Seasonal adjustments in body mass and thermogenesis in Mongolian gerbils (Meriones unguiculatus): the roles of short photoperiod and cold

Adjustments in energy metabolism of brown adipose tissue in heat-acclimated Kunming mice

The thermogenic capacity of brown adipose tissue (BAT) in rodents decreases with prolonged heat exposure. However, the underlying mechanisms are not well understood. In this study, Kunming mice were acclimated at 23 ± 1 °C and 33 ± 1 °C for four weeks each to examine the body heat balance and BAT alterations. Results showed that heat-acclimated Kunming mice exhibited reduced body mass and elevated body temperature. Additionally, they displayed lower resting metabolic rates, diminished non-shivering thermogenesis, and reduced BAT thermogenic function. Metabolically, there was a significant reduction in several key metabolites involved in energy metabolism in BAT, including thiamine pyrophosphate, citric acid, cis-Aconitate, isocitric acid, oxoglutaric acid, succinate, fumarate, L-Malic acid, oxaloacetate, flavin mononucleotide, nicotinamide adenine dinucleotide, and adenosine 5'-triphosphate. These findings suggest that BAT adapts to heat acclimation by regulating pathways related to pyruvate oxidation, tricarboxylic acid cycle, and oxidative phosphorylation, which may help maintain thermal homeostasis in Kunming mice.

Fat accumulation in striped hamsters (Cricetulus barabensis) reflects the temperature of prior cold acclimation

BACKGROUND

Proper adjustments of metabolic thermogenesis play an important role in thermoregulation in endotherm to cope with cold and/or warm ambient temperatures, however its roles in energy balance and fat accumulation remain uncertain. Our study aimed to investigate the effect of previous cold exposure (10 and 0 °C) on the energy budgets and fat accumulation in the striped hamsters (Cricetulus barabensis) in response to warm acclimation. The body mass, energy intake, resting metabolic rate (RMR) and nonshivering thermogenesis (NST), serum thyroid hormone levels (THs: T3 and T4), and the activity of brown adipose tissue (BAT), indicated by cytochrome c oxidase (COX) activity and uncoupling protein 1 (ucp1) expression, were measured following exposure to the cold (10 °C and 0 °C) and transition to the warm temperature (30 °C).

RESULTS

The hamsters at 10 °C and 0 °C showed significant increases in energy intake, RMR and NST, and a considerable reduction in body fat than their counterparts kept at 21 °C. After being transferred from cold to warm temperature, the hamsters consumed less food, and decreased RMR and NST, but they significantly increased body fat content. Interestingly, the hamsters that were previously exposed to the colder temperature showed significantly more fat accumulation after transition to the warm. Serum T3 levels, BAT COX activity and ucp1 mRNA expression were significantly increased following cold exposure, and were considerably decreased after transition to the warm. Furthermore, body fat content was negatively correlated with serum T3 levels, BAT COX activity and UCP1 expression.

CONCLUSION

The data suggest that the positive energy balance resulting from the decreased RMR and NST in BAT under the transition from the cold to the warm plays important roles in inducing fat accumulation. The extent of fat accumulation in the warm appears to reflect the temperature of the previous cold acclimation.

Warm spells in winter affect the equilibrium between winter phenotypes

Each phenotype is a product of the interaction of the genes and the environment. Although winter phenotype in seasonal mammals is heritable, its development may be modified by external conditions. In today's world, global climate change and increasing frequency of unpredictable weather events may affect the dynamic equilibrium between phenotypes. We tested the effect of changes in ambient temperature during acclimation to short photoperiod on the development of winter phenotypes in three generations of Siberian hamsters (Phodopus sungorus). Based on seasonal changes in fur colour, body mass, and expression of daily torpor we distinguished three different winter phenotypes: responding, non-responding, and partially-responding to short photoperiod. We found that warm spells in winter can increase the proportion of non-responding individuals in the population, while stable winter conditions can increase photoresponsiveness among the offspring of non-responders. We conclude that the polymorphism of winter phenotype is an inherent characteristic of the Siberian hamster population but the development of winter phenotype is not fixed but rather a plastic response to the environmental conditions.

Metabolic and microbial changes in light-vented bulbul during recent northward range expansion

Endotherms recently expanding to cold environments generally exhibit strong physiological acclimation to sustain high body temperature. During this process, gut microbes likely play a considerable role in host physiological functions, including digestion and thermogenesis. The light-vented bulbul Pycnonotus sinensis represents one such species. It used to be restricted to the Oriental realm but expanded its distribution range north to the Palearctic areas during the past few decades. Here, we explored the seasonal dynamics of the resting metabolic rate (RMR) and microbiota for local and newly colonized populations of the species. Our results showed that the mass-adjusted RMR and body mass were positively correlated with latitude variations in both seasons. Consistently, the gut microbiota showed a corresponding variation to the northern cold environments. In the two northern populations, the alpha diversity decreased compared with those of the two southern populations. Significant differences were detected in dominant phyla, such as Firmicutes, Bacteroidetes, Proteobacteria, and Desulfobacterota in both seasons. The core microbiota showed geographic differences in the winter, including the elevated relative abundance of 5 species in northern populations. Finally, to explore the link between microbial communities and host metabolic thermogenesis, we conducted a correlation analysis between microbiota and mass-adjusted RMR. We found that more genera were significantly correlated with mass-adjusted RMR in the wintering season compared to the breeding season (71 vs. 23). These results suggest that microbiota of the lighted-vented bulbul linked with thermogenesis in diversity and abundance under northward expansion.

The big brown bat (Eptesicus fuscus) reduces its body mass during winter in a tropical montane ecosystem of central Mexico

Most animals face changes in the availability of food and the environmental conditions in the places where they live. In response, they need to adjust their behavioral, physiological, and morphological traits. In temperate zones and high latitudes, bats increase their body mass (M b ) in autumn to store fat reserves and use them during hibernation. However, other small mammals decrease their M b prior to winter to reduce the energetic requirements of individuals. These changes are unknown for bats inhabiting other highly energetic demanding environments. We measured changes in M b of 84 non-reproductive males of Eptesicus fuscus inhabiting a tropical montane ecosystem in central Mexico over seasons. We also examined the relationship of bats’ M b with the minimum ambient temperature (T a , °C) and mean precipitation (mm). Bats presented an increase in M b from March to June, followed by a decrease from September to November and presented the lowest M b from November to March, in the dry-cold season. The results suggest that the pattern of changes in M b could be the result of two non-exclusive components related to the bats’ energy budget, the energetic demands experienced by the bats throughout the year and the morphological adaptations animals could display to reduce their energy requirements during the winter.

