Metabolic compensation during high energy output in fasting, lactating grey seals (Halichoerus grypus): metabolic ceilings revisited

Lactation is the most energetically expensive period for female mammals and is associated with some of the highest sustained metabolic rates (SusMR) in vertebrates (reported as total energy throughput). Females typically deal with this energy demand by increasing food intake and the structure of the alimentary tract may act as the central constraint to ceilings on SusMR at about seven times resting or standard metabolic rate (SMR). However, demands of lactation may also be met by using a form of metabolic compensation such as reducing locomotor activities or entering torpor. In some phocid seals, cetaceans and bears, females fast throughout lactation and thus cannot offset the high energetic costs of lactation through increased food intake. We demonstrate that fasting grey seal females sustain, for several weeks, one of the highest total daily energy expenditures (DEE; 7.4 x SMR) reported in mammals, while progressively reducing maintenance metabolic expenditures during lactation through means not explained by reduction in lean body mass or behavioural changes. Simultaneously, the energy-exported in milk is progressively increased, associated with increased lipoprotein lipase activity in the mammary gland, resulting in greater offspring growth. Our results suggest that females use compensatory mechanisms to help meet the extraordinary energetic costs of lactation. Additionally, although the concepts of SusMR and ceilings on total DEE may be somewhat different in fasting lactating species, our data on phocid seals demonstrate that metabolic ceilings on milk energy output, in general, are not constrained by the same kind of peripheral limitations as are other energy-consuming tissues. In phocid seals, the high ceilings on DEE during lactation, coupled with metabolic compensation, are undoubtedly important factors enabling shortened lactation.     

The energy costs of sexual dimorphism in mole-rats are morphological not behavioural

Different reproductive strategies of males and females may lead to the evolution of differences in their energetic costs of reproduction, overall energetic requirements and physiological performances. Sexual dimorphism is often associated with costly behaviours (e.g. large males might have a competitive advantage in fighting, which is energetically expensive). However, few studies of mammals have directly compared the energy costs of reproductive activities between sexes. We compared the daily energy expenditure (DEE) and resting metabolic rate (RMR) of males and females of two species of mole-rat, Bathyergus janetta and Georychus capensis (the former is sexually dimorphic in body size and the latter is not) during a period of intense digging when males seek females. We hypothesized that large body size might be indicative of greater digging or fighting capabilities, and hence greater mass-independent DEE values in males of the sexually dimorphic species. In contrast to this prediction, although absolute values of DEE were greater in B. janetta males, mass-independent values were not. No differences were apparent between sexes in G. capensis. By comparison, although RMR values were greater in B. janetta than G. capensis, no differences were apparent between the sexes for either species. The energy cost of dimorphism is most likely to be the cost of maintenance of a large body size, and not the cost of behaviours performed when an individual is large.     

Effects of feeding and habitat on resting metabolic rates of the Pacific walrus

Arctic marine mammals live in a rapidly changing environment due to the amplified effects of global warming. Pacific walruses (Odobenus rosmarus divergens) have responded to declines in Arctic sea-ice extent by increasingly hauling out on land farther from their benthic foraging habitat. Energy models can be useful for better understanding the potential implications of changes in behavior on body condition and reproduction but require behavior-specific metabolic rates. Here we measured the resting metabolic rates (RMR) of three captive, adult female Pacific walruses through breath-by-breath respirometry when fed and fasted resting out of water (sitting and lying down) and while fed resting in water. RMR in and out of water were positively related with pretrial energy intake when not fasted and 25% higher than RMR when walruses were fasted and out of water. Overall, RMR was higher than previously estimated for this species. Fasting RMR out of water was only 25% lower than subsurface swimming metabolic rates suggestive of relatively efficient swimming in adult females. Our results identify the importance of considering feeding status and species-specific differences in affecting metabolic costs. Further research is needed to better understand potential energetic costs of thermoregulation at temperatures experienced by wild walruses.     

Is resting metabolic rate related to reproductive output in an orb-web spider,Argiope radon?

