Heart rate and behavior of fur seals: implications for measurement of field energetics

Cardiorespiratory patterns of male South American sea lions (Otaria flavescens) resting on land

The goal of this study was to characterize the cardiorespiratory patterns of male South American sea lions (SASLs, Otaria flavescens) resting on land. We recorded respiratory and heart rate (n = 360 individuals studied) by observing the nostrils, chest movements and the impact of the heart on the thoracic wall. The sea lions breathe apneustically with a pause on inspiration, representing 74% of the respiratory cycle. The mean breathing frequency was 3.2 ± 1.0 breaths min-1, with a breathing cycle presenting periods of bradypneas, tachypneas, and long-term post-inspiratory pauses. The normal heart rate (nHR) was 73.4 ± 14.5 beats min-1 and no significant differences were observed between age classes. All animals showed variability in HR in relation to respiratory phases (Inspiration: 101.2 ± 18.4 beats min-1; post-inspiratory pause: 73.4 ± 14.5 beats min-1; expiration: 64.6 ± 17.7 beats min-1), consistent with respiratory sinus arrhythmia (RSA). The mean HR (measured during all respiratory phases) was 79.9 ± 22.7 beats min-1, and was significantly different between age classes. The total duration of respiratory cycle, and duration of both inspiration and expiration, decreased with an increment in ambient temperature, with no variation in the pause duration. Heart rate during pause and expiration was significantly higher during high temperatures. Similar changes in cardiorespiratory patterns have been reported in other pinnipeds. Our results showed ontogenetic differences in development and typical variations with environmental and behavioral variables.

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.

A Physio-Logging Journey: Heart Rates of the Emperor Penguin and Blue Whale

Physio-logging has the potential to explore the processes that underlie the dive behavior and ecology of marine mammals and seabirds, as well as evaluate their adaptability to environmental change and other stressors. Regulation of heart rate lies at the core of the physiological processes that determine dive capacity and performance. The bio-logging of heart rate in unrestrained animals diving at sea was infeasible, even unimaginable in the mid-1970s. To provide a historical perspective, I review my 40-year experience in the development of heart rate physio-loggers and the evolution of a digital electrocardiogram (ECG) recorder that is still in use today. I highlight documentation of the ECG and the interpretation of heart rate profiles in the largest of avian and mammalian divers, the emperor penguin and blue whale.

Fur seals do, but sea lions don't - cross taxa insights into exhalation during ascent from dives

Management of gases during diving is not well understood across marine mammal species. Prior to diving, phocid (true) seals generally exhale, a behaviour thought to assist with the prevention of decompression sickness. Otariid seals (fur seals and sea lions) have a greater reliance on their lung oxygen stores, and inhale prior to diving. One otariid, the Antarctic fur seal (Arctocephalus gazella), then exhales during the final 50-85% of the return to the surface, which may prevent another gas management issue: shallow-water blackout. Here, we compare data collected from animal-attached tags (video cameras, hydrophones and conductivity sensors) deployed on a suite of otariid seal species to examine the ubiquity of ascent exhalations for this group. We find evidence for ascent exhalations across four fur seal species, but that such exhalations are absent for three sea lion species. Fur seals and sea lions are no longer genetically separated into distinct subfamilies, but are morphologically distinguished by the thick underfur layer of fur seals. Together with their smaller size and energetic dives, we suggest their air-filled fur might underlie the need to perform these exhalations, although whether to reduce buoyancy and ascent speed, for the avoidance of shallow-water blackout or to prevent other cardiovascular management issues in their diving remains unclear. This article is part of the theme issue 'Measuring physiology in free-living animals (Part I)'.

