Behavioral and physiological significance of minimum resting metabolic rate in king penguins.

Because fasting king penguins (Aptenodytes patagonicus) need to conserve energy, it is possible that they exhibit particularly low metabolic rates during periods of rest. We investigated the behavioral and physiological aspects of periods of minimum metabolic rate in king penguins under different circumstances. Heart rate (f(H)) measurements were recorded to estimate rate of oxygen consumption during periods of rest. Furthermore, apparent respiratory sinus arrhythmia (RSA) was calculated from the f(H) data to determine probable breathing frequency in resting penguins. The most pertinent results were that minimum f(H) achieved (over 5 min) was higher during respirometry experiments in air than during periods ashore in the field; that minimum f(H) during respirometry experiments on water was similar to that while at sea; and that RSA was apparent in many of the f(H) traces during periods of minimum f(H) and provides accurate estimates of breathing rates of king penguins resting in specific situations in the field. Inferences made from the results include that king penguins do not have the capacity to reduce their metabolism to a particularly low level on land; that they can, however, achieve surprisingly low metabolic rates at sea while resting in cold water; and that during respirometry experiments king penguins are stressed to some degree, exhibiting an elevated metabolism even when resting.

Factorial aerobic scope is independent of temperature and primarily modulated by heart rate in exercising Murray cod (Maccullochella peelii peelii).

Several previous reports, often from studies utilising heavily instrumented animals, have indicated that for teleosts, the increase in cardiac output (Vb) during exercise is mainly the result of an increase in cardiac stroke volume (V(S)) rather than in heart rate (fH). More recently, this contention has been questioned following studies on animals carrying less instrumentation, though the debate continues. In an attempt to shed more light on the situation, we examined the heart rates and oxygen consumption rates (Mo2; normalised to a mass of 1 kg, given as Mo2kg) of six Murray cod (Maccullochella peelii peelii; mean mass+/-SE = 1.81+/-0.14 kg) equipped with implanted fH and body temperature data loggers. Data were determined during exposure to varying temperatures and swimming speeds to encompass the majority of the biological scope of this species. An increase in body temperature (Tb) from 14 degrees C to 29 degrees C resulted in linear increases in Mo2kg (26.67-41.78 micromol min(-1) kg(-1)) and fH (22.3-60.8 beats min(-1)) during routine exercise but a decrease in the oxygen pulse (the amount of oxygen extracted per heartbeat; 1.28-0.74 micromol beat(-1) kg(-1)). During maximum exercise, the factorial increase in Mo2kg was calculated to be 3.7 at all temperatures and was the result of temperature-independent 2.2- and 1.7-fold increases in fH and oxygen pulse, respectively. The constant factorial increases in fH and oxygen pulse suggest that the cardiovascular variables of the Murray cod have temperature-independent maximum gains that contribute to maximal oxygen transport during exercise. At the expense of a larger factorial aerobic scope at an optimal temperature, as has been reported for species of salmon and trout, it is possible that the Murray cod has evolved a lower, but temperature-independent, factorial aerobic scope as an adaptation to the largely fluctuating and unpredictable thermal climate of southeastern Australia.

The use of data loggers to determine the energetics and physiology of aquatic birds and mammals.

By deploying a data logger specifically designed for the purpose, it was possible to record heart rate, fH (beats/min), from free-ranging gentoo penguins, Pygoscelis papua, and Antarctic fur seals, Arctocephalus gazella, at the British Antarctic Survey Base at Bird Island. The heart rate data were then converted into oxygen consumption (VO2, mlO2 min-1 kg-1) and/or energy expenditure (W/kg) using equations that had been derived from calibration experiments. In these experiments the relationships between fH and VO2 were determined in animals at rest and while exercising at different levels on a treadmill or in a static water channel (penguins) and in a variable speed flume (California sea lions, Zalophus californianus, as surrogate fur seals) or in a static water channel (fur seals). The validity of using these relationships was tested by recording simultaneously fH, VO2 by direct respirometry and VCO2 by the doubly labelled water (DLW) technique in six penguins and in six California sea lions during 72 h and 24 h, respectively, at various levels of activity. For the penguins, both indirect methods gave mean algebraic errors within 2% of the measured VO2, whereas for the sea lions, the mean algebraic errors were 36.4% for the DLW method and 2.7% for the fH method. The range of errors was greater for the DLW method, in both species. Field data from 15 penguins indicate that the fH method provides data that are comparable to those obtained using the DLW method, but with the added advantage that they can be broken down into the energy costs for specific types of behaviour. The implanted data loggers also recorded the temperature of the abdominal cavity (T(ab)) and it was evident that this routinely decreased by approximately 2 degrees C during diving bouts and by a maximum of almost 5 degrees C. Such temperature decreases, particularly if representative of similar decreases in other tissues, may at least partly explain why the energy costs of travelling to and from the foraging site and of foraging itself are similar to those for penguins resting in water at 5 degrees C. Field data from 15 female fur seals indicate that, when the animals are ashore, there is good agreement between the values for mean energy expenditure obtained by the fH and DLW methods. However, when the animals are at sea, the values obtained by DLW are substantially greater than those obtained by fH. When the at-sea values are corrected for the apparent overestimations referred to above, there is good agreement between the data obtained using the two methods. The data derived from fH indicate that, as with the penguins, the energy expenditures during travelling to and from the foraging site and during foraging are similar to those of fur seals resting in water at 7 degrees C.

