Limits to sustained energy intake

Differences in the range of thermoneutral zone between mouse strains: potential effects on translational research

Laboratory mice are commonly used for studies emulating human metabolism. To render human energetics, their ratio of daily (DEE) to basal (BMR) energy expenditure of 1.7-1.8 should be maintained. However, the DEE/BMR ratio strongly depends on whether a given study using a mouse model is carried out above, or below the lower critical temperature (LCT) of the thermoneutral zone, which is rarely considered in translational research. Here, we used mice artificially selected for high or low rates of BMR along with literature data to analyze the effect of ambient temperature on possible systematic bias in DEE/BMR. We demonstrated that the estimated LCTs of mice from the high and low BMR lines differ by more than 7°C. Furthermore, the range of variation of LCTs of mouse strains used in translational research spans from 23 to 33°C. Differences between LCTs in our selected mice and other mouse strains are mirrored by differences in their DEE-to-BMR ratio, on average increasing it at the rate of 0.172°C-1 at temperatures below LCT. Given the wide range of LCTs in different mouse strains, we conclude that the energetic cost of thermoregulation may differ greatly for different mouse strains with a potentially large impact on translational outcomes. Thus, the LCT of a given mouse strain is an important factor that must be considered in designing translational studies.

Limits to sustained energy intake. XXXIII. Thyroid hormones play important roles in milk production but do not define the heat dissipation limit in Swiss mice

The limits to sustained energy intake set physiological upper boundaries that affect many aspects of human and animal performance. The mechanisms underlying these limits, however, remain unclear. We exposed Swiss mice to either supplementary thyroid hormones (THs) or the inhibitor methimazole during lactation at 21 or 32.5°C, and measured food intake, resting metabolic rate (RMR), milk energy output (MEO), serum THs and mammary gland gene expression of females, and litter size and mass of their offspring. Lactating females developed hyperthyroidism following exposure to supplementary THs at 21°C, but they did not significantly change body temperature, asymptotic food intake, RMR or MEO, and litter and mass were unaffected. Hypothyroidism, induced by either methimazole or 32.5°C exposure, significantly decreased asymptotic food intake, RMR and MEO, resulting in significantly decreased litter size and litter mass. Furthermore, gene expression of key genes in the mammary gland was significantly decreased by either methimazole or heat exposure, including gene expression of THs and prolactin receptors, and Stat5a and Stat5b. This suggests that endogenous THs are necessary to maintain sustained energy intake and MEO. Suppression of the thyroid axis seems to be an essential aspect of the mechanism by which mice at 32.5°C reduce their lactation performance to avoid overheating. However, THs do not define the upper limit to sustained energy intake and MEO at peak lactation at 21°C. Another, as yet unknown, factor prevents supplementary thyroxine exerting any stimulatory metabolic impacts on lactating mice at 21°C.

What are the Metabolic Rates of Marine Mammals and What Factors Impact this Value: A review

Over the past several decades, scientists have constructed bioenergetic models for marine mammals to assess potential population-level consequences following exposure to a disturbance, stressor, or environmental change, such as under the Population Consequences of Disturbance (pCOD) framework. The animal's metabolic rate (rate of energy expenditure) is a cornerstone for these models, yet the cryptic lifestyles of marine mammals, particularly cetaceans, have limited our ability to quantify basal (BMR) and field (FMR) metabolic rates using accepted 'gold standard' approaches (indirect calorimeter via oxygen consumption and doubly labeled water, respectively). Thus, alternate methods have been used to quantify marine mammal metabolic rates, such as extrapolating from known allometric relationships (e.g. Kleiber's mouse to elephant curve) and developing predictive relationships between energy expenditure and physiological or behavioral variables. To understand our current knowledge of marine mammal metabolic rates, we conducted a literature review (1900-2023) to quantify the magnitude and variation of metabolic rates across marine mammal groups. A compilation of data from studies using 'gold standard' methods revealed that BMR and FMR of different marine mammal species ranges from 0.2 to 3.6 and 1.1 to 6.1 x Kleiber, respectively. Mean BMR and FMR varied across taxa; for both measures odontocete levels were intermediate to higher values for otariids and lower values of phocids. Moreover, multiple intrinsic (e.g. age, sex, reproduction, molt, individual) and extrinsic (e.g. food availability, water temperature, season) factors, as well as individual behaviors (e.g. animal at water's surface or submerged, activity level, dive effort and at-sea behaviors) impact the magnitude of these rates. This review provides scientists and managers with a range of reliable metabolic rates for several marine mammal groups as well as an understanding of the factors that influence metabolism to improve the discernment for inputs into future bioenergetic models.

The Relevance of Time in Biological Scaling

Various phenotypic traits relate to the size of a living system in regular but often disproportionate (allometric) ways. These "biological scaling" relationships have been studied by biologists for over a century, but their causes remain hotly debated. Here, I focus on the patterns and possible causes of the body-mass scaling of the rates/durations of various biological processes and life-history events, i.e., the "pace of life". Many biologists have regarded the rate of metabolism or energy use as the master driver of the "pace of life" and its scaling with body size. Although this "energy perspective" has provided valuable insight, here I argue that a "time perspective" may be equally or even more important. I evaluate various major ways that time may be relevant in biological scaling, including as (1) an independent "fourth dimension" in biological dimensional analyses, (2) a universal "biological clock" that synchronizes various biological rates/durations, (3) a scaling method that uses various biological time periods (allochrony) as scaling metrics, rather than various measures of physical size (allometry), as traditionally performed, (4) an ultimate body-size-related constraint on the rates/timing of biological processes/events that is set by the inevitability of death, and (5) a geological "deep time" approach for viewing the evolution of biological scaling patterns. Although previously proposed universal four-dimensional space-time and "biological clock" views of biological scaling are problematic, novel approaches using allochronic analyses and time perspectives based on size-related rates of individual mortality and species origination/extinction may provide new valuable insights.

Temperature determines the shift of thermal neutral zone and influences thermogenic capacity in striped hamsters

The thermoneutral zone (TNZ) reflects the adaptation of mammals to their natural habitat. However, it remains unclear how TNZ shifts in response to variations in ambient temperature. To test the hypothesis that ambient temperature plays a key role in determining TNZ variations between seasons, we measured metabolic rate, body temperature, and cytochrome c oxidase (COX) activity of several visceral organs in striped hamsters (Cricetulus barabensis) either acclimated to semi-natural conditions over a year, or subjected to a gradual decrease in mean temperature from 30 ± 1°C to -15 ± 1°C. The TNZ range in striped hamsters differed seasonally, with a wider TNZ and a lower lower-critical temperature in winter compared to summer. The hamsters showed a considerable leftward shift of lower-critical temperature from 30°C to 20°C after the ambient temperature of acclimation from 30°C down to -15°C, whereas the upper-critical temperature of TNZ remained fixed at 32.5°C. The resting metabolic rate in thermoneutral zone (RMRt), nonshivering thermogenesis (NST), and COX activity of brown adipose tissue, liver, skeletal muscle, brain, and kidneys, increased significantly in hamsters acclimated at lower ambient temperatures. Following acute exposure to 5°C and -15°C, hamsters acclimated to 32.5°C had significantly lower maximal NST and lower serum thyroid tri-iodothyronine (T₃ ) levels compared to those kept at 23°C. These findings suggest that acclimation to the upper-critical temperature of TNZ impairs the hamsters' thermogenic capacity to cope with extreme cold temperature. Reduced ambient temperature was mainly responsible for the leftward shift of TNZ in striped hamsters, which reflects the adaptation to cold environments.

