The metabolic upper critical temperature of the human thermoneutral zone.

INTRODUCTION: The thermoneutral zone (TNZ) defines the range of ambient temperatures at which resting metabolic rate is at a minimum without sensible dry heat loss; the body not needing to defend its core temperature. The TNZ has been defined in a number of species yet surprisingly, in humans only its lower limit has been well characterised; indeed, it is not yet clear if there is an upper limit at which metabolic rate increases. AIM: To evaluate the evidence for a metabolic upper critical temperature to the thermoneutral zone in humans. METHODS: We synthesised current evidence about an upper limit to the human TNZ, highlighting the contradictions in the literature, and then discussed likely explanations for those contradictions. RESULTS: The data from relevant studies differ in terms of whether they indicate that the TNZ has an upper limit, and this was related to the fundamental type of heat exposure protocol employed. Those studies showing evidence for an upper limit associated that limit with a wide range of temperatures. CONCLUSIONS: We offer suggestions for future studies that should clarify the presence/absence of an upper limit to the TNZ and if present, where it lies.

Do method and species lifestyle affect measures of maximum metabolic rate in fishes?

The rate at which active animals can expend energy is limited by their maximum aerobic metabolic rate (MMR). Two methods are commonly used to estimate MMR as oxygen uptake in fishes, namely during prolonged swimming or immediately following brief exhaustive exercise, but it is unclear whether they return different estimates of MMR or whether their effectiveness for estimating MMR varies among species with different lifestyles. A broad comparative analysis of MMR data from 121 fish species revealed little evidence of different results between the two methods, either for fishes in general or for species of benthic, benthopelagic or pelagic lifestyles.

The interactions between temperature and activity levels in driving metabolic rate: theory, with empirical validation from contrasting ectotherms.

The rate of change in resting metabolic rate (RMR) as a result of a temperature increase of 10 °C is termed the temperature coefficient (Q10), which is often used to predict how an organism's total MR will change with temperature. However, this method neglects a potentially key component of MR; changes in activity level (and thus activity MR; AMR) with temperature may significantly alter the relationship between MR and temperature. The present study seeks to describe how thermal effects on total MR estimated from RMR-temperature measurements can be misleading when the contribution of activity to total MR is neglected. A simple conceptual framework illustrates that since the relationship between activity levels and temperature can be different to the relationship between RMR and temperature, a consistent relationship between RMR and total MR cannot be assumed. Thus the thermal effect on total MR can be considerably different to the thermal effect on RMR. Simultaneously measured MR and activity from three ectotherm species with differing behavioural and physiological ecologies were used to empirically examine how changes in temperature drive changes in RMR, activity level, AMR and the Q10 of MR. These species exhibited varied activity- and MR-temperature relationships, underlining the difficulty in predicting thermal influences on activity levels and total MR. These data support a model showing that thermal effects on total MR will deviate from predictions based solely on RMR; this deviation will depend upon the difference in Q10 between AMR and RMR, and the relative contribution of AMR to total MR. To develop mechanistic, predictive models for species' metabolic responses to temperature changes, empirical information about the relationships between activity levels, MR and temperature, such as reported here, is required. This will supersede predictions based on RMR alone.

A cross-over experiment to investigate possible mechanisms for lower BMIs in people who habitually eat breakfast.

BACKGROUND/OBJECTIVES: The body mass index (BMI) of breakfast eaters is frequently reported to be lower compared with that of breakfast skippers. This is not explained by differences in energy intakes, indicating there may be other mechanisms serving to drive this paradoxical association between breakfast and BMI. This study aimed to investigate the effect of eating breakfast versus morning fasting on measures predominantly of metabolism in lean and overweight participants who habitually eat or skip breakfast. SUBJECTS/METHODS: Participants (n=37) were recruited into four groups on the basis of BMI (lean and overweight) and breakfast habit (breakfast eater and breakfast skipper). Participants were randomly assigned to a breakfast experimental condition, breakfast eating or no breakfast, for 7 days and then completed the alternative condition. At the end of each breakfast experimental condition, measurements were made before and after a high carbohydrate breakfast of 2274 ± 777 kJ or a rest period. Resting metabolic rate, thermic effect of food (TEF), blood glucose, insulin and leptin levels were recorded. Hunger and 'morningness' were assessed and pedometers worn. RESULTS: Lean participants had lower fasting insulin levels (P=0.045) and higher insulin concentrations following breakfast (P=0.001). BMI and breakfast habit did not interact with the experimental breakfast condition, with the exception of hunger ratings; breakfast eaters were hungrier in the mornings compared with breakfast skippers in the no breakfast condition (P=0.001). CONCLUSIONS: There is little evidence from this study for a metabolic-based mechanism to explain lower BMIs in breakfast eaters.