Seasonal adjustments in body mass and basal thermogenesis in Chinese hwameis (Garrulax canorus): the roles of temperature and photoperiod

For small birds to survive during seasonal acclimatization in temperate zones, regulation of body mass and thermogenesis is crucial. To determine the role of temperature and photoperiod in seasonal changes in body mass and thermogenesis in Chinese hwameis (Garrulax canorus), we compared body mass, basal metabolic rate (BMR), energy intake and cellular metabolic capacity of the tissue (muscle) and/ or organs (liver, kidney, heart and small intestine) in seasonally acclimatized and laboratory acclimated hwameis. A significant seasonal influence on body mass and BMR (which peaked in winter) was found, and these variations were mirrored by exposing the housed birds to cold temperatures or a short photoperiod. The level of dry matter intake, gross energy intake and digestible energy intake were higher during winter, and in housed animals that were exposed to cold temperatures. These results suggest that by increasing energy intake and thermogenesis, Chinese hwameis can overcome winter thermoregulatory challenges. When compared with warm-acclimated birds, cold-acclimated birds displayed higher mass-specific and whole-organ state 4 respiration in the muscle, liver and kidney, and higher mass-specific and whole-organ cytochrome c oxidase activity in the liver. These data demonstrated that the cellular thermogenesis partly underpins basal thermoregulation in Chinese hwameis. Cold temperature and short photoperiod can be used as helpful environmental cues during seasonal acclimatization. However, the role of temperature is more significant compared with that of photoperiod in Chinese hwameis, the changes in energy metabolism and thermoregulation induced by temperature appear to be intensified by photoperiod.

Obligatory homeothermy of mesic adapted African striped mice, Rhabdomys pumilio, is governed by seasonal basal metabolism and year-round "thermogenic readiness" of brown adipose tissue

Small mammals undergo thermoregulatory adjustments in response to changing environmental conditions. Whereas small heterothermic mammals can employ torpor to save energy in the cold, homeothermic species must increase heat production to defend normothermia through the recruitment of brown adipose tissue (BAT). Here, we studied thermoregulatory adaptation in an obligate homeotherm, the African striped mouse (Rhabdomys pumilio), captured from a subpopulation living in a mesic, temperate climate with marked seasonal differences. Basal metabolic rate (BMR), non-shivering thermogenesis (NST) and summit metabolic rate (MSUM) increased from summer to winter, with NST and MSUM already reaching maximal rates in autumn, suggesting seasonal preparation to the cold. Typical of rodents, cold-induced metabolic rates positively correlate with BAT mass. Analysis of cytochrome c oxidase (COX) activity and UCP1 content, however, demonstrate that thermogenic capacity declines with BAT mass. This resulted in seasonal differences in NST being driven by changes in BMR. The increase in BMR is supported by a comprehensive anatomical analysis of metabolically active organs, revealing increased mass proportions in the cold season. The thermoregulatory response of R. pumilio is associated with the maintenance of body weight throughout the year (48.3±1.4 g), contrasting large summer-winter mass reductions often observed in Holarctic rodents. Collectively, bioenergetic adaptation of this Afrotropical rodent involves seasonal organ adjustments influencing BMR, combined with a constant thermogenic capacity dictated by trade-offs in thermogenic properties of BAT. Arguably, this high degree of plasticity was a response to unpredictable cold spells throughout the year. Consequently, the reliance on such a resource intensive thermoregulatory strategy may expose more energetic vulnerability in changing environments of food scarcity and extreme weather conditions due to climate change, with major ramifications for survival of the species.

Cold exposure prevents fat accumulation in striped hamsters refed a high-fat diet following food restriction

Background

In mammals, body mass lost during food restriction is often rapidly regained, and fat is accumulated when ad libitum feeding is resumed. Studies in small cold-acclimated mammals have demonstrated significant mobilization of fat deposits during cold exposure to meet the energy requirements of metabolic thermogenesis. However, no studies to our knowledge have examined the effect of cold exposure on fat accumulation during body mass recovery when refed ad libitum. In this study, striped hamsters restricted to 80% of their regular food intake were then refed ad libitum and exposed to one of three conditions: Intermittent cold temperature (5 °C) for 2 h per day (ICE-2 h/d), intermittent cold temperature (5 °C) for 12 h per day (ICE-12 h/d), or persistent cold exposure (PCE) for four weeks. We measured energy intake, fat deposit mass, serum thyroid hormone levels, and uncoupling protein 1 expression in brown adipose tissue.

Results

There was no significant effect of intermittent or persistent cold exposure on body mass regain, whereas energy intake increased significantly and total fat deposit decreased in the ICE-12 h/d and PCE groups compared to the ICE-2 h/d group and control group maintained at 23 °C (CON). In the ICE-12 h/d and PCE groups, hamsters had 39.6 and 38.3% higher serum 3,3′,5-triiodothyronine levels, respectively, and 81.6 and 71.3% up-regulated expression of uncoupling protein 1, respectively, in brown adipose tissue compared to their counterparts in the CON group. The rate of mitochondrial state III and state IV respiration O2 consumption and the activity of cytochrome c oxidase in BAT and liver were significantly higher in the ICE-12 h/d and PCE groups than in the ICE-2 h/d and CON groups.

Conclusions

Our findings suggest thyroid hormone-mediated heat production in brown adipose tissue and liver may be involved in preventing fat accumulation during refeeding in animals frequently or persistently exposed to cold conditions.

