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

Embryonic heart rate is higher in species that experience greater nest predation risk during incubation

Avian eggs develop outside of the female body, and therefore embryonic development is subject to multiple internal (physiological) and external (ecological) factors. Embryonic developmental rate has important consequences for survival. Within species, embryos that develop too quickly often experience deformities, disorders, or mortality, while embryos that develop slowly risk inviability and increase the time they are exposed to various sources of mortality in the nest. These contrasting forces may lead to interspecific variation in developmental rates. We investigated potential factors affecting embryonic heart rate (EHR), a proxy of development, across 14 passerine species in the field. More specifically, we investigated if nest predation risk, clutch size, seasonality, and egg volume influenced EHR. From previous research, we expected, and found, that EHR was positively associated with embryonic age and egg temperature. Species with greater nest predation risk had higher EHR, shorter incubation periods, and lower nest temperature variance. EHR increased as the season progressed and with egg volume, while EHR declined with clutch size. Bird species exhibit varying strategies to increase nestling and fledgling survival in response to predation risk, and these results suggest that variation in embryonic development may be related to species-specific differences in nest predation risk.

Heart rate as a proxy for estimating oxygen consumption rates in loggerhead turtles (Caretta caretta)

Heart rates of air-breathing diving animals can change on a short time scale due to the diving response during submergence. Heart rate is used frequently as a proxy for indirectly estimating metabolic rates on a fine time scale. However, most studies to date have been conducted on endothermic diving animals, and the relationships between metabolic rates and heart rates in ectothermic diving animals have not been well studied. Sea turtles are unique model organisms of diving ectotherms because they spend most of their life in the ocean and perform deep and/or long dives. In this study, we examined the relationship between heart rates and metabolic rates in captive loggerhead turtles, Caretta caretta, to estimate oxygen consumption rates during each dive based on heart rates. The oxygen consumption rates (V̇_O2: mlO₂ min⁻¹ kg⁻¹) and average heart rates (f_H: beats min⁻¹) were measured simultaneously in indoor tanks at water temperatures of 15–25°C. Our results showed that oxygen consumption rate was affected by heart rate and water temperature in loggerhead turtles. Based on the collected data, we formulated the model equation as V̇_O2=0.0124f_H+0.0047T_w - 0.0791. The equation can be used for estimating fine-scaled field metabolic rates in free-ranging loggerhead turtles. The results of this study will contribute to future comparative studies of the physiological states of ectothermic diving animals.

Summary: The relationship between oxygen consumption rate and heart rate in the loggerhead turtle.

Modeling Tissue and Blood Gas Kinetics in Coastal and Offshore Common Bottlenose Dolphins, Tursiops truncatus

Bottlenose dolphins (Tursiops truncatus) are highly versatile breath-holding predators that have adapted to a wide range of foraging niches from rivers and coastal ecosystems to deep-water oceanic habitats. Considerable research has been done to understand how bottlenose dolphins manage O2 during diving, but little information exists on other gases or how pressure affects gas exchange. Here we used a dynamic multi-compartment gas exchange model to estimate blood and tissue O2, CO2, and N2 from high-resolution dive records of two different common bottlenose dolphin ecotypes inhabiting shallow (Sarasota Bay) and deep (Bermuda) habitats. The objective was to compare potential physiological strategies used by the two populations to manage shallow and deep diving life styles. We informed the model using species-specific parameters for blood hematocrit, resting metabolic rate, and lung compliance. The model suggested that the known O2 stores were sufficient for Sarasota Bay dolphins to remain within the calculated aerobic dive limit (cADL), but insufficient for Bermuda dolphins that regularly exceeded their cADL. By adjusting the model to reflect the body composition of deep diving Bermuda dolphins, with elevated muscle mass, muscle myoglobin concentration and blood volume, the cADL increased beyond the longest dive duration, thus reflecting the necessary physiological and morphological changes to maintain their deep-diving life-style. The results indicate that cardiac output had to remain elevated during surface intervals for both ecotypes, and suggests that cardiac output has to remain elevated during shallow dives in-between deep dives to allow sufficient restoration of O2 stores for Bermuda dolphins. Our integrated modeling approach contradicts predictions from simple models, emphasizing the complex nature of physiological interactions between circulation, lung compression, and gas exchange.

Does the Treadmill Support Valid Energetics Estimates of Field Locomotion?

