Heart rate and energetics of free-ranging king penguins (Aptenodytes patagonicus)

Changes in the sizes of seabird populations are known to reflect variations in marine resources (Chastel et al.,1993; Montevecchi,1993; Boyd, 1996). Thus, there has been an increasing effort to use seabirds as indicators of variation in ocean resources and of global climatic changes(Le Maho et al., 1993; Le Maho, 1994). However, in order to give a predictive model that is as accurate as possible, it is necessary to estimate accurately the energy requirement of seabirds. Recent technologies have not only enabled a better understanding of the behaviour of free-ranging animals, e.g. via radio or satellite tracking(Bost et al., 1997; Butler et al., 1998) and time–depth recorders (Kooyman et al., 1980; Bevan et al.,1995), but have also made possible the acquisition of physiological data from heart rate and temperature data loggers (Bevan et al., 1997, 2002; Green et al., 2003).

King penguins breed and forage in the Antarctic Ocean, where primary and secondary productions are high (Huntley et al., 1991). Information on their energy expenditure and thus food consumption can be used in models of energy flux within the ecosystem(Croxall et al., 1999) or for inclusion within fisheries models(Croxall, 1984). To date,estimates of the field metabolic rate (FMR) have been made using data obtained either from captive birds in a metabolic chamber or in a water channel(Culik et al., 1996), or from free-ranging birds using the time–energy budget (TEB) and the doubly labelled water (DLW) methods (Kooyman et al., 1992a).

However, although DLW is still widely used to measure FMR(Speakman, 1998), it does not enable the determination of energetic costs associated with specific activities, without a detailed time budget(Butler et al., 2004). Heart rate (fh) has been proposed as an alternative indicator of the rate of oxygen consumption(V̇_O2; Butler, 1993). This technique has already been successfully used to monitor continuously the rate of energy expenditure in several species (Bevan et al., 1995, 2002; Boyd et al., 1999; Green et al., 2003). The advantages of this technique include a monitoring period of up to several months (Woakes et al., 1995),and the recorded fh can be divided into small time units to allow the investigation of energetic costs of integrated behaviour categories, such as foraging at sea and incubating ashore. In the present study, miniature implantable data loggers for simultaneous recording of depth and fh were utilised.

Froget et al. (2001)presented three models for estimating V̇_O2 of free-ranging king penguins and two of these involved the use of oxygen pulse(OP, the amount of oxygen consumed per heart beat). OP during rest (ROP) and during activity (AOP) were used according to the activity state of the birds. However, it is known that AOP is not usually constant over a range of activity levels (Butler, 1993), and it is now known that ROP is different between birds resting in air (i.e. when ashore) and resting in water (i.e. when at sea; Fahlman et al., 2004). Thus,the primary aim of the present study was to record fh and diving behaviour of free-ranging king penguins and, using the appropriate equation from Fahlman et al.(2004), to estimate the energy expenditure of the birds during the different phases of the foraging and incubating shifts. From these, it was expected to shed new light on the `king penguin's enigma' (Culik et al.,1996), which is that `king penguins seem to swim too fast and dive for too long, and too often, to be in agreement with our physiological models of diving' (Kooyman et al.,1992a).

Indeed, the aerobic dive limit (ADL) or diving lactate threshold (DLT),defined as the diving duration beyond which there is an increase in post-dive plasma lactate concentration (Kooyman,1989; Butler and Jones,1997) has not yet been measured in free-ranging king penguins. Thus a calculated ADL (cADL, which is the dive duration during which all usable oxygen would be exhausted; Kooyman,1989) is often used as an indicator of the DLT. Previous estimations of the cADL at 2 min for king penguins(Kooyman et al., 1992a; Culik et al., 1996) imply that 45% of all dives would exceed the cADL, which suggest that this calculation is inaccurate. Thus, an associated aim of the present study was to estimate V̇_O2 while foraging (diving) in free-ranging king penguins, and thus cADL.

The study was undertaken at the French station on Possession Island(46°25′S, 51°45′E), Crozet Archipelago during two consecutive austral summers (1998–1999 and 1999–2000). King penguins Aptenodytes patagonicus Miller 1778 used in the study were breeding at the colony of `La Baie du Marin', a few hundred metres from the research station. Twenty-seven penguins were captured 3–6 days after the egg exchange (the second egg exchange for a female or the third in the case of a male; see fig. 1 in Froget et al.,2001). They were each implanted with a custom-built fh- and depth-data logger (HRDDL: Woakes et al., 1995); 14 birds in 1998–1999 (mean mass ± s.e.m.=12.7±0.2 kg);13 birds in 1999–2000 (mean mass ± s.e.m.=12.2±0.3 kg). Several pairs of penguins were selected for observation and/or logger implantations. Both partners from each pair were marked using temporary plastic flipper bands, when the female laid its egg. The bands were removed at the end of the season or during the implantation(i.e. studied birds were not banded; see Froget et al., 1998).