Too big to study? The biologging approach to understanding the behavioural energetics of ocean giants

Wild animals are under selective pressure to optimise energy budgets; therefore, quantifying energy expenditure, intake and allocation to specific activities is important if we are to understand how animals survive in their environment. One approach toward estimating energy budgets has involved measuring oxygen consumption rates under controlled conditions and constructing allometric relationships across species. However, studying 'giant' marine vertebrates (e.g. pelagic sharks, whales) in this way is logistically difficult or impossible. An alternative approach involves the use of increasingly sophisticated electronic tags that have allowed recordings of behaviour, internal states and the surrounding environment of marine animals. This Review outlines how we could study the energy expenditure and intake of free-living ocean giants using this 'biologging' technology. There are kinematic, physiological and theoretical approaches for estimating energy expenditure, each of which has merits and limitations. Importantly, tag-derived energy proxies can hardly be validated against oxygen consumption rates for giant species. The proxies are thus qualitative, rather than quantitative, estimates of energy expenditure, and have more limited utilities. Despite this limitation, these proxies allow us to study the energetics of ocean giants in their behavioural context, providing insight into how these animals optimise their energy budgets under natural conditions. We also outline how information on energy intake and foraging behaviour can be gained from tag data. These methods are becoming increasingly important owing to the natural and anthropogenic environmental changes faced by ocean giants that can alter their energy budgets, fitness and, ultimately, population sizes.

Stroke effort and relative lung volume influence heart rate in diving sea lions

The dive response, bradycardia (decreased heart rate) and peripheral vasoconstriction, is the key mechanism allowing breath-hold divers to perform long-duration dives while actively swimming and hunting prey. This response is variable and modulated by factors such as dive duration, depth, exercise and cognitive control. This study assessed the potential role of exercise and relative lung volume in the regulation of heart rate (fH) during dives of adult female California sea lions instrumented with electrocardiogram (ECG), depth and tri-axial acceleration data loggers. A positive relationship between activity (minimum specific acceleration) and fH throughout dives suggested increased muscle perfusion associated with exercise. However, apart from late ascent, fH during dives was still less than or equal to resting fH (on land). In addition, the activity-fH relationship was weaker in long, deep dives consistent with prioritization of blood oxygen conservation over blood oxygen delivery to muscle in those dives. Pulmonary stretch receptor reflexes may also contribute to fH regulation as fH profiles generally paralleled changes in relative lung volume, especially in shallower dives and during early descent and late ascent of deeper dives. Overall, these findings support the concept that both exercise and pulmonary stretch receptor reflexes may influence the dive response in sea lions.

Drivers of the dive response in trained harbour porpoises (Phocoena phocoena)

Pronounced dive responses through peripheral vasoconstriction and bradycardia enables prolonged apnoea in marine mammals. For most vertebrates, the dive response is initiated upon face immersion, but little is known about the physical drivers of diving and surfacing heart rate in cetaceans whose faces are always mostly submerged. Using two trained harbour porpoises instrumented with an ECG-measuring DTAG-3, we investigate the initiation and progression of bradycardia and tachycardia during apnoea and eupnoea for varying levels of immersion. We show that paranasal wetting drives bradycardia initiation and progression, whereas apnoea leads to dive-level bradycardia eventually, but not instantly. At the end of dives, heart rate accelerates independently of lung expansion, perhaps in anticipation of surfacing; however, full tachycardia is only engaged upon inhalation. We conclude that breathing drives surface tachycardia, whereas blowhole wetting is an important driver of bradycardia; although, anticipatory/volitional modulation can overrule such responses to sensory inputs.

Locomotor muscle fibre heterogeneity and metabolism in the fastest large-bodied rorqual: the fin whale (Balaenoptera physalus)

From a terrestrial ancestry, the fin whale (Balaenoptera physalus) is one of the largest animals on Earth with a sprinter anti-predator strategy, and a characteristic feeding mode, lunge feeding, which involves bouts of high-intensity muscle activity demanding high metabolic output. We investigated the locomotor muscle morphology and metabolism of this cetacean to determine whether its muscle profile (1) explains this unique swimming performance and feeding behaviour, (2) is or is not homogeneous within the muscle, and (3) predicts allometric variations inherent to an extreme body size. A predominantly fast-glycolytic phenotype characterized the fin whale locomotor muscle, composed of abundant fast-twitch (type IIA) fibres with high glycolytic potential, low oxidative capacity, relatively small size, and reduced number of capillaries. Compared with superficial areas, deep regions of this muscle exhibited a slower and more oxidative profile, suggesting a division of labour between muscle strata. As expected, the fin whale locomotor muscle only expressed the two slowest myosin heavy chain isoforms (I and IIA). However, it displayed anaerobic (glycolytic) and aerobic (lipid-based metabolism) capabilities higher than would be predicted from the allometric perspective of its extreme body size. Relationships between muscle metabolism and body mass were fibre-type specific. The 'sprinter' profile of the fin whale swimming muscle, particularly of its superficial compartment, supports physiological demands during both high-speed swimming and the lunge, when energy expenditure reaches maximal or supramaximal levels. Comparatively, the slower and more oxidative profile of the deep compartment of this muscle seems to be well designed for sustained, low-intensity muscle activity during routine swimming.