SYNOPSIS

Quantifying animal energy expenditure during locomotion in the field is generally based either on treadmill measurements or on estimates derived from a measured proxy. Two common proxies are heart rate (ƒH) and dynamic body acceleration (accelerometry). Both ƒH and accelerometry have been calibrated extensively under laboratory conditions, which typically involve prompting the animal to locomote on a treadmill at different speeds while simultaneously recording its rate of oxygen uptake (V̇o2) and the proxy. Field estimates of V̇o2 during locomotion obtained directly from treadmill running or from treadmill-calibrated proxies make assumptions about similarities between running in the field and in the laboratory. The present study investigated these assumptions, focusing on humans as a tractable species. First we investigated experimentally if and how the rate of energy expenditure during treadmill locomotion differs to that during field locomotion at the same speeds, with participants walking and running on a treadmill, on tarmac, and on grass, while wearing a mobile respirometry system. V̇o2 was substantially higher during locomotion in both of the field conditions compared with on a level treadmill: 9.1% on tarmac and 17.7% on grass. Second, we included these data in a meta-analysis of previous, related studies. The results were influenced by the studies excluded due to particulars of the experiment design, suggesting that participant age, the surface type, and the degree of turning during field locomotion may influence by how much treadmill and field locomotion V̇o2 differ. Third, based on our experiments described earlier, we investigated the accuracy of treadmill-calibrated accelerometry and ƒH for estimating V̇o2 in the field. The mean algebraic estimate errors varied between 10% and 35%, with the ƒH associated errors being larger than those derived from accelerometry. The mean algebraic errors were all underestimates of field V̇o2, by around 10% for fH and varying between 0% and 15% for accelerometry. Researchers should question and consider how accurately a treadmill-derived proxy calibration of V̇o2 will estimate V̇o2 during terrestrial locomotion in free-living animals.

Measurement of flying and diving metabolic rate in wild animals: Review and recommendations

Animals' abilities to fly long distances and dive to profound depths fascinate earthbound researchers. Due to the difficulty of making direct measurements during flying and diving, many researchers resort to modeling so as to estimate metabolic rate during each of those activities in the wild, but those models can be inaccurate. Fortunately, the miniaturization, customization and commercialization of biologgers has allowed researchers to increasingly follow animals on their journeys, unravel some of their mysteries and test the accuracy of biomechanical models. I provide a review of the measurement of flying and diving metabolic rate in the wild, paying particular attention to mass loss, doubly-labelled water, heart rate and accelerometry. Biologgers can impact animal behavior and influence the very measurements they are designed to make, and I provide seven guidelines for the ethical use of biologgers. If biologgers are properly applied, quantification of metabolic rate across a range of species could produce robust allometric relationships that could then be generally applied. As measuring flying and diving metabolic rate in captivity is difficult, and often not directly translatable to field conditions, I suggest that applying multiple techniques in the field to reinforce one another may be a viable alternative. The coupling of multi-sensor biologgers with biomechanical modeling promises to improve precision in the measurement of flying and diving metabolic rate in wild animals.

The relationship between body temperature, heart rate, breathing rate, and rate of oxygen consumption, in the tegu lizard (Tupinambis merianae) at various levels of activity

The present study determined whether EEG and/or EMG recordings could be used to reliably define activity states in the Brazilian black and white tegu lizard (Tupinambis merianae) and then examined the interactive effects of temperature and activity states on strategies for matching O2 supply and demand. In a first series of experiments, the rate of oxygen consumption (VO2), breathing frequency (fR), heart rate (fH), and EEG and EMG (neck muscle) activity were measured in different sleep/wake states (sleeping, awake but quiet, alert, or moving). In general, metabolic and cardio-respiratory changes were better indictors of the transition from sleep to wake than were changes in the EEG and EMG. In a second series of experiments, the interactive effects of temperature (17, 27 and 37 °C) and activity states on fR, tidal volume (VT), the fraction of oxygen extracted from the lung per breath (FIO2-FEO2), fH, and the cardiac O2 pulse were quantified to determine the relative roles of each of these variables in accommodating changes in VO2. The increases in oxygen supply to meet temperature- and activity-induced increases in oxygen demand were produced almost exclusively by increases in fH and fR. Regression analysis showed that the effects of temperature and activity state on the relationships between fH, fR and VO2 was to extend a common relationship along a single curve, rather than separate relationships for each metabolic state. For these lizards, the predictive powers of fR and fH were maximized when the effects of changes in temperature, digestive state and activity were pooled. However, the best r(2) values obtained were 0.63 and 0.74 using fR and fH as predictors of metabolic rate, respectively.

