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.

Lactating SKH-1 furless mice prioritize own comfort over growth of their pups

Lactation is the most energetically demanding physiological process that occurs in mammalian females, and as a consequence of this energy expenditure, lactating females produce an enormous amount of excess heat. This heat is thought to limit the amount of milk a mother produces, and by improving heat dissipation, females may improve their milk production and offspring quality. Here we used SKH-1 hairless mice as a natural model of improved heat dissipation. Lactating mothers were given access to a secondary cage to rest away from their pups, and this secondary cage was kept either at room temperature (22 °C) in the control rounds or cooled to 8 °C in the experimental groups. We hypothesized that the cold exposure would maximize the heat dissipation potential, leading to increased milk production and healthier pups even in the hairless mouse model. However, we found the opposite, where cold exposure allowed mothers to eat more food, but they produced smaller weight pups at the end of lactation. Our results suggest that mothers prioritize their own fitness, even if it lowers the fitness of their offspring in this particular mouse strain. This maternal-offspring trade-off is interesting and requires future studies to understand the full interaction of maternal effects and offspring fitness in the light of the heat dissipation limitation.

Fur removal promotes an earlier expression of involution-related genes in mammary gland of lactating mice

Peak lactation occurs when milk production is at its highest. The factors limiting peak lactation performance have been subject of intense debate. Milk production at peak lactation appears limited by the capacity of lactating females to dissipate body heat generated as a by-product of processing food and producing milk. As a result, manipulations that enhance capacity to dissipate body heat (such as fur removal) increase peak milk production. We investigated the potential correlates of shaving-induced increases in peak milk production in laboratory mice. By transcriptomic profiling of the mammary gland, we searched for the mechanisms underlying experimentally increased milk production and its consequences for mother-young conflict over weaning, manifested by advanced or delayed involution of mammary gland. We demonstrated that shaving-induced increases in milk production were paradoxically linked to reduced expression of some milk synthesis-related genes. Moreover, the mammary glands of shaved mice had a gene expression profile indicative of earlier involution relative to unshaved mice. Once provided with enhanced capacity to dissipate body heat, shaved mice were likely to rear their young to independence faster than unshaved mothers.

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.

Manipulation of Heat Dissipation Capacity Affects Avian Reproductive Performance and Output

Animal life requires hard work but the ability to endure such workload appears to be limited. Heat dissipation limit (HDL) hypothesis proposes that the capacity to dissipate the excess of body heat during hard work may limit sustained energy use. Experimental facilitations of heat loss rate via feather-clipping in free-living birds seem to support HDL hypothesis but testing of HDL through laboratory experiments under controlled conditions is not reported. We employed a two-factorial experimental design to test HDL hypothesis by manipulating the capacity to dissipate heat through exposure of captive zebra finches (Taeniopygia guttata) to a cold and warm ambient temperature (14°C and 25°C), and through manipulation of the insulating layer of feathers around the brood patch in females (clipped and unclipped). To simulate foraging costs encountered in the wild we induced foraging effort by employing a feeding system that necessitated hovering to access food, which increased energetic costs of reproduction despite ad libitum conditions in captivity. We quantified the outcome of reproductive performance at the level of both parents, females, and offspring. Thermal limitations due to warm temperature appeared at the beginning of reproduction for both parents with lower egg-laying success, smaller clutch size and lower egg mass, compared to the cold. After hatching, females with an enhanced ability to dissipate heat through feather-clipping revealed higher body mass compared to unclipped females, and these clipped females also raised heavier and bigger nestlings. Higher levels for oxidative stress in plasma of females were detected prior to reproduction in warm conditions than in the cold. However, oxidative stress biomarkers of mothers were neither affected by temperature nor by feather-clipping during the reproductive activities. We document upregulation of antioxidant capacity during reproduction that seems to prevent increased levels of oxidative stress possibly due to the cost of female body condition and offspring growth. Our study on reproduction under laboratory-controlled conditions corroborates evidence in line with the HDL hypothesis. The link between temperature-constrained sustained performance and reproductive output in terms of quality and quantity is of particular interest in light of the current climate change, and illustrates the emerging risks to avian populations.

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.

