Limits to sustained energy intake VIII. Resting metabolic rate and organ morphology of laboratory mice lactating at thermoneutrality

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.

Effect of Different Ambient Temperatures on Reproductive Outcome and Stress Level of Lactating Females in Two Mouse Strains

Simple Summary

The optimal temperature for laboratory mice has been under discussion for some time. Current standard temperature is 20 °C–24 °C but it has been suggested to elevate the standard to 30 °C, which is the thermoneutral zone for mice. In this study, the effect of different cage temperatures (20 °C, 25 °C, 30 °C) on reproduction and stress hormone metabolite excretion was evaluated in lactating females of two commonly used mouse strains. Pup loss was higher, and weights of mothers and pups were reduced at 30 °C compared to the lower temperatures. In addition, pups showed increased tail length at weaning under the high temperature (30 °C). There was no difference in stress hormone metabolite excretion in mice between temperature groups. We could not show any detrimental effects of the lower or higher cage temperature on stress hormone metabolite excretion, but found decreased reproductive outcome under the higher temperature.

Abstract

Ambient temperature is an important non-biotic environmental factor influencing immunological and oncological parameters in laboratory mice. It is under discussion which temperature is more appropriate and whether the commonly used room temperature in rodent facilities of about 21 °C represents a chronic cold stress or the 30 °C of the thermoneutral zone constitutes heat stress for the animals. In this study, we selected the physiological challenging period of lactation to investigate the influence of a cage temperature of 20 °C, 25 °C, and 30 °C, respectively, on reproductive performance and stress hormone levels in two frequently used mouse strains. We found that B6D2F1 hybrid mothers weaned more pups compared to C57BL/6N mothers, and that the number of weaned pups was reduced when mothers of both strains were kept at 30 °C. Furthermore, at 30 °C, mothers and pups showed reduced body weight at weaning and offspring had longer tails. Despite pronounced temperature effects on reproductive parameters, we did not find any temperature effects on adrenocortical activity in breeding and control mice. Independent of the ambient temperature, however, we found that females raising pups showed elevated levels of faecal corticosterone metabolites (FCMs) compared to controls. Peak levels of stress hormone metabolites were measured around birth and during the third week of lactation. Our results provide no evidence of an advantage for keeping lactating mice in ambient temperatures near the thermoneutral zone. In contrast, we found that a 30 °C cage temperature during lactation reduced body mass in females and their offspring and declined female reproductive performance.

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.

Obesity: an evolutionary context

People completely lacking body fat (lipodystrophy/lipoatrophy) and those with severe obesity both show profound metabolic and other health issues. Regulating levels of body fat somewhere between these limits would, therefore, appear to be adaptive. Two different models might be contemplated. More traditional is a set point (SP) where the levels are regulated around a fixed level. Alternatively, dual-intervention point (DIP) is a system that tolerates fairly wide variation but is activated when critically high or low levels are breached. The DIP system seems to fit our experience much better than an SP, and models suggest that it is more likely to have evolved. A DIP system may have evolved because of two contrasting selection pressures. At the lower end, we may have been selected to avoid low levels of fat as a buffer against starvation, to avoid disease-induced anorexia, and to support reproduction. At the upper end, we may have been selected to avoid excess storage because of the elevated risks of predation. This upper limit of control seems to have malfunctioned because some of us deposit large fat stores, with important negative health effects. Why has evolution not protected us against this problem? One possibility is that the protective system slowly fell apart due to random mutations after we dramatically reduced the risk of being predated during our evolutionary history. By chance, it fell apart more in some people than others, and these people are now unable to effectively manage their weight in the face of the modern food glut. To understand the evolutionary context of obesity, it is important to separate the adaptive reason for storing some fat (i.e. the lower intervention point), from the nonadaptive reason for storing lots of fat (a broken upper intervention point). The DIP model has several consequences, showing how we understand the obesity problem and what happens when we attempt to treat it.

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.

The Ability to Dissipate Heat Is Likely to Be a More Important Limitation on Lactation in Striped Hamsters with Greater Reproductive Efforts under Warmer Conditions

The limitations on energy availability and outputs have been implied to have a profound effect on the evolution of many morphological and behavioral traits. It has been suggested that the reproductive performance of mammals is frequently constrained by intrinsic physiological factors, such as the capacity of the mammary glands to produce milk (the peripheral limitation [PL] hypothesis) or that of the body to dissipate heat (the heat dissipation limitation [HDL] hypothesis). Research on a variety of small mammals, however, has so far failed to provide unequivocal support for one hypothesis over the other. We tested the PL and HDL hypotheses in female striped hamsters (Cricetulus barabensis) with artificially manipulated litter sizes of two (three or four pups removed from natural litter size), five, eight (two or three pups added to natural litter size), and 12 (five to seven pups added to natural litter size) pups at ambient temperatures of 21° and 30°C. Energy intake and milk output of mothers, litter size, and litter mass were measured throughout lactation. Several markers indicating digestive enzyme activity and the gene expression of hypothalamic neuropeptides related to food intake were also measured. Food consumption and milk output increased with increasing litter size but reached a ceiling at 12 pups, causing 12-pup litters to have significantly lower litter mass and pup body mass than litters composed of fewer pups. Litter mass and maternal metabolic rate, milk output, maltase, sucrase, and aminopeptidase activity in the small intestine, and gene expression of hypothalamic orexigenic peptides were significantly lower at 30°C than at 21°C, and these differences were considerably more pronounced in 12-pup litters. These results suggest that PL and HDL can operate simultaneously but that the HDL hypothesis is probably more valid at warmer temperatures. Our results suggest that increased environmental temperatures in future climates may limit reproductive output through heat dissipation limits.

