Changes in body mass and fat reserves in pre-hibernating little brown bats (Myotis lucifugus)

Heterothermy, and the energetic consequences of huddling in small migrating passerine birds

The success of migration of small passerine birds depends largely on effective refueling at stopover sites. In our previous studies, we found that hypothermia facilitates accumulation of fuel at the beginning of a stopover. Later we found that blackcaps, Sylvia atricapilla, might further reduce their energy expenditure by huddling while at rest. Here, we report experimental results supporting our hypothesis that huddling is beneficial to small migrating passerines both from energetic and thermoregulatory points of view. To test this hypothesis we measured metabolic rates and body temperatures of seven blackcaps placed in respirometry chambers overnight, either solitarily or in groups of three or four at ambient temperatures of 5, 10, and 15°C. Concurring with our predictions, huddling blackcaps maintained higher body temperatures than did solitary birds, but had mass-specific metabolic rates lower by ∼30% than those of solitary individuals. Based on our previous studies, we estimated energy savings through huddling to be comparable to energy savings through hypothermia in solitary blackcaps and suggest that huddling may be an important way of saving energy for small passerine birds resting at migratory stopovers. At the same time it might offer the additional benefit of lower risk of predation. In this light, we predict that huddling occurs frequently in nature, leading to significant savings of energy, faster accumulation of fuel, presumably lower risk of becoming a prey, more successful migration, and eventually increased fitness.

Increased circulating leptin level inhibits folliculogenesis in vespertilionid bat, Scotophilus heathii

The present study investigates the mechanism by which obesity associated rise in leptin and insulin levels cause anovulation in vespertilionid bat, Scotophilus heathii. In the ovary of S. heathii, leptin and insulin receptors were mainly localized in interstitial and thecal cells and in the granulosa cells of primary follicles suggesting its possible role in androgen synthesis and follicular development. Adiposity associated increase in circulating leptin level down regulate ovarian LH-receptor expression and produce characteristic morphological changes in the antral follicles, such as hypertrophy of granulosa cells and a sharp decline in the rate of proliferation as well as apoptosis in the antral follicles. These follicles are referred as unique antral follicle. The in vitro study confirmed the in vivo findings that the high dose of leptin suppresses apoptosis and LH receptors. The present study thus showed that the adiposity associated increase in leptin during the first phase of follicular development inhibits folliculogenesis and simultaneously suppresses both follicular proliferation and apoptosis by reducing sensitivity to gonadotropin stimulation and decreasing circulating LH levels.

Fit females and fat polygynous males: seasonal body mass changes in the grey-headed flying fox

When females and males differ in their timing of maximum reproductive effort, this can result in sex-specific seasonal cycles in body mass. Such cycles are undoubtedly under strong selection, particularly in bats, where they affect flying ability. Flying foxes (Old World fruit bats, Pteropus spp.) are the largest mammals that can sustain powered flight and therefore face critical trade-offs in managing body reserves for reproduction, yet little is known about body mass dynamics in this group. I investigated body mass changes in relation to reproductive behaviour in a large colony of grey-headed flying foxes (Pteropus poliocephalus). In this polygynous mammal, females were predicted to maximise reproductive effort during lactation and males during the breeding season. As predicted, female body condition declined during the nursing period, but did not vary in relation to sexual activity. By contrast, males accumulated body reserves prior to the breeding season, but subsequently lost over 20% of their body mass on territory defence and courtship, and lost foraging opportunities as they also defended their day roost territories at night. Males in better condition had larger testes, particularly during territory establishment, prior to maximum sexual activity. Thus, the seasonality of female mass reflected the high metabolic load that lactation imposes on mothers. However, male mass followed a pattern akin to the "fatted male phenomenon", which is commonly observed in large polygynous mammals with seasonal reproduction, but not in bats. This shows the importance of body reserves for reproduction in flying foxes, despite their severe constraints on body mass.