Heart rate responses provide an objective evaluation of human disturbance stimuli in breeding birds

Intuition is a poor guide for evaluating the effects of human disturbance on wildlife. Using the endangered Yellow-eyed penguin, Megadyptes antipodes, as an example, we show that heart rate responses provide an objective tool to evaluate human disturbance stimuli and encourage the wider use of this simple and low-impact approach. Yellow-eyed penguins are a flagship species for New Zealand's wildlife tourism; however, unregulated visitor access has recently been associated with reduced breeding success and lower first year survival. We measured heart rate responses of Yellow-eyed penguins via artificial eggs to evaluate a range of human stimuli regularly occurring at their breeding sites. We found the duration of a stimulus to be the most important factor, with elevated heart rate being sustained while a person remained within sight. Human activity was the next important component; a simulated wildlife photographer, crawling slowly around during his stay, elicited a significantly higher heart rate response than an entirely motionless human spending the same time at the same distance. Stimuli we subjectively might perceive as low impact, such as the careful approach of a 'wildlife photographer', resulted in a stronger response than a routine nest-check that involved lifting a bird up to view nest contents. A single, slow-moving human spending 20 min within 2 m from the nest may provoke a response comparable to that of 10 min handling a bird for logger deployment. To reduce cumulative impact of disturbance, any human presence in the proximity of Yellow-eyed penguins needs to be kept at a minimum. Our results highlight the need for objective quantification of the effects of human disturbance in order to provide a sound basis for guidelines to manage human activity around breeding birds.

Does low daily energy expenditure drive low metabolic capacity in the tropical robin, Turdus grayi?

Temperate and tropical birds possess divergent life history strategies. Physiological parameters including energy metabolism correlate with the life history such that tropical species with a slower 'pace of life' have lower resting and maximal metabolic rates than temperate congeners. To better understand the physiological mechanisms underlying these differences, we investigated the relationship of metabolic capacity, muscle oxidative capacity and activity patterns to variation in life history patterns in American robins (Turdus migratorius), while resident in central North America and Clay-colored robins (Turdus grayi) resident in Panama. We measured summit metabolism [Formula: see text] in birds from both tropical and temperate habitats and found that the temperate robins have a 60 % higher metabolic capacity. We also measured the field metabolic rate (FMR) of free-living birds using heart rate (HR) telemetry and found that temperate robins' daily energy expenditure was also 60 % higher. Thus, [Formula: see text] and FMR both reflect life history differences between the species. Further, both species operate at a nearly identical ~50 % of their thermogenic capacity throughout a given day. As a potential mechanism to explain differences in activity and metabolic capacity, we ask whether oxidative properties of flight muscle are altered in accordance with life history variation and found minimal differences in oxidative capacity of skeletal muscle. These data demonstrate a close relationship between thermogenic capacity and daily activity in free-living birds. Further, they suggest that the slow pace of life in tropical birds may be related to the maintenance of low activity rather than functional capacity of the muscle tissue.

Acclimatization of seasonal energetics in northern cardinals (Cardinalis cardinalis) through plasticity of metabolic rates and ceilings

Northern cardinals (Cardinalis cardinalis) are faced with energetically expensive seasonal challenges that must be met to ensure survival, including thermoregulation in winter and reproductive activities in summer. Contrary to predictions of life history theory that suggest breeding metabolic rate should be the apex of energetic effort, winter metabolism exceeds that during breeding in several temperate resident bird species. By examining whole-animal, tissue and cellular function, we ask whether seasonal acclimatization is accomplished by coordinated phenotypic plasticity of metabolic systems. We measured summit metabolism (V(O(2),sum)), daily energy expenditure (DEE) and muscle oxidative capacity under both winter (December to January) and breeding (May to June) conditions. We hypothesize that: (1) rates of energy utilization will be highest in the winter, contrary to predictions based on life history theory, and (2) acclimatization of metabolism will occur at multiple levels of organization such that birds operate with a similar metabolic ceiling during different seasons. We measured field metabolic rates using heart rate telemetry and report the first daily patterns in avian field metabolic rate. Patterns of daily energy use differed seasonally, primarily as birds maintain high metabolic rates throughout the winter daylight hours. We found that DEE and V(O(2),sum) were significantly greater and DEE occurred at a higher fraction of maximum metabolic capacity during winter, indicating an elevation of the metabolic ceiling. Surprisingly, there were no significant differences in mass or oxidative capacity of skeletal muscle. These data, highlighting the importance of examining energetic responses to seasonal challenges at multiple levels, clearly reject life history predictions that breeding is the primary energetic challenge for temperate zone residents. Further, they indicate that metabolic ceilings are seasonally flexible as metabolic effort during winter thermoregulation exceeds that of breeding.

Dive behaviour impacts the ability of heart rate to predict oxygen consumption in Steller sea lions (Eumetopias jubatus) foraging at depth

The predictive relationship between heart rate (f(H)) and oxygen consumption (VO2) has been derived for several species of marine mammals swimming horizontally or diving in tanks to shallow depths. However, it is unclear how dive activity affects the f(H):VO2 relationship and whether the existing equations apply to animals diving to deeper depths. We investigated these questions by simultaneously measuring the f(H) and VO2 of Steller sea lions (Eumetopias jubatus) under different activity states (surface resting or diving), types of dives (single dives or dive bouts), and depths (10 or 40 m). We examined the relationship over dives only and also over dive cycles (dive + surface interval). We found that f(H) could only predict VO2 over a complete single dive cycle or dive bout cycle (i.e. surface intervals had to be included). The predictive equation derived for sea lions resting on the surface did not differ from that for single dive cycles. However, the equation derived over dive bout cycles (multiple dives + surface intervals) differed from those for single dive cycles or surface resting, with similar f(H) for multiple dive bout equations yielding higher predicted VO2 than that for single dive bout cycles (or resting). The f(H):VO2 relationships were not significantly affected by dive duration, dive depth, water temperature or cumulative food consumed under the conditions tested. Ultimately, our results demonstrate that f(H) can be used to predict activity-specific metabolic rates of diving Steller sea lions, but only over complete dive cycles that include a post-dive surface recovery period.