Impact of externally attached loggers on the diving behaviour of the king penguin.

The impact of relatively small externally attached time series recorders on some foraging parameters of seabirds was investigated during the austral summer of 1995 by monitoring the diving behaviour of 10 free-ranging king penguins (Aptenodytes patagonicus) over one foraging trip. Time-depth recorders were implanted in the abdominal cavities of the birds, and half of the animals also had dummy loggers attached on their backs. Although most of the diving behaviour was not significantly affected by the external loggers (P>0.05), the birds with externally attached loggers performed almost twice as many shallow dives, between 0 and 10 m depth, as the birds without external loggers. These shallow dives interrupted more frequently the deep-diving sequences in the case of birds with external loggers (percentage of deep dives followed by deep dives: 46% for birds with implants only vs. 26% for birds with an external attachment). Finally, the distribution pattern of the postdive durations plotted against the hour of the day was more heterogeneous for the birds with an external package. In addition, these penguins had extended surfacing times between two deep dives compared to birds without external attachments (P<0.0001). These results suggest the existence of an extra energy cost induced by externally attached loggers.

Implantable data logging system for heart rate and body temperature: its application to the estimation of field metabolic rates in Antarctic predators.

The metabolic rate of free-ranging animals is difficult to measure, but of great importance in understanding the interactions of a species with its environment. Heart rate can, if correctly validated and calibrated, give an estimate of metabolic rate, with both a fine time resolution and over long periods. The telemetry of heart rate is well documented, but is not appropriate over long ranges (possibly several thousands of kilometres) or for diving species. An implantable data logger has therefore been developed for the long term recording of heart rate and body temperature. The logger is built using hybrid and ASIC construction techniques, weighs 20 g and measures 55 x 24 x 6 mm. The device is programmable, and its solid-state memory holds over 70 days of data if, for example, heart rate is counted and stored every minute. Current consumption is 155 microA while logging, 50 microA during a programmable initial delay period, and less than 1 microA when the logger closes down after filling its memory. These loggers have been deployed for two field seasons in gentoo penguins, black-browed albatross and fur seals.

Recovery from swimming-induced hypothermia in king penguins: effects of nutritional condition.

We investigated changes in the rate of oxygen consumption (V O2) and body temperature of wild king penguins (Aptenodytes patagonicus) in different nutritional conditions during recovery after exposure to cold water. Over time, birds undertook an identical experiment three times, each characterized by different nutritional conditions: (1) having recently completed a foraging trip, (2) after fasting for many days, and (3) having been refed one meal after the fast. The experiments consisted of a 2-h session in a water channel followed by a period of recovery in a respirometer chamber on land. Refed birds recovered significantly more quickly than fed birds, in terms of both time to reach resting V O2 on land and time to reach recovery of lower abdominal temperature. Previous work found that when penguins are in cold water, abdominal temperatures decrease less in refed birds than in fed or fasted birds, suggesting that refed birds may be vasoconstricting the periphery while perfusing the gut region to access nutrients. This, alongside an increased resting [V O2], seems the most reasonable explanation for why refed birds recovered more quickly subsequent to cold-water exposure in this study; that is, vasoconstriction of the insulative periphery meant that they lost less heat generated by the body core.

Modeling the marine resources consumed in raising a king penguin chick: an energetics approach.

