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Summary 1.Allostatic load describes the interplay between energetic demand and availability and is highly context dependent, varying between seasons and life history stages. When energy demands exceed physiological set points modulated by glucocorticoid hormones, individuals may experience allostatic overload and transition between stages in sub-optimal physiological states. 2.Corticosterone, the major glucocorticoid hormone regulating energy expenditure in birds, is incorporated into growing feathers (CORTf), and it has been suggested that CORTf reflects long-term records of allostatic load during feather growth. However, relationships between allostatic load and CORTf have not been adequately evaluated. If such relationships exist, the use of CORTf to investigate cross-seasonal effects could provide novel insights into impacts of past allostatic load and/or overload events. 3.We tested whether experimental increases in daily workload during two adjacent life history stages would be reflected in CORTf levels, and examined if CORTf levels reflected either current energetic demand or allostatic load prior to feather growth. 4.Daily workloads in female mallard (Anas platyrhynchos) ducklings were increased over a 6-week period using physical obstacles and/or carrying back-mounted weights. We measured daily energy expenditure, growth, body mass, and CORTf in growing ducklings. Then, we induced feather moult and reapplied combinations of workload treatments for an additional 6 weeks to investigate whether effects of past energetic demands would be detected in future CORTf levels. 5.Ducklings confronted with higher workloads during development had reduced body mass, growth rates, and consequently higher daily energy expenditure and CORTf values compared to controls. When ducklings were fully developed, CORTf patterns in birds re-exposed to workload treatments reflected only current, rather than past, energetic demands. However, under allostatic overload conditions, past levels of CORTf were associated with CORTf in the subsequent moult. 6.Our study confirms the previously untested assumption that CORTf reflects energetic demand during the period of feather growth in a precocial bird. We show that allostatic overload conditions early in life, which temporarily supress growth, can be detected using CORTf; an event potentially missed in studies which rely solely on measures of body condition alone. 7.We suggest that CORTf can provide a valuable biomarker of allostatic load and overload conditions during the period of feather growth, but highlight how context should be considered for studies using CORTf to investigate influences of carry-over effects. Our study contributes to building a physiological foundation to inform interpretations of ecological patterns using CORTf. This article is protected by copyright. All rights reserved.

Summary 1. Reproduction is energetically expensive, and daily energy expenditure (DEE) often peaks during the period of rearing young. The ‘potentiation’ hypothesis predicts that high DEE needs to be sustained by a corresponding up-regulation of metabolic machinery; thus, a concomitant increase in the resting metabolic rate (RMR) is expected. Alternatively, the ‘compensation’ hypothesis predicts that DEE and RMR are regulated independently and animals may maintain low RMR to maximize the energy available for reproduction. This might particularly be the case if DEE was limited, for example, by extrinsic food supply or intrinsic physiological factors. 2. We tested these hypotheses in free-living seabirds by manipulating their energy demands (experimentally reduced or increased brood size) and supplies (providing supplemental food), and simultaneously measuring their DEE and RMR (by the doubly labelled water method and an indirect hormonal proxy, respectively). 3. In support of the ‘compensation’ hypothesis, metabolic rates were adjusted independently and in opposite directions with an increase in DEE and a decrease in the hormonal proxy for RMR in individuals rearing young compared to birds with removed broods. Energy expenditure of unfed birds with chicks appeared to be limited as experimental brood enlargement did not cause an increase in DEE. Supplemental feeding did not allow DEE to exceed this apparent limitation. 4. We propose that a reduction in the resting metabolism is a strategy to increase allocation of energy to reproduction when DEE is constrained and that this constraint is unlikely to be related to food supply.

Summary Energy expenditure is a trait of central importance in ecological and evolutionary theory. We examined the correlates of, the strength of selection on, and the heritability of, daily energy expenditure (DEE; kJ day−1) during lactation in free-ranging North American red squirrels (Tamiasciurus hudsonicus). Over 7 years, lactating squirrels with greater DEE had higher annual reproductive success (ARS; standardized selection gradient: β′ = 0·47; top 12% of published estimates). Surprisingly, positive fecundity selection on lactation DEE for increased ARS did not result because lactation DEE was correlated with typical measures of reproductive performance and/or investment. We found no evidence of costs of elevated lactation DEE acting through female survival, subsequent year lactation DEE or subsequent year reproduction. Lactation DEE was not significantly repeatable, and heritability was not significantly different from zero. Elevated lactation DEE enhances ARS through a link between DEE and an unidentified measure of maternal or environmental quality, but there is limited evolutionary potential for lactation DEE to respond to our documented selection.