1. Resting metabolic rate (RMR) is a fundamental feature of animal biology that reflects the baseline level of energy expenditure. There are two main strategies that can address energy demands; animals can reallocate energy from maintenance by reducing RMR to meet energy demands (compensation model) or they can increase intake rate by increasing metabolic activities (performance model). 2. Orb-web spiders are sit-and-wait foragers that typically reside at the centre of their web waiting to intercept prey. Given their sedentary resource acquisition strategy, it is predicted that lower RMR is favoured to reduce self-maintenance energetic costs and to allow greater allocation to oogenesis (i.e., egg sac development). 3. In this study, we tested temporal variation in RMR of female Argiope radon (Araneae: Araneidae) spiders in response to mating status. Then we tested the degree to which between-individual variation in the parental RMR relates to reproductive output and spiderling early life-history traits. 4. Despite the notable between-individual variation, we found a temporal consistency of RMR in the female spiders at early adulthood. Mated females significantly reduced their RMR by around 35% compared to their unmated stage which supports compensation model. However, there was a significant correlation between female RMR and mass of the egg sac in these spiders which is an evidence for performance model. 5. Our findings suggest that energy management in this species is a complex phenomenon, both strategies are in effect simultaneously at within- and between-individual level shaping the individuals' phenotype.     

Hypoxia impairs the digestive advantage of individual southern catfish (Silurus meridionalis) with high resting metabolic rates and postprandial metabolic responses

Empirical studies suggest that individuals with a high resting metabolic rate (RMR) are at an advantage under favourable conditions because they digest food rapidly and exhibit a greater growth potential. However, we hypothesised that high-RMR individuals have less energy available for digestion under hypoxia than they do under normoxia due to their relatively high maintenance cost. To test this hypothesis, we measured the RMR and postprandial metabolic responses of juvenile southern catfish, Silurus meridionalis, under normoxia and moderate hypoxia. The results provided the first evidence that (1) both the RMR and postprandial metabolic rate showed repeatability across different water [O₂] conditions and (2) the correlation between the RMR and postprandial metabolic traits differs with changes in environmental factors (water [O₂]). These findings suggested that the digestive advantage of individual southern catfish with a high RMR is impaired under hypoxia.     

Energetics during hatchling dispersal of the olive ridley turtle Lepidochelys olivacea using doubly labeled water

Studies of metabolism are central to the understanding of the ecology, behavior, and evolution of reptiles. This study focuses on one phase of the sea turtle life cycle, hatchling dispersal, and gives insight into energetic constraints that dispersal imposes on hatchlings. Hatchling dispersal is an energetically expensive phase in the life cycle of the olive ridley turtle Lepidochelys olivacea. Field metabolic rates (FMRs), determined using the doubly labeled water (DLW) method, for L. olivacea hatchlings digging out of their nest chamber, crawling at the sand surface, and swimming were five, four, and seven times, respectively, the resting metabolic rate (RMR). The cost of swimming was 1.5 and 1.8 times the cost of the digging and crawling phases, respectively, and we estimated that if L. olivacea hatchlings swim at frenzy levels, they can rely on yolk reserves to supply energy for only 3-6 d once they reach the ocean. We compared our RMR and FMR values by establishing an interspecific RMR mass-scaling relationship for a wide range of species in the order Testudines and found a scaling exponent of 1.06. This study demonstrates the feasibility of using the DLW method to estimate energetic costs of free-living sea turtle hatchlings and emphasizes the need for metabolic studies in various life-history stages.     

Seasonal variation in the metabolic rate and body composition of female grey seals: fat conservation prior to high-cost reproduction in a capital breeder?

Many animals rely on stored energy through periods of high energy demand or low energy availability or both. A variety of mechanisms may be employed to attain and conserve energy for such periods. Wild grey seals demonstrate seasonal patterns of energy storage and foraging behaviour that appear to maximize the allocation of energy to reproduction—a period characterized by both high energy demand and low food availability. We examined seasonal patterns in resting rates of oxygen consumption as a proxy for metabolic rate (RMR) and body composition in female grey seals (four adults and six juveniles), testing the hypothesis that adults would show seasonal changes in RMR related to the reproductive cycle but that juveniles would not. There was significant seasonal variation in rates of resting oxygen consumption of adult females, with rates being highest in the spring and declining through the summer months into autumn. This variation was not related to changes in water temperature. Adults increased in total body mass and in fat content during the same spring to autumn period that RMR declined. RMR of juveniles showed no clear seasonal patterns, but did increase with increasing mass. These data support the hypothesis that seasonal variation in RMR in female grey seals is related to the high costs of breeding.     

Morphological and physiological responses to altitude in deer mice Peromyscus maniculatus.