Accurate estimates of penguin energetics would represent an important contribution to our understanding of the trophodynamics of the Southern Ocean ecosystem and our ability to predict effects of environmental change on these species. We used the heart rate-rate of oxygen consumption technique to estimate rate of energy expenditure in adult king penguins raising a chick, in combination with data from the literature on changes in adult mass, chick energy requirements, and prey energy density. Our model estimated a variety of energetic costs and quantities of prey consumption related to raising a king penguin chick during the austral summer. The total energy requirements of a king penguin chick at the Crozet Archipelago from hatching until reaching a mass of 8 kg 90 d later is 271 MJ, representing the consumption of 38.4 kg of myctophid fish. A successfully breeding male requires 0.78 kg d(-1) of fish during the entirety of the incubation period and 1.14 kg d(-1) during the subsequent 90 d of chick rearing. Assuming the same energy requirements for females, the estimated 580,000 pairs of king penguins that breed successfully at Crozet each year, together with their chicks, consume a total of around 190,000 tons of fish during the incubation and summer rearing periods combined. If, due to depletion of fish stocks, the diet of breeders and chicks during the summer becomes identical to the typical diet of adults during the austral winter, the mass of prey required by both adults and chicks combined (where the chick still reaches 8 kg after 90 d) would increase by more than 25%.

Metabolism and thermoregulation during fasting in king penguins, Aptenodytes patagonicus, in air and water.

We measured oxygen consumption rate (Vo(2)) and body temperatures in 10 king penguins in air and water. Vo(2) was measured during rest and at submaximal and maximal exercise before (fed) and after (fasted) an average fasting duration of 14.4 +/- 2.3 days (mean +/- 1 SD, range 10-19 days) in air and water. Concurrently, we measured subcutaneous temperature and temperature of the upper (heart and liver), middle (stomach) and lower (intestine) abdomen. The mean body mass (M(b)) was 13.8 +/- 1.2 kg in fed and 11.0 +/- 0.6 kg in fasted birds. After fasting, resting Vo(2) was 93% higher in water than in air (air: 86.9 +/- 8.8 ml/min; water: 167.3 +/- 36.7 ml/min, P < 0.01), while there was no difference in resting Vo(2) between air and water in fed animals (air: 117.1 +/- 20.0 ml O(2)/min; water: 114.8 +/- 32.7 ml O(2)/min, P > 0.6). In air, Vo(2) decreased with M(b), while it increased with M(b) in water. Body temperature did not change with fasting in air, whereas in water, there were complex changes in the peripheral body temperatures. These latter changes may, therefore, be indicative of a loss in body insulation and of variations in peripheral perfusion. Four animals were given a single meal after fasting and the temperature changes were partly reversed 24 h after refeeding in all body regions except the subcutaneous, indicating a rapid reversal to a prefasting state where body heat loss is minimal. The data emphasize the importance in considering nutritional status when studying king penguins and that the fasting-related physiological changes diverge in air and water.

Do seasonal changes in metabolic rate facilitate changes in diving behaviour?

Macaroni penguins were implanted with data loggers to record heart rate (fH), abdominal temperature (Tab) and diving depth during their pre-moult trip (summer) and winter migration. The penguins showed substantial differences in diving behaviour between the seasons. During winter, mean and maximum dive duration and dive depth were significantly greater than during summer, but the proportion of dives within the calculated aerobic dive limit (cADL) did not change. Rates of oxygen consumption were estimated from fH. As winter progressed, the rate of oxygen consumption during dive cycles (sVO2DC)) declined significantly and mirrored the pattern of increase in maximum duration and depth. The decline in sVO2DC) was matched by a decline in minimum rate of oxygen consumption (sVO2min)). When sVO2min) was subtracted from sVO2DC), the net cost of diving was unchanged between summer and winter. We suggest that the increased diving capacity demonstrated during the winter was facilitated by the decrease in sVO2min). Abdominal temperature declined during winter but this was not sufficient to explain the decline in sVO2min). A simple model of the interactions between sVO2min), thermal conductance and water temperature shows how a change in the distribution of fat stores and therefore a change in insulation and/or a difference in foraging location during winter could account for the observed reduction in sVO2min) and hence sVO2DC).

Heart rate in humans during underwater swimming with and without breath-hold.