Phenotypic plasticity to chronic cold exposure in two species of Peromyscus from different environments

Effective thermoregulation is important for mammals, particularly those that remain winter-active. Adjustments in thermoregulatory capacity in response to chronic cold can improve capacities for metabolic heat production (cold-induced maximal oxygen consumption, [Formula: see text]), minimize rates of heat loss (thermal conductance), or both. This can be challenging for animals living in chronically colder habitats where necessary resources (i.e., food, O₂) for metabolic heat production are limited. Here we used lowland native white-footed mice (Peromyscus leucopus) and highland deer mice (P. maniculatus) native to 4300 m, to test the hypothesis that small winter-active mammals have evolved distinct cold acclimation responses to tailor their thermal physiology based on the energetic demands of their environment. We found that both species increased their [Formula: see text] after cold acclimation, associated with increases in brown adipose tissue mass and expression of uncoupling protein 1. They also broadened their thermoneutral zone to include lower ambient temperatures. This was accompanied by an increase in basal metabolic rate but only in white-footed mice, and neither species adjusted thermal conductance. Unique to highland deer mice was a mild hypothermia as ambient temperatures decreased, which reduced the gradient for heat loss, possibly to save energy in the chronically cold high alpine. These results highlight that thermal acclimation involves coordinated plasticity of numerous traits and suggest that small, winter-active mammals may adjust different aspects of their physiology in response to changing temperatures to best suit their energetic and thermoregulatory needs.

Variations of body mass and thermogenesis properties in Eothenomys olitor during cold acclimatization

The set-point hypothesis states that there is a biological control method in mammals that actively regulates weight toward a predetermined set weight for each individual, which may occur by regulation of energy intake or energy expenditure. In order to investigate the effects of low temperature on body mass regulation in Eothenomys olitor, body mass, body fat mass, food intake, resting metabolic rate (RMR), non-shivering thermogenesis (NST), serum leptin levels, morphology, biochemical indexes of liver and brown adipose tissue (BAT) and hypothalamic neuropeptide genes expression were measured during cold acclimatization. The results showed that there was no significant difference in body mass, but food intake, RMR and NST increased during cold acclimatization. Cytochrome c oxidase (COX) and α-glycerophosphate oxidase (α-GPO) activities in liver and BAT were significantly enhanced during cold acclimatization, and triiodothyronine (T3) and thyroxine (T4) levels in serum were significantly higher than those in the control group. Serum leptin levels decreased after cold acclimatization. Low temperature significantly increased the expression of neuropeptide Y (NPY) and agouti-related peptide (AgRP), while it decreased cocaine- and amphetamine-regulated transcript peptide (CART) and pro-opiomelanocortin (POMC) expressions. All of the above results suggested that body mass in E. olitor can remain relatively stable at low temperature, which conforms to the ‘set-point hypothesis’. However, the species showed differences with sympatric species, such as E. miletus, Apodemus chevrieri and Tupaia belangeri. Moreover, E. olitor can cope with low temperature by increasing its metabolic rate and thermogenesis properties.

Cecal microbial transplantation attenuates hyperthyroid-induced thermogenesis in Mongolian gerbils

Endothermic mammals have a high energy cost to maintain a stable and high body temperature (T_b , around 37°C). Thyroid hormones are a major regulator for energy metabolism and T_b . The gut microbiota is involved in modulating host energy metabolism. However, whether the interaction between the gut microbiota and thyroid hormones is involved in metabolic and thermal regulations is unclear. We hypothesized that thyroid hormones via an interaction with gut microbiota orchestrate host thermogenesis and T_b . l-thyroxine-induced hyperthyroid Mongolian gerbils (Meriones unguiculatus) increased resting metabolic rate (RMR) and T_b , whereas Methimazole-induced hypothyroid animals decreased RMR. Both hypothyroid and hyperthyroid animals differed significantly in faecal bacterial community. Hyperthyroidism increased the relative abundance of pathogenic bacteria, such as Helicobacter and Rikenella, and decreased abundance of beneficial bacteria Butyricimonas and Parabacteroides, accompanied by reduced total bile acids and short-chain fatty acids. Furthermore, the hyperthyroid gerbils transplanted with the microbiota from control donors increased type 2 deiodinase (DIO2) expression in the liver and showed a greater rate of decline of both serum T3 and T4 levels and, consequently, a more rapid recovery of normal RMR and T_b . These findings indicate that thyroid hormones regulate thermogenesis depending on gut microbiota and colonization with normal microbiota by caecal microbial transplantation attenuates hyperthyroid-induced thermogenesis. This work reveals the functional consequences of the gut microbiota-thyroid axis in controlling host metabolic physiology and T_b in endotherms.

Gut Microbiota and Host Thermoregulation in Response to Ambient Temperature Fluctuations

Ambient temperature (T_a ) is an important factor in shaping phenotypic plasticity. Plasticity is generally beneficial for animals in adapting to their environments. Gut microbiota are crucial in regulating host physiological and behavioral processes. However, whether the gut microbiota play a role in regulating host phenotypic plasticity under the conditions of repeated fluctuations in environmental factors has rarely been examined. We used intermittent T_a acclimations to test the hypothesis that the plasticity of gut microbiota confers on the host a metabolic adaptation to T_a fluctuations. Mongolian gerbils (Meriones unguiculatus) were acclimated to intermittent 5°C to 23°C, 37°C to 23°C or 23°C to 23°C conditions for 3 cycles (totally 3 months). Intermittent T_a acclimations induced variations in resting metabolic rate (RMR), serum thyroid hormones, and core body temperature (T_b ). We further identified that the β-diversity of the microbial community varied with T_a and showed diverse responses during the 3 cycles. Some specific bacteria were more sensitive to T_a and were associated with host dynamic metabolic plasticity during T_a acclimations. In addition, depletion of gut microbiota in antibiotic-treated gerbils impaired metabolic plasticity, particularly at low T_a , whereas supplementation with propionate as an energy resource improved the inhibited thermogenic capacity and increased the survival rate in the cold. These findings demonstrate that both gut microbiota and their host were more adaptive after repeated acclimations, and dynamic gut microbiota and their metabolites may confer host plasticity in thermoregulation in response to T_a fluctuations. It also implies that low T_a is a crucial cue in driving symbiosis between mammals and their gut microbiota during evolution.IMPORTANCE Whether gut microbiota play a role in regulating host phenotypic plasticity in small mammals living in seasonal environments has rarely been examined. The present study, through an intermittent temperature acclimation model, indicates that both gut microbiota and their host were more adaptive after repeated acclimations. It also demonstrates that dynamic gut microbiota confer host plasticity in thermoregulation in response to intermittent temperature fluctuations. Furthermore, low temperature seems to be a crucial cue in driving the symbiosis between mammals and their gut microbiota during evolution.