Role of leptin in seasonal adiposity associated changes in testicular activity of vespertilionid bat, Scotophilus heathi

The aim of this study was to investigate changes in circulating level of leptin in natural population of male bat, Scotophilus heathi in relation to seasonal changes in body mass, adipose tissue mass and testicular activity. The peak serum leptin level coincided with the period of maximum body mass due to heavy accumulation of adipose tissue. The in vivo study showed both inhibitory and stimulatory effects on testicular activity depending on circulating level of leptin. Moderate increase in circulating concentration of leptin coincided with the stimulatory effect on testicular activity in October, whereas peak circulating concentration of leptin coincided with suppressed testicular activity in December. Immunohistochemical and Western blot analysis confirmed expression of leptin receptors (Ob-R) in the testis, particularly in Leydig cells during different reproductive stages suggesting the direct action of leptin in regulation of testicular steroidogenesis. The in vitro study confirmed the in vivo observations of inhibitory effect of leptin on testosterone production through dose-dependent StAR expression in the testis. This study further showed that the leptin affects testicular activity through STAT3 mediated regulation of StAR expression. Thus, increased adiposity associated rise in circulating leptin level during winter dormancy impairs testicular activity by suppressing StAR mediated testosterone production in S. heathi. These results are suggestive of a link between the adipocyte derived leptin and testicular function.

White-nose syndrome fungus (Geomyces destructans) in bat, France

White-nose syndrome is caused by the fungus Geomyces destructans and is responsible for the deaths of >1,000,000 bats since 2006. This disease and fungus had been restricted to the northeastern United States. We detected this fungus in a bat in France and assessed the implications of this finding.

Grand challenges in migration biology

Billions of animals migrate each year. To successfully reach their destination, migrants must have evolved an appropriate genetic program and suitable developmental, morphological, physiological, biomechanical, behavioral, and life-history traits. Moreover, they must interact successfully with biotic and abiotic factors in their environment. Migration therefore provides an excellent model system in which to address several of the "grand challenges" in organismal biology. Previous research on migration, however, has often focused on a single aspect of the phenomenon, largely due to methodological, geographical, or financial constraints. Integrative migration biology asks 'big questions' such as how, when, where, and why animals migrate, which can be answered by examining the process from multiple ecological and evolutionary perspectives, incorporating multifaceted knowledge from various other scientific disciplines, and using new technologies and modeling approaches, all within the context of an annual cycle. Adopting an integrative research strategy will provide a better understanding of the interactions between biological levels of organization, of what role migrants play in disease transmission, and of how to conserve migrants and the habitats upon which they depend.

Seasonal variation in prey abundance influences habitat use by greater horseshoe bats (Rhinolophus ferrumequinum) in a temperate deciduous forest

The hypothesis that patterns of habitat selection of greater horseshoe bats ( Rhinolophus ferrumequinum (Schreber, 1774)) vary across seasons in a temperate deciduous forest was investigated. Variables associated with potentially important ecological factors for greater horseshoe bats (physical structure of shrub stratum, crown canopy, insect availability, lunar phase, and weather) were collected for different seasons, and 75 sampling sites were established in the Luotong Mountain Nature Reserve in northeast China. Insect abundance was highest in late summer and lowest in late autumn. Poisson generalized linear models showed that the activity of greater horseshoe bats was positively related to the height and density of shrub stratum in late summer, whereas the activity of greater horseshoe bats was associated with insect abundance in early and late autumn. During periods of intermediate prey abundance (early summer), the height and density of shrub stratum, as well as insect abundance, influenced the activity of greater horseshoe bats. Shrub stratum may provide shelter against predation for foraging greater horseshoe bats. These results support our prediction that there was a trade-off between importance of food and cover among seasons for foraging bats. These findings are useful for the conservation and management of greater horseshoe bats.