Heart rate was monitored, by way of radiotelemetry, from 6 male subjects of mean age (+/- SE) 24 +/- 1 years and of mean mass 73.5 +/- 2.5 kg. Measurements were made in a 25 m pool at a water temperature of 28 degrees C. Resting heart rate was 67 +/- 3.7 beats X min-1 and when the subjects submerged themselves completely in the pool, but remained inactive, there was a prompt, gradual reduction in heart rate which reached 48 +/- 2.6 beats X min-1 within 30 sec and 40 +/- 2.6 beats X min-1 within 59 +/- 5.6 sec (maximum duration). When they propelled themselves under water for 33 sec by kicking their legs and breathed through a snorkel tube, heart rate increased progressively to a value of 118 +/- 4.1 beats X min-1 at 28 sec. However, when they performed the same manoeuvre while holding their breath, there was an initial increase in heart rate to 106 +/- 5.7 beats X min-1 within the first 10 sec. This was followed by a decline in heart rate which was more rapid than that recorded during inactive submersion and which eventually reached 48 +/- 4.4 beats X min-1 at mean underwater duration of 33 +/- 1.8 sec. It is concluded that during the first 10-15 sec of underwater breath-hold swimming in humans, the cardiovascular response (as indicated by heart rate) is similar to that seen during a similar level of exercise while breathing air. From then on there is a progressively more intense bradycardia which is probably indicative of an oxygen conserving response consisting of reduced perfusion of most of the body except the heart, CNS and active locomotory muscles. The degree and rate of onset of this proposed oxygen conserving response are influenced by the intensity of the exercise performed while under water and whether or not the period of underwater breath-hold swimming is preceded by exercise.

Seasonal hypothermia in a large migrating bird: saving energy for fat deposition?

We have tested the hypothesis that a large (2 kg) migratory bird, such as the barnacle goose Branta leucopsis, becomes hypothermic before its autumn migration, when food is not scarce, but when it is necessary to conserve and/or store energy in the form of fat. Abdominal temperature (T(ab)) was measured in wild geese using an implanted data logger. Commencing a few days before and continuing until approximately 20 days after the start of their autumn migration, mean daily T(ab) fell progressively by 4.4 degrees C. Thus, it is suggested that, rather than increasing the rate of pre-migratory fattening, the energy saved as a result of this hypothermia reduces the rate at which fat is used and thus enables its more rapid replacement following and, possibly during, migration. The energy saved may also be used for the replacement of non-fat tissues such as the locomotory muscles and gastro-intestinal tract. These observations are the first of their kind from birds in their natural environment and, together with other data, demonstrate that hypothermia in endotherms is not necessarily related to extremely low environmental temperature, to shortage of food or to the resting phase of the daily cycle. The data also highlight the relationship between hypothermia and fat deposition over extended periods in relatively large, endothermic animals and may have some relevance to obesity in humans.

An implantable transmitter for monitoring heart rate and respiratory frequency in diving ducks.

An implantable telemetry transmitter is described for monitoring respiratory frequency and heart rate in animal physiological studies. The ECG is transmitted directly while the respiratory signal, derived from the temperature variation of the air in the respiratory tract, modulates the frequency of a subcarrier oscillator. A magnetic switch allows the implanted device to be switched on and off remotely. Details are given of the high packing density achieved by a modified cordwood method of construction. This system is being used to investigate the changes in heart rate and respiratory frequency associated with spontaneous diving in ducks, and typical results are presented.

Control of heart rate by carotid body chemoreceptors during diving in tufted ducks.

Previous work has shown that during forcible submersion of domestic ducks there is a gradual reduction in heart rate to 10-20% of its predive value after 45-60 s. Bilateral denervation of the carotid body chemoreceptors abolishes most of this bradycardia. By use of implanted radio transmitters it has been shown that in free-swimming tufted ducks, Aythya fuligula, there is an immediate reduction in heart rate, on spontaneous diving, from an elevated predive level. It then increases for a few seconds before it stabilizes at a level similar to that recorded when the duck is swimming fairly vigorously. The present study has shown that, following bilateral denervation of the carotid bodies, there is a significant increase in mean dive duration but no effect on the immediate reduction in heart rate on submersion. Heart rate is, however, significantly higher toward the end of spontaneous dives after carotid body denervation. Unlike the situation in mallards and their domesticated varieties, carotid body denervation has no effect on heart rate in tufted ducks during the first 40 s of forced dives. The carotid bodies therefore do not play the dominant role in cardiac control during submersion of diving ducks that has been suggested by work involving the forcible submersion of the mallard duck and its domesticated varieties.

Heart rate and aerobic metabolism in Humboldt penguins, Spheniscus humboldti, during voluntary dives.