It costs to be clean and fit: energetics of comfort behavior in breeding-fasting penguins

BACKGROUND

Birds may allocate a significant part of time to comfort behavior (e.g., preening, stretching, shaking, etc.) in order to eliminate parasites, maintain plumage integrity, and possibly reduce muscular ankylosis. Understanding the adaptive value of comfort behavior would benefit from knowledge on the energy costs animals are willing to pay to maintain it, particularly under situations of energy constraints, e.g., during fasting. We determined time and energy devoted to comfort activities in freely breeding king penguins (Aptenodytes patagonicus), seabirds known to fast for up to one month during incubation shifts ashore.

METHODOLOGY/PRINCIPAL FINDINGS

A time budget was estimated from focal and scan sampling field observations and the energy cost of comfort activities was calculated from the associated increase in heart rate (HR) during comfort episodes, using previously determined equations relating HR to energy expenditure. We show that incubating birds spent 22% of their daily time budget in comfort behavior (with no differences between day and night) mainly devoted to preening (73%) and head/body shaking (16%). During comfort behavior, energy expenditure averaged 1.24 times resting metabolic rate (RMR) and the corresponding energy cost (i.e., energy expended in excess to RMR) was 58 kJ/hr. Energy expenditure varied greatly among various types of comfort behavior, ranging from 1.03 (yawning) to 1.78 (stretching) times RMR. Comfort behavior contributed 8.8-9.3% to total daily energy expenditure and 69.4-73.5% to energy expended daily for activity. About half of this energy was expended caring for plumage.

CONCLUSION/SIGNIFICANCE

This study is the first to estimate the contribution of comfort behavior to overall energy budget in a free-living animal. It shows that although breeding on a tight energy budget, king penguins devote a substantial amount of time and energy to comfort behavior. Such findings underline the importance of comfort behavior for the fitness of colonial seabirds.

The heart rate method for estimating metabolic rate: review and recommendations

Under most circumstances heart rate (f(H)) is correlated with the rate of oxygen consumption (VO(2)) and hence the rate of energy expenditure or metabolic rate (MR). For over 60 years this simple principle has underpinned the use of heart rate to estimate metabolic rate in a range of animal species and to answer questions about their physiology, behaviour and ecology. The heart rate method can be applied both quantitatively and qualitatively. The quantitative approach is a two-stage process where firstly f(H) and MR are measured simultaneously under controlled conditions and a predictive calibration relationship derived. Secondly, measurements of heart rate are made and converted to estimates of MR using the calibration relationship. The qualitative approach jumps directly to the second stage, comparing estimates of f(H) under different circumstances and drawing conclusions about MR under the assumption that a relationship exists. This review describes the range of studies which have adopted either the quantitative or qualitative approach to estimating the MR of birds, mammals and reptiles. Studies have tended to focus on species, states and questions which are hard to measure, control or define using other techniques. For example, species studied include large, wide-ranging species such as ungulates, marine predators, and domestic livestock while research questions have concerned behaviours such as flight, diving and the effects of stress. In particular, the qualitative approach has applied to circumstances and/or species where it may be hard or impossible to derive a calibration relationship for practical reasons. The calibration process itself can be complex and a number of factors such as body mass, activity state and stress levels can affect the relationship between f(H) and VO(2). I recommend that a quantitative approach be adopted wherever possible but that this may entail deriving a calibration relationship which is practical and applicable, rather than the most accurate possible. I conclude with a series of recommendations for the application and development of this method.

Assessing the development and application of the accelerometry technique for estimating energy expenditure

A theoretically valid proxy of energy expenditure is the acceleration of an animal's mass due to the movement of its body parts. Acceleration can be measured by an accelerometer and recorded onto a data logging device. Relevant studies have usually derived a measure of acceleration from the raw data that represents acceleration purely due to movement of the animal. This is termed 'overall dynamic body acceleration' (ODBA) and to date has proved a robust derivation of acceleration for use as an energy expenditure proxy. Acceleration data loggers are generally easy to deploy and the measures recorded appear robust to slight variation in location and orientation. This review discusses important issues concerning the accelerometry technique for estimating energy expenditure and ODBA; deriving ODBA, calibrating ODBA, acceleration logger recording frequencies, scenarios where ODBA is less likely to be valid, and the power in recording acceleration and heart rate together. While present evidence suggests that ODBA may not quantify energy expenditure during diving by birds and mammals, several recent studies have assessed changes in mechanical work in such species qualitatively through variation in ODBA during periods of submergence. The use of ODBA in field metabolic studies is likely to continue growing, supported by its relative ease of use and range of applications.