Summary Oxidative stress is a potential proximal physiological cost of reproduction. Detecting this cost may be performed in several different ways – manipulating reproductive status, correlating natural variations in effort to oxidative stress or manipulating reproductive effort. Here, we manipulated reproductive status and studied oxidative stress due to natural and experimental variation in reproductive effort in Brandt's voles (Lasiopodomys brandtii), using a variety of markers and tissues. We measured markers of oxidative stress in lactating (raising 6 to 8 offspring) and non-reproductive voles (Experiment I) and found that a marker of oxidative protection [serum total-superoxide dismutase (SOD) activity] was reduced, and a marker of oxidative damage (protein carbonyls) was increased, in the serum, in lactating compared with non-reproductive voles. However, protein carbonyls in the liver were lower in lactating compared with non-reproductive voles, consistent with increased liver SOD activity. Lipid damage [malonaldehyde (MDA)] in both serum and liver was unrelated to reproductive status. We compared these markers of oxidative stress between natural large (n ≥ 9) and small litter sizes (n ≤ 5; Experiment II), and between manipulated large (11–13) and small litter sizes (2–3; Experiment III) and found that liver MDA and SOD activity was higher in voles with natural large compared with natural small litter sizes, but there were no differences in other markers. There was no effect of litter size on all measures when it was experimentally manipulated. The effects of reproductive status on oxidative stress were critically dependent on the exact markers and tissues used. The effects of natural variation in reproductive effort suggested that there might be an oxidative stress cost associated with large litter sizes, but this effect was not replicated in the experimentally manipulated litters.

Summary 1. Allocating a greater amount of limited resources, such as energy, to current reproduction can reduce the amount of energy available for somatic maintenance and can ultimately impair future breeding success or maternal survival (i.e. cost of reproduction hypothesis). Although there is some support for the cost of reproduction hypothesis in birds, few empirical studies of mammals have demonstrated a trade-off between current and future reproduction. More importantly, most studies testing ultimate costs have neglected to confirm that the proximate costs of reproduction are high. 2. We experimentally manipulated litter size in a wild population of Columbian ground squirrels for 2 years to examine both the proximate energetic and ultimate fitness (i.e. survival and breeding) costs of reproduction. We predicted that females raising augmented litters would have the highest rates of daily energy expenditure and as a result would experience lower survival rates or future fecundity. 3. Females raising augmented litters weaned more pups, had the highest litter masses at weaning, and had field metabolic rates that were almost 15 times greater than females raising control or reduced litters. Contrary to our prediction, there were no negative impacts of greater maternal investment and higher energy expenditure on the probability of maternal survival or future reproduction. 4. Pups from augmented litters grew more slowly during the lactation period were smaller at weaning and had a lower probability of survival over-winter. Thus, although females were capable of raising more young than they gave birth to without short-term costs of reduced survival or fecundity, our observations suggest that limitations to litter size are not due to a trade-off in the allocation of energy, but rather due to the reduced survival of offspring from larger litters. 5. Examining the proximate mechanisms hypothesized to underlie life-history trade-offs can be challenging but is critical for a comprehensive understanding of the evolution of life histories.