Late lactation in small mammals is a critically sensitive window of vulnerability to elevated ambient temperature

Predicted increases in global average temperature are physiologically trivial for most endotherms. However, heat waves will also increase in both frequency and severity, and these will be physiologically more important. Lactating small mammals are hypothesized to be limited by heat dissipation capacity, suggesting high temperatures may adversely impact lactation performance. We measured reproductive performance of mice and striped hamsters (Cricetulus barabensis), including milk energy output (MEO), at temperatures between 21 and 36 °C. In both species, there was a decline in MEO between 21 and 33 °C. In mice, milk production at 33 °C was only 18% of that at 21 °C. This led to reductions in pup growth by 20% but limited pup mortality (0.8%), because of a threefold increase in growth efficiency. In contrast, in hamsters, MEO at 33 °C was reduced to 78.1% of that at 21 °C, yet this led to significant pup mortality (possibly infanticide) and reduced pup growth by 12.7%. Hamster females were more able to sustain milk production as ambient temperature increased, but they and their pups were less capable of adjusting to the lower supply. In both species, exposure to 36 °C resulted in rapid catastrophic lactation failure and maternal mortality. Upper lethal temperature was lowered by 3 to 6 °C in late lactation, making it a critically sensitive window to high ambient temperatures. Our data suggest future heat wave events will impact breeding success of small rodents, but this is based on animals with a long history in captivity. More work should be performed on wild rodents to confirm these impacts.

Limits to sustained energy intake. XXXI. Effect of graded levels of dietary fat on lactation performance in Swiss mice

The heat dissipation limit theory predicts that lactating female mice consuming diets with lower specific dynamic action (SDA) should have enhanced lactation performance. Dietary fat has lower SDA than other macronutrients. Here we tested the effects of graded dietary fat levels on lactating Swiss mice. We fed females five diets varying in fat content from 8.3 to 66.6%. Offspring of mothers fed diets of 41.7% fat and above were heavier and fatter at weaning compared with those of 8.3 and 25% fat diets. Mice on dietary fat contents of 41.7% and above had greater metabolizable energy intake at peak lactation (8.3%: 229.4±39.6; 25%: 278.8±25.8; 41.7%: 359.6±51.5; 58.3%: 353.7±43.6; 66.6%: 346±44.7 kJ day-1), lower daily energy expenditure (8.3%: 128.5±16; 25%: 131.6±8.4; 41.7%: 124.4±10.8; 58.3%: 115.1±10.5; 66.6%: 111.2±11.5 kJ day-1) and thus delivered more milk energy to their offspring (8.3%: 100.8±27.3; 25%: 147.2±25.1; 41.7%: 225.1±49.6; 58.3%: 238.6±40.1; 66.6%: 234.8±41.1 kJ day-1). Milk fat content (%) was unrelated to dietary fat content, indicating that females on higher fat diets (>41.7%) produced more rather than richer milk. Mothers consuming diets with 41.7% fat or above enhanced their lactation performance compared with those on 25% or less, probably by diverting dietary fat directly into the milk, thereby avoiding the costs of lipogenesis. At dietary fat contents above 41.7% they were either unable to transfer more dietary fat to the milk, or they chose not to do so, potentially because of a lack of benefit to the offspring that were increasingly fatter as maternal dietary fat increased.

Limits to sustained energy intake. XXX. Constraint or restraint? Manipulations of food supply show peak food intake in lactation is constrained

Lactating mice increase food intake 4- to 5-fold, reaching an asymptote in late lactation. A key question is whether this asymptote reflects a physiological constraint, or a maternal investment strategy (a 'restraint'). We exposed lactating mice to periods of food restriction, hypothesizing that if the limit reflected restraint, they would compensate by breaching the asymptote when refeeding. In contrast, if it was a constraint, they would by definition be unable to increase their intake on refeeding days. Using isotope methods, we found that during food restriction, the females shut down milk production, impacting offspring growth. During refeeding, food intake and milk production rose again, but not significantly above unrestricted controls. These data provide strong evidence that asymptotic intake in lactation reflects a physiological/physical constraint, rather than restraint. Because hypothalamic neuropeptide Y (Npy) was upregulated under both states of restriction, this suggests the constraint is not imposed by limits in the capacity to upregulate hunger signalling (the saturated neural capacity hypothesis). Understanding the genetic basis of the constraint will be a key future goal and will provide us additional information on the nature of the constraining factors on reproductive output, and their potential links to life history strategies.

Not that hot after all: no limits to heat dissipation in lactating mice selected for high or low BMR

Heat dissipation has been suggested as a limit to sustained metabolic effort, e.g. during lactation, when overheating is a possible risk. We tested this hypothesis using mice artificially selected for high (H-BMR) or low (L-BMR) BMR that also differ with respect to parental effort. We used fixed sized cross-fostered families and recorded litter mass daily until the 14th day of lactation. Midway through the experiment (day 8th) half of randomly chosen mothers from each line type had fur from the dorsal body surface removed to increase their thermal conductance and facilitate heat dissipation. Our results showed that neither of the line types benefited from increasing their thermal conductance at peak lactation. On the contrary, growth of the litters reared by the L-BMR females was compromised. Thus, our results do not support the heat dissipation limitation hypothesis.