The thermal neutral zone is shifted during lactation in striped hamsters

The thermoneutral zone (TNZ), one of the most well-recognized concepts of thermal physiology of homeothermic organisms, is observed to differ between animal species, and may be associated with energy expenditure. However, the characteristics of the TNZ of lactating females, the stage of life history with typically the highest energy demands, remain unclear. In this study, we examined body mass, metabolic rate, TNZ and body composition, and milk energy output, in striped hamsters (Cricetulus barabensis, mean body mass: 29.1 ± 4.4g ranging from 20.0 to 36.6g) at peak lactation, and in hamsters raising small, medium, and large litter sizes throughout lactation. There was a significant downward shift in the lower critical temperature (LCT) of the TNZ in lactating hamsters (TNZ = 22.5-35 °C), resulting in a wider TNZ compared to non-reproductive females (TNZ = 27.5-32.5 °C). At peak lactation, hamsters raising large litter sizes had a considerably lower LCT and a wider TNZ compared to hamsters raising medium and small sized litters, whose upper critical temperature of the TNZ remain fixed. Compared to virgin hamsters, hamsters at peak lactation consumed 2.5 times more food, and had significantly higher energy expenditure corresponding to a significantly higher resting metabolic rate and milk output to meet the requirements of their offspring, which increased with litter size. The organs with the highest oxygen consumption rates, such as the liver, kidneys, and digestive tracts, were considerably heavier in lactating hamsters, particular in those raising large litter sizes, compared to virgin hamsters. The data show that the increased energy expenditure during lactation induces a substantial downward shift of the LCT, consequently resulting in a wider TNZ. The morphological plasticity of organs with high energy requirements is likely involved in this TNZ shift.

Effects of ambient temperatures between 5 and 35 oC on energy balance, body mass and body composition in mice

Background

Considerable attention is currently focused on the potential to switch on brown adipose tissue (BAT), or promote browning of white adipose tissue, to elevate energy expenditure and thereby reduce obesity levels. These processes are already known to be switched on by cold exposure. Yet humans living in colder regions do not show lower levels of obesity. This could be because humans shield themselves from external temperatures, or because the resultant changes in BAT and thermogenesis are offset by elevated food intake, or reductions in other components of expenditure.

Scope of Review

We exposed mice to 11 different ambient temperatures between 5 and 35 °C and characterized their energy balance and body weight/composition. As it got colder mice progressively increased their energy expenditure coincident with changes in thyroid hormone levels and increased BAT activity. Simultaneously, these increases in expenditure were matched by elevated food intake, and body mass remained stable. Nevertheless, within this envelope of unchanged body mass there were significant changes in body composition – with increases in the sizes of the liver and small intestine, presumably to support the greater food intake, and reductions in the level of stored fat – maximally providing about 10% of the total elevated energy demands.

Major Conclusions

Elevating activity of BAT may be a valid strategy to reduce fat storage even if overall body mass is unchanged but if it is mostly offset by elevated food intake, as found here, then the impacts may be small.

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Male and female mice were exposed to 11 different ambient temperatures between 5 and 35 °C.

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As it got colder mice increased both energy expenditure and food intake.

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Increased energy expenditure was coincident with increased THs and BAT activity.

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Stored fat was considerably reduced in colder conditions, providing about 10% of the elevated energy requirements.

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Elevating activity of BAT may be a valid strategy to reduce fat storage.

Larger guts and faster growth in mice selected for high basal metabolic rate

Postnatal growth in birds and mammals is the time of highest vulnerability and relatively high energy demands and therefore shapes the organisms' future outcomes. Several different factors might impose limitations on growth in juveniles, one of them being the efficiency of the digestive process and size of the gastrointestinal tract. We tested the gut size-growth rate relationship using a unique experimental model-mice from a selection experiment designed to produce two lines with divergent levels of basal metabolic rate (BMR): the high BMR (H-BMR) and low BMR (L-BMR) line types. These lines differ with respect to not only BMR, but also correlated traits-internal organ size and food intake. Applying a cross-fostering design and a thermoregulatory burden imposed by shaving the mothers, we demonstrated that the mass of intestine strongly affected the growth rate, with the H-BMR pups having larger intestines and growing faster, and with reduced growth rate of pups of both lines nursed by shaved L-BMR mothers. Our study also provides a functional link between high growth rate of neonates and high BMR of adults, partly reflecting metabolic costs of maintenance of their guts.

Testing the heat dissipation limitation hypothesis: basal metabolic rates of endotherms decrease with increasing upper and lower critical temperatures

Metabolic critical temperatures define the range of ambient temperatures where endotherms are able to minimize energy allocation to thermogenesis. Examining the relationship between metabolic critical temperatures and basal metabolic rates (BMR) provides a unique opportunity to gain a better understanding of how animals respond to varying ambient climatic conditions, especially in times of ongoing and projected future climate change. We make use of this opportunity by testing the heat dissipation limit (HDL) theory, which hypothesizes that the maximum amount of heat a species can dissipate constrains its energetics. Specifically, we test the theory’s implicit prediction that BMR should be lower under higher metabolic critical temperatures. We analysed the relationship of BMR with upper and lower critical temperatures for a large dataset of 146 endotherm species using regression analyses, carefully accounting for phylogenetic relationships and body mass. We show that metabolic critical temperatures are negatively related with BMR in both birds and mammals. Our results confirm the predictions of the HDL theory, suggesting that metabolic critical temperatures and basal metabolic rates respond in concert to ambient climatic conditions. This implies that heat dissipation capacities of endotherms may be an important factor to take into account in assessments of species’ vulnerability to climate change.

The evolution of body fatness: trading off disease and predation risk

Human obesity has a large genetic component, yet has many serious negative consequences. How this state of affairs has evolved has generated wide debate. The thrifty gene hypothesis was the first attempt to explain obesity as a consequence of adaptive responses to an ancient environment that in modern society become disadvantageous. The idea is that genes (or more precisely, alleles) predisposing to obesity may have been selected for by repeated exposure to famines. However, this idea has many flaws: for instance, selection of the supposed magnitude over the duration of human evolution would fix any thrifty alleles (famines kill the old and young, not the obese) and there is no evidence that hunter-gatherer populations become obese between famines. An alternative idea (called thrifty late) is that selection in famines has only happened since the agricultural revolution. However, this is inconsistent with the absence of strong signatures of selection at single nucleotide polymorphisms linked to obesity. In parallel to discussions about the origin of obesity, there has been much debate regarding the regulation of body weight. There are three basic models: the set-point, settling point and dual-intervention point models. Selection might act against low and high levels of adiposity because food unpredictability and the risk of starvation selects against low adiposity whereas the risk of predation selects against high adiposity. Although evidence for the latter is quite strong, evidence for the former is relatively weak. The release from predation ∼2-million years ago is suggested to have led to the upper intervention point drifting in evolutionary time, leading to the modern distribution of obesity: the drifty gene hypothesis. Recent critiques of the dual-intervention point/drifty gene idea are flawed and inconsistent with known aspects of energy balance physiology. Here, I present a new formulation of the dual-intervention point model. This model includes the novel suggestion that food unpredictability and starvation are insignificant factors driving fat storage, and that the main force driving up fat storage is the risk of disease and the need to survive periods of pathogen-induced anorexia. This model shows why two independent intervention points are more likely to evolve than a single set point. The molecular basis of the lower intervention point is likely based around the leptin pathway signalling. Determining the molecular basis of the upper intervention point is a crucial key target for future obesity research. A potential definitive test to separate the different models is also described.