Significance of variation in basal metabolic rate in laboratory mice for translational experiments

The basal metabolic rate (BMR) accounts for 60–70% of the daily energy expenditure (DEE) in sedentary humans and at least 50% of the DEE in laboratory mice in the thermoneutral zone. Surprisingly, however, the significance of the variation in the BMR is largely overlooked in translational research using such indices as physical activity level (PAL), i.e., the ratio of DEE/BMR. In particular, it is unclear whether emulation of human PAL in mouse models should be carried out within or below the thermoneutral zone. It is also unclear whether physical activity within the thermoneutral zone is limited by the capacity to dissipate heat generated by exercise and obligatory metabolic processes contributing to BMR. We measured PAL and spontaneous physical activity (SPA) in laboratory mice from two lines, divergently selected towards either high or low level of BMR, and acclimated to 30 °C (i.e., the thermoneutral zone), 23 or 4 °C. The mean PAL did not differ between both lines in the mice acclimated to 30 °C but became significantly higher in the low BMR mouse line at the lower ambient temperatures. Acclimation to 30 °C reduced the mean locomotor activity but did not affect the significant difference observed between the selected lines. We conclude that carrying out experiments within the thermoneutral zone can increase the consistency of translational studies aimed at the emulation of human energetics, without affecting the variation in physical activity correlated with BMR.

The effect of aggression II: Acclimation to a high ambient temperature reduces territorial aggression in male striped hamsters (Cricetulus barabensis)

Thyroid hormones have a profound influence on development, cellular differentiation and metabolism, and are also suspected of playing a role in aggression. We measured territorial aggression, body temperature (T_b) and serum thyroid hormones levels of male striped hamsters (Cricetulus barabensis) acclimated to either cold (5 °C), cool (21 °C) or hot (34 °C) ambient temperatures. The effects of methimazole on territorial aggression, food intake, metabolic rate and serum thyroid hormone levels, were also examined. Territorial aggression was significantly lower in male hamsters acclimated to the hot temperature compared to those acclimated to the cool or cold temperatures. T_b significantly increased during aggressive territorial interactions with intruders but did not significantly differ among the three temperature treatments. Serum T₃, T₄ and cortisol levels of hamsters acclimated to 34 °C were significantly lower than those acclimated to 21 °C. In addition to significantly reducing territorial aggression, treatment with methimazole also significantly reduced serum T₃ and T₄ levels, T_b and metabolic rate. These results suggest that exposure to high temperatures reduces the capacity of hamsters to dissipate heat causing them to lower their metabolic rate, which, in turn, causes them to reduce territorial aggression to prevent hyperthermia. The lower metabolic rate mediated by down-regulated thyroid hormones inhibits territorial aggression and could thereby determine the outcome of territorial conflicts.

Limits to sustained energy intake. XXXII. Hot again: dorsal shaving increases energy intake and milk output in golden hamsters (Mesocricetus auratus)

Golden hamsters have four times the body size of mice, raise very large litters and are required to produce large quantities of milk during the 18-day lactation period. We have previously proposed that they may be prone to being limited by their heat dissipation capacity. Studies where lactating females are shaved to elevate their heat dissipation capacity have yielded conflicting data so far. With their short pregnancy of ∼18 days, the large litters and the reported high skin temperatures, they may serve as an ideal model to elucidate the role of epilation for energy budgets in lactating mammals. We shaved one group of lactating females dorsally on the sixth day of lactation, and tested if the elevated heat dissipation capacity would enable them to have higher energy intakes and better food-to-milk conversion rates. Indeed, we observed that females from the shaved group had 6% higher body mass and 0.78°C lower skin temperature than control females during lactation. When focusing on the phase of peak lactation, we observed significantly higher (10%) gross energy intake of food and 23.4% more milk energy output in the shaved females, resulting in 3.3 g higher individual pup weights. We conclude that shaving off the females' fur, even though restricted to the dorsal surface, had large consequences on female energy metabolism in lactation and improved milk production and pup growth in line with our previous work on heat dissipation limitation. Our new data from golden hamsters confirm heat dissipation as a limiting factor for sustained metabolic rate in lactation in some small mammals and emphasise the large effects of a relatively small manipulation such as fur removal on energy metabolism of lactating females.