The data loggers (61 mm×24 mm×6 mm, 27 g, <0.3% of the body mass of king penguins) had 4 MB of memory and were able to record fh and hydrostatic pressure (to an accuracy of 1 m depth)and were programmed to store these measurements every 2 s. After programming,the loggers were encapsulated in wax and coated with medical grade silicone rubber. Prior to implantation, the logger was bathed in a cold sterilising solution for 2 h, and rinsed thoroughly with sterile water.

The implantation was performed 3–6 days after the egg exchange, so that the bird was settled on the egg and sufficiently motivated to continue incubating despite the disturbance caused by handling and implantation. A portable enclosure was placed over the bird before capture in order to protect its territory during the surgery. The enclosure consisted of wire mesh formed into a circular fence that was approximately the area of a king penguin territory (about 1.5 m diameter and 0.6 m high). The enclosure was also used to confine the bird to the territory it previously held, while awakening and recovering from anaesthesia, and to prevent other penguins (or sheathbills and skuas) from attacking it. 30 min prior the surgery, the bird was injected with 0.5 ml of Valium (Virbac, Centravet Plancoët, France) subcutaneously on the back. The penguin was captured by placing a sack over the adult and lifting both it and its egg. The egg was immediately replaced under the bird by a plaster egg that had previously been kept in an incubator at 38°C, to prevent breakage of the real one. This procedure generally produced little reaction from the bird. The real egg was kept in an incubator at 38°C until its replacement under the adult.

The birds were anaesthetised with halothane in a mixture of O₂:N₂O (1:1) administered via a hood placed over their head. Induction with 4.5% Halothane (Virbac) usually took 3–5 min. After induction, the bird was weighed on a load cell balance (accurate to±2 g) and placed on the operating table. The bird was then intubated,and the anaesthetic gas was altered to 1.5% halothane in pure O₂. A long-acting antibiotic (Oxytetrin LA; Virbac) was injected into the pectoral muscle. Some feathers and the down surrounding the brood patch were removed. The remaining feathers around the brood patch and the incision area were deflected and held away using tape. The brood patch, tape and the surrounding feathers were then swabbed with an iodine solution (Betadine; Virbac). The electrocardiogram (ECG) and rectal temperature were continuously monitored throughout the surgical procedures. Xylocaine (lignocaine + 2% adrenaline;Virbac) was injected subcutaneously along the proposed incision lines to prevent bleeding.

The initial skin incision was made in line with the body axis, starting approximately 3 cm above the brood patch, and was about 5 cm long. The second incision, through the abdominal muscles, was made at right angles to the first, enabling an opening of the abdominal cavity of 3 cm length. The HRDDL was inserted into the body cavity, one of the ECG electrodes was placed close to the apex of the heart, its position was checked using an endoscope, and the other was placed pointing in the opposite direction, below the brood patch. The HRDDL was sutured into position with silk thread. Each logger incorporates a transmitter that emits a click on each detected QRS wave of the ECG and a receiver was used to confirm that the logger was detecting and recording the ECG. Heart rate was calculated from its mean period, by dividing the total duration of n heart beat periods by n, over the sampling period.

After the surgery the bird, still asleep, was replaced together with a warm dummy egg within the colony in its enclosure. After full recovery from the anaesthetic (usually 4–5 h later), the enclosure was removed. 1–2 days after the surgery, the real egg was replaced under the bird. Attendance behaviour was then monitored during the following experimental period at least twice a day. 3–4 days after the equipped bird returned to the colony from its foraging trip, it was recaptured with its chick and weighed. Prior to the removal of the data logger, the bird was anaesthetised and carried to the base to be X-rayed, to check the position of the logger and of the ECG electrodes. The procedures for the removal of the logger and for the replacement of the bird in the colony after surgery were similar to those described for implantation.

Birds were initially divided in two groups depending on the year the data were obtained. Behavioural data were compared and, if no difference was detected, the data from the 2 years were pooled.

