activated receptor-γ (PPARγ) and the CCAAT/enhancer-binding proteins (C/EBPα and -β). Experimental studies performed in vitro and in vivo support the view that these factors act in a coordinated and sequential manner to control the various steps of adipogenesis (37, 29). Most studies of birds have investigated the lipogenic genes in chickens, because fatness is of primary importance in poultry breeding (2, 31). By contrast, our knowledge of adipose tissue development in wild, cold-adapted species is very limited despite the eco-physiological importance of this process. Elucidating the mechanisms of adipose tissue development in a context of massive energy constraints because of cold may also contribute to enlighten fundamental key regulatory steps.
The aim of the present study was to delineate the developmental phases of adipose tissue in Adélie penguin chicks from hatching to the first month of life, a critical period of tissue maturation leading to thermal emancipation. For that purpose, we measured the mRNA expression of the main adipogenic transcriptional factors, the growth factors known to be involved in adipogenesis and the functional markers of terminal differentiation that lead to the phenotype of mature adipocytes. The molecular events identified will be discussed in the context of growing chicks and correlated with plasma fuel concentrations.
MATERIALS AND METHODS
Ethical approval for all procedures was granted by the ethics committee of the French Polar Research Institute and by the Ministère de l'Environnement. Our experiments conformed to the Code of Ethics of Animal Experimentation in the Antarctic.
The present study was conducted at Dumont d'Urville, Adélie Land (66°07′S–140°00′E) in Antarctica and more precisely on Pointe Geologie archipelago where about 34,000 pairs of Adélie penguins (Pygoscelis adeliae) nest every year (18). Tissue and blood samples were collected during two successive summer seasons in 2005–2006 and 2006–2007. Adélie penguins generally lay two eggs, and the smallest chick does not usually survive. To minimize impact of our study on breeding success, we only used second chicks in the present study. Following our program authorization, 16 Adélie chicks were euthanized each year (4 chicks for each point of growth: hatching, 7, 15, and 30 days old) after fluothane anesthesia. All chicks were weighed before organs, muscles, subcutaneous and retroperitoneal adipose tissues were quickly excised, weighed, and immediately frozen in liquid nitrogen. All samples were stored at −80°C until RNA extraction.
The breeding season of Adélie penguins begins at the end of October. After a period of nest building and courtship, two eggs are generally laid by the females (1.8 ± 0.4) (1). The eggs hatch early in December after an incubation period of 32–38 days. Chick growth then depends exclusively on parental feeding up to the age of 2 mo when departure to sea occurs. Given the brevity of the Antarctic summer at high latitude, climatic changes and their impact on the abundance and location of marine resources largely influence the survival and growth of the chicks. It might be supposed that harsh climatic conditions impact biological responses during the first stages of their development. We have therefore collected the daily measurements of air temperature (°C) to appreciate the environmental conditions during chick growth.
Total blood (0.5 to 1 ml) was collected on heparin at the time of sacrifice. All samples were centrifuged at 5,000 g for 10 min. Triglycerides (PAP; BioMérieux, Marcy L'Etoile, France) and nonesterified fatty acids (NEFA, NEFA-C; WAKO, Neuss, Germany) were assayed using commercially available kits according to the manufacturer's recommendations. Plasma glucose was assayed using a glucometer (Accu-Check' Roche, Meylan, France). Plasma total 3,5,3′-triiodo-l-thyronine (T3) was measured using an ELISA kit (Calbiotech, Spring Valley, CA).
Histological analysis of adipose tissue.
Samples of subcutaneous adipose tissue from 1, 7, 15, and 30-day-old chicks, located in the ventral position along the pectoralis muscle, were immediately fixed in 4% paraformaldehyde, dehydrated, and embedded in paraffin. Sections of 7-μm thick were stained by hemalum, eosin, and saffron for histological evaluation.
Quantification of mRNA expression by semiquantitative RT-PCR.
RT-PCRs were performed in the biology laboratory of Adélie Land (Biomar) to avoid mRNA damage during the long trip back to France. Total RNA were extracted from frozen subcutaneous adipose tissue of Adélie chicks. Tissues were homogenized in Trizol reagent (Invitrogen-Life Technologies; Cergy Pontoise, France), and total RNA was isolated according to the manufacturer's protocol. Reverse transcription (RT) of 1 μg RNA was performed using 200 U of Maloney-murine leukemia virus-RT (Promega, Charbonnières les Bains, France). The cDNAs obtained were submitted to PCR amplification using 2.5 U of Taq DNA polymerase (Eurobio, Les Ullis, France) and 1 μM of forward and reverse primers (Invitrogen-Life Technologies). The primers used were defined according to chicken (Gallus gallus) gene sequences and are listed in Table 1.