Continuous growth through winter correlates with increased resting metabolic rate but does not affect daily energy budgets due to torpor use

Small mammals that are specialists in homeothermic thermoregulation reduce their self-maintenance costs of normothermy to survive the winter. By contrast, heterothermic ones that are considered generalists in thermoregulation can lower energy expenditure by entering torpor. It is well known that different species vary the use of their strategies to cope with harsh winters in temperate zones; however, little is still known about the intraspecific variation within populations and the associated external and internal factors. We hypothesized that yellow-necked mice Apodemus flavicollis decrease their resting metabolic rate (RMR) from autumn to winter, and then increase it during spring. However, since the alternative for seasonal reduction of RMR could be the development of heterothermy, we also considered the use of this strategy. We measured body mass (m_b), RMR, and body temperature (T_b) of mice during 2 consecutive years. In the 1st year, mice decreased whole animal RMR in winter, but did not do so in the 2nd year. All mice entered torpor during the 2nd winter, whereas only a few did so during the first one. Mice showed a continuous increase of m_b, which was steepest during the 2nd year. The relationship between RMR and m_b varied among seasons and years most likely due to different mouse development stages. The m_b gain at the individual level was correlated positively with RMR and heterothermy. This indicates that high metabolism in winter supports the growth of smaller animals, which use torpor as a compensatory mechanism. Isotope composition of mice hair suggests that in the 1st year they fed mainly on seeds, while in the 2nd, they likely consumed significant amounts of less digestible herbs. The study suggests that the use of specialist or generalist thermoregulatory strategies can differ with environmental variation and associated differences in developmental processes.

GnRH expression and cell proliferation are associated with seasonal breeding and food hoarding in Mongolian gerbils (Meriones unguiculatus)

Seasonal brain plasticity contributes to a variety of physiological and behavioral processes. We hypothesized that variations in GnRH expression and cell proliferation facilitated seasonal breeding and food hoarding. Here, we reported seasonal changes in sexual and social behavior, GnRH expression and brain cell proliferation, and the role of photoperiod in inducing seasonal breeding and brain plasticity in Mongolian gerbils (Meriones unguiculatus). The gerbils captured in April and July had more mature sexual development, higher exploratory behavior, and preferred novelty much more than those captured in September. Male gerbils captured in April and July had consistently higher GnRH expression than those captured in September. GnRH expression was also found to be suppressed by food-induced hoarding behavior in the breeding season. Both subadult and adult gerbils from April and July had higher cell proliferation in SVZ, hypothalamus and amygdala compared to those in September. However, adult gerbils captured in September preferred familiar objects, and no seasonal differences were found in cell proliferation in hippocampal dentate gyrus among the three seasons. The laboratory study showed that photoperiod alone did not alter reproductive traits, behavior, cell proliferation or cell survival in the detected brain regions. These findings suggest that the structural variations in GnRH expression in hypothalamus and cell proliferation in hypothalamus, amygdala and hippocampus are associated with seasonal breeding and food hoarding in gerbils. It gives a new insight into the proximate physiological and neural basis for these seasonal life-history traits of breeding and food hoarding in small mammals.

Brown adipose tissue plays thermoregulatory role within the thermoneutral zone in Mongolian gerbils (Meriones unguiculatus)

Brown adipose tissue (BAT) plays an important role in thermoregulation and many metabolic processes in small mammals, especially in cold adaptation. However, in warm adaptation, ambient temperature cannot directly activate BAT by sympathetic nervous system. Mongolian gerbils exhibit a wider thermoneutral zone (26.5-38.9 °C). We hypothesized that BAT atrophied near the lower critical temperature and further atrophied near the upper critical temperature. Male gerbils were acclimated to 23 °C, 27 °C or 37 °C, respectively, for 3 weeks. Results showed that regulatory non-shivering thermogenesis did not change in gerbils acclimated to 27 °C compared with 23 °C group, whereas it was reduced by 43.5% in gerbils acclimated to 37 °C. Bigger lipid droplet in BAT was observed in gerbils acclimated to 27 °C and 37 °C compared with 23 °C group, while the expression of uncoupling protein 1 and tyrosine hydroxylase was only reduced in gerbils acclimated to 37 °C. Further, thermoneutral acclimation did not change BAT thermogenesis by down-regulation of peroxisome proliferator-activated receptor gamma coactivator-1α, PR domain containing 16, peroxisome proliferator-activated receptor-α or peroxisome proliferator activated receptor-γ gene expression in BAT. In addition, body temperature was reduced in gerbils acclimated to 37 °C compared with 23 °C group, which was associated with a decreased resting metabolic rate and regulatory non-shivering thermogenesis. In conclusion, BAT does not atrophy near the lower critical temperature, whereas it atrophies near the upper critical temperature, suggesting that BAT may play thermoregulatory role within the TNZ in Mongolian gerbils.

Inter-group associations in Mongolian gerbils: Quantitative evidence from social network analysis

Animals often interact non-randomly with conspecifics, and association preferences can differ across life-history stages to maximize individuals' fitness. Mongolian gerbils (Meriones unguiculatus) are a social rodent that live in highly seasonal habitats and display seasonal fluctuations in population density, growth rate and the size of overlapped home ranges. Nevertheless, whether gerbils modify their social relationships at different life-history stages remains unknown. Here, we used social network analysis to examine whether social associations differ between the sexes and between life-history stages in a wild population of Mongolian gerbils. We quantified social attributes at both group level (assortativity) and individual level (social differentiation and degree, closeness and betweenness centrality); these attributes reflect individuals' social preferences and their potential influence on others in the network. We found that both male and female gerbils established fewer inter-group social connections during the food-hoarding season than during the breeding season, revealing constraints on sociality. Similarly, during the food-hoarding season, degree centrality and social differentiation increased significantly whereas closeness and betweenness centrality decreased significantly. Together, these results suggest that gerbils have relatively more partners and preferred associations and decreased influence over others in the network during the food-hoarding season. In addition, we found no significant difference in any of the social attribute between males and females, but there was a significant interaction effect between sex and season on degree, closeness and betweenness centrality. Our results demonstrate that Mongolian gerbils adjust their association strategies to adapt to the changes of life history. Such adjustments may balance the costs/benefits associated with survival and reproduction.