Heart rate as a predictor of energy expenditure in undisturbed fasting and incubating penguins

Heart rate (f(H)) measurement offers the possibility to monitor energy expenditure (EE) in wild animals if the EE/f(H) relationship for the species, physiological stages and activities of interest is known. This relationship has been extensively studied using oxygen consumption rate ( ) measurement in captive, repeatedly handled king penguins (Aptenodytes patagonicus). Unfortunately, the potential effects of stress on the observed relationships resulting from handling and confinement were not considered. This study is the first involving undisturbed animals, and determines the EE/f(H) relationship in naturally fasting and freely incubating or captivity-acclimatized male and female king penguins. EE determination was based on (1) the measurement of body mass loss during periods of phase II fasting, and (2) the calculation of its energy equivalent from changes in body composition, i.e. 23.9 kJ g(-1). f(H) levels in freely incubating and captivity-acclimatized birds were found to be 50-70% lower than those previously reported for resting king penguins during measurements. Significant EE/f(H) relationships were found in freely incubating and captive males and females (R(2)=0.59 to 0.84), with no difference observed between genders. The best overall relationship was obtained by including fasting duration (t, days) in the model: EE=818+43.7xf(H)+36.3t-1.4txf(H) (R(2)=0.91). This equation yielded EE estimates approximately 26% higher than the previously reported 'best' predictive equation in king penguins, and even more so when f(H) was low. This result suggests that stress induces a disproportionate increase of f(H) vs O(2) consumption, and that the use of EE/f(H) relationships obtained in stressed birds could lead to underestimated EE values.

Predicting the rate of oxygen consumption from heart rate in barnacle geese Branta leucopsis: effects of captivity and annual changes in body condition

Quantifying a relationship between heart rate (f(H)) and rate of oxygen consumption (V(O(2))) allows the estimation of V(O(2)) from f(H) recordings in free-ranging birds. It has been proposed that this relationship may vary throughout an animal's annual cycle, due to changes in physiological status. Barnacle geese, Branta leucopsis, provide an ideal model to test this hypothesis, as they exhibit significant intra-annual variability in body mass, body composition and abdominal temperature, even in captivity. Heart rate data loggers were implanted in 14 captive barnacle geese, and at six points in the year the relationship between f(H) and V(O(2)) was determined. The f(H)/V(O(2)) relationship was also determined in seven moulting wild barnacle geese to examine whether relationships from captive animals might be applicable to wild animals. In captive barnacle geese, the f(H)/V(O(2)) relationship was significantly different only between two out of the six periods when the relationship was determined (late September-early October and November). Accounting for changes in physiological parameters such as body mass, body composition and abdominal temperature did not eliminate this difference. The relationship between f(H) and V(O(2)) obtained from wild geese was significantly different from all of the relationships derived from the captive geese, suggesting that it is not possible to apply calibrations from captive birds to wild geese. However, the similarity of the f(H) and V(O(2)) relationship derived during moult in the captive geese to those during the remainder of the annual cycle implies it is not unreasonable to assume that the relationship between f(H)/V(O(2)) during moult in the wild geese is indicative of the relationship throughout the remainder of the annual cycle.

To fly or not to fly: high flight costs in a large sea duck do not imply an expensive lifestyle

A perennial question in ornithology is whether flight has evolved mostly to facilitate access to food or as an anti-predator strategy. However, flight is an expensive mode of locomotion and species using flight regularly are associated with an expensive lifestyle. Using heart rate (HR) data loggers implanted in 13 female common eiders (Somateria mollissima), our objective was to test the hypothesis that a high level of flight activity increases their energy budget. We used the long-term recording (seven months) of HR as an index of energy expenditure and the HR flight signature to compile all flight events. Our results indicate that the eider is one of the thriftiest volant birds with only 10 minutes of flight time per day. Consequently, we were not able to detect any effect of flight activity on their energy budget despite very high flight costs (123-149 W), suggesting that flight was controlled by energy budget limitations. However, the low flight activity of that species may also be related to their prey landscape requiring few or no large-scale movements. Nevertheless, we suggest that the (fitness) benefits of keeping flight ability in this species exceed the costs by allowing a higher survival in relation to predation and environmental harshness.