The amplified products were separated on agarose gels, and band intensity was quantified with a Kodak Digital Science TMID image analyze software. For each gene, band intensity was normalized against β-actin, a housekeeping gene that shows little change in adipose tissue of growing Adélie penguin chicks. PCR amplification of actin was performed using the same RT reaction as the target gene. In all PCR reactions, care was taken to use the appropriate number of amplification cycles to remain in the exponential phase of the amplification process and avoid saturation. Separate duplicate PCR reactions were used to verify the expression profile of each gene during chick growth. The effect of age was analyzed on samples obtained within the same PCR run and separated on the same gel containing samples from birds at different ages. Results from duplicated reactions were averaged and used for quantification.
The required amount of material was amplified by PCR with a “high-fidelity” Taq DNA polymerase (Invitrogen-Life Technologies) and was sequenced (Genoscreen, Lille, France) to confirm the specific amplification of the targeted cDNAs and transcripts.
Data are expressed as means ± SD. Statistical differences have been evaluated with a two-way ANOVA using StatView program of the MacIntosh system. A Fisher protected least-significant difference post hoc test was used for group comparisons. A P value <0.05 was considered statistically significant.
Body mass and morphological data.
Figure 1A gives a general survey of living conditions of Adélie penguin chicks. After birth, chicks are fully protected against climatic hazards by their parents. That protection is efficient as long as the body size of the chicks allows them to completely enter the brood pouch of the parent. Chicks are then progressively exposed to harsh climatic conditions from 15 to 30 days old, a critical period during which they must ensure their thermal autonomy, related in part to the amount of subcutaneous fat layer. We thus chose four different ages to study the development of subcutaneous white adipose tissue: hatching time (D1), day 7 (D7) characterized by optimal parental thermal protection, day 15 (D15) corresponding to partial exposure to ambient conditions, and day 30 (D30) when chicks are continuously exposed to cold environment. During their first month of life, the mean daily temperature remained below freezing (Fig. 1A). Despite the harsh climatic conditions, chick growth was remarkably rapid as body weight increased more than 20-fold within the first month of life showing that at least during that period, even the second chick is well fed (Fig. 1A). During that period, the mass of adipose tissue (subcutaneous plus retroperitoneal) increased rapidly. It was multiplied by 40 over the first month of life (Fig. 1B). Such increase in fat mass may likely imply both proliferation of adipose stem cells, differentiation into adipose cells, and hypertrophy of preexisting adipocytes. A close correlation has been found between subcutaneous and retroperitoneal adipose tissue mass (Fig. 1C). Subcutaneous adipose tissue was studied throughout the study on account of its higher abundance than retroperitoneal adipose tissue (18) and its greater role in thermal insulation.
Gene expression profile in adipose tissue from hatching to D30 of development.
To understand the molecular basis underlying adipogenesis, we first analyzed the expression of pro- and antiadipogenic transcription factors known to be involved in adipocyte differentiation. The relative abundance of PPARγ mRNA markedly increased at D15 of growth and remained high until D30 (Fig. 2). The same pattern was observed for the mRNA encoding C/EBPβ (Fig. 2). C/EBPα, a potent enhancer of adipocyte terminal differentiation, known to be expressed in parallel to PPARγ in 3T3-L1 cells, reached maximum levels at D7 and remained unchanged until D30 (Fig. 2). The GATA binding protein 3 (GATA3), a member of the GATA family of transcription factors involved in adipocyte developmental processes and characterizing preadipocytes, was specifically expressed at D7 and downregulated to basal levels thereafter (Fig. 2).