Determining feeding state and rate of mass change in insectivorous bats using plasma metabolite analysis

Insectivorous bats regularly experience dramatic and sometimes rapid changes in nutrient stores, yet our ability to study these changes has been limited by available techniques. Plasma metabolite analysis has proven effective for studying individual rates of mass change in birds but has not been validated for other taxa. We tested the effectiveness of plasma metabolite analysis by conducting a study with captive big brown bats (Eptesicus fuscus) and little brown bats (Myotis lucifugus) in the field. In the lab, we varied food availability to induce various rates of mass change. As predicted, individual rate of mass change was positively correlated with plasma triglyceride concentration, but there was no relationship with plasma beta-hydroxybutyrate concentration, whereas such a relationship has been found in birds. In the field, we collected blood samples from postlactating females as they emerged in the evening (fasted) and when they returned from feeding in the morning. Plasma triglyceride concentration was greater in fed bats than fasted bats, and the increase was less when rain limited foraging. Contrary to predictions, beta-hydroxybutyrate concentration was also greater in fed bats than fasted bats. Analysis of plasma triglyceride concentration provides a technique for assessing individual feeding state and rate of mass change of bats and will facilitate further study of bat nutritional ecology and energetics.

Effects of pulsatile secretion of growth hormone (GH) on fat deposition in human GH transgenic rats

Growth hormone (GH) is an endocrine regulator of glucose and lipid metabolism as well as body growth. GH levels are decreased and a unique pulsatile secretory pattern becomes obvious after puberty particularly in males. Coincidentally with this, males tend to deposit body fat. Experimental and clinical evidence has accumulated that obesity is associated with a decrease in GH levels. A strain of transgenic rats has been generated with severe obesity but normal nose-to-tail length, which has low circulating GH levels without pulsatility (human growth hormone (hGH) transgenic rats). The present review mainly focuses on recent and current work analysing the relationship between the occurrence of obesity and low GH levels and/or the absence of GH pulsatility in this transgenic animal model. This model has elevated blood glucose, non-esterified fatty acid, insulin and leptin levels associated with hyperphagia, suggesting that these rats also carry insulin- and leptin-resistant characteristics. hGH transgenic rats were subjected to a pair-feeding treatment to normalize food intake and chronic GH replacement to normalize GH levels. While the pair-feeding for 8 weeks successfully suppressed body-weight gain, the fat pad : body weight ratio remained very similar to freely-eating control hGH transgenic rats, which indicates the hyperphagia is not the sole contributor to the excess fat accumulation in this model. However, continuous elevation of peripheral hGH levels (approximately 2-fold) for 8 weeks by means of a slow-release vehicle resulted in a significant decrease in the fat mass : body weight ratios by 30 %. This GH treatment altered neither food intake nor body-weight gain. Thus, two characteristic phenotypes observed in the hGH transgenic rats, hyperphagia and obesity, seem to be closely related to GH levels and GH secretory pattern. This relationship might be working in the regulation of changes in seasonal body composition in wild animals.

Influence of climate and reproductive timing on demography of little brown myotis Myotis lucifugus

1. Estimating variation in demographic rates, such as survival and fecundity, is important for testing life-history theory and identifying conservation and management goals. 2. We used 16 years (1993-2008) of mark-recapture data to estimate age-specific survival and breeding probabilities of the little brown myotis Myotis lucifugus LeConte in southern New Hampshire, USA. Using Kendall & Nichols' (1995) full-likelihood approach of the robust design to account for temporary emigration, we tested whether survival and breeding propensity is influenced by regional weather patterns and timing of reproduction. 3. Our results demonstrate that adult female survival of M. lucifugus ranged from 0.63 (95% CL = 0.56, 0.68) to 0.90 (95% CL = 0.77, 0.94), and was highest in wet years with high cumulative summer precipitation. First-year survival [range: 0.23 (95% CL = 0.14, 0.35) to 0.46 (95% CL = 0.34, 0.57)] was considerably lower than adult survival and depended on pup date of birth, such that young born earlier in the summer (c. late May) had a significantly higher probability of surviving their first year than young born later in the summer (c. mid-July). Similarly, the probability of young females returning to the maternity colony to breed in the summer following their birth year was higher for individuals born earlier in the summer [range: 0.23 (95% CL = 0.08, 0.50) to 0.53 (95% CL = 0.30, 0.75)]. 4. The positive influence of early parturition on 1st-year survival and breeding propensity demonstrates significant fitness benefits to reproductive timing in this temperate insectivorous bat. 5. Climatic factors can have important consequences for population dynamics of temperate bats, which may be negatively affected by summer drying patterns associated with global climate change. 6. Understanding long-term demographic trends will be important in the face of a novel disease phenomenon (White-Nose Syndrome) that is associated with massive mortalities in hibernating bat species, including M. lucifugus, in the northeastern United States.