Understanding variation in metabolic rate

Metabolic rate reflects an organism's capacity for growth, maintenance and reproduction, and is likely to be a target of selection. Physiologists have long sought to understand the causes and consequences of within-individual to among-species variation in metabolic rates – how metabolic rates relate to performance and how they should evolve. Traditionally, this has been viewed from a mechanistic perspective, relying primarily on hypothesis-driven approaches. A more agnostic, but ultimately more powerful tool for understanding the dynamics of phenotypic variation is through use of the breeder's equation, because variation in metabolic rate is likely to be a consequence of underlying microevolutionary processes. Here we show that metabolic rates are often significantly heritable, and are therefore free to evolve under selection. We note, however, that ‘metabolic rate’ is not a single trait: in addition to the obvious differences between metabolic levels (e.g. basal, resting, free-living, maximal), metabolic rate changes through ontogeny and in response to a range of extrinsic factors, and is therefore subject to multivariate constraint and selection. We emphasize three key advantages of studying metabolic rate within a quantitative genetics framework: its formalism, and its predictive and comparative power. We make several recommendations when applying a quantitative genetics framework: (i) measuring selection based on actual fitness, rather than proxies for fitness; (ii) considering the genetic covariances between metabolic rates throughout ontogeny; and (iii) estimating genetic covariances between metabolic rates and other traits. A quantitative genetics framework provides the means for quantifying the evolutionary potential of metabolic rate and why variance in metabolic rates within populations might be maintained.

Comparative analyses of basal rate of metabolism in mammals: data selection does matter

Basal rate of metabolism (BMR) is a physiological parameter that should be measured under strictly defined experimental conditions. In comparative analyses among mammals BMR is widely used as an index of the intensity of the metabolic machinery or as a proxy for energy expenditure. Many databases with BMR values for mammals are available, but the criteria used to select metabolic data as BMR estimates have often varied and the potential effect of this variability has rarely been questioned. We provide a new, expanded BMR database reflecting compliance with standard criteria (resting, postabsorptive state; thermal neutrality; adult, non-reproductive status for females) and examine potential effects of differential selectivity on the results of comparative analyses. The database includes 1739 different entries for 817 species of mammals, compiled from the original sources. It provides information permitting assessment of the validity of each estimate and presents the value closest to a proper BMR for each entry. Using different selection criteria, several alternative data sets were extracted and used in comparative analyses of (i) the scaling of BMR to body mass and (ii) the relationship between brain mass and BMR. It was expected that results would be especially dependent on selection criteria with small sample sizes and with relatively weak relationships. Phylogenetically informed regression (phylogenetic generalized least squares, PGLS) was applied to the alternative data sets for several different clades (Mammalia, Eutheria, Metatheria, or individual orders). For Mammalia, a 'subsampling procedure' was also applied, in which random subsamples of different sample sizes were taken from each original data set and successively analysed. In each case, two data sets with identical sample size and species, but comprising BMR data with different degrees of reliability, were compared. Selection criteria had minor effects on scaling equations computed for large clades (Mammalia, Eutheria, Metatheria), although less-reliable estimates of BMR were generally about 12-20% larger than more-reliable ones. Larger effects were found with more-limited clades, such as sciuromorph rodents. For the relationship between BMR and brain mass the results of comparative analyses were found to depend strongly on the data set used, especially with more-limited, order-level clades. In fact, with small sample sizes (e.g. <100) results often appeared erratic. Subsampling revealed that sample size has a non-linear effect on the probability of a zero slope for a given relationship. Depending on the species included, results could differ dramatically, especially with small sample sizes. Overall, our findings indicate a need for due diligence when selecting BMR estimates and caution regarding results (even if seemingly significant) with small sample sizes.

Energy Homeostasis in Monotremes

In 1803, the French anatomist Étienne Geoffroy Saint-Hilaire decided that the newly described echidna and platypus should be placed in a separate order, the monotremes, intermediate between reptiles and mammals. The first physiological observations showed monotremes had low body temperatures and metabolic rates, and the consensus was that they were at a stage of physiological development intermediate between "higher mammals" and "lower vertebrates." Subsequent studies demonstrated that platypuses and echidnas are capable of close thermoregulation in the cold although less so under hot conditions. Because the short-beaked echidna Tachyglossus aculeatus, may show very large daily variations in body temperature, as well as seasonal hibernation, it has been suggested that it may provide a useful model of protoendotherm physiology. Such analysis is complicated by the very significant differences in thermal relations between echidnas from different climates. In all areas female echidnas regulate Tb within 1°C during egg incubation. The lactation period is considered to be the most energetically expensive time for most female mammals but lactating echidnas showed no measurable difference in field metabolic rate from non-lactating females, while the lactation period is more than 200 days for Kangaroo Island echidnas but only 150 days in Tasmania. In areas with mild winters echidnas show reduced activity and shallow torpor in autumn and early winter, but in areas with cold winters echidnas enter true hibernation with Tb falling as low as 4.5°C. Monotremes do not possess brown adipose tissue and maximum rates of rewarming from hibernation in echidnas were only half those of marmots of the same mass. Although echidnas show very large seasonal variations in fat stores associated with hibernation there is no relationship between plasma leptin and adiposity. Leptin levels are lowest during post-reproductive fattening, supporting suggestions that in evolutionary terms the anorectic effects of leptin preceded the adiposity signal. BMR of platypuses is twice that of echidnas although maximum metabolism is similar. High levels of thyroid hormones in platypuses may be driving metabolism limited by low body temperature. Monotremes show a mosaic of plesiomorphic and derived features but can still inform our understanding of the evolution of endothermy.

Consequences of Fatherhood in the Biparental California Mouse (Peromyscus californicus): Locomotor Performance, Metabolic Rate, and Organ Masses

Although effects of motherhood on mothers have been well documented in mammals, the effects of fatherhood on fathers are not well known. We evaluated effects of being a father on key metabolic and performance measures in the California mouse, Peromyscus californicus. California mice are genetically monogamous in the wild, and fathers show similar parental behavior to mothers, with the exception of lactation. To investigate the impact of fatherhood on fathers, focal males were paired with an intact female (breeding males), a tubally ligated female (nonbreeding males), or another male (virgins). Starting 3-5 d after the birth of each breeding pair's first litter, males were tested for locomotor performance (maximum sprint speed, treadmill endurance), basal metabolic rate (BMR), and maximum oxygen consumption ([Formula: see text]). At the end of the 11-d test period, mice were euthanized, hematocrit was determined, and organs were weighed. Speed, endurance, and [Formula: see text] were significantly repeatable between two replicate measurement days but did not differ among groups, nor did BMR. Breeding males had significantly larger hind limb muscles than did nonbreeding males, whereas virgin males had heavier subcutaneous fat pads than did nonbreeding and breeding males. Several correlations were observed at the level of individual variation (residuals from ANCOVA models), including positive correlations for endurance with [Formula: see text], [Formula: see text] with testes mass, and some of the digestion-related organs with each other. These results indicate that fatherhood may not have pronounced performance, metabolic, or morphological effects on fathers, at least under standard laboratory conditions and across a single breeding cycle.

Limits to sustained energy intake. XXIII. Does heat dissipation capacity limit the energy budget of lactating bank voles?