We also investigated potential growth factors known to stimulate adipogenesis (Fig. 3). Because of the difficulty of determining the concentrations of circulating hormones in Adélie penguins, we used an indirect approach by measuring the expression of their receptors in adipose tissue. We first investigated the mRNA abundance of insulin-like growth factor-I receptor (IGF-1R), as insulin may likely promote differentiation through these receptors (20). There was a significant increase in IGF-1R mRNA relative abundance at D15 and D30 compared with D1 and D7 (Fig. 3). This increase occurred in parallel with a peak of IGF-1 mRNA at D15 (Fig. 3), suggesting a possible autoparacrine action of fat-derived growth factors in the adipogenesis of Adélie chicks. As T3 and growth hormone (GH) are essential for growth, differentiation, and maintenance of metabolic homeostasis, we explored the expression levels of T3 receptor-α (T3Rα) and GH receptor (GHR) in adipose tissue of Adélie chicks. T3Rα mRNA relative abundance was already high at hatching, but despite trends, there was no significant posthatching change in expression (Fig. 3). The expression profile of mRNA encoding GHR was more variable as it was increased at D1 and D15 compared with D7 and D30 (Fig. 3) suggesting a peculiar role of GH at these specific ages.
Finally, we studied the expression of genes encoding proteins characteristic of adipose cell differentiation including fatty acid synthase (FAS), lipoprotein lipase (LPL), fatty acid binding protein (FABP), and adiponectin (Fig. 4). FAS and LPL mRNA relative abundances markedly increased at D15 and D30, while FABP reached maximal levels at D30 (Fig. 4). The expression of mRNA encoding adiponectin gradually increased between D7 and D30 (Fig. 4).
The sequences obtained for each Adélie penguin cDNA studied were compared with the corresponding chicken and mouse gene sequences, and the degree of similarity is summarized in Table 2. Sequence similarities were found between 81–95% with chicken corresponding sequences confirming the specificity of the amplifications.
Plasma substrate levels.
As shown in Table 3, plasma NEFA concentration was not modified whatever the age, while plasma triglycerides and glucose increased from D1 (P < 0.05). From D7 to D30 plasma triglycerides and glucose remained stable. Plasma T3 concentration slightly decreased from hatching to D30 with a significant effect of age observed at D30 (P < 0.05).
The present study has identified for the first time the molecular events potentially involved in adipose tissue development during the first month of life of Adélie penguins.
Rapid development of adipose tissue involves adipogenic genes.
Changes in tissue weight indicate that adipose tissue develops rapidly during the posthatching growing period. There is little doubt that adipose tissue growth results from both an increase in cell number and cell size. Indeed, as shown in Fig. 5, adipose cell size significantly increased during the first month of life. The rapid growth of fat stores until D15 (Fig. 1B), when chicks benefited from the maximal thermal protection by their parents, suggests that adipose tissue development is favored by chick intense feeding and energy saving allowed by parental care. It follows that adipose tissue development precedes the marked cold exposure that occurs after thermal emancipation, i.e., after D15.
Factors responsible for adipogenesis have never been studied in wild birds. Much of our knowledge on the transcription control of adipogenesis comes from studies of cultured mouse 3T3-L1 (38, 28) and from a few in vitro studies using cultured chicken preadipocytes (9, 15, 25). These studies generally state that the early expression of C/EBPβ and -δ triggers the expression of C/EBPα and PPARγ, which coordinately activate the transcription of genes that rise to the mature adipocyte phenotype. Our results point out a striking difference with this general consensus sequence of promoter activation. Indeed, we found a precocious activation of C/EBPα (D7, Fig. 2) preceding that of PPARγ and C/EBPβ (D15, Fig. 2). This observation in Adélie penguin in vivo differs from the in vitro differentiation of cultured chicken preadipocytes showing that PPARγ mRNA expression level is rapidly increased before C/EBPα and -β gene activation (25). It is not clear as to whether this relates to differences between in vivo vs. in vitro situations and/or species differences, but a precocious activation of C/EBPα preceding that of other regulating proteins was also reported during pig fetal development without the expression of C/EBPβ and -δ (17, 22). Although C/EBPα expression alone is not sufficient to induce adipocyte development in culture (17), it may represent a necessary step that is conserved during evolution and found in birds and mammals.
Nevertheless, the present data also underline a marked upregulation of PPARγ in parallel with the final maturation of adipocytes. It is also consistent with the observation that PPARγ is not expressed or is expressed at low levels in preadipocytes and is turned on during differentiation, prior to the expression of most adipocyte genes, many of which contain PPAR-binding sites (reviewed in Refs. 3 and 35). This is in keeping with the observation that PPARγ plays an important role in the regulation of fat deposition (31) and the recent demonstration that transient transfection with silencing PPARγ mRNA inhibits the differentiation of chicken preadipocytes (41). Nevertheless, present data are strengthening the notion that although PPARγ may be sufficient to trigger the adipogenic program, C/EBPα is required for many aspects of adipocyte differentiation and maturation (16).