Activity-specific metabolic rates for diving, transiting, and resting at sea can be estimated from time-activity budgets in free-ranging marine mammals

Time and energy are the two most important currencies in animal bioenergetics. How much time animals spend engaged in different activities with specific energetic costs ultimately defines their likelihood of surviving and successfully reproducing. However, it is extremely difficult to determine the energetic costs of independent activities for free-ranging animals. In this study, we developed a new method to calculate activity-specific metabolic rates, and applied it to female fur seals. We attached biologgers (that recorded GPS locations, depth profiles, and triaxial acceleration) to 12 northern (Callorhinus ursinus) and 13 Antarctic fur seals (Arctocephalus gazella), and used a hierarchical decision tree algorithm to determine time allocation between diving, transiting, resting, and performing slow movements at the surface (grooming, etc.). We concomitantly measured the total energy expenditure using the doubly-labelled water method. We used a general least-square model to establish the relationship between time-activity budgets and the total energy spent by each individual during their foraging trip to predict activity-specific metabolic rates. Results show that both species allocated similar time to diving (~29%), transiting to and from their foraging grounds (~26-30%), and resting (~8-11%). However, Antarctic fur seals spent significantly more time grooming and moving slowly at the surface than northern fur seals (36% vs. 29%). Diving was the most expensive activity (~30 MJ/day if done non-stop for 24 hr), followed by transiting at the surface (~21 MJ/day). Interestingly, metabolic rates were similar between species while on land or while slowly moving at the surface (~13 MJ/day). Overall, the average field metabolic rate was ~20 MJ/day (for all activities combined). The method we developed to calculate activity-specific metabolic rates can be applied to terrestrial and marine species to determine the energetic costs of daily activities, as well as to predict the energetic consequences for animals forced to change their time allocations in response to environmental shifts.

New insights into the cardiorespiratory physiology of weaned southern elephant seals (Mirounga leonina)

Southern elephant seal (Mirounga leonina) pups must strike a balance between conserving energy during their post-weaning fast and simultaneously developing diving abilities to attain nutritional independence. Little is known about environmental influences on cardiorespiratory patterns, hence energy use, throughout the 6 week fast. Continuous heart rates were recorded for free-ranging, newly weaned southern elephant seals using heart rate time-depth recorders for 5-9 days at Sub-Antarctic Macquarie Island, during October 1994 (n = 1), 1995 (n = 4) and 1996 (n = 1). Daytime observations of respiration and behaviour were made throughout. We present the first instance of synchronous heart rate traces recorded simultaneously for individual weaners. Generalized additive models revealed that a sinusoidal pattern of diurnal heart rate elevation and nocturnal depression was evident in all seals and, on at least one occasion, a conspicuous break in this pattern coincided with an extreme cold weather event. Seals in this study were capable of considerable cardiorespiratory control and regularly demonstrated bradycardia during periods of resting apnoea. Apnoeic duration ranged from 33 to 291 s (mean 134 s). Apnoeic heart rates (mean 67 ± 15 beats min(-1), range 40-114 beats min(-1)) were on average 19.7% lower than those exhibited during periods of eupnoea (mean 83 ± 15 beats min(-1), range 44-124 beats min(-1)). The early development of the cardiorespiratory response is characterized by arrhythmic heart and respiration rates. The strong temporal patterns observed are being driven by the opposing requirements of maximizing time spent fasting in order to develop diving capabilities and of maximizing departure mass. This pilot study has highlighted a potentially large effect of ambient weather conditions on newly weaned southern elephant seal cardiorespiratory activity. Given the increasing westerlies and more erratic and increasing storminess associated with the Southern Annular Mode predicted in the Southern Ocean, the patterns observed here warrant further investigation.