Understanding factors limiting sustained metabolic rate (SusMR) is a central issue in ecological physiology. According to the heat dissipation limit (HDL) theory, the SusMR at peak lactation is constrained by the maternal capacity to dissipate body heat. To test that theory, we shaved lactating bank voles (Myodes glareolus) to experimentally elevate their capacity for heat dissipation. The voles were sampled from lines selected for high aerobic exercise metabolism (A; characterized also by increased basal metabolic rate) and unselected control lines (C). Fur removal significantly increased the peak-lactation food intake (mass-adjusted least square means ± s.e.; shaved: 16.3 ± 0.3 g day(-1), unshaved: 14.4 ± 0.2 g day(-1); P<0.0001), average daily metabolic rate (shaved: 109 ± 2 kJ day(-1), unshaved: 97 ± 2 kJ day(-1); P<0.0001) and metabolisable energy intake (shaved: 215 ± 4 kJ day(-1), unshaved: 185 ± 4 kJ day(-1); P<0.0001), as well as the milk energy output (shaved: 104 ± 4 kJ day(-1); unshaved: 93 ± 4 kJ day(-1); P=0.021) and litter growth rate (shaved: 9.4 ± 0.7 g 4 days(-1), unshaved: 7.7 ± 0.7 g 4 days(-1); P=0.028). Thus, fur removal increased both the total energy budget and reproductive output at the most demanding period of lactation, which supports the HDL theory. However, digestive efficiency was lower in shaved voles (76.0 ± 0.3%) than in unshaved ones (78.5 ± 0.2%; P<0.0001), which may indicate that a limit imposed by the capacity of the alimentary system was also approached. Shaving similarly affected the metabolic and reproductive traits in voles from the A and C lines. Thus, the experimental evolution model did not reveal a difference in the limiting mechanism between animals with inherently different metabolic rates.

Mice that are resistant to diet-induced weight loss have greater food anticipatory activity and altered melanocortin-3 receptor (MC3R) and dopamine receptor 2 (D2) gene expression

Diet-induced weight loss varies considerably between individuals, but the mechanisms driving these individual differences remain largely unknown. Here we investigated whether key neuropeptides involved in the regulation of energy balance or reward systems were differentially expressed in mice that were prone or resistant to caloric restriction (CR) induced weight loss. Mice (n=30 males and n=34 females) were fed 70% of their own baseline ad libitum intake for 25days, after which their brains were collected and expression of various neuropeptides were investigated and compared between the 10 male and 10 female mice that showed the greatest (high weight loss, HWL) or lowest weight loss (LWL) (n=40 in total). HWL mice showed a differential neuropeptide profile to LWL in both sexes, characterised by increased expression of neuropeptide Y (NPY), agouti-related peptide (AgRP), leptin receptor (ObRb), and melanocortin 3 receptor (MC3R) in the arcuate nucleus. No changes in the expression of fat mass and obesity related gene (FTO) or suppressor of cytokine signalling 3 (Socs3) were observed. Levels of dopamine D2 receptor were decreased in the nucleus accumbens in HWL compared to LWL mice. HWL mice showed a stronger increase in food anticipatory activity (FAA) in response to CR than LWL mice. These results indicate that the mice prone to diet-induced weight loss experienced greater hunger, potentially driving their elevated FAA.

Effect of reproduction on the consistency of the between-line type divergence in laboratory mice selected on Basal metabolic rate

Artificial selection experiments are an effective tool for testing evolutionary hypotheses, because they allow one to separate genetic and environmental variances of the phenotype. However, it is unclear whether trait divergence typically selected early in life persists over an animal's life and altered physiological states, such as reproduction. Here we analyzed the long-term consistency of the between-line type divergence in basal metabolic rate (BMR) selected at 12 wk of age in laboratory mice. We measured BMR in nonreproducing and reproducing females at the age of 22 wk and then at 27 wk of age. Our results show that within both the reproducing group and the control group, the between-line type separation in BMR is consistently retained over time and reproductive status. Metabolically active internal organs (heart, liver, kidneys, and small intestine) also consistently differed in size between the two line types with no significant long-term effect of reproduction. The observed consistency of the between-line type divergence in BMR suggests the existence of the persistent effect of the selection on metabolic traits applied early in life. Moreover, BMR variation achieved by means of artificial selection is considerably higher than that found in natural/unmanipulated populations. The latter may therefore be characterized by insufficient variance to statistically resolve correlations involving BMR.

Evolutionary perspectives on the obesity epidemic: adaptive, maladaptive, and neutral viewpoints

The prevalence of obesity in modern societies has two major contributory factors-an environmental change that has happened in historical times and a genetic predisposition that has its origins in our evolutionary history. Understanding both aspects is complex. From an evolutionary perspective, three different types of explanation have been proposed. The first is that obesity was once adaptive and enabled us to survive (or sustain fecundity) through periods of famine. People carrying so-called thrifty genes that enabled the efficient storage of energy as fat between famines would be at a selective advantage. In the modern world, however, people who have inherited these genes deposit fat in preparation for a famine that never comes, and the result is widespread obesity. The key problem with this, and any other adaptive scenario, is to understand why, if obesity was historically so advantageous, many people did not inherit these thrifty genes and in modern society are able to remain slim, despite the environmental change favoring fat storage. The second type of explanation is that obesity is not adaptive and may never even have existed in our evolutionary past, but it is favored today as a maladaptive by-product of positive selection on some other trait. An example of this type of explanation is the suggestion that obesity results from variation in brown adipose tissue thermogenesis. Finally, a third class of explanation is that most mutations in the genes that predispose us to obesity are neutral and have been drifting over evolutionary time--so-called drifty genes, leading some individuals to be obesity prone and others obesity resistant. In this article, I review the current evidence for and against these three different scenarios and conclude that the thrifty gene hypothesis is untenable but the other two ideas may provide a cogent explanation of the modern obesity phenomenon.

Limits to sustained energy intake. XXII. Reproductive performance of two selected mouse lines with different thermal conductance

Maximal sustained energy intake (SusEI) appears limited, but the factors imposing the limit are disputed. We studied reproductive performance in two lines of mice selected for high and low food intake (MH and ML, respectively), and known to have large differences in thermal conductance (29% higher in the MH line at 21°C). When these mice raised their natural litters, their metabolisable energy intake significantly increased over the first 13 days of lactation and then reached a plateau. At peak lactation, MH mice assimilated on average 45.3 % more energy than ML mice (222.9±7.1 and 153.4±12.5 kJ day-1, N=49 and 24, respectively). Moreover, MH mice exported on average 62.3 kJ day-1 more energy as milk than ML mice (118.9±5.3 and 56.6±5.4 kJ day-1, N= subset of 32 and 21, respectively). The elevated milk production of MH mice enabled them to wean litters (65.2±2.1 g) that were on average 50.2% heavier than litters produced by ML mothers (43.4±3.0 g), and pups that were on average 27.2% heavier (9.9±0.2 and 7.8±0.2 g, respectively). Lactating mice in both lines had significantly longer and heavier guts compared to non-reproductive mice. However, inconsistent with the central limit hypothesis, the ML mice had significantly longer and heavier intestines than MH mice. An experiment where the mice raised litters of the opposing line demonstrated that lactation performance was not limited by offspring growth capacity. Our findings are consistent with the idea that the SusEI at peak lactation is constrained by the capacity of the mothers to dissipate body heat.