To counteract the action of transcription factors that promote adipogenesis, members of the GATA-binding family, which are zinc finger DNA-binding proteins involved in developmental processes, rather act as adipogenic repressors. Previous studies indicated that GATA3 is expressed in preadipocytes and is downregulated during adipocyte differentiation (38). Accordingly, we found a high abundance of GATA3 mRNA early at D7 that may correspond to an early proliferation of new preadipocytes. The return of mRNA encoding GATA3 back to basal level after D7 suggests that preadipocytes have then been engaged into adipocyte differentiation.
Implication of endocrine factors during adipogenesis.
Adipocyte differentiation can be induced by several factors, such as IGF-1, GH, and T3 and (40, 26, 12) acting on specific receptors expressed by adipocytes. Present results indicate an increased abundance of mRNA encoding IGF-1R, IGF-1, and GHR at D15. Such time-related change in expression, at least at the mRNA level, is consistent with adipose cell differentiation at that stage (Figs. 3 and 5). The marked upregulation of IGF-1 by D15 is in accordance with a major role of the peptide as a regulator of cell proliferation regulating adipose tissue growth and differentiation of preadipocytes into adipocytes (6). Interestingly, the concomitant expression of IGF-1 and IGF-1R observed at D15 suggests an additional autocrine/paracrine effect of the peptide. The correlation between the changes in IGF-1 and GHR mRNA favors a causal link consistent with the known GH-activated expression and secretion of IGF-1 in preadipocytes (14). The activity of GH may thus result from both a direct action of the hormone on its receptors and an indirect action mediated by IGF-1. On account of the stimulatory role of GH on the pool of adipocyte precursor cells capable of differentiating into mature adipocytes (6), it is postulated that the high level of GHR expression found at hatching contributes to an early phase of proliferation occurring during the first week posthatching. Altogether, present results emphasize that the proliferation, differentiation, and metabolism of adipose tissue may be highly regulated by the GH/IGF-1 system in Adélie penguins.
The thyroid hormone T3 is also critical for the growth and differentiation of a number of tissues including adipose tissue. The high levels of circulating T3 after hatching may contribute to early activation of adipose tissue development. Thereafter, the lowering of plasma T3 levels after D15 is likely to reduce metabolic rate and thus spare energy substrates that would be available for storage in adipose tissue. Most T3 effects are mediated by interactions with nuclear T3 receptors (T3R), which are ligand-dependent transcriptional factors that positively or negatively regulate T3 responsive genes (4). In birds, cDNAs encoding at least two T3Rs (α and β) have been isolated in chickens (33, 36) and ducks (21). A recent study (43) demonstrating that a knock-in mutation in the TRα isoform reduced adipogenesis and PPARγ expression, prompted us to explore TRα expression during the adipogenesis of Adélie chicks. Present results indicated high but rather constant relative abundance of TRα mRNA from D1 to D30 (Fig. 3). This in vivo observation is in keeping with in vitro findings indicating that during the adipogenesis of 3T3-L1, TRα is constitutively expressed in preadipocytes as well as in mature adipocytes (43).
Late expression of functional markers of matured adipocytes.
The apparition of functional markers of maturation, such as LPL, FAS from D15, FABP, and adiponectin from D30 (Fig. 4) finalizes the program of adipogenesis in the Adélie chicks. Indeed, LPL that is responsible for the hydrolysis of circulating triglycerides into free fatty acids and glycerol, is known to be an early marker of adipose cell maturation and will contribute to the storage of circulating lipids arising from food intake rich in lipids or endogenous lipogenesis. Despite the fact that much of the regulation of the LPL occurs at the posttranscriptional level (10, 27), present results clearly indicate a surge in the relative abundance of LPL mRNA at that point of adipose tissue development. This is in agreement with previous observations of increased LPL gene transcription during adipogenesis in rat (28) and 3T3-L1 adipocytes (32). Similarly, transcription of the LPL gene in the heart increased 10-fold in rat pups (34). Although de novo lipogenesis in birds mainly occurs in liver (5), mRNA encoding FAS, a lipogenic enzyme involved in the synthesis of long-chain fatty acids, is significantly expressed in white adipose tissue as shown in Fig. 3. The increases in both LPL and FAS with the same time course between D15 and D30 strongly suggest the differentiation of preadipocytes into mature adipocytes. Such increase is also congruent with chick thermal emancipation that occurs around D15 and allows both parents to actively feed their offsprings and bring them sufficient food energy for fat storage.