Individuals within a species, living across a wide range of habitats, often display a great deal of phenotypic plasticity for organ mass and function. We investigated the extent to which changes in organ mass are variable, corresponding to environmental demand, across an altitudinal gradient. Are there changes in the mass of oxygen delivery organs (heart and lungs) and other central processing organs (gut, liver, kidney) associated with an increased sustainable metabolic rate that results from decreased ambient temperatures and decreased oxygen availability along an altitudinal gradient? We measured food intake, resting metabolic rate (RMR), and organ mass in captive deer mice (Peromyscus maniculatus bairdii) at three sites from 1,200 to 3,800 m above sea level to determine whether energy demand was correlated with organ mass. We found that food intake, gut mass, and cardiopulmonary organ mass increased in mice living at high altitudes. RMR was not correlated with organ mass differences along the altitudinal gradient. While the conditions in this study were by no means extreme, these results show that mice living at high altitudes have higher levels of energy demand and possess larger cardiopulmonary and digestive organs than mice living at lower altitudes.     

Seasonal intake responses in the nectar-feeding bat Glossophaga soricina

Food intake in nectar-feeding animals is affected by food quality, their energetic demands, and the environmental conditions they face. These animals increase their food intake in response to a decrease in food quality, a behavior named “intake response”. However, their capacity to achieve compensatory feeding, in which they maintain a constant flux of energy, could be constrained by physiological processes. Here we evaluated how both a seasonal change in environmental conditions and physiological constraints affected the food ingestion in the bat Glossophaga soricina. We measured food intake rate during both the wet/warm and dry/cool seasons at sucrose solutions ranging from 146 to 1,022 mmol L⁻¹. We expected that food intake and metabolic demands would be greater during the dry/cool season. Bats ingested ~20% more food in the dry/cool than in the wet/warm season. Regardless of season, bats were unable to achieve a constant flux of energy when facing the different sugar concentrations that we used in our experiments. This suggests that the rate of food intake is physiologically constrained in G. soricina. Using the digestive capacity of bats we modeled their food intake. The analytic model we used predicts that digestive limitations to ingest energy should have an important effect on the ecology of this species.     

Energetic trade-offs between immunity and reproduction in male Japanese quail (Coturnix coturnix)

We investigated a postulated trade-off between reproduction and immune function by comparing the energetic costs of an immune response with phytohemagglutinin challenge (or injection) in castrated (low testosterone [T]) and intact (high T) Japanese Quail (Coturnix coturnix). Intact birds had higher resting metabolic rate (RMR) and significantly lower immune response than castrates. RMR of intact birds did not change in response to an immune challenge, suggesting that maintenance of reproductive tissues and associated high T is both immunosuppressive and energetically costly. Despite having a greater immune response than intact quail, castrates had a lower pre-challenge RMR than intact birds and paradoxically tended to decrease RMR during an immune challenge. This paradox may be because of pro-inflammatory cytokines that are released during immune responses. Cytokines promote energy conservation through malaise and soporific behaviors, possibly explaining the co-occurrence of a relatively strong immune response and a decrease in nocturnal RMR in castrates. The lower immune response in intact birds may not elicit as great a response of pro-inflammatory cytokines owing to an already elevated RMR from reproductive state, thus reducing any effect on RMR. The suppressed immune response and elevated RMR in intact birds may be because of T; however, we cannot separate the effects of T per se from the metabolic requirements of reproductive tissues.     

Energetics and life-history of bats in comparison to small mammals

Mammals can be aligned along a slow-fast life-history continuum and a low–high metabolic rate continuum based on their traits. Small non-volant mammals occupy the fast/high end in both continua with high reproductive rates and short life spans linked with high mass-specific metabolic rates. Bats occupy the high end of the metabolic continuum, but the slow end of the life-history continuum with low reproductive rates and long life spans. Typically, both continua are linked, and similar life-history traits of species are reflected in more similar metabolic rates. We therefore hypothesized that metabolic rates are similar in species with similar life-history traits. Resting metabolic rates (RMR) were measured for three ecologically and morphologically similar sympatric bat species (Myotis nattereri, M. bechsteinii, and Plecotus auritus; Vespertilionidae) and compared to data from other similar-sized, temperate insectivorous mammals with other life-history strategies. The bat species share similar life-histories and RMRs, both of which differ from the remaining mammals and therefore supporting our hypothesis. To verify that bats are similar in RMR, two energetically contrasting periods were compared. RMRs in post-lactating females did not differ between bat species. It was, however, positively correlated with parasite load in both Myotis species. However, RMRs differed during energy-demanding pregnancy where M. nattereri had the significantly lowest RMR, suggesting metabolic compensation as an energy-saving strategy. We conclude that the energy requirements of bat species with similar life-history traits resemble each other during periods of low energetic demands and are more similar to each other than to other small temperate mammals.     