Regulation of intestinal growth in response to variations in energy supply and demand

The growth of the intestine requires energy, which is known to be met by catabolism of ingested nutrients. Paradoxically, during whole body energy deficit including calorie restriction, the intestine grows in size. To understand how and why this happens, we reviewed data from several animal models of energetic challenge. These were bariatric surgery, cold exposure, lactation, dietary whey protein intake and calorie restriction. Notably, these challenges all reduced the adipose tissue mass, altered hypothalamic neuropeptide expression and increased intestinal size. Based on these data, we propose that the loss of energy in the adipose tissue promotes the growth of the intestine via a signalling mechanism involving the hypothalamus. We discuss possible candidates in this pathway including data showing a correlative change in intestinal (ileal) expression of the cyclin D1 gene with adipose tissue mass, adipose derived-hormone leptin and hypothalamic expression of leptin receptor and the pro-opiomelanocortin gene. The ability of the intestine to grow in size during depletion of energy stores provides a mechanism to maximize assimilation of ingested energy and in turn sustain critical functions of tissues important for survival.

Approaches for testing hypotheses for the hypometric scaling of aerobic metabolic rate in animals

Hypometric scaling of aerobic metabolism [larger organisms have lower mass-specific metabolic rates (MR/g)] is nearly universal for interspecific comparisons among animals, yet we lack an agreed upon explanation for this pattern. If physiological constraints on the function of larger animals occur and limit MR/g, these should be observable as direct constraints on animals of extant species and/or as evolved responses to compensate for the proposed constraint. There is evidence for direct constraints and compensatory responses to O2 supply constraint in skin-breathing animals, but not in vertebrates with gas-exchange organs. The duration of food retention in the gut is longer for larger birds and mammals, consistent with a direct constraint on nutrient uptake across the gut wall, but there is little evidence for evolving compensatory responses to gut transport constraints in larger animals. Larger placental mammals (but not marsupials or birds) show evidence of greater challenges with heat dissipation, but there is little evidence for compensatory adaptations to enhance heat loss in larger endotherms, suggesting that metabolic rate (MR) more generally balances heat loss for thermoregulation in endotherms. Size-dependent patterns in many molecular, physiological, and morphological properties are consistent with size-dependent natural selection, such as stronger selection for neurolocomotor performance and growth rate in smaller animals and stronger selection for safety and longevity in larger animals. Hypometric scaling of MR very likely arises from different mechanisms in different taxa and conditions, consistent with the diversity of scaling slopes for MR.

Prey productivity and predictability drive different axes of life-history variation in carnivorous marsupials

Variation in life-history strategies has usually been characterized as a single fast-slow continuum of life-history variation, in which mean lifespan increases with age at maturity as reproductive output at each breeding event declines. Analyses of plants and animals suggest that strategies of reproductive timing can vary on an independent axis, with iteroparous species at one extreme and semelparous species at the other. Insectivorous marsupials in the Family Dasyuridae have an unusually wide range of life-history strategies on both purported axes. We test and confirm that reproductive output and degree of iteroparity are independent in females across species. Variation in reproductive output per episode is associated with mean annual rainfall, which predicts food availability. Position on the iteroparity-semelparity axis is not associated with annual rainfall, but species in regions of unpredictable rainfall have longer maximum lifespans, more potential reproductive events per year, and longer breeding seasons. We suggest that these two axes of life-history variation arise because reproductive output is limited by overall food availability, and selection for high offspring survival favours concentrated breeding in seasonal environments. Longer lifespans are favoured when reproductive opportunities are dispersed over longer periods in environments with less predictable food schedules.

Limits to sustained energy intake XXIX: the case of the golden hamster (Mesocricetus auratus)

Golden hamster females have the shortest known gestation period among placental mammals and at the same time raise very large litters of up to 16 offspring, which are born in a naked and blind state and are able to pick up food from days 12-14 only. We quantified energy metabolism and milk production in female golden hamsters raising offspring under cold (8°C), normal (22°C) and hot (30°C) ambient temperature conditions. We monitored energy intake, subcutaneous body temperature, daily energy expenditure, litter size and pup masses over the course of lactation. Our results show that, in line with the concept of heat dissipation limitation, female golden hamsters had the largest energy intake under the coldest conditions and a significantly lower intake at 30° (partial for influence of ambient temperature: F2,403=5.6; p= 0.004). Metabolisable energy intake as well as milk energy output showed the same pattern and were significantly different between the temperatures (partial for milk energy production: F1,40= 86.4; p<0.0001). With consistently higher subcutaneous temperatures in the reproductive females (F1,813= 36.77; p<0.0001) compared to baseline females. These data suggest that raising offspring in golden hamsters comes at the cost of producing large amounts of body heat up to a level constraining energy intake, similar to that observed in some laboratory mice. Notably, we observed that females seemed to adjust litter size according to their milk production with the smallest litters (3.4±0.7 pups) being raised by hot exposed mothers. Future research is needed to unravel the mechanism by which females assess their own milk production capabilities and how this may be linked to litter size at different ambient temperatures. Golden hamsters reach 8-10 times resting metabolic rate (RMR) when raising offspring under cold conditions, which is compatible with the findings from laboratory mice and other rodents.

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.

Ambient temperature affects postnatal litter size reduction in golden hamsters

BACKGROUND

To better understand how different ambient temperatures during lactation affect survival of young, we studied patterns of losses of pups in golden hamsters (Mesocricetus auratus) at different ambient temperatures in the laboratory, mimicking temperature conditions in natural habitats. Golden hamsters produce large litters of more than 10 young but are also known to wean fewer pups at the end of lactation than they give birth to. We wanted to know whether temperature affects litter size reductions and whether the underlying causes of pup loss were related to maternal food (gross energy) intake and reproductive performance, such as litter growth. For that, we exposed lactating females to three different ambient temperatures and investigated associations with losses of offspring between birth and weaning.