Heart rate and aerobic metabolism have been recorded from three Humboldt penguins, Spheniscus humboldti, freely diving on a freshwater pond (9 X 4.6 X 2.7 m deep), using an implanted radiotransmitter and an open circuit respirometer. Oxygen uptake at mean dive duration (46.2s) was 26% greater than the resting value, but the difference was not statistically significant. Heart rate was also similar to the resting value. It is concluded that voluntary dives of penguins are completely aerobic and that oxygen stores are sufficient to allow metabolism to continue at the rate estimated in the present study for 2.27 min during voluntary submersion. This is longer than that calculated for tufted ducks, probably because the penguins are more efficient at underwater locomotion and because they are almost neutrally buoyant.

Diving behaviour and heart rate in tufted ducks (Aythya fuligula).

Diving behaviour and heart rate were monitored in tufted ducks diving under circumstances which simulated various environmental conditions such as feeding under ice in winter. When distance to food was increased on a covered outdoor pond, dive duration increased proportionately, but it was calculated that time available for feeding was reduced during the longer-distance 'extended' dives. There was a gradual reduction in heart rate to 77.3 +/- 13.8 beats min-1, which is significantly lower than the resting value of 121.1 +/- 14.1 beats min-1, during the course of extended dives, suggesting that the ducks could gradually switch over to a 'classical' oxygen-conserving response during these prolonged voluntary dives. The duration of the pre-dive preparatory period was positively correlated with dive distance. When the ducks were briefly unable to resurface during an otherwise normal feeding dive in an indoor tank, a situation which may occur if they become disoriented under ice, there was an immediate switch to a full bradycardia. Reduction in heart rate during these 'enclosed' dives occurred only when the ducks were apparently aware of the situation and the rate of onset of bradycardia was very similar to that previously observed during involuntary submersion of tufted ducks. Minimum heart rate was the same at 46 beats min-1 after 15 s of enclosed dives and after 30 s of involuntary submersions, despite the differences in levels of activity in the two situations.

The synchronization of ventilation and locomotion in horses (Equus caballus).

Ciné film and synchronized records of respiratory flow were obtained from Thoroughbred racehorses cantering on a treadmill at speeds of 9 and 11 m s-1. Horses and some other galloping and hopping mammals link their breathing and locomotion, taking exactly one breath per stride. Three theoretical mechanisms by which the movements of locomotion might drive ventilation are considered. (i) Flexion of the lumbosacral joint and the resulting forward sweep of the pelvis pushes the viscera against the diaphragm. However, back flexion lags behind ventilation at 11 m s-1 and could not exclusively drive ventilation at this speed. (ii) Loading of the thorax by the impact of the forelimbs with the ground might force air out of the lungs. If the respiratory system were damped sufficiently to perform as this mechanism requires, the work of driving ventilation would make up approximately 15% of the total work of running. In comparison with other estimates of the work of ventilation this seems improbably high. (iii) The observed phase relationship between displacements of the viscera, caused by the accelerations of the body during running, and respiratory airflow is not consistent with a tuned visceral piston mechanism driving breathing. Thus, it would seem likely that back flexion is likely to contribute towards driving ventilation but loading of the thorax and the visceral piston mechanism do not.

Heart rate as an indicator of oxygen consumption: influence of body condition in the king penguin.

The use of heart rate to estimate field metabolic rate has become a more widely used technique. However, this method also has some limitations, among which is the possible impact that several variables such as sex, body condition (i.e. body fat stores) and/or inactivity might have on the relationship between heart rate and rate of oxygen consumption. In the present study, we investigate the extent to which body condition can affect the use of heart rate as an indicator of the rate of oxygen consumption. Twenty-two breeding king penguins (Aptenodytes patagonicus) were exercised on a variable-speed treadmill. These birds were allocated to four groups according to their sex and whether or not they had been fasting. Linear regression equations were used to describe the relationship between heart rate and the rate of oxygen consumption for each group. There were significant differences between the regression equations for the four groups. Good relationships were obtained between resting and active oxygen pulses and an index of the body condition of the birds. Validation experiments on six courting king penguins showed that the use of a combination of resting oxygen pulse and active oxygen pulse gave the best estimate of the rate of oxygen consumption V(O2). The mean percentage error between predicted and measured V(O2) was only +0.81% for the six birds. We conclude that heart rate can be used to estimate rate of oxygen consumption in free-ranging king penguins even over a small time scale (30 min). However, (i) the type of activity of the bird must be known and (ii) the body condition of the bird must be accurately determined. More investigations on the impact of fasting and/or inactivity on this relationship are required to refine these estimates further.

Estimation of the rate of oxygen consumption of the common eider duck (Somateria mollissima), with some measurements of heart rate during voluntary dives.