The data were prepared and initially analysed using a purpose-written computer program in the Labview programming package (version 5.0, National Instruments, Austin, TX, USA). Further analyses were performed with the statistical package MINITAB 12.22 for Windows (Minitab Inc.) and Excel 97(Microsoft Corp.). Diving bouts were visually determined; a bout was deemed to have started as soon as three deep dives (below 10 m) were interspaced by a surface duration shorter than 10 min (Boyd et al., 1994) and it ended as soon as the surface duration was longer than 10 min. In king penguins, diving bouts are usually easily determined visually from the depth trace (see Fig. 1).

The conversion of V̇_O2 to W was achieved assuming that 1 ml O₂ s^–1=19.8 W when the animal was ashore (fasting and post-absorptive: respiratory quotient, RQ=0.7; Culik et al., 1996; Schmidt-Nielsen, 1997) and that 1 ml O₂ s^–1=18.9 W when the animal was at sea(Bevan et al., 2002, for gentoo penguins).

All values (other than V̇_O2est) are presented as means ± s.e.m. of the mean values from each animal. After verifying that the data were normally distributed and of equal variances using a Kolmogorov–Smirnov normality test and F-test,respectively, a Student's t-test was used to compare the significance of any difference between the means of two populations. One-way analysis of variance (ANOVA) with Tukey's HSD post-hoc test were used when more than two populations were compared. Results were considered significant at P<0.05. Woolf's test for differences and Z-tests were used to compare estimates of V̇_O2.

Of the 27 birds implanted with data loggers, 25 were recaptured. Depth data were not recovered from four birds, due to failure of the pressure sensor. In the remaining 21 birds, ECG was not recorded accurately in six birds, due to movement of the ECG electrodes. And finally, five data loggers failed to record any data, possibly due to deficient waterproofing of the electronics. Thus, simultaneous measurements of depth and fh were successfully recorded from ten birds. Altogether, fh and diving data were analysed from a total of 66 days ashore and 188 days at sea.

Finally, in three of the ten birds (D99, W99 and U00) there were, due to programming problems, slight discrepancies (2–4 min over 15 days)between the recordings of fh and depth, thus the data from these animals were excluded for the fine scale analysis of the cardiac response to diving. For this purpose a total of 10 343 dives was analysed from the remaining seven birds.

Fig. 1 illustrates fh over a 6 day period recorded from a bird at the transition between the incubation and the foraging shift. Note that departure from the colony and diving behaviour are clearly discernible from the fh trace.

Birds left the colony on average 9±2 days after the implantation of the HRDDLs when their mate returned from the sea. The average duration of the foraging shift of the implanted birds was 16±2 days in 1998–1999 and 15±1 days in 1999–2000. In the second year, the mean duration of the foraging shifts of implanted birds was compared with that of non-implanted birds (18±5 days, N=13, P>0.05), at the same stage of reproduction and in close proximity to implanted birds. There was no significant difference between the two groups.

Due to sampling rate and the resolution of the pressure sensor, only dives for periods longer than 4 s and deeper than 6 m were analysed. The deepest dive depth recorded was 257 m (for a duration of 7 min 42 s). General details of diving behaviour are given in Table 1. There was a significant positive correlation for the relationship of dive duration to depth (depth=0.6×duration – 56, r²=0.82, P<0.05). There was a clear diurnal dive pattern, with a mixture of deep long and shallow short dives from dawn(between 05:00 h and 06:00 h) to dusk (around 21:00 h–22:00 h), while at night activity was limited to shallow short dives. This bimodal distribution of dive depth gave peaks at 10 m and 70 m and a related bimodal distribution of dive duration (2 and 3.5 min, Fig. 2). Thus, it was possible to establish two groups of dives: long and deep (>3 min and >40 m) and short and shallow (<3 min and <40 m). 80% of the inter-dive intervals were less than 2.5 min.

The frequency distribution of fh clearly differed depending upon whether the birds were ashore or at sea. When the birds were ashore, fh ranged between 50 and 150 beats min^–1, whereas while they were at sea, fhranged between 50 and 260 beats min^–1, showing peaks at 80–90 beats min^–1 when ashore and 120–130 beats min^–1 when at sea (Fig. 3). The mean fh of individual birds over the recording periods ranged from 69.0 to 119.2 beats min^–1 (mean fh=87.2±4.3 beats min^–1) when the animals were ashore and from 110.9 beats min^–1 to 156.3 (mean fh=130.8±5.1 beats min^–1) when they were at sea (Table 2). The overall mean fh of the free-ranging penguins was 111.1±0.3 beats min^–1. All of the fh values monitored from the birds in the wild were within the range of fh recorded during the calibration experiments (from 47 beats min^–1 to 308 beats min^–1; Fahlman et al.,2004).