Summary 1. Despite the central importance of the rate of energy expenditure in the lives of animals, the major drivers of within-species variation in energy expenditure remain uncertain, largely because most intraspecific studies focus on one or only a few potential determinants of expenditure. 2. Here, we examine the determinants of daily energy expenditure (DEE) in free-ranging female North American red squirrels (Tamiasciurus hudsonicus Erxleben) occupying a highly seasonal environment. By relating variation in 260 measurements of DEE from 176 individuals to key sources of seasonal (reproductive and foraging stages), environmental (resources and air temperature) and individual (body mass and individual identity) variation, our comprehensive analysis examines the relative importance of DEE predictors that have been more commonly examined in isolation. 3. Red squirrels demonstrated extensive variation in DEE with 5th (177 kJ per day) and 95th (660 kJ per day) percentile DEE levels that would correspond to mammals on an interspecific scale ranging in mass from 148 to 1120 g. 4. Seasonal stage differences accounted for most variation in DEE, with high expenditure during lactation and autumn hoarding, and very low expenditure during winter. Contrary to interspecific studies, energy expenditure increased with increasing ambient temperature and it was weakly related to body mass in all seasons except for winter. High resource availability was associated with reduced energy expenditure in winter, but elevated expenditure during lactation and hoarding. 5. Collectively, these results highlight substantial intraspecific variation in energy expenditure, most of which can be explained by a combination of seasonal stages and environmental conditions, and fundamental differences in the importance and direction of determinants of energy expenditure when examined at the intra- versus the interspecific level.

Summary 1. As understanding of the energetic costs of reproduction in birds and mammals continues to improve, oxidative stress is an increasingly cited example of a non-energetic cost of reproduction that may serve as a proximal physiological link underlying life-history trade-offs. 2. Here, we provide the first study to measure daily energy expenditure (DEE) and oxidative damage in a wild population. We measured both traits on eastern chipmunks (Tamias striatus) and assessed their relationships with age, reproductive status, litter size and environmental conditions. 3. We found that both physiological traits were correlated with environmental characteristics (e.g. temperature, seasons). DEE tended to increase with decreasing temperature, while oxidative damage was lower in spring, after a winter of torpor expression, than in autumn. We also found that DEE decreased with age, while oxidative damage was elevated in young individuals, reduced in animals of intermediate age and tended to increase at older age. 4. After controlling for age and environmental variables, we found that both female DEE and oxidative damage increased with litter size, although the latter increased weakly. 5. Our results corroborate findings from laboratory studies but highlight the importance of considering environmental conditions, age and reproductive status in broader analyses of the causes and consequences of physiological costs of reproduction in wild animals.

Summary 1. Resting metabolic rate (RMR) varies considerably among and within species. Two central questions in physiological ecology are whether values of RMR are repeatable and whether an association exists between RMR and fitness. 2. First, we investigated the repeatability of RMR in food hoarding, juvenile, North American red squirrels (Tamiasciurus hudsonicus Erxleben). Second, we explored links between RMR and survival. A low RMR may enhance survival if it reduces winter expenditure costs and/or allows more energy to be allocated towards autumn food hoarding. Alternately, a high RMR may enhance survival if it enables juveniles to hoard more food by increasing the throughput of energy available for investment in hoarding activities. 3. Resting metabolic rate adjusted for body mass, was repeatable in both males and females (r = 0·77) over a short-term (mean 24·3 days) but only among females (r = 0·72) over a long-term interval (mean 192 days). 4. Heavier juveniles and those with a lower RMR relative to their body mass were more likely to survive over-winter. Multiple selection models found significant selection for a decreased RMR (β′ = −0·56 ± 0·16) and increased mass (β′ = 0·69 ± 0·17). Survivors also tended to have more food stored within their hoard. 5. A low RMR relative to body mass and large body mass may have allowed individuals to minimize the expenditure costs related to a larger body mass, while maximizing thermal inertia.

Summary 1. The regulation of energy expenditure in relation to food availability and its consequences for individual fitness in free-ranging animals are poorly understood. Increased daily energy expenditure (DEE) may be viewed as the result of two different processes: expenditure may be forced upwards by low food availability (forcing hypothesis) or enabled to increase by high levels of food resources (enabling hypothesis). Several studies have suggested long-term fitness costs due to increased mortality as a trade-off to increased DEE. 2. We examined the relationship between energy expenditure and an indirect measure of food availability, and the short-term fitness consequences associated with changes in DEE in a small, Arctic seabird, the little auk (Alle alle). We measured DEE of 43 parent little auks by the doubly labelled water method during two consecutive breeding seasons and inferred food availability from plasma concentrations of the stress hormone corticosterone (CORT). 3. We found that DEE was elevated by 26·7% in the year with reduced levels of CORT, indicating that little auks responded to increased food availability by increasing their DEE. These results support the enabling hypothesis. Elevated DEE was presumably caused by increased parental effort as reflected by higher chick provisioning rates and larger chick meals, and was associated with fitness benefits in terms of enhanced current reproductive success. 4. Contrary to earlier studies, our data did not indicate adverse effects associated with elevated DEE; there was no negative relationship between DEE and the probability of adults returning to the colony the following year. Instead, adult return rate was positively related to body mass, with lower return rates when food was limited. 5. These results suggest that ecological consequences associated with limited resource availability may outweigh possible direct negative physiological effects of elevated DEE.