RESULTS

Overall, around one third of pups per litter disappeared, obviously consumed by the mother. Such litter size reductions were greatest at 30 °C, in particular during the intermediate postnatal period around peak lactation. Furthermore, litter size reductions were generally higher in larger litters. Maternal gross energy intake was highest at 5 °C suggesting that mothers were not limited by milk production and might have been able to raise a higher number of pups until weaning. This was further supported by the fact that the daily increases in litter mass as well as in the individual pup body masses, a proxy of mother's lactational performance, were lower at higher ambient temperatures.

CONCLUSIONS

We suggest that ambient temperatures around the thermoneutral zone and beyond are preventing golden hamster females from producing milk at sufficient rates. Around two thirds of the pups per litter disappeared at high temperature conditions, and their early growth rates were significantly lower than at lower ambient temperatures. It is possible that these losses are due to an intrinsic physiological limitation (imposed by heat dissipation) compromising maternal energy intake and milk production.

Hybrid Dysfunction Expressed as Elevated Metabolic Rate in Male Ficedula Flycatchers

Studies of ecological speciation are often biased towards extrinsic sources of selection against hybrids, resulting from intermediate hybrid morphology, but the knowledge of how genetic incompatibilities accumulate over time under natural conditions is limited. Here we focus on a physiological trait, metabolic rate, which is central to life history strategies and thermoregulation but is also likely to be sensitive to mismatched mitonuclear interactions. We measured the resting metabolic rate of male collared, and pied flycatchers as well as of naturally occurring F1 hybrid males, in a recent hybrid zone. We found that hybrid males had a higher rather than intermediate metabolic rate, which is indicative of hybrid physiological dysfunction. Fitness costs associated with elevated metabolic rate are typically environmentally dependent and exaggerated under harsh conditions. By focusing on male hybrid dysfunction in an eco-physiological trait, our results contribute to the general understanding of how combined extrinsic and intrinsic sources of hybrid dysfunction build up under natural conditions.

Limits to sustained energy intake XXV: milk energy output and thermogenesis in Swiss mice lactating at thermoneutrality

Previous studies at 21 °C and 5 °C suggest that in Swiss mice sustained energy intake (SusEI) and reproductive performance are constrained by the mammary capacity to produce milk. We aimed to establish if this constraint also applied at higher ambient temperature (30 °C). Female Swiss mice lactating at 30 °C had lower asymptotic food intake and weaned lighter litters than those at 21 °C. Resting metabolic rate, daily energy expenditure, milk energy output and suckling time were all lower at 30 °C. In a second experiment we gave mice at 30 °C either 6 or 9 pups to raise. Female performance was independent of litter size, indicating that it is probably not controlled by pup demands. In a third experiment we exposed only the mother, or only the offspring to the elevated temperature. In this case the performance of the mother was only reduced when she was exposed, and not when her pups were exposed, showing that the high temperature directly constrains female performance. These data suggest that at 30 °C SusEI and reproductive performance are likely constrained by the capacity of females to dissipate body heat, and not indirectly via pup demands. Constraints seem to change with ambient temperature in this strain of mouse.

The high cost of reproduction in sea otters necessitates unique physiological adaptations

Superimposed on inherently high basal metabolic demands, the additional energetic requirements of reproduction can push female sea otters beyond physiological limits. Indeed, the resulting energy imbalance contributes to disproportionately high rates of mortality at the end of lactation in this species. To examine and quantify metabolic changes associated with reproduction, we measured the resting metabolic rate (RMR) of a female sea otter across gestation, lactation and non-reproductive periods. Concurrently, measurements were made on a non-breeding control female. Our results suggest that RMR declines during gestation. Conversely, RMR increases during lactation, reaches a peak at 3-4 months postpartum, and remains elevated until weaning. Combining these direct measurements with published data, we found the cost of pup rearing to be significantly higher than previously estimated. High baseline energy demands and limited energy reserves, combined with significant lactation and pup rearing costs, appear to necessitate metabolic and thermal lability during key reproductive stages.

Heat dissipation does not suppress an immune response in laboratory mice divergently selected for basal metabolic rate (BMR)

The capacity for heat dissipation is considered to be one of the most important constraints on rates of energy expenditure in mammals. To date, the significance of this constraint has been tested exclusively under peak metabolic demands, such as during lactation. Here, we used a different set of metabolic stressors, which do not induce maximum energy expenditures and yet are likely to expose the potential constraining effect of heat dissipation. We compared the physiological responses of mice divergently selected for high (H-BMR) and low basal metabolic rate (L-BMR) to simultaneous exposure to the keyhole limpet haemocyanin (KLH) antigen and high ambient temperature (Ta). At 34°C (and at 23°C, used as a control), KLH challenge resulted in a transient increase in core body temperature (Tb) in mice of both line types (by approximately 0.4°C). Warm exposure did not produce line-type-dependent differences in Tb (which was consistently higher by ca. 0.6°C in H-BMR mice across both Ta values), nor did it result in the suppression of antibody synthesis. These findings were also supported by the lack of between-line-type differences in the mass of the thymus, spleen or lymph nodes. Warm exposure induced the downsizing of heat-generating internal organs (small intestine, liver and kidneys) and an increase in intrascapular brown adipose tissue mass. However, these changes were similar in scope in both line types. Mounting a humoral immune response in selected mice was therefore not affected by ambient temperature. Thus, a combined metabolic challenge of high Ta and an immune response did not appreciably compromise the capacity to dissipate heat, even in the H-BMR mice.