Effect of age on energy storage during prehibernation swarming in little brown bats (Myotis lucifugus)

At temperate latitudes insectivorous bats face substantial nutritional demands prior to hibernation. As temperature decreases and availability of insect prey declines, bats must deposit nutrient stores for hibernation. The use of torpor allows bats to limit energy expenditures resulting in a net energy gain despite decreased energy intake. However, subadult bats have lower initial fat stores than adults and may have greater difficulty depositing sufficient nutrient stores to survive the winter. We used plasma metabolite analysis to determine the fueling performance of little brown bats ( Myotis lucifugus (LeConte, 1831)) during swarming to see if subadults compensated for the increased challenges by increased feeding throughout the swarming period. During the period of our study (August and September), adult bats gained mass, while subadults lost mass. There was, however, no difference in nutrient intake of the age groups as indicated by plasma metabolite concentrations. The number of bats using torpor while roosting in the hibernaculum by day increased exponentially coincident with the onset of mating and a decrease in nutrient intake. The results are consistent with wild bats using torpor to minimize energy expenditure and compensate for lower nutrient intake. The difference in mass change for adults and subadults despite the same nutrient intake indicates that subadults incur greater energetic costs.

Bird or bat: comparing airframe design and flight performance

Birds and bats have evolved powered flight independently, which makes a comparison of evolutionary 'design' solutions potentially interesting. In this paper we highlight similarities and differences with respect to flight characteristics, including morphology, flight kinematics, aerodynamics, energetics and flight performance. Birds' size range is 0.002-15 kg and bats' size range is 0.002-1.5 kg. The wingbeat kinematics differ between birds and bats, which is mainly due to the different flexing of the wing during the upstroke and constraints by having a wing of feathers and a skin membrane, respectively. Aerodynamically, bats appear to generate a more complex wake than birds. Bats may be more closely adapted for slow maneuvering flight than birds, as required by their aerial hawking foraging habits. The metabolic rate and power required to fly are similar among birds and bats. Both groups share many characteristics associated with flight, such as for example low amounts of DNA in cells, the ability to accumulate fat as fuel for hibernation and migration, and parallel habitat-related wing shape adaptations.

Effect of food availability and leptin on the physiology and hypothalamic gene expression of the golden spiny mouse: a desert rodent that does not hoard food

Food availability and quality in desert habitats are spatially and temporally unpredictable, and animals face periods of food shortage. The golden spiny mouse ( Acomys russatus) is an omnivorous desert rodent that does not hoard food, requiring it to withstand such periods by physiological means alone. In response to food restriction, plasma leptin concentrations, core body temperature, and energy expenditure of the spiny mouse decrease significantly after 24 h, and most spiny mice are able to maintain their body mass to ∼85% of ad libitum for a prolonged period of time. Both 1-day food deprivation and long-term food restriction had a significant effect on body mass and plasma leptin concentrations, which decreased significantly with a high correlation, as well as on the orexigenic agouti-related protein, which increased significantly as a result of the 24-h food deprivation; and on neuropeptide Y (NPY), in which the increase was more pronounced under long-term food restriction. Food restriction and food deprivation had no effect, however, on the anorexigenic pro-opiomelanocortin and cocaine and amphetamine-related transcript. Leptin administration to food-restricted spiny mice did not affect food intake or the rate of decrease in body mass, indicating that it cannot overcome the drive to eat when food is scarce. However, it did result in a significant decrease in NPY levels, and the spiny mice spent less time at low body temperatures compared with PBS-treated golden spiny mice. These results show that in food-restricted golden spiny mice, leptin affects thermogenesis, but not food consumption, and suggest that the thermoregulatory effects of leptin are mediated by NPY.