The relationship between heart rate (f(H)) and rate of oxygen consumption (V(O2)) was established for a marine diving bird, the common eider duck (Somateria mollissima), during steady-state swimming and running exercise. Both variables increased exponentially with speed during swimming and in a linear fashion during running. Eleven linear regressions of V(O2) (ml kg(-1 )min(-1)) on f(H) (beats min(-1)) were obtained: five by swimming and six by running the birds. The common regression was described by V(O2)=10.1 + 0.15f(H) (r(2)=0.46, N=272, P<0.0001). The accuracy of this relationship for predicting mean V(O2) was determined for a group of six birds by recording f(H) continuously over a 2-day period and comparing estimated V(O2) obtained using the common regression with (i) V(O2) estimated using the doubly labelled water technique (DLW) and (ii) V(O2) measured using respirometry. A two-pool model produced the most accurate estimated V(O2) using DLW. Because of individual variability within mean values of V(O2) estimated using both techniques, there was no significant difference between mean V(O2) estimated using f(H) or DLW and measured V(O2) values (P>0.2), although individual errors were substantially less when f(H) was used rather than DLW to estimate V(O2). Both techniques are, however, only suitable for estimating mean V(O2) for a group of animals, not for individuals. Heart rate and behaviour were monitored during a bout of 63 voluntary dives by one female bird in an indoor tank 1.7 m deep. Tachycardia occurred both in anticipation of and following each dive. Heart rate decreased before submersion but was above resting values for the whole of the dive cycle. Mean f(H) at mean dive duration was significantly greater than f(H) while swimming at maximum sustainable surface speeds. Heart rate was used to estimate mean V(O2) during the dive cycle and to predict aerobic dive limit (ADL) for shallow dives.

Relationship between heart rate and oxygen consumption during steady-state swimming in California sea lions.

Heart rate (fH) and rate of oxygen uptake (VO2) were measured in six subadult California sea lions Zalophus californianus while they were at rest and while they were swimming for 15 min at controlled speeds of up to 1.4 m s-1 and pulling loads of up to 3 kg. There was a good linear relationship between fH and VO2 in all six sea lions. The slopes of the individual regression lines varied between 2.66 and 4.36 beats ml-1 O2 kg-1, the intercepts varied between 48.2 and 63.0 beats min-1 and r2 varied between 0.82 and 0.93. The mean relationship for all six sea lions is fH = (57.4 +/- 2.0) + (3.58 +/- 0.23) VO2, r2 = 0.89 +/- 0.01. The mean of the lowest VO2 values was 5.1 +/- 0.4 ml min-1 kg-1 and the mean of the highest VO2 values was 26.9 +/- 1.9 ml min-1 kg-1. The means of the lowest and highest values of fH were less extreme, being 72 +/- 3 beats min-1 and 155 +/- 5 beats min-1, respectively. It is concluded that, by using data storage devices and grouped data, fH could be used in otariids as an indicator of aerobic metabolism under field conditions, in particular for breeding females during the period of lactation.

Energetics of diving in macaroni penguins.

Heart rate (fH), abdominal temperature (T(ab)) and diving depth were measured in thirteen free-ranging breeding female macaroni penguins. Measurement of these variables allowed estimation of the mass-specific rate of oxygen consumption (V(O(2))) while diving and investigation of the physiological adjustments that might facilitate the diving behaviour observed in this species. In common with other diving birds, macaroni penguins showed significant changes in fH associated with diving, and these variables accounted for 36% of the variation in dive duration. When V(O(2)) was calculated for dives of different durations, 95.3% of dives measured were within the calculated aerobic dive limit (cADL) for this species. Mean fH for all complete dive cycles was 147+/-6 beats min(-1). When this fH is used to estimate (O(2)) of 26.2+/-1.4 ml min(-1) kg(-1) then only 92.8% of dives measured were within the cADL. Significant changes in abdominal temperature were not detected within individual dives, though the time constant of the measuring device used may not have been low enough to record these changes if they were present. Abdominal temperature did decline consistently during bouts of repeated diving of all durations and the mean decrease in T(ab) during a diving bout was 2.32+/-0.2 degrees C. There was a linear relationship between bout duration and the magnitude of this temperature drop. There was no commensurate increase in dive duration during dive bouts as T(ab) declined, suggesting that macaroni penguins are diving within their physiological limits and that factors other than T(ab) are important in determining the duration of dives and dive bouts. Lowered T(ab) will in turn facilitate lower metabolic rates during diving bouts, but it was not possible in the present study to determine the importance of this energy saving and whether it is occurs actively or passively.