Coevolution of body size and metabolic rate in vertebrates: a life-history perspective

Despite many decades of research, the allometric scaling of metabolic rates (MRs) remains poorly understood. Here, we argue that scaling exponents of these allometries do not themselves mirror one universal law of nature but instead statistically approximate the non-linearity of the relationship between MR and body mass. This 'statistical' view must be replaced with the life-history perspective that 'allows' organisms to evolve myriad different life strategies with distinct physiological features. We posit that the hypoallometric allometry of MRs (mass scaling with an exponent smaller than 1) is an indirect outcome of the selective pressure of ecological mortality on allocation 'decisions' that divide resources among growth, reproduction, and the basic metabolic costs of repair and maintenance reflected in the standard or basal metabolic rate (SMR or BMR), which are customarily subjected to allometric analyses. Those 'decisions' form a wealth of life-history variation that can be defined based on the axis dictated by ecological mortality and the axis governed by the efficiency of energy use. We link this variation as well as hypoallometric scaling to the mechanistic determinants of MR, such as metabolically inert component proportions, internal organ relative size and activity, cell size and cell membrane composition, and muscle contributions to dramatic metabolic shifts between the resting and active states. The multitude of mechanisms determining MR leads us to conclude that the quest for a single-cause explanation of the mass scaling of MRs is futile. We argue that an explanation based on the theory of life-history evolution is the best way forward.

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.

Exposure to artificial wind increases energy intake and reproductive performance of female Swiss mice (Mus musculus) in hot temperatures

High temperatures and heatwaves are rapidly emerging as an important threat to many aspects of physiology and behavior in females during lactation. The body's capacity to dissipate heat is reduced by high ambient temperatures, increasing the risk of hyperthermia. Exposure to wind, a pervasive environmental factor for most terrestrial animals, is known to increase heat loss, but its effects on the reproductive performance of small mammals remains unclear. In the present study, the effects of wind on the energy budgets, resting metabolic rate and milk energy output (MEO) were measured in lactating Swiss mice at 21 and 32.5°C. Females kept at 32.5°C had a significantly lower resting metabolic rate, food intake and MEO, and lighter offspring, than those kept at 21°C. However, exposure to wind increased the asymptotic food intake of females kept at 32.5°C by 22.5% (P<0.01), their MEO by 20.7% (P<0.05) and their litter mass by 17.6% (P<0.05). The body temperature of females kept at 32.5°C was significantly higher during lactation than that of females kept at 21°C, but this difference was reduced by exposure to wind. These findings suggest that exposure to wind considerably improves reproductive performance, increasing the fitness of small mammals while undergoing hot temperatures during heatwaves.

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.

Exposure to hot temperatures during lactation stunted offspring growth and decreased the future reproductive performance of female offspring

Among the important aspects of climate change, exposure to high temperatures (heat waves) is rapidly emerging as an important issue, in particular for female mammals during lactation. High temperatures adversely impact ability to dissipate heat, which has negative effects on reproductive output. The cumulative effects on growth of F1 offspring after weaning and future reproductive performance of offspring remain uncertain. In this study, the F1 mice that weaned from mothers lactating at 21°C and 32.5°C were housed at 21°C from day 19 till 56 of age; during which food intake and body mass were measured. The F1 adult females that had been weaned at the two temperatures were bred and then both exposed to 32.5°C during lactation. Energy intake, milk output and litter size and mass were determined. The F1 adults weaned at 32.5°C consumed less food and had lower body mass than their counterparts weaned at 21°C. Several visceral organs or reproductive tissues were significantly lower in mass in F1 weaned at 32.5°C than at 21°C. The exposure to 32.5°C significantly decreased energy intake, milk output and litter mass in F1 adult females during lactation. The F1 adult females weaned at 32.5°C produced less milk and raised lighter pups than those previously weaned at 21°C. The data suggest that transient exposure to hot temperature during lactation has long-lasting impacts on the offspring, including stunted growth and decreases in future reproductive performance when adult. This indicates that the offspring of females previously experiencing hot temperatures have a significant fitness disadvantage.