As adipocytes acquire the machinery that is necessary for lipid transport and synthesis, they can also synthesize specific proteins such as leptin, resistin, and adiponectin. The crucial role of adiponectin, which exerts pleiotropic insulin-sensitizing effects prompted us to investigate the occurrence of its mRNA during adipogenesis. Our results show that adiponectin mRNA was detected very early after hatching, and the relative abundance of the transcript gradually increased up to 30 days in parallel with adipocyte differentiation (Fig. 4). On account of the potential role of this fat-derived peptide in carbohydrate and lipid metabolism of avian species (24), such gradual rise in adiponectin expression may possibly contribute to the maintenance of a high blood glucose concentration during the first month of life (Table 3). Further, adiponectin was also suggested to act as an autocrine factor in adipose tissues by promoting cell proliferation and differentiation from preadipocytes into adipocytes, augmenting programmed gene expression responsible for adipogenesis and increasing lipid content and insulin responsiveness of the glucose transport system in adipocytes (13). More experiments are required to clarify the physiological role of adiponectin in penguins.
Proposed scheme for the adipogenic process in Adélie chick.
We have tentatively summarized the identified major events that may chronologically underlie the morphological transformation of adipocytes precursor cells into mature adipocytes as they appear on histological sections of subcutaneous adipose tissue of Adélie chicks (Fig. 5). At hatching time, adipose tissue was a loose connective tissue in which it is rather difficult to identify preadipocytes. It is suggested that GH and T3 play early activating roles at that stage to favor early proliferative steps of preadipocyte precursors. The peak expression of GATA3, a marker of preadipocytes, at D7 implies that it can be a key event at that stage of adipocyte development, possibly in relation with the expression of C/EBPα. This timely controlled upregulation of GATA3 strongly suggests an active phase of preadipocyte formation during the first week posthatching. Adipocyte-like cells were visually present from D7 with other cell types including probably fibroblasts, macrophages, and endothelial cells, but major differentiation and growth of preadipocytes may occur at D15 in connection with several factors such as IGF-1, PPARγ, and C/EBPβ, gradually leading to functional maturation of metabolic features of adipocytes, including lipid storage (LPL, FAS) and late endocrine functions by D30. Accordingly, the size of adipocytes was markedly enlarged until D30 where they appeared as large white empty-looking cells.
In conclusion, the present study has described for the first time the molecular events that may drive the rapid ontogenesis of adipose tissue during the first month posthatching in Adélie penguin chicks. A sequential scheme of gene activation that is slightly different from in vitro studies is proposed. Future studies will clearly have to reconcile animal physiology and cell culture information.
Perspectives and Significance
Given the brevity of the summer breeding season in Antarctica, Adélie chick growth becomes a true race against the clock. Any parameter, such as climatic conditions and food availability that affects rapid chick growth and building up of energy reserves, is detrimental to juvenile survival during that early period and later at departure to sea when a massive thermogenic effort is superimposed on molting energetic cost and fasting. The first weeks posthatching, when adipose tissue development mainly occurs, are therefore of critical importance for Adélie penguin survival. It is noteworthy that chick mortality is mainly observed during that early period in tight link with changes in environment and food supply. Although global warming has generated ecological changes that increase the accessibility to rich food waters leading to increased Adélie populations (42), a slight perturbation of Antarctic ecosystem may punctually cause dramatic damages in seabird populations. Individual capacities to both develop important fat pads and later use these reserves represent metabolic adaptations that could contribute to select birds able to overcome the energetic challenges of Antarctic life. The physiological factors that control the molecular events triggering early fat development deserve to be more fully investigated and in particular those that enable energy sparing when the necessity to conserve and/or store energy as fat is of overriding importance. In this context, the example of barnacle geese that become hypothermic just before their autumn migration in relation to intense fat deposition (8) is of primary interest. There is, therefore, much to be learned about the relationship between metabolic rate, body temperature, fat deposition, and adipose tissue development in birds, particularly in those species such as penguins that naturally undergo large developmental and seasonal changes in their fat stores.
This work was funded by a grant from the French polar institute (program 131). B. Rey was a receipt of a fellowship from the French Ministère de l'Enseignement Supérieur et de la Recherche.
We are grateful to the members of the 56th and 57th mission in Adélie Land and to the French Polar Research Institute for their technical and logistical assistance.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2008 the American Physiological Society