Beyond mtDNA: nuclear gene flow suggests taxonomic oversplitting in the little brown bat (Myotis lucifugus)

In southern Alberta and north-central Montana, there is substantial mtDNA sequence divergence between two groups of the little brown bat, Myotis lucifugus (LeConte, 1831), previously thought to be subspecies ( Myotis lucifugus lucifugus and Myotis lucifugus carissima ) but recently hypothesized to be species. We tested this hypothesis using population genetic techniques. Using nuclear microsatellite markers (10 loci), we found a lack of differentiation between these two groups of bats (ascribed based on mitochondrial hypervariable region II sequence), suggesting interbreeding was sympatric. Our findings add to the recent discovery that M. l. lucifugus haplotypes are found throughout the range previously thought to be only M. l. carissima, suggesting widespread sympatry and extensively mixed gene pools, thus refuting the cryptic species hypothesis. Clinal morphology and individual variation demonstrated the impossibility to differentiate groups based on original subspecies definitions. The lack of geographic and morphological boundaries, in addition to the likelihood that the interbreeding observed in this study is occurring across western North America, suggests that no line can be drawn between these two groups. We thus suggest that the carissima subspecies designation be dropped. This study highlights the importance of investigating nuclear gene flow in widely sympatric animals suspected of being cryptic genetic species, and has important implications for applications of the DNA Barcoding Project.

Energy availability influences microclimate selection of hibernating bats

Many species hibernate to conserve energy during periods of low food and water availability. It has long been assumed that the optimal hibernation strategy involves long, deep bouts of torpor that minimize energy expenditure. However, hibernation has ecological (e.g. decreased predator avoidance) and physiological (e.g. sleep deprivation) costs that must be balanced with energy savings; therefore, individuals possessing sufficient energy reserves may reduce their use of deep torpor. We tested the hypothesis that energy (fat)availability influences temperature selection of two fat-storing bat species during hibernation. We predicted that individuals with small energy reserves would select colder temperatures for hibernation in order to minimize energy expenditure, while individuals with larger energy reserves would choose warmer temperatures to minimize the costs of hibernation. Results from our field experiment indicate that little brown myotis (Myotis lucifugus)hibernating in warm microclimates were significantly heavier than individuals hibernating in cooler microclimates. To determine if energy availability was mediating this relationship, we limited fatty acid availability with mercaptoacetate (MA) and quantified its effect on torpid metabolic rate(TMR) and thermal preference of big brown bats (Eptesicus fuscus). Administration of MA caused a 43% drop in TMR at 10°C and caused bats to choose significantly colder temperatures for hibernation. Our results suggest that fat-storing bats minimize torpor expression using both physiological and behavioral mechanisms.

Relationship between delayed embryonic development and metabolic factors and fat deposition in fruit bat Cynopterus sphinx

The present study was undertaken in the fruit bat Cynopterus sphinx, which breeds twice in quick succession at Varanasi, India. Its gestation period varies significantly in the two successive pregnancies of the year owing to delayed embryonic development during the first (winter) pregnancy. The primary aim of the present study was to determine the role of metabolic factors in delayed embryonic development in the fruit bat C. sphinx. Variation in bodyweight, fat deposition, oxygen (O2) consumption rate, basal metabolic rate (BMR), body temperature (Tb) and hepatic succinate dehydrogenase (SDH) activity, along with circulating levels of thyroid hormones (tri-iodothyronine and thyroxine), were examined as metabolic factors during the two successive pregnancies in C. sphinx. The increase in bodyweight observed in November was due to accumulation of white adipose tissue in the posterior abdominal region. A significant decline in O2 consumption rate, BMR, Tb and SDH activity was found in early winter in November–December, which coincides closely with the period of fat accumulation and with the period of delayed embryonic development in C. sphinx. A significantly higher O2 consumption rate, BMR, Tb and SDH activity was noted during the second pregnancy in, when embryonic development was relatively faster. Thyroid hormone levels were high during the period of embryonic delay compared with levels during the remaining months. The results of the present study suggest that the delayed embryonic development in C. sphinx during early winter may be due to a low O2 consumption rate, BMR, Tb and SDH activity in November–December. The energy saved by suppressing embryonic development in this species may be advantageous for fat accumulation. Increased thyroid hormone levels during the early winter period might facilitate fat accumulation in C. sphinx.