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.

The roles of metabolic thermogenesis in body fat regulation in striped hamsters fed high-fat diet at different temperatures

The metabolic thermogenesis plays important roles in thermoregulation, and it may be also involved in body fat regulation. The thermogenesis of brown adipose tissue (BAT) is largely affected by ambient temperature, but it is unclear if the roles in body fat regulation are dependent on the temperature. In the present study, uncoupling protein 1 (ucp1)-based BAT thermogenesis, energy budget and body fat content were examined in the striped hamsters fed high fat diet (HF) at cold (5°C) and warm (30°C) temperatures. The effect of 2, 4-dinitrophenol (DNP), a chemical uncoupler, on body fat was also examined. The striped hamsters showed a notable increase in body fat following the HF feeding at 21°C. The increased body fat was markedly elevated at 30°C, but was significantly attenuated at 5°C compared to that at 21°C. The hamsters significantly increased energy intake at 5°C, but consumed less food at 30°C relative to those at 21°C. Metabolic thermogenesis, indicated by basal metabolic rate, UCP1 expression and/or serum triiodothyronine levels, significantly increased at 5°C, but decreased at 30°C compared to that at 21°C. A significant decrease in body fat content was observed in DNP-treated hamsters relative to the controls. These findings suggest that the roles of metabolic thermogenesis in body fat regulation largely depend on ambient temperature. The cold-induced enhancement of BAT thermogenesis may contribute the decreased body fat, resulting in a lean mass. Instead, the attenuation of BAT thermogenesis at the warm may result in notable obesity.

How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms

Adaptive explanations for both high and low body mass-independent basal metabolic rate (BMR) in endotherms are pervasive in evolutionary physiology, but arguments implying a direct adaptive benefit of high BMR are troublesome from an energetic standpoint. Here, we argue that conclusions about the adaptive benefit of BMR need to be interpreted, first and foremost, in terms of energetics, with particular attention to physiological traits on which natural selection is directly acting. We further argue from an energetic perspective that selection should always act to reduce BMR (i.e., maintenance costs) to the lowest level possible under prevailing environmental or ecological demands, so that high BMR per se is not directly adaptive. We emphasize the argument that high BMR arises as a correlated response to direct selection on other physiological traits associated with high ecological or environmental costs, such as daily energy expenditure (DEE) or capacities for activity or thermogenesis. High BMR thus represents elevated maintenance costs required to support energetically demanding lifestyles, including living in harsh environments. BMR is generally low under conditions of relaxed selection on energy demands for high metabolic capacities (e.g., thermoregulation, activity) or conditions promoting energy conservation. Under these conditions, we argue that selection can act directly to reduce BMR. We contend that, as a general rule, BMR should always be as low as environmental or ecological conditions permit, allowing energy to be allocated for other functions. Studies addressing relative reaction norms and response times to fluctuating environmental or ecological demands for BMR, DEE, and metabolic capacities and the fitness consequences of variation in BMR and other metabolic traits are needed to better delineate organismal metabolic responses to environmental or ecological selective forces.

Implication of metabolomic profiles to wide thermoneutral zone in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) have evolved a wide thermoneutral zone (26.5-38.9 °C) and high upper critical temperature, and appear to have a high tolerance for heat exposure. Here, we use a metabolomic approach to measure global metabolite profiles for gerbils between lower (27 °C) and upper critical temperatures (38 °C) to investigate the role of metabolomic characterization in maintaining basal metabolic rates within a wide thermoneutral zone. We found that in serum and liver, 14 and 19 metabolites were significantly altered, respectively. In the aerobic respiration-related tricarboxylic cycle (TCA), 5 intermediates (isocitric acid, cis-aconitic acid, α-ketoglutaric acid, fumaric acid and malic acid) were increased in serum in 38 °C animals; however, no such increase was found in the liver. A stable level of hepatic TCA cycle intermediates may be related to the steady state of aerobic respiration at 38 °C. Metabolomic results also revealed that acute heat exposure caused increased oxidative stress and low molecular weight antioxidants in Mongolian gerbils. Increased methionine and 2-hydroxybutyrate suggest an accelerated synthesis of glutathione. Increased urate and its precursors, inosine and hypoxanthine, were detected at 38 °C. Glucuronate, threonate and oxalate involved in ascorbate synthesis and degradation were increased in serum at 38 °C. In conclusion, although dramatic metabolomic variation was found, a stable hepatic TCA cycle may contribute to maintaining a constant basal metabolic rate within a wide thermoneutral zone in Mongolian gerbils.

Role of thermal physiology and bioenergetics on adaptation in tree shrew (Tupaia belangeri): the experiment test

Ambient conditions, as temperature and photoperiod, play a key role in animals’ physiology and behaviors. To test the hypothesis that the maximum thermal physiological and bioenergetics tolerances are induced by extreme environments in Tupaia belangeri. We integrated the acclimatized and acclimated data in several physiological, hormonal, and biochemical markers of thermogenic capacity and bioenergetics in T. belangeri. Results showed that T. belangeri increased body mass, thermogenesis capacity, protein contents and cytochrome c oxidase (COX) activity of liver and brown adipose tissue in winter-like environments, which indicated that temperature was the primary signal for T. belangeri to regulate several physiological capacities. The associated photoperiod signal also elevated the physiological capacities. The regulations of critical physiological traits play a primary role in meeting the survival challenges of winter-like condition in T. belangeri. Together, to cope with cold, leptin may play a potential role in thermogenesis and body mass regulation, as this hormonal signal is associated with other hormones. The strategies of thermal physiology and bioenergetics differs between typical Palearctic species and the local species. However, the maximum thermal physiology and bioenergetic tolerance maybe is an important strategy to cope with winter-like condition of T. belangeri.