Energetic costs of parasitism in the Cape ground squirrel Xerus inauris

Parasites have been suggested to influence many aspects of host behaviour. Some of these effects may be mediated via their impact on host energy budgets. This impact may include effects on both energy intake and absorption as well as components of expenditure, including resting metabolic rate (RMR) and activity (e.g. grooming). Despite their potential importance, the energy costs of parasitism have seldom been directly quantified in a field setting. Here we pharmacologically treated female Cape ground squirrels (Xerus inauris) with anti-parasite drugs and measured the change in body composition, the daily energy expenditure (DEE) using doubly labelled water, the RMR by respirometry and the proportions of time spent looking for food, feeding, moving and grooming. Post-treatment animals gained an average 19g of fat or approximately 25kJd-1. DEE averaged 382kJd-1 prior to and 375kJd-1 post treatment (p>0.05). RMR averaged 174kJd-1 prior to and 217kJd-1 post treatment (p<0.009). Post-treatment animals spent less time looking for food and grooming, but more time on feeding. A primary impact of infection by parasites could be suppression of feeding behaviour and, hence, total available energy resources. The significant elevation of RMR after treatment was unexpected. One explanation might be that parasites produce metabolic by-products that suppress RMR. Overall, these findings suggest that impacts of parasites on host energy budgets are complex and are not easily explained by simple effects such as stimulation of a costly immune response. There is currently no broadly generalizable framework available for predicting the energetic consequences of parasitic infection.     

Resting and swimming metabolic rates in juvenile walruses (Odobenus rosmarus)

Changes in Arctic ice conditions have raised concerns regarding potential impacts on energy expenditure and food requirements of walruses. Modeling the repercussions of environmental changes requires accurate species-specific measures of bioenergetic expenditures. This is particularly true for walruses, who have a unique anatomy and foraging ecology from other pinnipeds. This study measured resting metabolic rate (RMR) and subsurface swimming metabolism in two juvenile walruses over a 13-month period. The walruses had relatively low RMR compared to studies of other young pinnipeds. RMR was greater for the male than the female, as expected given its larger size; the reverse was true on a mass-specific basis. There was also considerable variability in RMR for each walrus during the year that could not be accounted for by changes in body mass. Metabolism while swimming was about twice RMR, and locomotor costs were higher than generally predicted for other marine mammals. The lower calculated swimming efficiency may reflect the fact that walruses are not “high velocity” pursuit predators. The estimates of metabolic expenditure obtained in this study for young walruses are invaluable for quantifying the energetic consequences of behavioral changes induced by environmental shifts in the wild.     

Immune activity elevates energy expenditure of house sparrows: a link between direct and indirect costs?

The activation of an immune response is beneficial for organisms but may also have costs that affect fitness. Documented immune costs include those associated with acquisition of special nutrients, as well as immunopathology or autoimmunity. Here, we test whether an experimental induction of the immune system with a non-pathological stimulant can elevate energy turnover in passerine birds. We injected phytohaemagglutinin (PHA), a commonly used mitogen that activates the cell-mediated immune response, into the wing web of house sparrows, Passer domesticus. We then examined energetic costs resulting from this immune activity and related those costs to other physiological activities. We found that PHA injection significantly elevated resting metabolic rate (RMR) of challenged sparrows relative to saline controls. We calculated the total cost of this immune activity to be ca. 4.20 kJ per day (29% RMR), which is equivalent to the cost of production of half of an egg (8.23 kJ egg(-1)) in this species. We suggest that immune activity in wild passerines increases energy expenditure, which in turn may influence important life-history characteristics such as clutch size, timing of breeding or the scheduling of moult.     