Limits to sustained energy intake XXIV: impact of suckling behaviour on the body temperatures of lactating female mice

The objective of this study was to investigate the potential causes of high body temperature (Tb) during lactation in mice as a putative limit on energy intake. In particular we explored whether or not offspring contributed to heat retention in mothers while suckling. Tb and physical activity were monitored in 26 female MF1 mice using intraperitoneally implanted transmitters. In addition, maternal behaviour was scored each minute for 8 h d(-1) throughout lactation. Mothers that raised larger litters tended to have higher Tb while nursing inside nests (P < 0.05), suggesting that nursing offspring may have influenced heat retention. However, Tb during nursing was not higher than that recorded during other behaviours. In addition, the highest Tb during the observation period was not measured during nursing behaviour. Finally, there was no indication that mothers discontinued suckling because of a progressive rise in their Tb while suckling. Tb throughout lactation was correlated with daily increases in energy intake. Chronic hyperthermia during lactation was not caused by increased heat retention due to surrounding offspring. Other factors, like metabolic heat produced as a by-product of milk production or energy intake may be more important factors. Heat dissipation limits are probably not a phenomenon restricted to lactation.

CB1 receptor blockade counters age-induced insulin resistance and metabolic dysfunction

The endocannabinoid system can modulate energy homeostasis by regulating feeding behaviour as well as peripheral energy storage and utilization. Importantly, many of its metabolic actions are mediated through the cannabinoid type 1 receptor (CB1R), whose hyperactivation is associated with obesity and impaired metabolic function. Herein, we explored the effects of administering rimonabant, a selective CB1R inverse agonist, upon key metabolic parameters in young (4 month old) and aged (17 month old) adult male C57BL/6 mice. Daily treatment with rimonabant for 14 days transiently reduced food intake in young and aged mice; however, the anorectic response was more profound in aged animals, coinciding with a substantive loss in body fat mass. Notably, reduced insulin sensitivity in aged skeletal muscle and liver concurred with increased CB1R mRNA abundance. Strikingly, rimonabant was shown to improve glucose tolerance and enhance skeletal muscle and liver insulin sensitivity in aged, but not young, adult mice. Moreover, rimonabant‐mediated insulin sensitization in aged adipose tissue coincided with amelioration of low‐grade inflammation and repressed lipogenic gene expression. Collectively, our findings indicate a key role for CB1R in aging‐related insulin resistance and metabolic dysfunction and highlight CB1R blockade as a potential strategy for combating metabolic disorders associated with aging.

Plasticity in gastrointestinal morphology and enzyme activity in lactating striped hamsters (Cricetulus barabensis)

In small mammals marked phenotypic plasticity of digestive physiology has been shown to make it easier to cope with the energetically stressful periods, such as lactation. It has been proposed that the capacity of the gut to digest and absorb food is not the factor limiting to sustained energy intake (SusEI) during peak lactation. In this study, plasticity in energy intake and gastrointestinal morphology was examined in striped hamsters at different stages of reproduction and raising litters of different sizes. Mechanisms associated with digestive enzymes and neuroendocrine hormones underpinning the plasticity were also examined. The females significantly increased energy intake, digestibility, masses of digestive tracts and activity of stomach pepsin and maltase, sucrase and aminopeptidase of small intestine in peak lactation compared to the non-productive and post-lactating periods. Further, the females raising large litters significantly increased energy intake, digestibility, gastrointestinal mass and activity of digestive enzymes, and weaned heavier offspring compared with those nursing small and medium litters, indicating that the significant plasticity of digestive physiology increased reproductive performance. The agouti-related protein (AgRP) mRNA expression in the hypothalamus was up-regulated significantly in the females raising large litters relative to those raising small litters. Serum leptin levels, hypothalamus neuropeptide Y (NPY), or anorexigenic neuropeptides (pro-opiomelanocortin / cocaine- and amphetamine-regulated transcript, POMC / CART) mRNA expression did not differ among the females raising small, medium and large litters, indicating that leptin levels in lactation might only reflect a state of energy balance rather than being the prime driver of hyperphagia. Some hypothalamic neuropeptides, such as NPY, POMC and CART, would be involved in the limits to the SusEI during lactation.

Oxidative Damage Does Not Occur in Striped Hamsters Raising Natural and Experimentally Increased Litter Size

Life-history theory assumes that animals can balance the allocation of limited energy or resources to the competing demands of growth, reproduction and somatic maintenance, while consequently maximizing their fitness. However, somatic damage caused by oxidative stress in reproductive female animals is species-specific or is tissue dependent. In the present study, several markers of oxidative stress (hydrogen peroxide, H₂O₂ and malonadialdehyde, MDA) and antioxidant (catalase, CAT and total antioxidant capacity, T-AOC) were examined in striped hamsters during different stages of reproduction with experimentally manipulated litter size. Energy intake, resting metabolic rate (RMR), and mRNA expression of uncoupling protein 1 (UCP₁) in brown adipose tissue (BAT) and UCP₃ in skeletal muscle were also examined. H₂O₂ and MDA levels did not change in BAT and liver, although they significantly decreased in skeletal muscle in the lactating hamsters compared to the non-reproductive group. However, H₂O₂ levels in the brain were significantly higher in lactating hamsters than non-reproductive controls. Experimentally increasing litter size did not cause oxidative stress in BAT, liver and skeletal muscle, but significantly elevated H₂O₂ levels in the brain. CAT activity of liver decreased, but CAT and T-AOC activity of BAT, skeletal muscle and the brain did not change in lactating hamsters compared to non-reproductive controls. Both antioxidants did not change with the experimentally increasing litter size. RMR significantly increased, but BAT UCP₁ mRNA expression decreased with the experimentally increased litter size, suggesting that it was against simple positive links between metabolic rate, UCP₁ expression and free radicals levels. It may suggest that the cost of reproduction has negligible effect on oxidative stress or even attenuates oxidative stress in some active tissues in an extensive range of animal species. But the increasing reproductive effort may cause oxidative stress in the brain, indicating that oxidative stress in response to reproduction is tissue dependent. These findings provide partial support for the life-history theory.