Variation in body composition of female big brown bats (Eptesicus fuscus) during lactation

Most small mammals support the nutritional requirements of milk production by increasing food intake. However, when nutrient intake is low, maternal body reserves may be mobilized to maintain adequate milk output. We examined patterns of body composition, including dry matter, fat, protein, and mineral content in big brown bats, Eptesicus fuscus, during lactation. Concentrations of fat and phosphorus were markedly lower in lactating mothers during week three of lactation than during the first two weeks, but these constituents rebounded to previous levels in the fourth and fifth week. Rapid recovery from fat depletion suggests that females are able to adjust to changes in demands for energy. The decrease in phosphorus during mid-lactation suggests bone demineralization, but an interspecific comparison of adult concentrations of minerals prevalent in bone suggests that mineral concentrations may never reach critically low levels in reproductively active females.

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

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

Thermal physiology of pregnant and lactating female and male long-eared bats, Nyctophilus geoffroyi and N. gouldi

During roosting in summer, reproductive female bats appear to use torpor less frequently and at higher body temperatures (Tb) than male bats, ostensibly to maximise offspring growth. To test whether field observations result from differences in thermal physiology or behavioural thermoregulation during roosting, we measured the thermoregulatory response and energetics of captive pregnant and lactating female and male long-eared bats (Nyctophilus geoffroyi 8.9 g and N. gouldi 11.5 g) during overnight exposure to a constant ambient temperature (Ta) of 15 degrees C. Bats were captured 1-1.5 h after sunset and measurements began at 21:22+/-0:36 h. All N. geoffroyi entered torpor commencing at 23:47+/-01:01 h. For N. gouldi, 10/10 males, 9/10 pregnant females and 7/8 lactating females entered torpor commencing at 01:10+/-01:40 h. The minimum Tb of torpid bats was 15.6+/-1.1 degrees C and torpid metabolic rate (TMR) was reduced to 0.05+/-0.02 ml O2 g(-1) h(-1). Sex or reproductive condition of either species did not affect the timing of entry into torpor (F=1.5, df=2, 19, P=0.24), minimum TMR (F=0.21, df=4, 40, P=0.93) or minimum Tb (F=0.76, df=5, 41, P=0.58). Moreover, sex or reproductive condition did not affect the allometric relationship between minimum resting metabolic rate and body mass (F=1.1, df=4, 37, P=0.37). Our study shows that under identical thermal conditions, thermal physiology of pregnant and lactating female and male bats are indistinguishable. This suggests that the observed reluctance by reproductive females to enter torpor in the field is predominantly because of ecological rather than physiological differences, which reflect the fact that females roost gregariously whereas male bats typically roost solitarily.

Histochemical localization of enzymes and lipids in the ovary of a vespertilionid bat, Scotophilus heathi, during the reproductive cycle

The present study describes seasonal changes in delta5 3beta hydroxysteroid dehydrogenase (3beta-HSD), glusose-6 phosphates dehydrogenase (G-6-PD), and lipids in the ovary of a vespertilionid bat, Scotophilus heathi. Total lipids and 3beta-HSD activity are restricted to thecal and interstitial cells of the ovary. The total lipids, 3beta-HSD, and G-6-PD significantly increase during recrudescence, and remain high during winter dormancy and breeding as compared to the other reproductive phases. High incidence of lipids and enzyme activity in interstitial cells during the breeding period and at the time of ovulation clearly suggests that these cells are actively involved in steroidogenesis. A decline in enzymes and lipid activity during winter dormancy, which correlates with the declining levels of steroidogenesis, might be the factors responsible for prolonged survival of the Graafian follicle in the ovary of S. heathi.