The demands of incubation and avian clutch size

We reviewed information on the demands of incubation to examine whether these could influence the optimal clutch size of birds. The results indicate that appreciable metabolic costs of incubation commonly exist, and that the incubation of enlarged clutches can impose penalties on birds. In 23 studies on 19 species, incubation metabolic rate (IMR) was not elevated above the metabolic rate of resting non-incubating birds (RMR), but contrary to the physiological predictions of King and others, IMR was greater than RMR in 15 studies on 15 species. Across species, IMR was substantially above basal metabolic rate (BMR), averaging 1.606 × BMR. Of six studies on three species performed under thermo-neutral conditions, none found IMR to be in excess of RMR. IMRs measured exclusively within the thermo-neutral zone averaged only 1.08 × BMR contrasting with the significantly higher figure of 1.72 × BMR under wider conditions. 16 of 17 studies on procellariiforms found IMR below RMR, indicating a significant difference between this and other orders. We could find no other taxonomic, or ecological factors which had clear effects on IMR. Where clutch size was adjusted experimentally during incubation, larger clutches were associated with: significantly lower percentage hatching success in 11 of 19 studies; longer incubation periods in eight of ten studies; greater loss of adult body condition in two of five studies; and higher adult energy expenditure in eight of nine studies. Given that incubation does involve metabolic costs and given that the demands of incubation increase sufficiently with clutch size to affect breeding performance, we propose that the optimal clutch size of birds may in part by shaped by the number of eggs the parents can afford to incubate.     

Substitution of heat from exercise and digestion by ducks diving for mussels at varying depths and temperatures

Diving birds can lose significant body heat to cold water, but costs can be reduced if heat from exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. Potential for substitution depends jointly on the rate of heat loss, the rate of heat produced by exercise, and the level of HIF. To explore these interactions, we measured oxygen consumption by lesser scaup ducks (Aythya affinis) diving to depths of 1.2 and 2 m at thermoneutral (23°C) and sub-thermoneutral (18 and 8°C) temperatures. Birds dove while fasted and when feeding on blue mussels (Mytilus edulis). Substitution occurred if HIF or costs of diving above resting metabolic rate (RMR) were lower at 18 or 8°C than at 23°C, indicating reduction in the thermoregulatory part of RMR. For fasted scaup diving to 1.2 m, substitution from exercise heat was not apparent at either 18 or 8°C. At 2 m depth, dive costs above RMR were reduced by 5% at 18°C and by 40% at 8°C, indicating substitution. At 1.2 m depth (with voluntary intake of only 14–17% of maintenance requirements), HIF did not differ between temperatures, indicating no substitution. However, at 2 m (intake 13–25% of maintenance), substitution from HIF was 23% of metabolizable energy intake at 18°C and 22% at 8°C. These results show that even with low HIF due to low intake rates, substitution from HIF can add to substitution from the heat of exercise.     

Meeting the energy demands of reproduction in female koalas,<Emphasis Type="Italic"> Phascolarctos cinereus</Emphasis>: evidence for energetic compensation

Koalas are generally considered to be limited by their ability to acquire energy from their diet of Eucalyptus foliage and have the lowest mass-specific peak lactational energy output measured in any mammal to date. This study considered the energetics and sources of energy utilised for reproduction in free-ranging female koalas. Energy requirements and foliage intake were greater in both lactating and non-lactating females in winter than summer, presumably due to demands of thermoregulation. Koalas met the peak energy requirements of lactation primarily by a 36% increase in their intake of foliage. Metabolic energy expenditure (field metabolic rate, 1778 kJ.day^–1 for a 6.25-kg female at the time of peak lactation) was not elevated during lactation. This was due to compensation for part of their lactational demands by reduction of another, non-reproductive, component of their energy budget. The observed energetic compensation was probably due primarily to substitution of the waste heat from the metabolic costs of milk production and increased heat increment of feeding for thermoregulatory energy expenditure. There may also have been energetic compensation by reduction of some aspect of maintenance metabolism. Such energetic compensation, together with the strategy of spreading lactation over a long period, minimises the magnitude of lactational energy demands on koalas, and thus the increase in daily food intake required during lactation. As the nutritional requirements of females at peak lactation are the highest of any members of the population, low reproductive requirements effectively increase the types and amount of habitat able to support koala populations.Abbreviations FMR field metabolic rate - HIF heat increment of feeding - RMR resting metabolic rate - _O2 rate of oxygen consumptionCommunicated by I.D. Hume     