Energy expenditure during activity in the American lobster Homarus americanus: Correlations with body acceleration.

How animals manage time and expend energy has implications for survivorship. Being able to measure key metabolic costs of animals under natural conditions is therefore an important tool in behavioral ecology. One method for estimating activity-specific metabolic rate is via derived measures of acceleration, often 'overall dynamic body acceleration' (ODBA), recorded by an instrumented acceleration logger. ODBA has been shown to correlate well with rate of oxygen consumption (V˙o2) in a range of species during activity in the laboratory. This study devised a method for attaching acceleration loggers to decapod crustaceans and then correlated ODBA against concurrent respirometry readings to assess accelerometry as a proxy for activity-specific energy expenditure in a model species, the American lobster Homarus americanus. Where the instrumented animals exhibited a sufficient range of activity levels, positive linear relationships were found between V˙o2 and ODBA over 20min periods at a range of ambient temperatures (6, 13 and 20°C). Mixed effect linear models based on these data and morphometrics provided reasonably strong predictive power for estimating activity-specific V˙o2 from ODBA. These V˙o2-ODBA calibrations demonstrate the potential of accelerometry as an effective predictor of behavior-specific metabolic rate of crustaceans in the wild during periods of activity.

Turn costs change the value of animal search paths.

The tortuosity of the track taken by an animal searching for food profoundly affects search efficiency, which should be optimised to maximise net energy gain. Models examining this generally describe movement as a series of straight steps interspaced by turns, and implicitly assume no turn costs. We used both empirical- and modelling-based approaches to show that the energetic costs for turns in both terrestrial and aerial locomotion are substantial, which calls into question the value of conventional movement models such as correlated random walk or Lévy walk for assessing optimum path types. We show how, because straight-line travel is energetically most efficient, search strategies should favour constrained turn angles, with uninformed foragers continuing in straight lines unless the potential benefits of turning offset the cost.

Comparative energetics of mammalian locomotion: humans are not different.

Debates about the evolution of human bipedality sometimes include discussion on the energy costs of terrestrial locomotion of extinct and extant hominins. However, comparative analyses of hominin transport costs conducted to date have been limited and potentially misinforming, in part because they fail to consider phylogenetic history. In the present study, we compare the measured costs of pedestrian locomotion in humans and the estimated costs for Australopithecus afarensis (an early bipedal hominin), to a database of locomotory costs for mammals. Using data for 81 species of mammal, we demonstrate significant phylogenetic signal in both log-transformed body mass (logMass) and log-transformed net cost of transport (logNCOT), but no phylogenetic signal in residuals of the relationship between logNCOT and logMass. We then used this relationship to generate a prediction line for NCOT based on body mass, and compared this prediction with published measured data for NCOT of running and walking in humans, and estimated NCOT of walking in A. afarensis. The cost of human walking was 25% lower than predicted, while the cost of running was 27% higher. The cost of A. afarensis walking was 32% lower than predicted. However, all of these data points fall within the 95% prediction interval for mammals, indicating that they are not significantly lower or higher than predicted for other mammals of similar mass. Moreover, the difference between humans and our closest living relative the common chimpanzee is comparable to differences between other similarly closely related species. We therefore conclude that there is no evidence from metabolic data that humans, or A. afarensis, have/had a reduced energy cost of pedestrian locomotion compared to other mammals in general.

Assessing the validity of the accelerometry technique for estimating the energy expenditure of diving double-crested cormorants Phalacrocorax auritus.