Increased energy expenditure but decreased stress responsiveness during molt

Baseline and stress-induced corticosterone (CORT), heart rate (fH), and energy expenditure were measured in eight captive European starlings Sturnus vulgaris during and following a prebasic molt. The fH and oxygen consumption (V O2 ) were measured simultaneously across a range of heart rates, and energy expenditure (kJ/d) was then calculated from data. Energy expenditure and fH were strongly and positively correlated in each individual. Baseline fH and energy expenditure were significantly higher during molt. Molting starlings expended 32% more energy over 24 h than nonmolting birds, with the most significant increase (60%) occurring at night, indicating a substantial energetic cost to molt. Furthermore, the cardiac and metabolic responses to stress during molt were different than during nonmolt. Birds were subjected to four different 30-min acute stressors. The fH and CORT responses to these stressors were generally lower during molt. Although restraint caused a 64% increase in daily energy expenditure during nonmolt, no other stressor caused a significant increase in energy expenditure. Overall, our data suggest that molt is not only energetically expensive but that it also alters multiple stress response pathways. Furthermore, most acute stressors do not appear to require a significant increase in energy expenditure.

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.

Acceleration versus heart rate for estimating energy expenditure and speed during locomotion in animals: tests with an easy model species, Homo sapiens

An important element in the measurement of energy budgets of free-living animals is the estimation of energy costs during locomotion. Using humans as a particularly tractable model species, we conducted treadmill experiments to test the validity of tri-axial accelerometry loggers, designed for use with animals in the field, to estimate rate of oxygen consumption (VO2: an indirect measure of metabolic rate) and speed during locomotion. The predictive power of overall dynamic body acceleration (ODBA) obtained from loggers attached to different parts of the body was compared to that of heart rate (fH). When subject identity was included in the statistical analysis, ODBA was a good, though slightly poorer, predictor of VO2 and speed during locomotion on the flat (mean of two-part regressions: R2=0.91 and 0.91, from a logger placed on the neck) and VO2 during gradient walking (single regression: R2=0.77 from a logger placed on the upper back) than was fH (R2=0.96, 0.94, 0.86, respectively). For locomotion on the flat, ODBA was still a good predictor when subject identity was replaced by subject mass and height (morphometrics typically obtainable from animals in the field; R2=0.92 and 0.89) and a slightly better overall predictor than fH (R2=0.92 and 0.85). For gradient walking, ODBA predicted VO2 more accurately than before (R2=0.83) and considerably better than did fH (R2=0.77). ODBA and fH combined were the most powerful predictor of VO2 and speed during locomotion. However, ODBA alone appears to be a good predictor and suitable for use in the field in particular, given that accelerometry traces also provide information on the timing, frequency and duration of locomotion events, and also the gait being used.

Improving the Precision and Accuracy for Estimating Energy Expenditure Using the Heart Rate Method

The "heart rate technique" is commonly used to estimate the rate of oxygen consumption (a proxy for energy expenditure) of free-ranging animals. However, a major limitation of this technique is that interindividual variability in the relationship between heart rate (f(H)) and rate of oxygen consumption (Vo2) generates large errors of estimation when the technique is applied to individual free-ranging animals. In this study, we present a new analysis technique that takes advantage of the observation that the f(H) or Vo2 relationships between individuals are frequently parallel and differ only in elevation. This technique offers superior accuracy and precision of Vo2 estimates, reducing the coefficient of variability from 18% to 9% for individual animals in an example application in macaroni penguins. This approach enables application of the heart rate technique to deduce the energetic strategies of individual animals.

Onshore energetics in penguins: Theory, estimation and ecological implications

Penguins are known to have high pedestrian locomotory costs in comparison to other cursorial birds, but the ecological consequences of this difference have received limited attention. Here we present a method for the accurate estimation of onshore energetics based on measurements of body mass, simple morphometrics and distance moved. The method is shown to be similarly accurate to other field-based estimates of energy expenditure, but has the advantage of logistical simplicity. King penguins spend 30-50% of their time ashore and may walk distances of several kilometres to and from their breeding colonies. However, in such cases the total energetic cost of pedestrian locomotion is estimated to be only 1.0% of the energy expended whilst ashore. Thus, despite a high instantaneous cost, pedestrian locomotion is a small and possibly negligible component of total energy turnover in king penguins.

How accurately can we estimate energetic costs in a marine top predator, the king penguin?