Effects of reproduction on immuno-suppression and oxidative damage, and hence support or otherwise for their roles as mechanisms underpinning life history trade-offs, are tissue and assay dependent

Life history parameters appear to be traded off against each other, but the physiological mechanisms involved remain unclear. One hypothesis is that potentially energetically costly processes such as immune function and protection from oxidative stress may be compromised during reproductive attempts because of selective resource allocation. Lower temperatures also impose energy costs, and hence allocation decisions might be more pronounced when animals are forced to reproduce in the cold. Here, we experimentally tested whether reproduction at different ambient temperatures was associated with elevated oxidative stress and suppressed immune function in Mongolian gerbils (Meriones unguiculatus). Using a variety of different markers for both immune function and oxidative stress, we found that some measures of immune function (serum bactericidal capacity and size of the thymus) were significantly suppressed, while some measures of oxidative protection [serum superoxide dismutase (SOD) activity and glutathione peroxidase (GPx) activity] were also reduced, and a marker of oxidative damage (protein carbonyls in serum) was increased in lactating compared with non-reproductive gerbils. These changes were in line with the selective resource allocation predictions. However, the phytohaemagglutinin response and serum total immunoglobulin (IgG) were not suppressed, and other markers of oxidative damage [malondialdehyde (MDA) (TBARS) and protein carbonyls in the liver] were actually lower in lactating compared with non-reproductive gerbils, consistent with increased levels of SOD activity and total antioxidant capacity in the liver. These latter changes were opposite of the expectations based on resource allocation. Furthermore, other measures of protection (GPx levels in the liver and protein thiols in both serum and liver) and damage [MDA (TBARS) in serum] were unrelated to reproductive status. Ambient temperature differences did not impact on these patterns. Collectively, our results indicated that the inferred effects of reproduction on immunosuppression and oxidative damage, and hence support or otherwise for particular physiological mechanisms that underpin life history trade-offs, are critically dependent on the exact markers and tissues used. This may be because during reproduction individuals selectively allocate protection to some key tissues, but sacrifice protection of others.

Limits to sustained energy intake. XX. Body temperatures and physical activity of female mice during lactation

Lactating animals consume greater amounts of food than non-reproductive animals, but energy intake appears to be limited in late lactation. The heat dissipation limit theory suggests that the food intake of lactating mice is limited by the capacity of the mother to dissipate heat. Lactating mice should therefore have high body temperatures (Tb), and changes in energy intake during lactation should be reflected by variation in Tb. To investigate these predictions, 26 mice (Mus musculus) were monitored daily throughout lactation for food intake, body mass, litter size and litter mass. After weaning, 21 days postpartum, maternal food intake and body mass were monitored for another 10 days. Maternal activity and Tb were recorded every minute for 23 h a day using implanted transmitters (vital view). Energy intake increased to a plateau in late lactation (days 13-17). Daily gain in pup mass declined during this same period, suggesting a limit on maternal energy intake. Litter size and litter mass were positively related to maternal energy intake and body mass. Activity levels were constantly low, and mice with the largest increase in energy intake at peak lactation had the lowest activity. Tb rose sharply after parturition and the circadian rhythm became compressed within a small range. Tb during the light period increased considerably (1.1 ° C higher than in baseline), and lactating mice faced chronic hyperthermia, despite their activity levels in lactation being approximately halved. Average Tb increased in relation to energy intake as lactation progressed, but there was no relationship between litter size or litter mass and the mean Tb at peak lactation. These data are consistent with the heat dissipation limit theory, which suggests performance in late lactation is constrained by the ability to dissipate body heat.

Limits to sustained energy intake. XVI. Body temperature and physical activity of female mice during pregnancy

Lactation is the most energy-demanding phase of mammalian reproduction, and lactation performance may be affected by events during pregnancy. For example, food intake may be limited in late pregnancy by competition for space in the abdomen between the alimentary tract and fetuses. Hence, females may need to compensate their energy budgets during pregnancy by reducing activity and lowering body temperature. We explored the relationships between energy intake, body mass, body temperature and physical activity throughout pregnancy in the MF1 mouse. Food intake and body mass of 26 females were recorded daily throughout pregnancy. Body temperature and physical activity were monitored every minute for 23 h a day by implanted transmitters. Body temperature and physical activity declined as pregnancy advanced, while energy intake and body mass increased. Compared with a pre-mating baseline period, mice increased energy intake by 56% in late pregnancy. Although body temperature declined as pregnancy progressed, this served mostly to reverse an increase between baseline and early pregnancy. Reduced physical activity may compensate the energy budget of pregnant mice but body temperature changes do not. Over the last 3 days of pregnancy, food intake declined. Individual variation in energy intake in the last phase of pregnancy was positively related to litter size at birth. As there was no association between the increase in body mass and the decline in intake, we suggest the decline was not caused by competition for abdominal space. These data suggest overall reproductive performance is probably not constrained by events during pregnancy.

Limits to sustained energy intake. XVII. Lactation performance in MF1 mice is not programmed by fetal number during pregnancy

Several studies have suggested that lactation performance may be programmed by the number of fetuses during pregnancy, whereas other studies indicate that processes during lactation are more important. As gestation litter size and litter size in lactation are usually strongly correlated, separating the roles of pregnancy and lactation in lactation performance is difficult. To break this link, we experimentally manipulated litter size of MF1 mice to five or 16 pups per litter by cross-fostering. Litter size and mass at birth were recorded on day 1 of lactation prior to litter size manipulation. Maternal body mass and food intake, litter size and litter mass were measured daily throughout. After weaning, the potential differential utilisation of body tissues of the mothers was investigated. Relationships between maternal mass and food intake, including asymptotic daily food intake at peak lactation, offspring traits and other maternal parameters suggested that the number of fetuses the females had carried during pregnancy had no effect on lactation performance. Litter mass increases depended only on maternal food intake, which was highly variable between individuals, but was independent of fetal litter size. The sizes of key organs and tissues like the liver and alimentary tract were not related to maximal food intake at peak lactation or to fetal litter size, but the masses of the pelage, mammary glands and retroperitoneal fat pad were. These data suggest that while growth of the mammary glands and associated structures may be initiated in gestation, and vary in relation to the number of placentas, the ultimate sizes and activities of the tissues depends primarily on factors during lactation.

Limits to sustained energy intake. XIV. Heritability of reproductive performance in mice

Limits to sustained energy intake (SusEI) are important because they constrain many aspects of animal performance. Individual variability in SusEI may be imposed by genetic factors that are inherited from parents to offspring. Here, we investigated heritability of reproductive performance in MF1 mice. Food intake, milk energy output (MEO) and litter mass were measured in mothers (F0) and daughters (F1) that were raising litters of 10 pups. Cross-fostering was designed so that half of each litter consisted of biological offspring and the rest came from one unrelated female (i.e. fostered pups). Food intake increased linearly during early lactation and reached a plateau during late lactation (day 9–13, called the asymptotic food intake, FIAS, equivalent to SusEI). Parent–offspring regression showed that FIAS, MEO and litter mass were all positively and significantly related between mothers and their biological daughters, but no significant relationships were found between the same traits for mothers and fostered daughters. FIAS at peak lactation was significantly correlated to adult food intake and body mass when the mice were 6 months old and not lactating. In conclusion, a large part of the variation in FIAS could be explained by genetic variation or maternal effects in pregnancy whereas non-genetic maternal effects in lactation were negligible. As a consequence, biological daughters of mothers with high reproductive performance (i.e. high milk production and hence higher litter mass at weaning) had a better reproductive performance themselves, independent of the mother that raised them during lactation.