The effects of handling and anesthetic agents on the stress response and carbohydrate metabolism in northern elephant seals

Free-ranging animals often cope with fluctuating environmental conditions such as weather, food availability, predation risk, the requirements of breeding, and the influence of anthropogenic factors. Consequently, researchers are increasingly measuring stress markers, especially glucocorticoids, to understand stress, disturbance, and population health. Studying free-ranging animals, however, comes with numerous difficulties posed by environmental conditions and the particular characteristics of study species. Performing measurements under either physical restraint or chemical sedation may affect the physiological variable under investigation and lead to values that may not reflect the standard functional state of the animal. This study measured the stress response resulting from different handling conditions in northern elephant seals and any ensuing influences on carbohydrate metabolism. Endogenous glucose production (EGP) was measured using [6-(3)H]glucose and plasma cortisol concentration was measured from blood samples drawn during three-hour measurement intervals. These measurements were conducted in weanlings and yearlings with and without the use of chemical sedatives--under chemical sedation, physical restraint, or unrestrained. We compared these findings with measurements in adult seals sedated in the field. The method of handling had a significant influence on the stress response and carbohydrate metabolism. Physically restrained weanlings and yearlings transported to the lab had increased concentrations of circulating cortisol (F(11, 46) = 25.2, p<0.01) and epinephrine (F(3, 12) = 5.8, p = 0.01). Physical restraint led to increased EGP (t = 3.1, p = 0.04) and elevated plasma glucose levels (t = 8.2, p<0.01). Animals chemically sedated in the field typically did not exhibit a cortisol stress response. The combination of anesthetic agents (Telazol, ketamine, and diazepam) used in this study appeared to alleviate a cortisol stress response due to handling in the field without altering carbohydrate metabolism. Measures of hormone concentrations and metabolism made under these conditions are more likely to reflect basal values.

No energetic cost of anthropogenic disturbance in a songbird

Anthropogenic or natural disturbances can have a significant impact on wild animals. Therefore, understanding when, how and what type of human and natural events disturb animals is a central problem in wildlife conservation. However, it can be difficult to identify which particular environmental stressor affects an individual most. We use heart rate telemetry to quantify the energy expenditure associated with different types of human-mediated and natural disturbances in a breeding passerine, the white-eyed vireo (Vireo griseus). We fitted 0.5g heart rate transmitters to 14 male vireos and continuously recorded heart rate and activity for two days and three nights on a military installation. We calibrated heart rate to energy expenditure for five additional males using an open-flow, push-through respirometry system showing that heart rate predicted 74 per cent of energy expenditure. We conducted standardized disturbance trials in the field to experimentally simulate a natural stressor (predator presence) and two anthropogenic stressors. Although birds initially showed behavioural and heart rate reactions to some disturbances, we could not detect an overall increase in energy expenditure during 1- or 4-hours disturbances. Similarly, overall activity rates were unaltered between control and experimental periods, and birds continued to perform parental duties despite the experimental disturbances. We suggest that vireos quickly determined that disturbances were non-threatening and thus showed no (costly) physiological response. We hypothesize that the lack of a significant response to disturbance in vireos is adaptive and may be representative of animals with fast life histories (e.g. short lifespan, high reproductive output) so as to maximize energy allocation to reproduction. Conversely, we predict that energetic cost of human-mediated disturbances will be significant in slow-living animals.

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.

Heart rate regulation and extreme bradycardia in diving emperor penguins

To investigate the diving heart rate (f(H)) response of the emperor penguin (Aptenodytes forsteri), the consummate avian diver, birds diving at an isolated dive hole in McMurdo Sound, Antarctica were outfitted with digital electrocardiogram recorders, two-axis accelerometers and time depth recorders (TDRs). In contrast to any other freely diving bird, a true bradycardia (f(H) significantly <f(H) at rest) occurred during diving [dive f(H) (total beats/duration)=57+/-2 beats min(-1), f(H) at rest=73+/-2 beats min(-1) (mean +/- s.e.m.)]. For dives less than the aerobic dive limit (ADL; duration beyond which [blood lactate] increases above resting levels), dive f(H)=85+/-3 beats min(-1), whereas f(H) in dives greater than the ADL was significantly lower (41+/-1 beats min(-1)). In dives greater than the ADL, f(H) reached extremely low values: f(H) during the last 5 mins of an 18 min dive was 6 beats min(-1). Dive f(H) and minimum instantaneous f(H) during dives declined significantly with increasing dive duration. Dive f(H) was independent of swim stroke frequency. This suggests that progressive bradycardia and peripheral vasoconstriction (including isolation of muscle) are primary determinants of blood oxygen depletion in diving emperor penguins. Maximum instantaneous surface interval f(H) in this study is the highest ever recorded for emperor penguins (256 beats min(-1)), equivalent to f(H) at V(O(2)) max., presumably facilitating oxygen loading and post-dive metabolism. The classic Scholander-Irving dive response in these emperor penguins contrasts with the absence of true bradycardia in diving ducks, cormorants, and other penguin species.