King penguins (Aptenodytes patagonicus) are one of the greatest consumers of marine resources. However, while their influence on the marine ecosystem is likely to be significant, only an accurate knowledge of their energy demands will indicate their true food requirements. Energy consumption has been estimated for many marine species using the heart rate-rate of oxygen consumption (f(H) - V(O2)) technique, and the technique has been applied successfully to answer eco-physiological questions. However, previous studies on the energetics of king penguins, based on developing or applying this technique, have raised a number of issues about the degree of validity of the technique for this species. These include the predictive validity of the present f(H) - V(O2) equations across different seasons and individuals and during different modes of locomotion. In many cases, these issues also apply to other species for which the f(H) - V(O2) technique has been applied. In the present study, the accuracy of three prediction equations for king penguins was investigated based on validity studies and on estimates of V(O2) from published, field f(H) data. The major conclusions from the present study are: (1) in contrast to that for walking, the f(H) - V(O2) relationship for swimming king penguins is not affected by body mass; (2) prediction equation (1), log(V(O2) = -0.279 + 1.24log(f(H) + 0.0237t - 0.0157log(f(H)t, derived in a previous study, is the most suitable equation presently available for estimating V(O2) in king penguins for all locomotory and nutritional states. A number of possible problems associated with producing an f(H) - V(O2) relationship are discussed in the present study. Finally, a statistical method to include easy-to-measure morphometric characteristics, which may improve the accuracy of f(H) - V(O2) prediction equations, is explained.

Predicting rate of oxygen consumption from heart rate while little penguins work, rest and play

The relationship between heart rate (f(H)) and rate of oxygen consumption (V(.)O2) was investigated under changing conditions of ambient temperature, digestive state and exercise state in the little penguin (Eudyptula minor). Both f(H) and V(.)O2 were recorded simultaneously from 12 little penguins while they each (a) rested and exercised within their reported thermo-neutral zone (TNZ), (b) rested and exercised below their reported TNZ and (c) digested a meal of sardines within their reported TNZ. Contrary to our expectations, we found that minimum V(.)O2 did not vary between the two temperatures used. Comparison with values from the literature suggests that both minimum V(.)O2 and the extent of the TNZ in this species may vary along a latitudinal gradient. Furthermore, while minimum V(.)O2 was unchanged at the lower temperature, minimum f(H) was significantly higher, suggesting a hitherto undescribed cardiac response to lowered ambient temperature in an avian species. This response was maintained when the penguins exercised within and below their apparent TNZ as f(H) was significantly greater in cold conditions for a given level of V(.)O2. Furthermore, both f(H) and V(.)O2 were slightly but significantly elevated for a given walking speed during exercise at the lower temperature. This suggests that the penguins may have been close to their TNZ and that the measures employed to counteract heat loss while at rest may have been compromised during exercise. There was no significant difference in the relationship between f(H) and V(.)O2 while the penguins were inactive ina post-digestive state or inactive and digesting a meal within their TNZ, though both of these relationships were significantly different from that during exercise. This suggests that while digestion has no effect on the f(H)/V(.)O2 relationship, for little penguins at least, it is of little value in deriving a predictive relationship for application to active free-ranging animals.

Allometric estimation of metabolic rate from heart rate in penguins

Studies of the relationship between heart rate (f(H)) and rate of oxygen consumption (V(.) (O(2))), which are then used to predict field metabolic rate, frequently fail to incorporate body mass as a predictive variable. This is a potentially important omission in the study of animals whose body mass fluctuates substantially during their annual cycle. In an attempt further to improve estimates of field metabolic rate from f(H), we re-evaluated data on M(b), f(H) and V(.) (O(2)) from previous studies of macaroni penguins (Eudyptes chrysolophus) and king penguins (Aptenodytes patagonicus) and derived a new relationship to integrate these three quantities. This relationship is at least as accurate and precise as previously determined relationships. We applied this same principle to published data on 11 of the 20 recognised penguin taxa to derive a relationship to predict V(.) (O(2)) from f(H) and M(b) in penguins of any species. This result has interesting implications in terms of reducing the logistical burden in studies of field metabolic rate.

Cardiovascular adjustments during locomotion in penguins

The high costs of airborne flapping flight are reflected in a difference between the oxygen pulse (OP; amount of oxygen consumed per heart beat) during flight and that during terrestrial locomotion, as well as a difference in the relationship between heart rate (fH) and rate of oxygen consumption ([Formula: see text]O2). We tested and failed to accept the hypothesis that there would be similar differences in the fH–[Formula: see text]O2and [Formula: see text]O2–OP relationships during swimming under water and during walking for macaroni penguins (Eudyptes chrysolophus (Brandt, 1837)). We suggest that this may be a result of the penguins having to overcome physical forces different from those experienced by volant birds and possibly a response to limited access to oxygen while porpoising or diving. This result has important implications for those who wish to use fHto estimate [Formula: see text]O2and hence metabolic rate in the field for this and similar species. To test the utility of the relationship, we compared our results with data obtained previously for this species, and we simultaneously measured [Formula: see text]O2in a respirometer and estimated [Formula: see text]O2from fHfor 48 h. There was no significant difference in the fH–[Formula: see text]O2or [Formula: see text]O2–OP relationships between the two studies or between [Formula: see text]O2estimated from fHand that measured using respirometry.