Effect of melatonin different time administration on the development of diet-induced obesity in rats

In recent years much attention has been paid for study of the melatonin use possibilities for improving obesity comorbidities. The aim of our study was to determine the influence of melatonin different time treatment on body weight changes of dietinduced obesity in rats. The administration by gavage of melatonin in dose 30 mg/kg for 7 weeks had the potential to decrease visceral fat weight, Lee index (both after morning and evening treatment) and body weight gain rate (only after evening dose).

Photoperiod induced obesity in the Brandt's vole (Lasiopodomys brandtii): a model of 'healthy obesity'?

Brandt's voles have an annual cycle of body weight and adiposity. These changes can be induced in the laboratory by manipulation of photoperiod. In the present study, male captive-bred Brandt's voles aged 35 days were acclimated to a short day (SD) photoperiod (8L:16D) for 70 days. A subgroup of individuals (n=16) were implanted with transmitters to monitor physical activity and body temperature. They were then randomly allocated into long day (LD=16L:8D) (n=19, 8 with transmitters) and SD (n=18, 8 with transmitters) groups for an additional 70 days. We monitored aspects of energy balance, glucose and insulin tolerance (GTT and ITT), body composition and organ fat content after exposure to the different photoperiods. LD voles increased in weight for 35 days and then re-established stability at a higher level. At the end of the experiment LD-exposed voles had greater white adipose tissue mass than SD voles (P=0.003). During weight gain they did not differ in their food intake or digestive efficiency; however, daily energy expenditure was significantly reduced in the LD compared with SD animals (ANCOVA, P<0.05) and there was a trend to reduced resting metabolic rate RMR (P=0.075). Physical activity levels were unchanged. Despite different levels of fat storage, the GTT and ITT responses of SD and LD voles were not significantly different, and these traits were not correlated to body fatness. Hence, the photoperiod-induced obesity was independent on disruptions to glucose homeostasis, indicating a potential adaptive decoupling of these states in evolutionary time. Fat content in both the liver and muscle showed no significant difference between LD and SD animals. How voles overcome the common negative aspects of fat storage might make them a useful model for understanding the phenomenon of 'healthy obesity'.

The increase in fat content in the warm-acclimated striped hamsters is associated with the down-regulated metabolic thermogenesis

It has been well known that metabolic thermogenesis plays an important role in the thermoregulation of small mammals under different temperatures, while its role in fat accumulation is far from clear. In the present study, several physiological, hormonal, and biochemical measures indicative of metabolic thermogenesis were measured in the weaning striped hamsters after acclimated to a warm condition (30°C) for 1, 3 and 4months. The warm-acclimated groups significantly decreased energy intake, and simultaneously decreased nonshivering thermogenesis compared to those housed at 21°C. Body fat content increased by 29.9%, 22.1% and 19.6% in the hamsters acclimated to 1, 3 or 4months, respectively relative to their counterparts maintain at 21°C (P<0.05). The cytochrome c oxidase (COX) activity of brain, liver, heart and skeletal muscle, and the ratio of serum tri-iodothyronine to thyroxine significantly decreased in warm-acclimated groups compared with 21°C group. COX activity and uncoupling protein 1 (UCP1) mRNA expression of brown adipose tissue (BAT) were significantly down-regulated under the warm conditions. COX activity of BAT, liver, heart and muscle were significantly negatively correlated with body fat content, and the correlation between UCP1 expression and body fat content tended to be negative. These findings suggest that the decrease in the energy spent on metabolic thermogenesis plays an important role in the fat accumulation. The attenuation of COX and UCP1-based BAT activity may be involved in body fat accumulation in animals under warm conditions.

Effects of random food deprivation and refeeding on energy metabolism, behavior and hypothalamic neuropeptide expression in Apodemus chevrieri

Maintaining adaptive control of behavior and physiology is the main strategy used by animals in responding to changes of food resources. To investigate the effects of random food deprivation (FD) and refeeding on energy metabolism and behavior in Apodemus chevrieri, we acclimated adult males to FD for 4weeks, then refed them ad libitum for 4weeks (FD-Re group). During the period of FD, animals were fed ad libitum for 4 randomly assigned days each week, and deprived of food the other 3days. A control group was fed ad libitum for 8weeks. At 4 and 8weeks we measured body mass, thermogenesis, serum leptin levels, body composition, gastrointestinal tract morphology, behavior and hypothalamic neuropeptide expression. At 4weeks, food intake, gastrointestinal mass, neuropeptide Y (NPY) and agouti-related protein (AgRP) mRNA expressions increased and thermogenesis, leptin levels, pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART) expressions decreased in FD compared with controls. FD also showed more resting behavior and less activity than the controls on ad libitum day. There were no differences between FD-Re and controls at 8weeks, indicating significant plasticity. These results suggested that animals can compensate for unpredictable reduction in food availability by increasing food intake and reducing energy expended through thermogenesis and activity. Leptin levels, NPY, AgRP, POMC, and CART mRNA levels may also regulate energy metabolism. Significant plasticity in energy metabolism and behavior was shown by A. chevrieri over a short timescale, allowing them to adapt to food shortages in nutritionally unpredictable environments.

Leptin regulates energy intake but fails to facilitate hibernation in fattening Daurian ground squirrels (Spermophilus dauricus)

Body fat storage before hibernation affects the timing of immergence in Daurian ground squirrels (Spermophilus dauricus). Leptin is an adipose signal and plays vital role in energy homeostasis mainly by action in brain. To test the hypothesis that leptin plays a role in facilitating the process of hibernation, squirrels were administrated with recombinant murine leptin (1μg/day) through intracerebroventricular (ICV) injection for 12 days during fattening. From day 7 to 12, animals were moved into a cold room (5±1°C) with constant darkness which functioned as hibernaculum. Energy intake, body mass and core body temperature (Tb) were continuously monitored throughout the course of experiment. Resting metabolic rate (RMR) was measured under both warm and cold conditions. At the end of leptin administration, we measured the serum concentration of hormones related to energy regulation, mRNA expression of hypothalamic neuropeptides and uncoupling protein 1 (UCP1) levels in brown adipose tissue (BAT). Our results showed that during leptin administration, the cumulative food intake and increase of body mass were suppressed while Tb and RMR were unaltered. The proportion of torpid squirrels was not different between two groups. At the end of leptin administration, the expressions of hypothalamic neuropeptide Y and agouti gene-related protein were suppressed. There were no differences in UCP1 mRNA expression or protein content in BAT between groups. Our data suggest that leptin can affect energy intake via hypothalamic neuropeptides, but is not involved in the initiation of hibernation in fattening Daurian ground squirrels.