Effect of food restriction on energy budget in warm-acclimated striped hamsters

The capacity of small mammals to sustain periods of food shortage largely depends on the adaptive regulation of energy budget in response to the decrease in food supply. In addition to food availability, ambient temperature (Ta) is an important factor affecting the rates of both energy intake and expenditure. To examine the effect of Ta on energy strategy and the capacity to sustain food shortage, striped hamsters were exposed to a warm condition (30°C) and were then restricted to 70% of ad libitum food intake. Body mass, energy intake and expenditure and physiological markers indicative of thermogenesis were measured. Warm exposure had no effect on body mass and digestibility, but decreased energy intake, basal metabolic rate and maximum nonshivering thermogenesis. The mitochondria protein content, cytochrome c oxidase activity and uncoupling protein 1 level of brown adipose tissue were significantly lower in hamsters at 30°C than at 21°C. Food restriction induced a significant decrease in body mass, but the decreased body mass was attenuated at 30°C relative to 21°C. This suggests that striped hamsters could not compensate for the limited food supply by decreasing daily energy expenditure at 21°C, whereas they could at 30°C. The significant reductions in the rates of metabolism and thermogenesis in warm-acclimated hamsters increase the capacity to cope with food shortage. Although, it remains uncertain whether this response represents some generalized evolutionary adaptation, the Ta-dependent adjustment in the capacity to survive food restriction may reflect that warm acclimation plays an important role in adaptive regulation of both physiology and behavior in response to the variations of food availability.

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.

High-saturated fat-sucrose feeding affects lactation energetics in control mice and mice selectively bred for high wheel-running behavior

Feeding a diet high in fat and sucrose (HFS) during pregnancy and lactation is known to increase susceptibility to develop metabolic derangements later in life. A trait for increased behavioral activity may oppose these effects, since this would drain energy from milk produced to be made available to the offspring. To investigate these interactions, we assessed several components of behavioral energetics during lactation in control mice (C) and in mice of two lines selectively bred for high wheel-running activity (S1, S2) subjected to a HFS diet or a low-fat (LF) diet. Energy intake, litter growth, and milk energy output at peak lactation (MEO; assessed by subtracting maternal metabolic rate from energy intake) were elevated in HFS-feeding dams across all lines compared with the LF condition, an effect that was particularly evident in the S dams. This effect was not preceded by improved lactation behaviors assessed between postnatal days 1 and 7 (PND 1-7). In fact, S1 dams had less high-quality nursing, and S2 dams showed poorer pup retrieval than C dams during PND 1-7, and S dams had generally higher levels of physical activity at peak lactation. These data demonstrate that HFS feeding increases MEO underlying increased litter and pup growth, particularly in mice with a trait for increased behavioral physical activity.

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. XXI. Effect of exposing the mother, but not her pups, to a cold environment during lactation in mice

The capacity of females to dissipate heat may constrain sustained energy intake during lactation. However, some previous experiments supporting this concept have confounded the impact of temperature on the mothers with the impact on the pups. We aimed to separate these effects in lactating laboratory mice (MF1 strain) by giving the mothers access to cages at two ambient temperatures (10 and 21°C) joined by a tube. Food was available only in the cold cage, but females could also choose go to this cage to cool down while their pups were housed in the warmer cage. Control animals had access to the same configuration of cages but with both maintained at 21°C. We hypothesised that if females were limited by heat dissipation, alleviating the heat load by providing a cool environment would allow them to dissipate more heat, take in more food, generate more milk and hence wean heavier litters. We measured maternal energy budgets and monitored time courses of core body temperature and physical activity. To minimise the variance in energy budgets, all litters were adjusted to 12 (±1) pups. Females in the experimental group had higher energy intake (F1,14=15.8, P=0.0014) and higher assimilated energy (F1,13=10.7, P=0.006), and provided their pups with more milk (F1,13=6.65, P=0.03), consistent with the heat dissipation limit theory. Yet, despite keeping demand constant, mean pup growth rates were similar (F1,13=0.06, P=0.8); thus, our data emphasise the difficulties of inferring milk production indirectly from pup growth.

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. 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.