Limits to sustained energy intake. XV. Effects of wheel running on the energy budget during lactation

The capacity of animals to dissipate heat may constrain sustained energy intake during lactation. We examined these constraints at peak lactation in MF1 mice that had ad libitum access to food, or that had to run a pre-set target on running wheels to obtain ad libitum access to food. The voluntary distance run decreased sharply during pregnancy and peak lactation. When lactating females were provided with 80% of their estimated food requirements, and had to run pre-set distances of 2, 4 or 6 km before given access to additional ad libitum food, most of them did not complete the running target during late lactation and the mice with the highest targets failed to reach their targets earlier in lactation. There were consequently significant group differences in asymptotic food intake (2 km, 16.97±0.40 g day−1; 4 km, 14.29±0.72 g day−1; and 6 km, 12.65±0.45 g day−1) and weaned litter masses (2 km, 71.11±2.39 g; 4 km, 54.63±4.28 g and 6 km, 47.18±2.46 g). When the females did run sufficiently to gain ad libitum food access, their intake did not differ between the different distance groups or from controls that were not required to run. Thus, despite being physically capable of running the distances, mice could not exercise sufficiently in lactation to gain regular ad libitum access to food, probably because of the risks of hyperthermia when combining heat production from exercise with thermogenesis from lactation.

Standard metabolic rate is associated with gestation duration, but not clutch size, in speckled cockroaches Nauphoeta cinerea

Metabolic rate varies significantly between individuals, and these differences persist even when the wide range of biotic and abiotic factors that influence metabolism are accounted for. It is important to understand the life history implications of variation in metabolic rate, but they remain poorly characterised despite a growing body of work examining relationships between metabolism and a range of traits. In the present study we used laboratory-bred families (one sire to three dams) of Nauphoeta cinerea (Olivier) (speckled cockroaches) to examine the relationship between standard metabolic rate (SMR) and reproductive performance (number of offspring and gestation duration). We show that SMR is negatively associated with female gestation duration. Age at mating is negatively associated with gestation duration for females, and mass is negatively associated with the average gestation duration of the females a male was mated with. In addition to the results in the current literature, the results from the present study suggest that the association between metabolism and life history is more complex than simple relationships between metabolism and various fitness traits. Future work should consider longitudinal, ontogenetic as well as selective and quantitative genetic breeding approaches to fully examine the associations between metabolism and fitness.

Factors predicting individual variability in diet-induced weight loss in MF1 mice

The effectiveness of caloric restriction (CR) as a treatment for obesity varies considerably between individuals. Reasons for this interindividual variation in weight loss in response to CR may lie in pre-existing individual differences and/or individual differences in compensatory responses. Here we studied the responses of 127 MF1 mice to 30% CR over four weeks, and investigated whether pre-existing differences or compensatory changes in body temperature, resting metabolic rate (RMR) and behavior explained the variation observed in body mass (BM) and fat mass (FM) changes. Mice showed considerable variation in BM loss (36-1%), and in the type of tissue lost (FM or fat free mass, FFM). About 50% of the variation in BM and FM loss could be predicted by pre-existing differences in food intake, RMR, and general activity, where BM loss was greater when food intake was lower and activity and RMR were higher. Compensatory changes in activity and body temperature together explained ~50% of the variation in BM and FM loss in both sexes. In models incorporating baseline variables and compensatory changes, food intake, and activity were the strongest predictors of weight loss in both sexes; i.e., lower baseline food intake and increased changes in activity resulted in greater BM and FM loss. Interestingly, increased baseline activity was a significant predictor of weight loss independent of compensatory changes in activity. Identifying factors involved in individual variability in weight loss may give insights into the mechanisms that underlie this variability, and is important to develop individually tailored weight-management strategies.

Repletion of TNFα or leptin in calorically restricted mice suppresses post-restriction hyperphagia

The causes of post-restriction hyperphagia (PRH) represent a target for drug-based therapies to prevent obesity. However, the factors causing PRH are poorly understood. We show that, in mice, the extent of PRH was independent of the time under restriction, but depended on its severity, suggesting that PRH was driven by signals from altered body composition. Signals related to fat mass were important drivers. Circulating levels of leptin and TNFα were significantly depleted following caloric restriction (CR). We experimentally repleted their levels to match those of controls, and found that in both treatment groups the level of PRH was significantly blunted. These data establish a role for TNFα and leptin in the non-pathological regulation of energy homeostasis. Signals from adipose tissue, including but not limited to leptin and TNFα, regulate PRH and might be targets for therapies that support people engaged in CR to reduce obesity.

Negative correlation between milk production and brown adipose tissue gene expression in lactating mice

It has been proposed that the performance of lactating animals is limited by the capacity of the female to dissipate body heat – the heat dissipation limit (HDL) theory. This theory predicts that milk production might be constrained not by intrinsic properties of the mammary glands but rather by competitive heat production such as thermogenesis in brown adipose tissue (BAT). To test this prediction, we measured the expression of genes linked to thermogenesis in BAT of lactating laboratory mice. The applicability of BAT gene expression to reflect thermogenic activity of BAT was confirmed by a positive relationship between expression levels of several BAT genes (summarised by the first principal component following principal component analysis) and daily energy expenditure in virgin mice. Milk production at peak lactation was strongly and negatively associated with the expression of thermogenic genes in BAT. Downregulation of these genes during lactation was correlated with low levels of circulating leptin and high levels of circulating prolactin. Our results are consistent with the HDL theory. However, we cannot discount the converse interpretation that milk production may reduce BAT activity. If the reduction in BAT activity does facilitate increased milk production, then reducing the heat generated by competitive processes may be a more productive route to increase lactational performance than attempts to improve mammary gland performance in isolation from the other body systems.

Metabolomics of aerobic metabolism in mice selected for increased maximal metabolic rate

Maximal aerobic metabolic rate (MMR) is an important physiological and ecological variable that sets an upper limit to sustained, vigorous activity. How the oxygen cascade from the external environment to the mitochondria may affect MMR has been the subject of much interest, but little is known about the metabolic profiles that underpin variation in MMR. We tested how seven generations of artificial selection for high mass-independent MMR affected metabolite profiles of two skeletal muscles (gastrocnemius and plantaris) and the liver. MMR was 12.3% higher in mass selected for high MMR than in controls. Basal metabolic rate was 3.5% higher in selected mice than in controls. Artificial selection did not lead to detectable changes in the metabolic profiles from plantaris muscle, but in the liver amino acids and tricarboxylic acid cycle (TCA cycle) metabolites were lower in high-MMR mice than in controls. In gastrocnemius, amino acids and TCA cycle metabolites were higher in high-MMR mice than in controls, indicating elevated amino acid and energy metabolism. Moreover, in gastrocnemius free fatty acids and triacylglycerol fatty acids were lower in high-MMR mice than in controls. Because selection for high MMR was associated with changes in the resting metabolic profile of both liver and gastrocnemius, the result suggests a possible mechanistic link between resting metabolism and MMR. In addition, it is well established that diet and exercise affect the composition of fatty acids in muscle. The differences that we found between control lines and lines selected for high MMR demonstrate that the composition of fatty acids in muscle is also affected by genetic factors.