Variation in the reproductive rate of bats

In many respects, bats have relatively slow life histories. However, the reproductive rate of bats (i.e., the proportion of females that reproduce in any breeding season) has not been critically examined. We compiled data on the reproductive rates of bats to test predictions based on life-history theory. Among 257 samples from 103 species, reproductive rate varied considerably and was typically under 100%. Temperate-zone species had significantly lower and more variable reproductive rates than did tropical species. Reproductive rate also varied among families, with species in the Vespertilionidae having particularly high rates. As predicted based on life-history theory, reproductive rate was negatively correlated with longevity, and among vespertilionids, species with larger litters had higher reproductive rates. Thus, the data suggest that bats have relatively slow reproductive rates and, as in other life-history traits, fall at the "slow" end of the fast–slow life-history continuum found among mammals. Female bats, especially those in temperate regions, appear to adjust their allocation of resources to reproduction, and at times forego reproduction, perhaps in relation to their body condition, prey availability, and weather conditions.

Changes in baseline and stress-induced glucocorticoid levels during the active period in free-ranging male and female little brown myotis, Myotis lucifugus (Chiroptera: Vespertilionidae)

Baseline and stress-responsive glucocorticoid (GC) levels were characterized during the active period in free-ranging male and reproductive female little brown myotis (Myotis lucifugus). Bats were trapped and blood was sampled within 3 min of capture at two maternity sites during the summer and at one swarming site prior to hibernation in New England. Both GC hormones, cortisol and corticosterone, were detected, with cortisol accounting for an average of approximately 95% of total circulating GCs. Samples collected at the dusk emergence and after the first return from feeding showed significant seasonal differences across the active period (early pregnancy, mid-to-late pregnancy, lactation [and comparable mid-summer times for males], and pre-hibernation) within and between each sex. Elevated baseline values were found in mid-to-late pregnancy females at emergence, and in both males and females at the swarming site compared to other groups. Female GC values during mid-to-late pregnancy and during the pre-hibernation period were greater than those for males. Significantly higher GC levels following 15 min of restraint were exhibited by all animals in the summer and prior to hibernation. There was little variation between groups or sexes in the total GC levels reached following restraint. Taken together, these results suggest that: (1) GCs may be involved in the increased feeding and/or fat deposition characteristic of pregnancy and the pre-hibernation period, (2) GCs may be related to mating and to the generally increased levels of activity that occur during the pre-hibernation period, and (3) regardless of sex or reproductive condition, all animals maximally respond to restraint stress.

Body mass changes in male Daubenton's bats Myotis daubentonii (Chiroptera, Vespertilionidae) during the seasonal activity period

Body mass changes of adult and juvenile male Daubenton’s bats were studied in a summer habitat in Hesse (Germany) in the years 1998-2001. Animals from the resident population were regularly captured along the flight path from April to October, and body mass was determined using a digital balance. Body mass of adult individuals differed significantly among fortnightly periods (Kruskal-Wallis ANOVA by ranks: H = 141.75, p < 0.001). Body mass increased significantly from the second half of April (median: 7.0 g) to the first half of May (median: 7.8 g). Between the second half of May and the first half of August, median body mass fluctuated between 7.8 g and 8.3 g. From the second half of July (median: 7.9 g) to the second half of August (median: 8.4 g), and the second half of September (median: 9.3 g) body mass again significantly increased. The highest median body mass (9.6 g) was recorded in the second half of October. Median body mass of juvenile males ranged between 6.5 g in the first half of July and 8.5 g in the second half of October. Differences in body mass over the studied period were significant (H = 21.70, p < 0.01). During the months July, August, and September, body mass of adult males was significantly higher than that of juveniles, whereas in October the difference was no longer of statistical significance. It is suggested that the decrease in body mass observed in some adult males in September was related to their reproductive activity.