Limits to sustained energy intake. XIX. A test of the heat dissipation limitation hypothesis in Mongolian gerbils (Meriones unguiculatus)

We evaluated factors limiting lactating Mongolian gerbils (Meriones unguiculatus) at three temperatures (10, 21 and 30°C). Energy intake and daily energy expenditure (DEE) increased with decreased ambient temperature. At peak lactation (day 14 of lactation), energy intake increased from 148.7±5.7 kJ day(-1) at 30°C to 213.1±8.2 kJ day(-1) at 21°C and 248.7±12.3 kJ day(-1) at 10°C. DEE increased from 105.1±4.0 kJ day(-1) at 30°C to 134.7±5.6 kJ day(-1) at 21°C and 179.5±8.4 kJ day(-1) at 10°C on days 14-16 of lactation. With nearly identical mean litter sizes, lactating gerbils at 30°C exported 32.0 kJ day(-1) less energy as milk at peak lactation than those allocated to 10 or 21°C, with no difference between the latter groups. On day 14 of lactation, the litter masses at 10 and 30°C were 12.2 and 9.3 g lower than those at 21°C, respectively. Lactating gerbils had higher thermal conductance of the fur and lower UCP-1 levels in brown adipose tissue than non-reproductive gerbils, independent of ambient temperature, suggesting that they were attempting to avoid heat stress. Thermal conductance of the fur was positively related to circulating prolactin levels. We implanted non-reproductive gerbils with mini-osmotic pumps that delivered either prolactin or saline. Prolactin did not influence thermal conductance of the fur, but did reduce physical activity and UCP-1 levels in brown adipose tissue. Transferring lactating gerbils from warm to hot conditions resulted in reduced milk production, consistent with the heat dissipation limit theory, but transferring them from warm to cold conditions did not elevate milk production, consistent with the peripheral limitation hypothesis, and placed constraints on pup growth.

Physiological adaptations to reproduction. II. Mitochondrial adjustments in livers of lactating mice

Reproduction imposes significant costs and is characterized by an increased energy demand. As a consequence, individuals adjust their cellular structure and function in response to this physiological constraint. Because mitochondria are central to energy production, changes in their functional properties are likely to occur during reproduction. Such changes could cause adjustments in reactive oxygen species (ROS) production and consequently in oxidative stress levels. In this study, we investigated several mechanisms involved in energy production, including mitochondrial respiration at different steps of the electron transport system (ETS) and related the results to citrate synthase activity in the liver of non-reproductive and reproductive (two and eight pups) female house mice at peak lactation. Whereas we did not find differences between females having different litter sizes, liver mitochondria of reproductive females showed lower ETS activity and an increase in mitochondrial density when compared with the non-reproductive females. Although it is possible that these changes were due to combined processes involved in reproduction and not to the relative investment in lactation, we propose that the mitochondrial adjustment in liver might help to spare substrates and therefore energy for milk production in the mammary gland. Moreover, our results suggest that these changes lead to an increase in ROS production that subsequently upregulates antioxidant defence activity and decreases oxidative stress.

Measuring energy metabolism in the mouse - theoretical, practical, and analytical considerations

The mouse is one of the most important model organisms for understanding human genetic function and disease. This includes characterization of the factors that influence energy expenditure and dysregulation of energy balance leading to obesity and its sequelae. Measuring energy metabolism in the mouse presents a challenge because the animals are small, and in this respect it presents similar challenges to measuring energy demands in many other species of small mammal. This paper considers some theoretical, practical, and analytical considerations to be considered when measuring energy expenditure in mice. Theoretically total daily energy expenditure is comprised of several different components: basal or resting expenditure, physical activity, thermoregulation, and the thermic effect of food. Energy expenditure in mice is normally measured using open flow indirect calorimetry apparatus. Two types of system are available – one of which involves a single small Spartan chamber linked to a single analyzer, which is ideal for measuring the individual components of energy demand. The other type of system involves a large chamber which mimics the home cage environment and is generally configured with several chambers/analyzer. These latter systems are ideal for measuring total daily energy expenditure but at present do not allow accurate decomposition of the total expenditure into its components. The greatest analytical challenge for mouse expenditure data is how to account for body size differences between individuals. This has been a matter of some discussion for at least 120 years. The statistically most appropriate approach is to use analysis of covariance with individual aspects of body composition as independent predictors.

Basal metabolic rate is positively correlated with parental investment in laboratory mice

The assimilation capacity (AC) hypothesis for the evolution of endothermy predicts that the maternal basal metabolic rate (BMR) should be positively correlated with the capacity for parental investment. In this study, we provide a unique test of the AC model based on mice from a long-term selection experiment designed to produce divergent levels of BMR. By constructing experimental families with cross-fostered litters, we were able to control for the effect of the mother as well as the type of pup based on the selected lines. We found that mothers with genetically determined high levels of BMR were characterized by higher parental investment capacity, measured as the offspring growth rate. We also found higher food consumption and heavier visceral organs in the females with high BMR. These findings suggested that the high-BMR females have higher energy acquisition abilities. When the effect of the line type of a foster mother was controlled, the pup line type significantly affected the growth rate only in the first week of life, with young from the high-BMR line type growing more rapidly. Our results support the predictions of the AC model.

Determinants of intra-specific variation in basal metabolic rate

Basal metabolic rate (BMR) provides a widely accepted benchmark of metabolic expenditure for endotherms under laboratory and natural conditions. While most studies examining BMR have concentrated on inter-specific variation, relatively less attention has been paid to the determinants of within-species variation. Even fewer studies have analysed the determinants of within-species BMR variation corrected for the strong influence of body mass by appropriate means (e.g. ANCOVA). Here, we review recent advancements in studies on the quantitative genetics of BMR and organ mass variation, along with their molecular genetics. Next, we decompose BMR variation at the organ, tissue and molecular level. We conclude that within-species variation in BMR and its components have a clear genetic signature, and are functionally linked to key metabolic process at all levels of biological organization. We highlight the need to integrate molecular genetics with conventional metabolic field studies to reveal the adaptive significance of metabolic variation. Since comparing gene expressions inter-specifically is problematic, within-species studies are more likely to inform us about the genetic underpinnings of BMR. We also urge for better integration of animal and medical research on BMR; the latter is quickly advancing thanks to the application of imaging technologies and ‘omics’ studies. We also suggest that much insight on the biochemical and molecular underpinnings of BMR variation can be gained from integrating studies on the mammalian target of rapamycin (mTOR), which appears to be the major regulatory pathway influencing the key molecular components of BMR.