Energetic cost of bot fly parasitism in free-ranging eastern chipmunks

The energy and nutrient demands of parasites on their hosts are frequently invoked as an explanation for negative impacts of parasitism on host survival and reproductive success. Although cuterebrid bot flies are among the physically largest and most-studied insect parasites of mammals, the only study conducted on metabolic consequences of bot fly parasitism revealed a surprisingly small effect of bot flies on host metabolism. Here we test the prediction that bot fly parasitism increases the resting metabolic rate (RMR) of free-ranging eastern chipmunks (Tamias striatus), particularly in juveniles who have not previously encountered parasites and have to allocate energy to growth. We found no effect of bot fly parasitism on adults. In juveniles, however, we found that RMR strongly increased with the number of bot fly larvae hosted. For a subset of 12 juveniles during a year where parasite prevalence was particularly high, we also compared the RMR before versus during the peak of bot fly prevalence, allowing each individual to act as its own control. Each bot fly larva resulted in a ~7.6% increase in the RMR of its host while reducing juvenile growth rates. Finally, bot fly parasitism at the juvenile stage was positively correlated with adult stage RMR, suggesting persistent effects of bot flies on RMR. This study is the first to show an important effect of bot fly parasitism on the metabolism and growth of a wild mammal. Our work highlights the importance of studying cost of parasitism over multiple years in natural settings, as negative effects on hosts are more likely to emerge in periods of high energetic demand (e.g. growing juveniles) and/or in harsh environmental conditions (e.g. low food availability).     

Seasonal variability in otariid energetics: implications for the effects of predators on localized prey resources

Otariids, like other wild mammals, contend with a wide variety of energetic demands across seasons. However, due to the cryptic behaviors of this marine group, few studies have been able to examine longitudinal energetic costs or the potential impact of these costs on seasonal or annual prey requirements. Here we evaluated the changes in energy demand and intake of female California sea lions (Zalophus californianus) during reproductive (n=2 sea lions) and nonreproductive (n=3) periods. Monthly measurements included resting metabolic rate, blood hormone levels, body condition (blubber thickness and body mass), and caloric intake for adult sea lions throughout molting, late pregnancy, lactation, and postweaning. We found that maintenance energy demands decreased from 32.0 to 23.1 MJ d(-1) before pupping, remaining stable at 19.4+/-0.6 MJ d(-1) during lactation and postweaning. Energy intake rates to meet these demands showed marked changes with activity level and the reproductive cycle, reaching a peak intake of 3.6 times baseline levels during lactation. Translating this into prey demands, we find that 20,000 reproductively active females on San Nicolas Island rookeries would maximally require 4,950 metric tons of Pacific whiting during a month of the breeding season. This localized impact is reduced significantly with postbreeding dispersal and demonstrates the importance of considering spatial and temporal factors driving the energetic requirements of predators when designing marine protected areas.     

Fuel, fasting, fear: routine metabolic rate and food deprivation exert synergistic effects on risk-taking in individual juvenile European sea bass

1. Individuals of the same species often exhibit consistent differences in metabolic rate, but the effects of such differences on ecologically important behaviours remain largely unknown. In particular, it is unclear whether there is a cause-and-effect relationship between metabolic rate and the tendency to take risks while foraging. Individuals with higher metabolic rates may need to take greater risks while foraging to obtain the additional food required to satisfy their energy requirements. Such a relationship could be exacerbated by food deprivation if a higher metabolic demand also causes greater mass loss and hunger. 2. We investigated relationships among metabolic rate, risk-taking and tolerance of food deprivation in juvenile European sea bass. Individual fish were tested for risk-taking behaviours following a simulated predator attack, both before and after a 7-day period of food deprivation. The results were then related to their routine metabolic rate (RMR), which was measured throughout the period of food deprivation. 3. The amount of risk displayed by individual fish before food deprivation showed no relationship with RMR. After food deprivation, however, the amount of risk among individuals was positively correlated with RMR. In general, most fish showed an increase in risk-taking after food deprivation, and the magnitude of the increase in risk-taking was correlated with the rate of individual mass loss during food deprivation, which was itself strongly correlated with RMR. 4. The observation that RMR was related to risk-taking behaviour after food deprivation, but not before, suggests that although RMR can influence risk-taking, the strength of the relationship is flexible and context dependent. The effects of RMR on risk-taking may be subtle or non-existent in regularly feeding animals, but may lead to variability in risk-taking among individuals when food is scarce or supply is unpredictable. This synergistic relationship between RMR and food deprivation could lead to an increased likelihood of being predated for individuals with a relatively high intrinsic energy demand during times when food is scarce.