Milk energy output in Swiss mice throughout the first, second, third and fourth lactation events

Most studies on the factors limiting sustained energy intake (SusEI) during peak lactation period have been performed in females at the 1st lactation event. However, an inconsistent change in SusEI is observed between the 1st and 2nd lactation event. Thus, the limits to SusEI may be associated with reproductive experiences, but the effects of reproductive experiences on SusEI or reproductive output remain unclear. Here, food intake, reproductive output, suckling behaviour and serum prolactin levels were measured in female Swiss mice throughout the 1st, 2nd, 3rd and 4th lactation periods. Asymptotic food intake was significantly elevated during the 2nd lactation period relative to that observed during the 1st lactation period. Females in the 2nd lactation period exported significantly more energy in milk than those in the 1st lactation event and consequently raised larger litters with heavier litters at weaning. This was inconsistent with the prediction of the peripheral limitation hypothesis, but also did not provide support for the heat dissipation limitation hypothesis. Neither food intake nor reproductive output, indicative of litter size, litter mass and milk energy output (MEO), was different between the 1st, 3rd and 4th lactation event. Differences in suckling behaviour and serum prolactin levels were not significant between the four lactation events. Correlations of prolactin levels with asymptotic food intake, MEO and mammary gland mass were only observed in females during the 1st lactation period. This may suggest that prolactin is not a key factor in stimulating milk production when the mammary glands work at their maximum during the peak lactation period.

Why marathon migrants get away with high metabolic ceilings: towards an ecology of physiological restraint

Animals usually are not willing to perform at levels, or for lengths of time, of which they should be maximally capable. In stating this, exercise performance and inferred capacity are gauged with respect to body size and the duration of particular levels of energy expenditure. In such relative terms, the long-term metabolic ceiling of ca. 7 times basal metabolic rate in challenged but energy-balanced individuals may be real and general, because greater performance over long periods requires larger metabolic machinery that is ever more expensive to maintain. Avian marathon migrants relying on stored fuel (and therefore not in energy balance) that work for 9 consecutive days at levels of 9-10 times basal metabolic rate are exceptional performers in terms of the 'relative expenditure' on 'duration of a particular activity' curve nevertheless. Here I argue that metabolic ceilings in all situations (energy balanced or not) have their origin in the fitness costs of high performance levels due to subsequently reduced survival, which then precludes the possibility of future reproduction. The limits to performance should therefore be studied relative to ecological context (which includes aspects such as pathogen pressure and risk of overheating), which determines the severity of the survival punishment of over-exertion. I conclude that many dimensions of ecology have determined at which performance levels (accounting for time) individual animals, including human athletes, begin to show physiological restraint. Using modern molecular techniques to assay wear and tear, in combination with manipulated work levels in different ecological contexts, might enable experimental verification of these ideas.

Energy budget during lactation in striped hamsters at different ambient temperatures

The combination of two stressors, lactation and cold, is suggested to be an excellent model for testing the factors limiting sustained energy intake (SusEI). Limits to SusEI during peak lactation may be imposed peripherally by the capacity of mammary glands to produce milk or may be driven by the ability of animals to dissipate body heat. To distinguish between the two mechanisms, body mass change, food intake, reproductive output (using litter size and mass) and serum prolactin (PRL) levels were measured in striped hamsters lactating at 23, 30 and 5°C. Resting metabolic rate (RMR) during late lactation was also measured. Female hamsters lactating at 5°C showed significantly lower change in body mass, but had higher food intake and RMR than females at 23 and 30°C. Asymptotic food intake averaged 14.6±0.4, 14.5±0.7 and 16.2±0.5 g d–1 for females at 23, 30 and 5°C, respectively. The females at 5°C had 11.4% higher asymptotic food intake than females at 23 and 30°C (F2,51=3.3, P&lt;0.05, Tukey's HSD, P&lt;0.05). No significant differences in litter size and PRL levels were observed between the three groups; however, litter mass at 5°C was lower by 19.7 and 19.8% than litter mass at 23 and 30°C on day 19 of lactation (F2,51=3.5, P&lt;0.05, Tukey's HSD, P&lt;0.05). Differences in the above parameters between 23 and 30°C were not significant. Litter mass was positively correlated with asymptotic food intake (23°C, r=0.60, P&lt;0.05; 30°C, r=0.94, P&lt;0.01; 5°C, r=0.77, P&lt;0.01). These data suggested that females lactating at cold temperatures increased food intake to compensate for additional energy demands for thermogenesis, but they might not be capable of exporting more energy as milk to the pups, indicating a possible consistency with the peripheral hypothesis. However, the present results do not considerably distinguish the peripheral limitation hypothesis from the heat dissipation limits hypothesis.

Limits to sustained energy intake. XIII. Recent progress and future perspectives

Several theories have been proposed to explain limits on the maximum rate at which animals can ingest and expend energy. These limits are likely to be intrinsic to the animal, and potentially include the capacity of the alimentary tract to assimilate energy – the ‘central limitation’ hypothesis. Experimental evidence from lactating mice exposed to different ambient temperatures allows us to reject this and similar ideas. Two alternative ideas have been proposed. The ‘peripheral limitation’ hypothesis suggests that the maximal sustained energy intake reflects the summed demands of individual tissues, which have their own intrinsic limitations on capacity. In contrast, the ‘heat dissipation limit’ (HDL) theory suggests that animals are constrained by the maximal capacity to dissipate body heat. Abundant evidence in domesticated livestock supports the HDL theory, but data from smaller mammals are less conclusive. Here, we develop a novel framework showing how the HDL and peripheral limitations are likely to be important in all animals, but to different extents. The HDL theory makes a number of predictions – in particular that there is no fixed limit on sustained energy expenditure as a multiple of basal metabolic rate, but rather that the maximum sustained scope is positively correlated with the capacity to dissipate heat.

Effects of leptin infusion during peak lactation on food intake, body composition, litter growth, and maternal neuroendocrine status in female Brandt's voles (Lasiopodomys brandtii)

During lactation, female small mammals frequently reduce their fat reserves to very low levels. The function of this reduction is unclear, as calculations suggest that the contribution of the withdrawn energy from fat to the total energy balance of lactation is trivial. An alternative hypothesis is that reducing fat leads to a reduction in circulating adipokines, such as leptin, that play a role in stimulating the hyperphagia of lactation. We investigated the role of circulating leptin in lactation by repleting leptin levels using miniosmotic pumps during the last 7 days of lactation in Brandt's voles (Lasiopodomys brandtii), a model small wild mammal we have extensively studied in the context of lactation energy demands. Repletion of leptin resulted in a dose-dependent reduction of body mass and food intake in lactating voles. Comparisons to nonreproducing individuals suggests that the reduced leptin in lactation, due to reduced fat stores, may account for ∼16% of the lactational hyperphagia. Reduced leptin in lactation may, in part, cause lactational hyperphagia via stimulatory effects on hypothalamic orexigenic neuropeptides (neuropeptide Y and agouti-related peptide) and inhibition of the anorexigenic neuropeptide (proopiomelanocortin). These effects were reversed by the experimental repletion of leptin. There was no significant effect of leptin treatment on daily energy expenditure, milk production or pup growth, but leptin repletion did result in a reversal of the suppression of uncoupling protein-1 levels in brown adipose tissue, indicating an additional role for reducing body fat and leptin during peak lacation.

Peak energy turnover in lactating European hares: a test of the heat dissipation limitation hypothesis

It has been suggested that maximum sustained metabolic rate (SusMR) in mammals as reached, for instance, during lactation, is due to a limited capacity for heat dissipation. Here, we experimentally tested whether heat dissipation limitation (HDL) also constrains energy turnover in lactating European hares. Experimentally, we made use of the fact that hares nurse their young only once per day, which allowed us to keep females and young either at the same or at different ambient temperatures. During the last lactation week (week 4) females kept at thermoneutrality (22 degrees C), irrespective of the cold load of their young, had significantly lower rates of metabolisable energy intake (MEI) than cold-exposed mothers (5 degrees C), as predicted by the HDL hypothesis. However, in week 2 of lactation females at thermoneutrality rearing cold-exposed young were able to increase MEI to levels indistinguishable from those of cold-exposed females. Thus, even at thermoneutral temperature females reached maximum rates of energy turnover, which was inconsistent with the HDL hypothesis. We conclude that SusMR in lactating European hares typically results not from physiological constraints but from an active restriction of their energy turnover in order to maximise lifetime reproductive success.