Summary 1. Measures of the field metabolic rate of marine mammals are extremely difficult to make but they are essential for assessing the impacts of marine mammals on prey populations, and for assessing dive performance in relation to aerobic limits. 2. The doubly labelled water (DLW) method is an isotope-based technique for the estimation of the CO 2 production, and hence energy expenditure, of free-living animals. Estimates of field metabolic rate (FMR) from DLW in pinnipeds to date are extremely high and at the upper range for most mammals. DLW has previously been validated in pinnipeds but logistical difficulties meant for these validations were less than ideal, and it has been hypothesised that DLW may overestimate FMR in these animals. 3. To test this hypothesis, we used DLW and simultaneous open-flow respirometry to determine the daily energy expenditures (DEE) of wild grey seals ( Halichoerus grypus ) held temporarily in a captive facility, during 4‐5 days of simulated foraging at sea. Comparing DEE from DLW and respirometry, we found that DLW predicted DEE accurately (average difference between the two estimates was 0·7% SD = 17% n = 31), but as with validations of other species there was a large range of individual errors (from ‐39% to +44%). 4. The results dispel the doubts surrounding the use of DLW as a field method for estimating energy expenditure in grey seals, and by implication other pinnipeds, and simultaneously open a series of questions about their ability to maintain surprisingly high metabolic rates for protracted periods. 5. We make a number of recommendations for future studies of pinniped FMR using DLW. We suggest use of the Speakman two-pool calculation will be most appropriate. Studies should aim for enrichment levels as high as economically feasible but to at least 150 p.p.m. above background for the O 2 isotope. Measurement periods should be extended between one and two half-lives (5‐10 days for a typical foraging seal). We also encourage the calculation and presentation of estimates of precision in estimates of FMR.

Summary 1Mammals usually derive energy from metabolizing fat and glycogen stores combined with exogenous food. Nectarivorous bats mostly consume a diet low in both fat and proteins but rich in simple carbohydrates. Metabolizing exogenous carbohydrates directly to fuel their high mass-specific metabolic rate would save the energetic costs of lipogenesis and gluconeogenesis for nectarivorous bats. Therefore, we expected nectarivorous bats to switch to exogenous carbohydrates rapidly when available and use them predominantly instead of fat or glycogen. 2We first investigated the rate of fractional incorporation of dietary sugars into the pool of metabolized substrates in Glossophaga soricina by measuring the change in 13C enrichment of exhaled CO2 (δ13Cbreath) when animals were fed glucose, fructose or sucrose that was isotopically distinct from their normal diet. Second, we performed a diet-switch experiment to estimate the turnover rate of fat tissue. 3When fed with sugars, the δ13Cbreath converged quickly on the isotope signature of the ingested sugars, indicating an almost exclusive use of dietary carbohydrates. The time for a 50% carbon isotope exchange in exhaled CO2 equalled 9, 13 and 14 min for fructose, glucose and sucrose, respectively. Nectarivorous bats fuelled 82% of their metabolism with exogenous carbohydrate when fed with fructose, 95% when fed with glucose and 77% when fed with sucrose. Bats depleted 50% of their fat stores each day. 4Although nectarivorous bats consumed most of their body fat each day, this was still barely enough to sustain their diurnal metabolism. The fractional incorporation rates of dietary sugars into the pool of metabolized substrates in G. soricina are the fastest rates ever found in a mammal.