Over the past few years, acceleration-data loggers have been used to provide calibrated proxies of energy expenditure: the accelerometry technique. Relationships between rate of oxygen consumption and a derivation of acceleration data termed "overall dynamic body acceleration" (ODBA) have now been generated for a range of species, including birds, mammals, and amphibians. In this study, we examine the utility of the accelerometry technique for estimating the energy expended by double-crested cormorants Phalacrocorax auritus to undertake a dive cycle (i.e., a dive and the subsequent pause at the surface before another dive). The results show that ODBA does not calibrate with energy expenditure in diving cormorants, where energy expenditure is calculated from measures of oxygen uptake during surface periods between dives. The possible explanations include reasons why energy expenditure may not relate to ODBA but also reasons why oxygen uptake between dives may not accurately represent energy expenditure during a dive cycle.

Flow-through respirometry applied to chamber systems: pros and cons, hints and tips.

Flow-through respirometry is a powerful, accurate methodology for metabolic measurement that is applicable to organisms spanning a body mass range of many orders of magnitude. Concentrating on flow-through respirometry that utilizes a chamber to contain the experimental animals, we describe the most common flow measurement and control methodologies (push, pull and stop-flow) and their associated advantages and disadvantages. Objective methods for calculating air flow rates through the chamber, based on the body mass and taxon of the experimental organism, are presented. Techniques for removing the effect of water vapor dilution, including the direct measurement of water vapor pressure and mathematical compensation for its presence, are described and evaluated, as are issues surrounding the analysis of one or both of the respiratory gases (oxygen and carbon dioxide), and issues related to the mathematical correction of wash-out phenomena (response correction). Two important biomedical applications of flow-through respirometry (metabolic phenotyping and room calorimetry) are discussed in detail, and we conclude with a list of suggestions aimed primarily at investigators starting out in applying flow-through respirometry.

An explanation for enhanced perceptions of attractiveness after alcohol consumption.

Acute alcohol consumption increases ratings of attractiveness to faces. This may help to explain increased frequencies of sexual encounters during periods of alcohol intoxication. At least in part, such increased attraction may be the result of alcohol consumption decreasing ability to detect bilateral asymmetry, presumably because of the reductions in the levels of visual function. We tested the hypotheses that acute alcohol consumption decreases ability to detect asymmetry in faces and reduces preference for symmetrical faces over asymmetrical faces. Twenty images of a pair of faces and then 20 images of a single face were displayed on a computer, one at a time. Participants were instructed to state which face of each of the face pairs displayed was most attractive and then whether the single face being displayed was symmetrical or not. Data were collected near campus bars at Roehampton University. Sixty-four self-selecting students who undertook the study were classified as either sober (control) or intoxicated with alcohol. For each face pair or single face displayed, participant response was recorded and details of the alcohol consumption of participants that day were also obtained. Sober participants had a greater preference for symmetrical faces and were better at detecting whether a face was symmetrical or otherwise, supporting the hypotheses. A further, unexpected finding was that males made fewer mistakes than did females when determining whether individual faces were asymmetrical. The reduced ability of inebriated people to perceive asymmetry may be an important mechanism underlying the higher ratings of facial attractiveness they give for members of the opposite sex and hence their increased frequency of mate choice.

Estimating energy expenditure of animals using the accelerometry technique: activity, inactivity and comparison with the heart-rate technique.

Several methods have been used to estimate the energy expenditure of free-ranging animals. A relatively new technique uses measures of dynamic body acceleration as a calibrated proxy for energy expenditure and has proved an excellent predictor of energy expenditure in active animals. However, some animals can spend much of their time inactive and still expend energy at varying rates for a range of physiological processes. We tested the utility of dynamic body acceleration to estimate energy expenditure during a range of active (locomotion, eating) and inactive (digesting, thermoregulating) behaviours exhibited by domestic chickens. We also compared this technique with the more established heart-rate method for estimating energy expenditure. During activity, the error of estimation using body acceleration was very similar to that from the heart-rate method. Importantly, our results also showed that body acceleration can be used to estimate energy expenditure when birds are inactive. While the errors surrounding these estimates were greater than those during activity, and those made using the heart-rate method, they were less than those made using interspecific allometric equations. We highlight the importance of selecting a methodology that is appropriate for the life-history of the subject animal. We suggest that, to achieve the greatest possible accuracy and precision when estimating energy expenditure in free-ranging animals, the two techniques should be combined, and both heart rate (f(H)) and dynamic body acceleration could be included as covariates in predictive models. Alternatively, measures of acceleration can be used to ascertain which behaviour is being exhibited at each moment and hence which predictive model should be applied.

