Summary 1.Measuring the metabolic rate of animals in the field (FMR) is central to the work of ecologists in many disciplines. In this article we discuss the pros and cons of the two most commonly used methods for measuring FMR. 2. Both methods are constantly under development, but at the present time can only accurately be used to estimate the mean rate of energy expenditure of groups of animals. The doubly labelled water method (DLW) uses stable isotopes of hydrogen and oxygen to trace the flow of water and carbon dioxide through the body over time. From these data, it is possible to derive a single estimate of the rate of oxygen consumption ( ) for the duration of the experiment. The duration of the experiment will depend on the rate of flow of isotopes of oxygen and hydrogen through the body, which in turn depends on the animal's size, ranging from 24 h for small vertebrates to up to 28 days in Humans. 3. This technique has been used widely, partly as a result of its relative simplicity and potential low cost, though there is some uncertainty over the determination of the standard error of the estimate of mean . 4. The heart rate ( f H ) method depends on the physiological relationship between heart rate and . 5. If these two quantities are calibrated against each other under controlled conditions, f H can then be measured in free-ranging animals and used to estimate . 6. The latest generation of small implantable data loggers means that it is possible to measure f H for over a year on a very fine temporal scale, though the current size of the data loggers limits the size of experimental animals to around 1 kg. However, externally mounted radio-transmitters are now sufficiently small to be used with animals of less than 40 g body mass. This technique is gaining in popularity owing to its high accuracy and versatility, though the logistic constraint of performing calibrations can make its use a relatively extended process.

Summary 1.Breeding female Macaroni Penguins ( Eudyptes chrysolophus ) were implanted with heart rate, temperature and depth data loggers in order to estimate their rate of energy expenditure from heart rate. 2. The estimated average daily metabolic rates (ADMR) of birds during the brood and crèche phases of the breeding season were 8·17 ± 0·44 W kg − 1 and 8·24 ± 0·41 W kg − 1 , respectively. There was no significant difference between these estimates and the pooled ADMR for all birds was 8·22 ± 0·38 W kg − 1 . 3.Metabolic rate while the penguins were at-sea was 9·03 ± 0·39 W kg − 1 and this was significantly greater than the metabolic rate of 6·27 ± 0·38 W kg − 1 while they were on-shore. 4. Females undertook shorter foraging trips during the crèche phase than during the brood phase. 5. When considered as a breeding pair, Macaroni Penguins expend more energy in raising their chick during the crèche phase of the breeding season as the male assists in provisioning the chick only at this time.

Macaroni penguins were implanted with data loggers to record heart rate (fH), abdominal temperature (T ab ) 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 (sV O ∑ DC ) declined significantly and mirrored the pattern of increase in maximum duration and depth. The decline in sV O ∑ DC was matched by a decline in minimum rate of oxygen consumption (sV O ∑ min ). When sV O ∑ min was subtracted from sV O ∑ DC , 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 sV O ∑ min .Abdominal temperature declined during winter but this was not sufficient to explain the decline in sV O ∑ min . A simple model of the interactions between sV O ∑ min , 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 sV O ∑ min and hence sV O ∑ DC .Key words: macaroni penguin, Eudyptes chrysolophus, diving, seasonal change, oxygen consumption, thermoregulation, cADL. Summary IntroductionDo seasonal changes in metabolic rate facilitate changes in diving behaviour?