Variation in basal metabolic rate and activity in relation to reproductive condition and photoperiod in white-footed mice (Peromyscus leucopus)

A naturally variable life-history trait with underlying physiological variation is the photoperiodic response of many temperate-zone rodents, including white-footed mice (Peromyscus leucopus (Rafinesque, 1818)). Male P. leucopus were obtained from a short photoperiod responsive (R) line, artificially selected for reproductive suppression in short-day conditions (SD) and a nonresponsive (NR) line selected for reproductive maturity in SD. We tested for variation in metabolic rate between lines in SD and long-day conditions (LD). NR mice consumed 34% more food than R mice, without concomitant increase in body mass in SD. Basal metabolic rate (BMR) was found to be significantly greater in NR than R mice, and NR mice were found to engage in significantly more spontaneous (daily) locomotor activity. Energy-use estimates based on 24 h respirometry matched closely the level of intake reported for individual mice. The increased BMR and average daily metabolic rate in NR mice was correlated with testis size, but not with major central organs or digestibility. No significant difference in BMR or activity was found in mice from the same lines held in LD. Elevated intake in SD mice appears to be associated with differences in fertility and not other aspects of physiology in the respective lines.

Rates of maximum food intake in young northern fur seals (Callorhinus ursinus) and the seasonal effects of food intake on body growth

Accurate estimates of food intake and its subsequent effect on growth are required to understand the interaction between an animal’s physiology and its biotic environment. We determined how food intake and growth of six young northern fur seals ( Callorhinus ursinus (L., 1758)) responded seasonally to changes in food availability. Animals were given unrestricted access to prey for 8 h·day–1 on either consecutive days or on alternate days only. We found animals offered ad libitum food on consecutive days substantially increased their food intake over normal “training” levels. However, animals that fasted on alternate days were unable to compensate by further increasing their levels of consumption on subsequent feeding days. Absolute levels of food intake were highly consistent during winter and summer trials (2.7–2.9 kg·day–1), but seasonal differences in body mass meant that fur seals consumed more food relative to their body mass in summer (~27%) than in winter (~20%). Despite significant increases in absolute food intake during both seasons, the fur seals did not appear to efficiently convert this additional energy into mass growth, particularly in the winter. These seasonal differences in conversion efficiencies and estimates of maximum intake rates can be used to generate physiologically realistic predictions about the effect of changes in food availability on an individual fur seal, as well as the consequences for an entire population.

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.

Dietary supplementation with phytosterol and ascorbic acid reduces body mass accumulation and alters food transit time in a diet-induced obesity mouse model

Previous research indicates that animals fed a high fat (HF) diet supplemented with disodium ascorbyl phytostanyl phosphate (DAPP) exhibit reduced mass accumulation when compared to HF control. This compound is a water-soluble phytostanol ester and consists of a hydrophobic plant stanol covalently bonded to ascorbic acid (Vitamin C). To provide insight into the mechanism of this response, we examined the in vivo effects of a high fat diet supplemented with ascorbic acid (AA) in the presence and absence of unesterified phytosterols (PS), and set out to establish whether the supplements have a synergistic effect in a diet-induced obesity mouse model. Our data indicate that HF diet supplementation with a combination of 1% w/w phytosterol and 1% w/w ascorbic acid results in reduced mass accumulation, with mean differences in absolute mass between PSAA and HF control of 10.05%; and differences in mass accumulation of 21.6% (i.e. the PSAA group gained on average 21% less mass each week from weeks 7-12 than the HF control group). In our previous study, the absolute mass difference between the 2% DAPP and HF control was 41%, while the mean difference in mass accumulation between the two groups for weeks 7-12 was 67.9%. Mass loss was not observed in animals supplemented with PS or AA alone. These data suggest that the supplements are synergistic with respect to mass accumulation, and the esterification of the compounds further potentiates the response. Our data also indicate that chronic administration of PS, both in the presence and absence of AA, results in changes to fecal output and food transit time, providing insight into the possibility of long-term changes in intestinal function related to PS supplementation.

Is there a linkage between metabolism and personality in small mammals? The root vole (Microtus oeconomus) example

Significant inter-individual variation in the rate of animal metabolism is a widespread phenomenon that has started to accumulate general interest. Here we follow recent calls to focus on linkage between the variation in energy metabolism and animal personality. By using wild caught root voles as a study species, we examined the relationship between the behavioral patterns (assessed in open field test) and resting metabolic rate (RMR), both of which are known to show large individual differences and intra-individual consistency in voles. Our results showed only a weak relationship between personality traits and metabolism, since the most parsimonious model (according to AICc) explaining RMR included only body mass and season as factors (explaining 84.8% of variation in RMR). However, the next two alternative models (within ΔAICc=2) also included the personality trait reflecting proactive behaviors (PC1) in addition to body mass, sex and season (85.2 and 85.8% of RMR variance explained, respectively). In all, our study does not provide compelling support for recent ideas of close linkage between behavior and metabolism. Still, our study highlights that even in the case of wild caught individuals, when behavior and metabolism often carry effects of both intrinsic and extrinsic conditions, the potential metabolic effects of varying energetically costly behaviors cannot be neglected.

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.