Estimating the rate of oxygen consumption during submersion from the heart rate of diving animals

How animals manage their oxygen stores during diving and other breath-hold activities has been a topic of debate among physiologists for decades. Specifically, while the behavior of free-ranging diving animals suggests that metabolism during submersion must be primarily aerobic in nature, no studies have been able to determine their rate of oxygen consumption during submersion (Vo(2)d) and hence prove that this is the case. In the present study, we combine two previously used techniques and develop a new model to estimate Vo(2)d accurately and plausibly in a free-ranging animal and apply it to data for macaroni penguins (Eudyptes chrysolophus) as an example. For macaroni penguins at least, Vo(2)d can be predicted by measuring heart rate during the dive cycle and the subsequent surface interval duration. Including maximum depth of the dive improves the accuracy of these predictions. This suggests that energetically demanding locomotion events within the dive combine with the differing buoyancy and locomotion costs associated with traveling to depth to influence its cost in terms of oxygen use. This will in turn effect the duration of the dive and the duration of the subsequent recovery period. In the present study, Vo(2)d ranged from 4 to 28 ml.min(-1).kg(-1), indicating that, at least as far as aerobic metabolism was concerned, macaroni penguins were often hypometabolic, with rates of oxygen consumption usually below that for this species resting in water (25.6 ml.min(-1).kg(-1)) and occasionally lower than that while resting in air (10.3 ml.min(-1).kg(-1)).

Juvenile Southern Elephant Seals Exhibit Seasonal Differences in Energetic Requirements and Use of Lipids and Protein Stores

Growing juvenile animals undergo many morphological, physiological, and behavioural changes that influence their energetic requirements, patterns of energy use, and ultimately, their survival and reproductive success. We examined changes in mass loss and body composition of juvenile southern elephant seals (1- and 2-yr-olds) during their two annual haul-outs. At the start and end of the midyear and molt haul-outs, we caught, weighed, and measured 41 and 14 seals, respectively. We measured blubber depth using ultrasound to estimate body composition (lean and adipose tissue mass). Using energy densities of the adipose and lean tissue, we calculated total, lean, and adipose mass changes and energy expenditure. While molting, juvenile seals used more energy than during the midyear, which is related to the increased use of lean tissue for hair and skin regeneration. The amount of energy used increases with mass as individuals mature. We found sexual differences in energy use where females retained greater fat reserves than males by utilizing more lean tissue. These differences are most likely related to haul-out function and behavior, growth, and earlier development of females toward sexual maturity.

Cardiorespiratory and metabolic reactions during entrance into torpor in dormice, Glis glis

Dormice voluntarily enter torpor at ambient temperatures ranging between 0-28 degrees C. This study describes heart rate, ventilation frequency, O2-consumption (defined as metabolic rate), CO2-production and body temperature during entrance into torpor. Their temporal relationship was analysed during the time course of metabolic depression at different ambient temperatures. Body temperature and heart rate were measured in unrestrained dormice with implanted transmitter. Ventilation frequency was monitored by total body plethysmography or infrared video monitoring. To compare entries into torpor at different T(a) these periods were distinguished into four different phases: the resting phase prior to torpor, the phase of pre-torpor adjustments, the reduction phase and the phase of steady state torpor. In the pre-torpor phase, dormice increased their ventilation, metabolic rate and heart rate, indicating that the torpid state is initiated by an enhanced metabolic activity for about an hour. This was followed by a rapid reduction of ventilation, metabolism and heart rate, which reached their minimum values long before body temperature completed its decline. The results of the present study show that the entrance into torpor is caused by an active respiratory, cardiac and metabolic depression.

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.