When the animals were ashore, no circadian pattern of fh was observed (see Fig. 1). On the other hand,when at sea (Fig. 4), there was a clear pattern, with a decline of fh at dawn (from 121.8±2.0 beats min^–1 at 00:00 h down to 113.8±1.4 beats min^–1 at 05:30 h), when the animals start to forage actively (indicated by the increase of mean depth). During the daytime, fh remained at a reasonably steady level (around 122 beats min^–1), while the birds regularly dived to greater depths (mean diving depth around 66 m). Then, around 21:00 h and after diving depth had begun to decrease, fh increased, to reach a peak of 135.6±2.2 beats min^–1 at approximately 23:00 h,before returning to the midnight value.

When dives were grouped according to their duration separated by 10 s intervals (e.g. all dives between 130 to 139 s were grouped as dives of 134.5 s), a rapid decrease in fh was observed in all categories,reaching a minimum 4 s after submergence(Fig. 5). During long dives(>3 min), fh then showed an increase for the subsequent 6 s, before steadily decreasing to a value similar to that measured when the birds were ashore (lowest fh, 82.1±5.4 beats min^–1; mean ± s.e.m.). During the shorter dives (<3 min), this secondary increase in fh was not observed, and fh was maintained at the low level achieved in the first 6 s ofthe dive, which was also similar to that measured in birds resting ashore (lowest fh, 88.8±5.2 beats min^–1; mean ± s.e.m.; see Fig. 5). Mean fh during diving, for all dives, was significantly greater than these values, but almost 30% below the value between bouts(Table 2). When fh was plotted relative to the time of surfacing, there was no difference between short and long dives. Both showed a slight anticipatory tachycardia, with the largest increase around 4 s prior to surfacing. There was no significant difference between fhpre- and post-dive. Similarly, in the 10 s prior to the dive, fh did not vary significantly between short and long dives.

Heart rate values between dives, during the dive and over the dive-cycle(dive + post dive interval, 126±4.2 beats min^–1) were not significantly different between long and short dives. For dives longer than 3 min, however, there were significant negative correlations between dive duration and (a) mean dive fh(r²=0.78, P<0.001), (b) mean dive-cycle fh (r²=0.83, P<0.001), and(c) mean post-dive interval fh(r²=0.78, P<0.001).

The mean fh during all diving bouts (123.5±3.1 beats min^–1; Table 2) was 40% lower than the mean fh recorded during the hour following the diving bout (168.7±10.2 beats min^–1; paired t-test, t=5.3, P<0.01, N=8). The mean fh during diving bouts was also 20% lower than the mean fh over the entire subsequent inter-bout interval (141.6±8.8 beats min^–1; paired t-test, t=2.64, P<0.05, N=8). The mean fh during the hour following a diving bout was, on average, 20% higher than mean fh over the entire inter-bout interval (paired t-test, t=12.01, P<0.001, N=8).

Estimated V̇_O2of birds was calculated according to equations 1 and 2A from Fahlman et al.(2004) as appropriate, and then converted to estimated field metabolic rate FMR_est(Table 3). When at sea,FMR_est was 83% greater than when the birds were ashore. During diving bouts (and dive cycles), MR_est was 73% greater than when the birds were ashore and MR_est during the total period between bouts,although 20% greater, was not significantly different from that during diving bouts. However, during the first hour following a diving bout,MR_est was 52% greater than that during a diving bout and 33%greater than that during the remainder of the interbout interval.

To avoid using atypical data associated with the recovery of the birds from the surgery, the first 48 h after implantation were not used in subsequent data analysis (see Bevan et al.,1995). Furthermore, there was no significant difference in the duration of the foraging shifts in implanted and non-implanted birds. This suggests that the implantation had no long-term behavioural or physiological effects on the penguins. The foraging pattern of the king penguins used in the present study was similar to that described by other authors(Kooyman et al., 1992a; Culik et al., 1996; Charrassin et al., 1998), with deep diving activity limited to the daylight period, and diving activity at night reduced in frequency and limited to shallow depth. This daily diving pattern has been discussed extensively by the above authors, and probably reflects the photoperiodic vertical migration of their main prey (the myctophid fish species Electrona carlsbergii, Frefftichtys andersonniand Protomyctophum tensoni; Cherel and Ridoux 1992),and/or the light sensitivity of the eyes of penguins(Wilson et al., 1993).