Brown adipose tissue: physiological function and evolutionary significance

In modern eutherian (placental) mammals, brown adipose tissue (BAT) evolved as a specialized thermogenic organ that is responsible for adaptive non-shivering thermogenesis (NST). For NST, energy metabolism of BAT mitochondria is increased by activation of uncoupling protein 1 (UCP1), which dissipates the proton motive force as heat. Despite the presence of UCP1 orthologues prior to the divergence of teleost fish and mammalian lineages, UCP1's significance for thermogenic adipose tissue emerged at later evolutionary stages. Recent studies on the presence of BAT in metatherians (marsupials) and eutherians of the afrotherian clade provide novel insights into the evolution of adaptive NST in mammals. In particular studies on the 'protoendothermic' lesser hedgehog tenrec (Afrotheria) suggest an evolutionary scenario linking BAT to the onset of eutherian endothermy. Here, we review the physiological function and distribution of BAT in an evolutionary context by focusing on the latest research on phylogenetically distinct species.

Environmental metabolomics reveal geographic variation in aerobic metabolism and metabolic substrates in Mongolian gerbils (Meriones unguiculatus)

Mongolian gerbils (Meriones unguiculatus) have a large-scale distribution in northern China. Geographic physiological variations which related to energy and water metabolism are critical to animals' local adaptation and distribution. However, the underlying biochemical mechanism of such variation and its role in adaptation remains largely unknown. We used GC-MS metabolomics approach to investigate the biochemical adaptation of Mongolian gerbils from xeric (desert), transition (desert steppe) and mesic (typical steppe) environments. Gerbils in desert population had lower resting metabolic rate (RMR) and total evaporative water loss (TEWL) than mesic population. Serum metabolomics revealed that concentrations of five tricarboxylic acid cycle intermediates (citrate, cis-aconitate, α-ketoglutarate, fumarate and malate) were lower in desert population than mesic population. Gastrocnemius metabolomics and citrate synthase activity analysis showed a lower concentration of citrate and lower citrate synthase activity in desert population. These findings suggest that desert dwelling gerbils decrease RMR and TEWL via down-regulation of aerobic respiration. Gastrocnemius metabolomics also revealed that there were higher concentrations of glucose and glycolytic intermediates, but lower concentrations of lipids, amino acids and urea in desert population than mesic population. This geographic variation in metabolic substrates may enhance metabolic water production per oxygen molecule for desert population while constraining aerobic respiration to reduce RMR and TEWL.

The role of leptin in striped hamsters subjected to food restriction and refeeding

Food restriction (FR) and refeeding (Re) have been suggested to impair body mass regulation and thereby making it easier to regain the lost weight and develop over-weight when FR ends. However, it is unclear if this is the case in small mammals showing seasonal forging behaviors. In the present study, energy budget, body fat and serum leptin level were measured in striped hamsters that were exposed to FR-Re. The effects of leptin on food intake, body fat and genes expressions of several hypothalamus neuropeptides were determined. Body mass, fat content and serum leptin level decreased during FR and then increased during Re. Leptin supplement significantly attenuated the increase in food intake during Re, decreased genes expressions of neuropepetide Y (NPY) and agouti-related protein (AgRP) of hypothalamus and leptin of white adipose tissue (WAT). Hormone-sensitive lipase (HSL) gene expression of WAT increased in leptin-treated hamsters that were fed ad libitum, but decreased in FR-Re hamsters. This indicates that the adaptive regulation of WAT HSL gene expression may be involved in the mobilization of fat storage during Re, which partly contributes to the resistance to FR-Re-induced overweight. Leptin may be involved in the down regulations of hypothalamus orexigenic peptides gene expression and consequently plays a crucial role in controlling food intake when FR ends.

Seasonal changes of body mass and energy budget in striped hamsters: the role of leptin

Proper adjustments of physiology and behavior are required for small mammals to cope with seasonal climate change. The aim of this study was to examine the role of leptin in the regulation of body mass and energy budget in striped hamsters. We first investigated seasonal changes in body mass, energy budget, and serum leptin levels in hamsters acclimated to outdoor natural daylight and ambient temperature. Then we assessed the effect of leptin administration on energy budget, serum lipoprotein lipase (LPL) and hepatic lipase (HL) activities, and gene expression of uncoupling protein 1 (UCP1) in brown adipose tissue and of hypothalamic neuropeptides associated with the regulation of energy balance in hamsters maintained at 21° and 5°C. Hamsters showed constant body mass throughout the four seasons but significantly increased food intake and thermogenesis in winter, compared to summer. Minimum body fat was observed in winter, and minimum serum leptin was found in autumn. Hamsters housed at 5°C showed higher energy intake, upregulated gene expression of UCP1 and hormone-sensitive lipase, and lower fat content and LPL and HL activity than the animals maintained at 21°C. Leptin administration had no effect on energy intake but increased maximal thermogenic capacity, as indicated by upregulated UCP1 gene expression at both 21° and 5°C. Body fat and activity of LPL and HL were decreased in hamsters treated with leptin. The results suggest that leptin plays an important role in the seasonal regulation of thermogenic capacity and body composition in striped hamsters. Leptin may be involved in increasing maximal thermogenesis in the cold rather than acting as a starvation signal to increase energy intake.