Ascent exhalations of Antarctic fur seals: a behavioural adaptation for breath-hold diving?

Novel observations collected from video, acoustic and conductivity sensors showed that Antarctic fur seals consistently exhale during the last 50-85% of ascent from all dives (10-160 m, n > 8000 dives from 50 seals). The depth of initial bubble emission was best predicted by maximum dive depth, suggesting an underlying physical mechanism. Bubble sound intensity recorded from one seal followed predictions of a simple model based on venting expanding lung air with decreasing pressure. Comparison of air release between dives, together with lack of variation in intensity of thrusting movement during initial descent regardless of ultimate dive depth, suggested that inhaled diving lung volume was constant for all dives. The thrusting intensity in the final phase of ascent was greater for dives in which ascent exhalation began at a greater depth, suggesting an energetic cost to this behaviour, probably as a result of loss of buoyancy from reduced lung volume. These results suggest that fur seals descend with full lung air stores, and thus face the physiological consequences of pressure at depth. We suggest that these regular and predictable ascent exhalations could function to reduce the potential for a precipitous drop in blood oxygen that would result in shallow-water blackout.

Metabolic regulation in diving birds and mammals

Ducks, fur seals, Weddell seals and probably most cetaceans seem to be able to dive and remain aerobic for durations that are consistent with their elevated stores of usable oxygen and their metabolic rate while diving being similar to that when they are resting at the surface of the water. Ducks, in fact, have a high metabolic rate while diving, mainly because of their large positive buoyancy, but other species have relatively low buoyancy, are better streamlined and use lift-based rather than drag-based propulsion. However, species such as the larger penguins, grey seals and elephant seals seem to achieve the impossible by performing a substantial proportion of their dives for periods longer than would be expected on the above assumptions, and yet remaining aerobic. The logical conclusion is that during such dives these species reduce their metabolic rate below the resting level (hypometabolism) and, in some of them, there is a regional reduction in body temperature (hypothermia) which may contribute to the reduction in metabolic rate.

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.

Heart rate and energy expenditure of incubating wandering albatrosses: basal levels, natural variation, and the effects of human disturbance

We studied the changes in heart rate (HR) associated with metabolic rate of incubating and resting adult wandering albatrosses (Diomedea exulans) on the Crozet Islands. Metabolic rates of resting albatrosses fitted with external HR recorders were measured in a metabolic chamber to calibrate the relationship between HR and oxygen consumption (V̇O2) (V̇O2=0.074×HR+0.019, r2=0.567, P&lt;0.001, where V̇O2 is in ml kg–1 min–1 and HR is in beats min–1). Incubating albatrosses were then fitted with HR recorders to estimate energy expenditure of albatrosses within natural field conditions. We also examined the natural variation in HR and the effects of human disturbance on nesting birds by monitoring the changes in HR. Basal HR was positively related to the mass of the individual. The HR of incubating birds corresponded to a metabolic rate that was 1.5-fold (males) and 1.8-fold (females) lower than basal metabolic rate (BMR) measured in this and a previous study. The difference was probably attributable to birds being stressed while they were held in the metabolic chamber or wearing a mask. Thus, previous measurements of metabolic rate under basal conditions or for incubating wandering albatrosses are likely to be overestimates. Combining the relationship between HR and metabolic rate for both sexes, we estimate that wandering albatrosses expend 147 kJ kg–1 day–1 to incubate their eggs. In addition, the cost of incubation was assumed to vary because (i) HR was higher during the day than at night, and (ii) there was an effect of wind chill (&lt;0°C) on basal HR. The presence of humans in the vicinity of the nest or after a band control was shown to increase HR for extended periods (2–3 h), suggesting that energy expenditure was increased as a result of the disturbance. Lastly, males and females reacted differently to handling in terms of HR response: males reacted more strongly than females before handling, whereas females took longer to recover after being handled.

Diving beyond the limits

Some free-ranging birds and mammals dive for periods that substantially exceed those for which their usable O(2) stores are estimated to last. The mechanisms that extend the duration of aerobic diving include marked reductions in blood flow (and hence O(2) delivery) to certain organs and tissues, passive gliding and, most probably, regional hypothermia.

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 V̇O2. The mean percentage error between predicted and measured V̇O2 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 (30min). 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.