Accelerometry to Estimate Energy Expenditure during Activity: Best Practice with Data Loggers.

Measurement of acceleration can be a proxy for energy expenditure during movement. The variable overall dynamic body acceleration (ODBA), used in recent studies, combines the dynamic elements of acceleration recorded in all three dimensions to measure acceleration and hence energy expenditure due to body movement. However, the simplicity of ODBA affords it limitations. Furthermore, while accelerometry data loggers enable measures to be stored, recording at high frequencies represents a limit to deployment periods as a result of logger memory and/or battery exhaustion. Using bantam chickens walking at different speeds in a respirometer while wearing an accelerometer logger, we investigated the best proxies for rate of oxygen consumption (Vo(2)) from a range of different models using acceleration. We also investigated the effects of sampling acceleration at different frequencies. The best predictor of Vo(2) was a multiple regression including individual measures of dynamic acceleration in each of the three dimensions. However, R(2) was relatively high for ODBA as well and also for certain measures of dynamic acceleration in single dimensions. The aforementioned are single variables, therefore easily derived onboard a data logger and from which a simple calibration equation can be derived. For calibrations of Vo(2) against ODBA, R(2) was consistent as sampling number decreased down to 600 samples of each acceleration channel per ODBA data point, beyond which R(2) tended to be considerably lower. In conclusion, data storage can be maximized when using acceleration as a proxy for Vo(2) by consideration of reductions in (1) number of axes measured and (2) sampling frequency.

The relationship between oxygen consumption and body acceleration in a range of species.

The ability to measure the energy expenditure of free-ranging animals is of great importance but the techniques available each have their limitations. Recently, as an alternative to more established techniques, an integrated measure of body acceleration termed overall dynamic body acceleration (ODBA) has been used as a calibrated proxy for rate of oxygen consumption (V(O(2))) and hence metabolic rate. The present study tested the potential of this technique, firstly by expanding the range of species for which the V(O(2))-ODBA relationship has been defined and secondly by undertaking a validation exercise to explore the accuracy of predictions made using ODBA. V(O(2))-ODBA relationships during terrestrial locomotion were established for several bipedal and quadrupedal endotherms and compiled with similar relationships previously determined in other species. A model incorporating all of these species showed that ODBA is an excellent predictor of V(O(2)) but there is variation in the V(O(2))-ODBA relationship between species, and further variation within some species. Including measurements such as body mass and structural size in prediction equations might further improve the predictive power of the 'ODBA technique' and eliminate species-specific differences. In the validation exercise, estimate errors were calculated for the species-specific predictive equations. The use of ODBA to estimate V(O(2)) was valid across all species examined and may show a greater potential for estimating energy expenditure for individual animals than other techniques.

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

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

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.

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

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

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)).

Can wild common marmosets (Callithrix jacchus) solve the parallel strings task?

Patterned string tasks are a test of perceptual capacity and the understanding of means-end connections. Primates can solve complex forms of this task in laboratories. However, this may not indicate the level of such cognition that is commonly employed in the wild, where decision-making time is often short and distractions such as predator avoidance and competition between conspecifics are often prevalent. The current study tests whether wild common marmosets (Callithrix jacchus) can successfully complete the simplest form of the patterned string task, parallel strings, while in their natural environment. Although 12 out of 13 marmosets could successfully complete the task, in previous laboratory-based studies on primates, the errors at this task by all primate species tested were consistently lower than in the present study. This is probably explained by the added difficulties imposed by the natural setting of the task in the present study, exemplified by a significant increase in observed vigilance behaviour by subject animals prior to attempts at the task that were unsuccessful. The undertaking of such tasks by common marmosets in situ probably provides a more reasonable representation of the levels of cognitive capacity expressed by this species in the wild than do laboratory-based studies of the task.