Energy budget, behavior and leptin in striped hamsters subjected to food restriction and refeeding

Food restriction induces a loss of body mass that is often followed by rapid regaining of the lost weight when the restriction ends, consequently increasing a risk of development of obesity. To determine the physiological and behavioral mechanisms underlining the regaining, striped hamsters were restricted to 85% of initial food intake for 4 weeks and refed ad libitum for another 4 weeks. Changes in body mass, energy budget, activity, body composition and serum leptin level were measured. Body mass, body fat mass and serum leptin level significantly decreased in food-restricted hamsters, and increased when the restriction ended, showing a short “compensatory growth” rather than over-weight or obesity compared with ad libitum controls. During restriction, the time spent on activity increased significantly, which was opposite to the changes in serum leptin level. Food intake increased shortly during refeeding, which perhaps contributed to the rapid regaining of body mass. No correlation was observed between serum leptin and energy intake, while negative correlations were found in hamsters that were refed for 7 and 28 days. Exogenous leptin significantly decreased the time spent on activity during food restriction and attenuated the increase in food intake during refeeding. This suggests that low leptin in restricted animals may function as a starvation signal to induce an increase in activity behavior, and high leptin likely serves as a satiety signal to prevent activity during refeeding. Leptin may play a crucial role in controlling food intake when the restriction ends, and consequently preventing overweight.

Environmental challenges and physiological solutions: comparative energetic daily rhythms of field mice populations from different ecosystems

Daily and seasonal variations in physiological characteristics of mammals can be considered adaptations to temporal habitat variables. Across different ecosystems, physiological adjustments are expected to be sensitive to different environmental signals such as changes in photoperiod, temperature or water and food availability; the relative importance of a particular signal being dependent on the ecosystem in question. Energy intake, oxygen consumption (VO₂) and body temperature (T_b) daily rhythms were compared between two populations of the broad-toothed field mouse Apodemus mystacinus, one from a Mediterranean and another from a sub-Alpine ecosystem. Mice were acclimated to short-day (SD) ‘winter’ and long-day (LD) ‘summer’ photoperiods under different levels of salinity simulating osmotic challenges. Mediterranean mice had higher VO₂ values than sub-Alpine mice. In addition, mice exposed to short days had higher VO₂ values when given water with a high salinity compared with mice exposed to long days. By comparison, across both populations, increasing salinity resulted in a decreased T_b in SD- but not in LD-mice. Thus, SD-mice may conserve energy by decreasing T_b during (‘winter’) conditions which are expected to be cool, whereas LD-mice might do the opposite and maintain a higher T_b during (‘summer’) conditions which are expected to be warm. LD-mice behaved to reduce energy expenditure, which might be considered a useful trait during ‘summer’ conditions. Overall, increasing salinity was a clear signal for Mediterranean-mice with resultant effects on VO₂ and T_b daily rhythms but had less of an effect on sub-Alpine mice, which were more responsive to changes in photoperiod. Results provide an insight into how different populations respond physiologically to various environmental challenges.

Seasonal and photoperiodic effects on lipid droplet size and lipid peroxidation in the brown adipose tissue of bank voles (Myodes glareolus)

Seasonal changes in lipid droplet size and lipid peroxidation in the brown adipose tissue (BAT) of wild bank voles were examined. In addition, a role of photoperiod in these changes was studied; bank voles were held from the birth under long photoperiod (LP) for 12 weeks, and then half of them was transferred to short photoperiod (SP) for 6 weeks and another one remained under LP. In the wild bank voles the absolute BAT weight was seasonally constant, while the significant differences in the lipid droplet size were observed. The smallest lipid droplets (mean, 11 μm(2)) were seen in winter; they increased by 30 % in spring and reached the highest size (24 μm(2)) in summer. Lipid peroxidation in the BAT did not differ significantly between the seasons, although high intraseason variation of this process was noted. The laboratory experiment revealed that the size of lipid droplets was determined by photoperiod; SP induced 13-fold decrease, and continuous exposure to LP brought about a further 2.5-fold increase in the size of lipid droplets. Conversely, a significant decrease in lipid peroxidation was seen in LP bank voles in comparison with the SP animals. The data indicate that short photoperiod is responsible for the small size of lipid droplets in the BAT of bank voles during winter, which may be a necessary requirement for high thermogenic capacity of the tissue. Photoperiod appears also to affect lipid peroxidation in the BAT of these animals.

Effect of cold exposure on energy budget and thermogenesis during lactation in Swiss mice raising large litters

In Swiss mice sustained energy intake (SusEI) during peak lactation has been previously suggested to be constrained by the capacity of the mammary glands to produce milk, supporting the “peripheral limitation” hypothesis. Here we experimentally examined if SusEI in these mice was not only limited peripherally but also constrained by the ability to dissipate heat. Female Swiss mice were provided with additional offspring above their natural litter sizes and were maintained during lactation either in warm (23°C) or cold (5°C) conditions. Food intake, thermogenesis, litter size and mass, and the weight of the mammary glands were measured. No differences were observed in asymptotic food intake at peak lactation, litter mass and thermogenesis between females raising litters of different size. Cold-exposed females increased food intake and thermogenic capacity, but weaned significantly smaller and lighter litters with smaller pup sizes compared with females in warm conditions. The weight of the mammary glands did not differ between warm and cold-exposed females, but within temperatures was positively related to litter mass. These data suggested that cold exposure increased food intake, but had no effect on the capacity of the mammary glands to secret milk because they were already working maximally in the females raising larger litters. The factors causing this limit in the mammary capacity remain elusive.

Gerbils

The gerbil is usually nonaggressive and is one of the easiest rodents to maintain and handle. Its disposition, curious nature, relative freedom from naturally occurring infectious diseases, and adaptability to its environment have contributed to its popularity as a laboratory animal. Gerbils are found in deserts and semiarid geographical regions of the world. The Mongolian gerbils that are available today originated from 20 pairs of captured animals that were maintained in 1935 in a closed, random-bred colony at the Kitasato Institute in Japan. Gerbils have several unique anatomical and physiological features. Mature gerbils are smaller than rats, but larger than mice. Mongolian gerbils are attracted to saliva and use salivary cues to discriminate between siblings and nonsiblings, and females use oral cues in the selection of sociosexual partners. Gerbils have been used as experimental models in a number of areas of biomedical research. Gerbils are excellent subjects for laboratory animal research as they are susceptible to bacterial, viral, and parasitic pathogens that affect humans and other species. Gerbils may have spontaneous seizures secondary to stress such as handling, cage change, abrupt noises, or changes in the environment. Cystic ovaries are seen commonly in female gerbils over 1 year of age. Gerbils have unique characteristics, which make them appropriate for a number of animal models. Classically, gerbils have been used in research involving stroke, parasitology, infectious diseases, epilepsy, brain development and behavior, and hearing.