Summary 1Free-living animals make complex decisions associated with optimizing energy and nutrient intake. In environments where ambient temperatures fall below the thermoneutral zone, homeotherms must choose whether or not to forage, how long and what to forage for, and whether or not to perform activities that conserve energy. 2Huddling in groups has long been thought of as a possible means of conserving energy. Laboratory studies have shown that at low ambient temperatures individuals in groups expend less energy than individuals by themselves. However, studies have yet to demonstrate that thermoregulatory savings can have an impact on the overall daily energy expenditure (DEE) of free-living animals. 3Here we show that, in the laboratory, African Four-Striped Grass Mice (Rhabdomys pumilio) expend less energy per individual in large groups than smaller groups. We also show that when free-living groups were experimentally reduced to one-half of their original size, DEE and water turnover increased by 19% and 37%, respectively. 4The magnitudes of the reduction in free-living DEE were comparable with calculated energy savings from the laboratory. One of the reasons why this species may sometimes occur in groups is that energetic benefits can be gained through huddling in habitats in which food and water are scarce.

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

Summary 1 A comparison was made of the daily energy expenditure (DEE), resting metabolic rate (RMR) and water turnover (WTO) of two populations of Common Spiny Mice Acomys cahirinus from north- and south-facing slopes (NFS and SFS) of the same valley, which represented ‘Mediterranean’ and ‘desert’ habitats, respectively. 2 An examination was made as to whether these physiological characteristics differed between mice that had been in the laboratory (outdoor conditions) for 2 months compared with mice captured from the field. 3 Mice from the field had greater RMR values than mice in the laboratory and NFS mice had greater RMR values than SFS mice. In the field, NFS individuals had greater DEE values than SFS individuals. Mass-specific RMR values of SFS mice were 20% less than the allometrically predicted value, whereas those of NFS were not. WTO and sustained metabolic scope (DEE/RMR) were lower in the field than in the laboratory. 4 The results indicate that physiological capabilities are phenotypically plastic, as differences exist between the field and laboratory and between NFS and SFS mice. Furthermore, they highlight the importance of using field studies for understanding the link between energetics and physiological adjustment to environmental conditions.

1. Three experiments were performed, using laboratory mice (Mus musculus) as a model species, to evaluate the potential of using measurements of carbon isotope ratios in expired CO 2 for tracing diets. 2. Breath 13 C signatures of mice fed a constant diet (-21.4‰ ± 0.35) reflected their diet, but were depleted by on average -5.7‰. Body mass, sex and age were independent and significant factors correlated with the variability of 13 C enrichment in respired CO 2 . 3. Breath 13 C signatures from starved mice (7 h) were lower than unstarved mice by 2.0‰. Subsequently when starved mice were fed a small meal of a new diet, breath 13 C signatures approached those of the new diets within 15 min, returning to preingestion levels after 105 min. 4. After a permanent diet switch 13 C values of breath were not asymptotic within 6 days, possibly because of use of fat reserves during the daytime carrying an isotopic memory of the previous diet. Hence, individual breath 13 C signatures may vary according to nutritive state and previous dietary history. 5. Interindividual variability was measured at 3.3‰. The implications are that large samples of individuals will be required to distinguish between diets of different populations where the isotopic difference between their diets was small - for example, that expected between herbivorous and carnivorous diets. However, breath would be suitable for distinguishing between dietary intakes of individuals for food types that are isotopically more distinct - such as between C 3 and C 4 plants.

1. The daily energy expenditure of free-living adult male Wood Mice, Apodemus sylvaticus, from deciduous woodland and maritime sand-dune was estimated using the doubly labelled water (DLW) technique and time–energy budget models. 2. Method of calculation had a small effect on the calculated energy demands. There were no significant differences in energy expenditure across sites or seasons despite large differences in home range among mice from different sites. Mice in winter, and on the sand-dunes, had lower body masses, which offset the effects of larger home ranges and colder temperatures. 3. The daily energy expenditure for a Wood Mouse of mean body mass (20·5 g) was 67·9 kJ day–1 (n = 19) This was comparable to other similar size rodents (slightly higher), and did not deviate significantly from allometric predictions. 4. Variation in daily energy expenditure among individuals was not explained by ambient temperature, home range size, body mass, change in body mass, time spent active and the distance or speed of travel. 5. Sustainable metabolic rates approximated to five times basal metabolism and were within the range previously reported but higher than most other estimates of free-living energy demands. 6. Rates of water influx and efflux were significantly greater for Wood Mice on the sand-dunes (influx, 9·3 ± 1·14 ml day–1, efflux, 10·2 ± 1·10 ml day–1) than for animals in the woodland (influx, 6·3 ± 0·48 ml day–1, efflux, 6·2 ± 0·47 ml day–1). This was probably due to differences in the water content of their diets (seed in woodland and invertebrate on the sand-dunes). 7. The time–energy budget models yielded estimates of energy expenditure considerably lower than the doubly labelled water estimate. Potential reasons for this discrepancy are discussed.