Limits to sustainable energy budget during lactation in the striped hamster (Cricetulus barabensis) raising litters of different size

A female animal appears to approach an upper limit to the rate of sustained energy intake/metabolic rate (SusEI/MR) during lactation. However, different species of animals may respond differently to the sustainable limit. Here, we measured energy budget during lactation in female striped hamsters raising litters of natural size (Con), and females whose litter size was manipulated during early lactation to support fewer or more pups (minus pups, MP or plus pups, PP). The striped hamsters significantly decreased their body mass and increased food intake from early to late lactation; and MP females had lower weight loss and food intake than the control and PP females. Litter size of the PP group decreased significantly over the period of lactation, and pups were weaned at a similar weight to that of the controls. MP females supported a significantly lower litter mass throughout lactation compared with the control and PP females, but during late lactation the pups from the MP group were significantly heavier. Resting metabolic rate (RMR) did not differ significantly between the three groups and the gross energy intake during peak lactation was 5.0 ×, 4.2 × and 5.0 × RMR for the control, MP and PP females, respectively. Female striped hamsters reached a plateau in food intake at around 14 g/d during peak lactation, which might signify a limit of SusEI at 5.0 × RMR. However, it was not possible to determine whether the limitation on SusEI was imposed centrally by the capacity of the gastrointestinal tract to process food, peripherally by the capacity of the mammary gland to produce milk, or by the capacity of animals to dissipate heat.

Milk energy output during peak lactation in shaved Swiss mice

The limits to sustainable energy intake (SusEI) are very important because they define an envelope within which many aspects of animal performance are constrained. It has previously been suggested that Swiss mice may be constrained peripherally by the mammary gland, in contrast to the heat dissipation limits hypothesis. To distinguish between the two ideas, we dorsally shaved Swiss mice at early lactation, and examined the energy intake, resting metabolic rate (RMR), litter size and mass, milk energy output (MEO), serum prolactin levels (PRL) and suckling behavior of shaved mothers and non-shaved controls. Dorsal fur removal significantly increased energy intake and RMR, but did not have significant effects on litter mass, MEO, PRL and suckling behavior. These data were inconsistent with the heat dissipation limitation hypothesis and provided support for the peripheral limitation hypothesis, i.e. SusEI was more likely peripherally caused by the capacity of the mammary gland to produce milk. The inconsistent responses to sustainable limits suggested that the limitations on SusEI during peak lactation might be not the same in all species or even between different strains of mice.

Limits to sustained energy intake XII: is the poor relation between resting metabolic rate and reproductive performance because resting metabolism is not a repeatable trait?

Many studies have investigated the consequences of individual variation in resting metabolic rate at thermoneutrality (RMRt) on reproductive performance. Despite strong theoretical reasons for expecting such an association, results have generally been disappointing. A fundamental assumption of these studies is that RMRt is a repeatable trait. We examined repeatability of RMRt in female MF1 mice over short (15 days apart; N=238) and long intervals (110 days apart; N=33). In the long-term experiment, after the first RMRt measurement, females were separated in two groups: the first was kept virgin (N=16); the second was allowed to breed (N=17) and measured 15 days after they had weaned their pups. We also examined the association between RMRt and reproduction. We used Pearson's correlation (r) and intraclass correlation coefficients (rho) to estimate repeatability. There was a strong effect of body mass on RMRt for all measurements. Over the short interval, repeatability was significant for body mass (r=0.86; rho=0.86), RMRt (r=0.68; rho=0.68,) and residual-RMRt (r=0.58; rho=0.58). Over long intervals, repeatability of residual-RMRt was high in virgin females (r=0.59; rho=0.60), but not in the breeders (r=0.38; rho=0.39); body mass was repeatable only for non-breeders measured by r (r=0.55). There was no significant correlation between RMRt or residual-RMRt and litter size or litter mass. In conclusion, RMRt and residual-RMRt are highly repeatable traits in virgin MF1 female mice. The lack of association between non-reproductive RMRt and reproductive performance in MF1 mice does not come about because of its poor repeatability.

Effect of fur removal on the thermal conductance and energy budget in lactating Swiss mice

The limits to sustained energy intake (SusEI) are important because they determine the ceiling restricting all the competing physiological processes. A recent hypothesis is that SusEI is constrained by the capacity to dissipate heat. However some previous data for Swiss mice are inconsistent with this hypothesis. To examine the role of limits to heat dissipation on SusEI, the body temperature, thermal conductance and lactation performance were measured in dorsally shaved Swiss mice. Shaving reduces external insulation and the heat dissipation limitation hypothesis predicts such animals should be capable of eating more food and raising heavier litters. Shaved mice had a significantly higher thermal conductance and a faster reduction in body temperature following noradrenaline injection. At peak lactation, shaved mice spent more time in feeding behaviour, and increased food intake above that observed in non-shaved controls, indicating that limits on SusEI might be imposed by the capacity to dissipate heat. However, shaved females did not spend more time suckling their pups, and did not raise heavier litters, which is inconsistent with the expectations of the heat dissipation limitation hypothesis. The strong correlations between resting, feeding and suckling behaviour at peak lactation suggested that there might be a trade-off in the time distribution between the behavioural patterns. These data suggest that limits on performance may be set at different levels in different strains or species. In MF1 mice studied previously the limit on milk production imposed by maximal mammary secretion capability may lie above that for heat dissipation, hence when the latter was increased the mice produced more milk and raised heavier litters. In Swiss mice the opposite might be the case. Hence when the heat dissipation capacity was increased this did not translate into heavier litters, i.e. supporting the peripheral limitation hypothesis. Further work in a range of additional species or strains will be necessary to establish whether the more normal condition is for SusEI in animals during late lactation to be set by combined peripheral demands or by the heat dissipation capacity.

Shifts in metabolic demands in growing altricial nestlings illustrate context-specific relationships between basal metabolic rate and body composition

Basal metabolic rate (BMR) in animals is interpreted as reflecting the size and metabolic intensity of energy-consuming tissues. However, studies investigating relationships between the mass of specific organs and interindividual variation in BMR have produced inconsistent patterns with regard to which organs have the largest impact on BMR variation. Because of the known flexibility in organ mass and metabolic intensity within individual organs, relationships between BMR and body-composition variables are bound to be context specific. Altricial nestlings are excellent models to illustrate this phenomenon because of the extreme variation in body composition occurring during growth. Using European starlings at three age classes, we studied changes in body composition together with its effect on variation in resting metabolic rate (RMR) in order to highlight the context-specific nature of these relationships. Our data suggest a transition in metabolic costs during growth in starling nestlings. During the linear phase of growth, energy is mainly consumed by tissue-synthesis processes, with fast-growing organs having a large influence on RMR. In the plateau phase of growth, the energy expenditure is transferred to functional costs, with high-intensity organs having a predominant effect on RMR variation. Our data illustrates the context-specific nature of organ mass-metabolic rate correlations, which complicates inter- and intraspecific comparisons of BMR. In the future, such comparisons must be done while taking the physiological state of the study animal into account.