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

The main objective of this study was to determine heart rate (fh) and the energetic costs of specific behaviours of king penguins while ashore and while foraging at sea during their breeding period. In particular, an estimate was made of the energetic cost of diving in order to determine the proportion of dives that may exceed the calculated aerobic dive limit (cADL; estimated usable O2 stores/estimated rate of oxygen consumption during diving). An implanted data logger enabled fh and diving behaviour to be monitored from 10 free-ranging king penguins during their breeding period. Using previously determined calibration equations, it was possible to estimate rate of oxygen consumption (VO2) when the birds were ashore and during various phases of their foraging trips. Diving behaviour showed a clear diurnal pattern, with a mixture of deep (>40 m), long (>3 min) and shallow (<40 m), short (<3 min) dives from dawn to dusk and shallow, short dives at night. Heart rate during dive bouts and dive cycles (dive + post-dive interval) was 42% greater than that when the birds were ashore. During diving, fh was similar to the 'ashore' value (87+/-4 beats min(-1)), but it did decline to 76% of the value recorded from king penguins resting in water. During the first hour after a diving bout, fh was significantly higher than the average value during diving (101+/-4 beats min(-1)) and for the remainder of the dive bout. Rates of oxygen consumption estimated from these (and other) values of fh indicate that when at sea, metabolic rate (MR) was 83% greater than that when the birds were ashore [3.15 W kg(-1) (-0.71, +0.93), where the values in parentheses are the computed standard errors of the estimate], while during diving bouts and dive cycles, it was 73% greater than the 'ashore' value. Although estimated MR during the total period between dive bouts was not significantly different from that during dive bouts [5.44 W kg(-1) (-0.30, +0.32)], MR during the first hour following a dive bout was 52% greater than that during a diving bout. It is suggested that this large increase following diving (foraging) activity is, at least in part, the result of rewarming the body, which occurs at the end of a diving bout. From the measured behaviour and estimated values of VO2, it was evident that approximately 35% of the dives were in excess of the cADL. Even if VO2 during diving was assumed to be the same as when the birds were resting on water, approximately 20% of dives would exceed the cADL. As VO2 during diving is, in fact, that estimated for a complete dive cycle, it is quite feasible that VO2 during diving itself is less than that measured for birds resting in water. It is suggested that the regional hypothermia that has been recorded in this species during diving bouts may be at least a contributing factor to such hypometabolism.

Does Basal Metabolic Rate Contain a Useful Signal? Mammalian BMR Allometry and Correlations with a Selection of Physiological, Ecological, and Life‐History Variables

Basal metabolic rate (BMR, mL O2 h(-1)) is a useful measurement only if standard conditions are realised. We present an analysis of the relationship between mammalian body mass (M, g) and BMR that accounts for variation associated with body temperature, digestive state, and phylogeny. In contrast to the established paradigm that BMR proportional to M3/4, data from 619 species, representing 19 mammalian orders and encompassing five orders of magnitude variation in M, show that BMR proportional to M2/3. If variation associated with body temperature and digestive state are removed, the BMRs of eutherians, marsupials, and birds do not differ, and no significant allometric exponent heterogeneity remains between orders. The usefulness of BMR as a general measurement is supported by the observation that after the removal of body mass effects, the residuals of BMR are significantly correlated with the residuals for a variety of physiological and ecological variables, including maximum metabolic rate, field metabolic rate, resting heart rate, life span, litter size, and population density.

Predicting metabolic rate from heart rate in juvenile Steller sea lions Eumetopias jubatus

The validity of using heart rate to estimate energy expenditure in free-ranging Steller sea lions Eumetopias jubatus was investigated by establishing whether there is a relationship between heart rate (fH) and oxygen consumption rate ((O(2))) in captive sea lions while swimming and resting. Four trained Steller sea lions (2 males and 2 females; mass 87.4-194.4 kg; age 16 months-3 years) were each equipped with a datalogger and two dorsal subcutaneous electrodes to record electrocardiograms from which fH was calculated. (O(2)) (measured using open-circuit respirometry) was simultaneously recorded while the previously fasted animals were at rest within an enclosed dry metabolic chamber or while they swam in an enclosed swim mill against water currents of various speeds (0-1.5 m s(-1)). The mean regression equation describing the relationship between fH (beats min(-1)) and (O(2)) (ml h(-1) kg(-0.60)) for all four animals was (O(2)) =(71.3f(H)+/-4.3)-(1138.5+/-369.6) (means +/- S.E.M.) (r(2)=0.69, P<0.01). The relationship demonstrated between fH and (O(2)) while fasting suggests that heart rate can potentially be used to monitor energy consumption in free-ranging Steller sea lions. However, a short-term feeding experiment revealed no significant increase in heart rate following a 6 kg or 12 kg meal to match the observed increase in rate of oxygen consumption. This suggests that heart rate may not accurately reflect energy consumption during digestion events. Additional research should be conducted to further elucidate how the relationship between heart rate and oxygen consumption is affected by such factors as digestive state, stress and age.