1. The energy budgets of lactating and non-reproductive female Brown Long-Eared Bats fed primarily on noctuid moths (≈ 27·2 kJ g–1) were constructed and compared in flight enclosures in captivity. 2. The average dry food consumption of non-reproductive individuals was 1·8 g bat–1 day–1 (gross energy intake = 48 kJ day–1). The average food consumption throughout days 10–35 of lactation was 2·0 g bat–1 day–1 (gross energy intake = 53 kJ day–1). Lactating females obtained six times more energy from increased food consumption than from mobilization of fat stores, compared with non-reproductive bats. 3. Milk export, calculated using the difference in water turnover between lactating and non-reproductive bats (measured using 2H turnover) averaged 2·6 ml bat–1 day–1 (22·9 kJ day–1). This was similar to the average milk intake of sucklings estimated from 3H turnover (22·9 kJ day–1). 4. Energy available for respiration from food and mobilization of fat stores was 18·2 kJ day–1 for lactating females compared with 36·8 kJ day–1 for non-reproductive females. In comparison, respiratory daily energy expenditure (DEE) of lactating and non-reproductive bats, measured by doubly labelled water (DLW), was 21·3 kJ day–1 and 23·6 kJ day–1, respectively. Hence, there was a discrepancy between respiratory DEE (measured by DLW) and net available energy estimates for non-reproductive bats but not for lactating bats. 5. Respiratory DEE for lactating bats was equal to or less than that of non-reproductive females, suggesting they used compensatory mechanisms in their energy budgets in lactation.

1. Three experiments were performed on white mice (strain MF1) to establish if there was social or physiological suppression of metabolism, mediated by elevated ambient levels of CO2, when animals were in close contact. In the first two experiments the metabolism of two mice were each measured solitarily and paired together in the same chamber, both with and without a partition that allowed visual, auditory and olfactory communication but restricted physical contact. 2. Comparisons were made between the sum of the two solitary measurements, and the observed measurements of paired individuals. In the first experiment air flow was low and ambient CO2 was always at levels predicted to produce suppression (> 0·2%), independent of whether the animals were measured alone or paired. 3. The paired mice had greater metabolism than expected from the sum of their solitary metabolic rates whether the partition was present or absent. This increase was due to increased activity of the animals when in pairs. This experiment indicated that when CO2 levels were maintained in the range 0·2–0·5% there was no social suppression of metabolism. 4. In experiment 2 metabolic rates were measured of solitary mice exposed to subthreshold levels of CO2 (≈ 0·12%) and paired mice that experienced levels above the supposed threshold (≈ 0·24%). Again the observed metabolism of the paired mice exceeded the sum of their metabolic rates when solitary. The increase was also attributable to greater activity of the animals in pairs. 5. This experiment indicated no social suppression effect when CO2 varied in the range 0·12–0·24%. Moreover there was no significant difference between the oxygen consumptions of mice in experiment 2 compared with experiment 1, indicating that independent of the social context there was no suppressive effect of increases in CO2 concentration over the range 0·12–0·5% CO2. 6. Finally, in experiment 3, when two mice were placed in sequential separate chambers (allowing no social contact), their summed metabolism did not differ from the sum of their solitary measurements when a CO2 absorber was placed in the airflow linking the chambers. When the CO2 absorber was absent the observed metabolism of the pair exceeded the sum of the solitary measurements. 7. Overall, across all three experiments no evidence was found for either a social or a physiological suppression of metabolism mediated via CO2 (0·12–0·5%).