Frequency analyses of the dive durations are consistent with those described elsewhere (Kooyman et al.,1992a; Culik et al.,1996): up to 35% of all dives (deeper than 8 m) were of longer duration than 3.5 min, and 80% of interdive intervals were shorter than 2.5 min.

The hypothesis developed by Kooyman et al.(1992a) that king penguins accumulate lactic acid during long dives and pay off the `oxygen debt' by increasing surface times or surface frequency is not supported by the present data. Since a surface time of 2.5 min is theoretically inadequate for the metabolism of lactate after exceeding the DLT(Kooyman et al., 1992a), we reach a similar conclusion to Culik et al.(1996), that these birds must almost always dive within their DLT. This means that previous estimations of the cADL of 2 min (Kooyman et al.,1992a; Culik et al.,1996) must be inaccurate. On the other hand, it is possible, at least on some occasions, that any accumulated lactate could be metabolised by the locomotory muscles and/or heart during relatively short, shallow dives and/or when the birds are travelling to different foraging locations(Butler, 2004).

Few studies have investigated the variation of fhduring natural dives of birds or mammals. Most of the available data are either from captive animals during forced(Scholander 1940) or voluntary submersion (Millard et al.,1973; Butler and Woakes, 1975, 1984; Bevan et al., 1992) or from animals in their natural environment but diving through an artificial ice hole (Kooyman et al., 1992b). To our knowledge, only three published studies so far have investigated the full range of changes in fh during diving in free-ranging marine birds (Bevan et al., 1997, 2002; Green et al., 2003). In the present study, two different cardiac responses to the dive could be observed,and from the diving behaviour it was possible to define two distinct types of dive: short, shallow dives and deep, long dives (i.e. there were no long,shallow dives). Interestingly, this segregation of the dives between short and long dives is associated with a difference in temporal changes of fh during the dive. During the first 6 s of the dive,there was a rapid reduction in fh, similar to that measured in unrestrained freely diving captive tufted ducks(Bevan and Butler, 1992). Then,depending on the duration of the dive, fh either increased in the subsequent 6 s if the bird was going to perform a long and deep dive,then progressively decreased to a rate similar to the fhwhen the birds were ashore. If the dive was short and shallow, fh stayed low, and again stabilised around the level of fh recorded when birds were ashore. However, both the values for lowest fh during diving were significantly(approximately 25%) lower than that for king penguins resting in water(114±7.2, N=5; Fahlman et al., 2004).

The differences in fh during the first 6 s of submersion in the long and short dives may relate to the effort involved during this period. Sato et al.(2002) indicate that king penguins have to work harder at the beginning of deep dives than at the beginning of shallow dives to overcome the increased buoyancy associated with inhaling more air prior to the longer, deeper dives. However, as depth increases, the buoyant force would decrease due to compression of the air-filled spaces between the feathers, thus progressively reducing energy requirement.

Mean values of heart rate when ashore and during diving bouts (see Table 2) were similar to those obtained for gentoo penguins (90 and 140 beats min^–1,respectively; Bevan et al.,2002).

When a bird is at its nest incubating, it needs only to supply energy for its maintenance metabolism (basal metabolic rate, BMR), any thermoregulatory costs associated with the maintenance of the egg or the chick at its optimal temperature, and any territorial defence activity. Croxall(1982) estimated in a number of penguin species that the energy cost of incubation was 1.2–1.4 times the BMR, which is similar to that found in the present study (1.3× BMR;calculated from the formula of Ellis,1984). The estimated, mass-specific metabolic rate of king penguins ashore [3.15 W kg^–1 (–0.71, +0.93), where the values within parentheses are the computed s.e.e.; see Table 3], is within the range of data obtained by Kooyman et al.(1992a), who measured values between 2.4 and 4.8 W kg^–1 using doubly labelled water, and similar to the resting metabolic rate (RMR) obtained from king penguins in a respirometer (2.8±0.1 W kg^–1, Le Maho and Despin, 1976;3.0±0.3 W kg^–1, Barré, 1980;3.7±0.1 W kg^–1, Froget et al., 2001). More interestingly, fh, and thus FMR_est, was at its lowest when the birds were at sea during the first part of the foraging trip(travelling before the first diving bout), and during a diving bout(Table 3). There was then a large increase in fh during the first hour following the end of a diving bout. This may have been the result of the increased rate of energy expenditure associated with the rewarming of the body that occurs at the end of a diving bout (Handrich et al.,1997).

At sea, when the bird was at the surface, it was not possible to distinguish between periods when the bird was resting and those when it was porpoising or travelling from one foraging patch to another. However,mass-specific MR_est of the birds when at sea [5.77 W kg^–1 (–0.37, +0.40)] was 25% greater than that of birds resting in water (4.6 W kg^–1; Culik et al., 1996; Fahlman et al., 2004),indicating that these animals were quite active when at sea. This level of metabolism included all activities while the bird was at sea, such as the period when the birds were travelling to and from the colony, and activity at night when diving was reduced.

It is of interest to note that at sea, mass-specific FMR_est from the present study is 42% lower than the value of 9.95±0.33 W kg^–1 obtained by Kooyman et al.(1992a) using DLW. This illustrates the apparent overestimation of MR of aquatic birds and mammals when at sea by the DLW method (Butler et al., 2004).

The mass-specific V̇_O2est during diving bouts was determined in the present study to be 17.3 (–0.95,+1.01) ml min^–1 kg^–1. When tested against the validation data in Froget et al.(2001), equation 2A in Fahlman et al. (2004) gives an average algebraic overestimation of 2%. The available oxygen stores in diving king penguins are estimated to be between 45 ml kg^–1(Ponganis et al., 1999) and 58 ml kg^–1 (Kooyman,1989). Using the estimated V̇_O2 during diving bouts and the upper value of usable O₂ stores given above, the calculated aerobic dive limit (cADL) is 3.4 min and approximately 35% of all dives exceed this value (Fig. 2).

If our estimates of V̇_O2 are correct,this implies that king penguins are performing most of their foraging dives(those greater than 3 min duration) by exceeding their cADL. Unfortunately,with the exception of those on tufted ducks (see Woakes and Butler, 1983), no study so far has been able to determine V̇_O2 during diving itself. The rate of oxygen consumption estimated over a complete dive cycle,as in the present study, is the closest estimate that exists for free-ranging king penguins. Thus either V̇_O2 during diving is much lower or/and the oxygen stores are much larger than estimated. In fact, as Butler (2000) points out, even if the V̇_O2 during diving is the same as that when the birds are resting in water (14.1 ml min^–1 kg^–1, Culik et al., 1996; and 15.3±1.3 ml min^–1 kg^–1, Fahlman et al., 2004), maximum cADL would be 3.8–4.1 min and still approximately 20% of the dives would exceed this duration (Fig. 2). It is of course possible that V̇_O2 during diving itself, i.e. when the bird is actually submerged, is lower than when resting in water (hypometabolism), and one possible mechanism by which this could be achieved is regional hypothermia (Handrich et al., 1997).

Even though there is debate over the physiological significance of such hypothermia (Ponganis et al.,2003), Bevan et al.(2002) calculated that a drop in body temperature of only 2.4°C would reduce resting V̇_O2 in water to such a level that all of the dives of gentoo penguins would be within their cADL. This is very pertinent to the present discussion because, like the king penguin, over 20% of the dives of gentoo penguins exceed their cADL, even if V̇_O2 during diving is assumed to be the same as that when resting in water(Bevan et al., 2002).

List of symbols and abbreviations

 
• ADL

  aerobic dive limit

 
• AOP

  active oxygen pulse

 
• BMR

  basal metabolic rate

 
• cADL

  calculated aerobic dive limit

 
• DLT

  diving lactate threshold

 
• DLW

  doubly labelled water

 
• ECG

  electrocardiogram

 
• fh

  heart rate

 
• FMR

  field metabolic rate

 
• FMR_est

  estimated field metabolic rate

 
• HRDDL

  heart rate and depth data logger

 
• MR

  metabolic rate

 
• MR_est

  estimated metabolic rate

 
• OP

  oxygen pulse

 
• ROP

  resting oxygen pulse

 
• RQ

  respiratory quotient

 
• s.e.e.

  standard error of the estimate

 
• s.e.m.

  standard error of the mean

 
• TEB

  time–energy budget

 
• V̇ _O2

  rate of oxygen consumption

 
• V̇ _O2est

  estimated rate of oxygen consumption

This study was supported by grants from the French Institute of Polar Research (IFRTP Program 131) and from the Natural Environmental Research Council of the UK. G.F. was supported by a Marie Curie Fellowship from the EU(grant ERBFMBICT972460) The authors wish to thank Dr Jean Patrice Robin together with the members of the 34th and 35th over-wintering missions on Crozet Island for their assistance on the field and the crew of the Marion Dufresnes for logistical support.