The role of energy availability in Mammalian hibernation: a cost-benefit approach

Hibernation is widely regarded as an adaptation to seasonal energy shortage, but the actual influence of energy availability on hibernation patterns is rarely considered. Here we review literature on the costs and benefits of torpor expression to examine the influence that energy may have on hibernation patterns. We first establish that the dichotomy between food- and fat-storing hibernators coincides with differences in diet rather than body size and show that small or large species pursuing either strategy have considerable potential scope in the amount of torpor needed to survive winter. Torpor expression provides substantial energy savings, which increase the chance of surviving a period of food shortage and emerging with residual energy for early spring reproduction. However, all hibernating mammals periodically arouse to normal body temperatures during hibernation. The function of these arousals has long been speculated to involve recovery from physiological costs accumulated during metabolic depression, and recent physiological studies indicate these costs may include oxidative stress, reduced immunocompetence, and perhaps neuronal tissue damage. Using an optimality approach, we suggest that trade-offs between the benefits of energy conservation and the physiological costs of metabolic depression can explain both why hibernators periodically arouse from torpor and why they should use available energy to minimize the depth and duration of their torpor bouts. On the basis of these trade-offs, we derive a series of testable predictions concerning the relationship between energy availability and torpor expression. We conclude by reviewing the empirical support for these predictions and suggesting new avenues for research on the role of energy availability in mammalian hibernation.

Climate-mediated energetic constraints on the distribution of hibernating mammals

To predict the consequences of human-induced global climate change, we need to understand how climate is linked to biogeography. Energetic constraints are commonly invoked to explain animal distributions, and physiological parameters are known to vary along distributional gradients. But the causal nature of the links between climate and animal biogeography remain largely obscure. Here we develop a bioenergetic model that predicts the feasibility of mammalian hibernation under different climatic conditions. As an example, we use the well-quantified hibernation energetics of the little brown bat (Myotis lucifugus) to parameterize the model. Our model predicts pronounced effects of ambient temperature on total winter energy requirements, and a relatively narrow combination of hibernaculum temperatures and winter lengths permitting successful hibernation. Microhabitat and northern distribution limits of M. lucifugus are consistent with model predictions, suggesting that the thermal dependence of hibernation energetics constrains the biogeography of this species. Integrating projections of climate change into our model predicts a pronounced northward range expansion of hibernating bats within the next 80 years. Bioenergetics can provide the simple link between climate and biogeography needed to predict the consequences of climate change.

Seasonal changes in circulating steroid concentration and their correlation with the ovarian activity in the female Indian sheath-tailed bat, Taphozous longimanus

The relationship between ovarian activity and circulating steroid concentration was studied in the female sheath-tailed bat, Taphozous longimanus. T. longimanus breeds twice in rapid succession during the year at Varanasi, India. Ovarian recrudescence was observed during September, and antral follicles were first observed during the month of October. Circulating androstenedione concentration showed an increase beginning in October, reaching a peak in December. This increase in androstenedione concentration correlated with the period of heavy accumulation of adipose tissue and increase in body mass. Antral follicles grow slowly during the period of high circulating androstenedione concentration from October to December. There was a sharp decline in androstenedione concentration during January. Simultaneously with the decline in androstenedione concentration, a sharp increase in size of the antral follicle and circulating estradiol concentration was noticed. Soon thereafter, one of the follicles ruptures, followed by fertilization and the commencement of the first pregnancy. During the second pregnancy, antral follicles first appeared during late pregnancy in March in the contralateral ovary that lacked the corpus luteum. One follicle developed rapidly and quickly attained a preovulatory stage in April. This is reflected in a sharp increase in estradiol concentration during this period. Ovulation was observed immediately following the first pregnancy in May. During this period, androstenedione concentration remained low. The results of the present study suggest that high androstenedione concentration during October to December (winter dormancy) may be responsible for slow follicular development and delays ovulation in T. longimanus. It is further hypothesized that the geographical variation in reproductive pattern of T. longimanus could be due to variation in the duration of fat storage and associated changes in the androstenedione concentration.

Dissociation of leptin secretion and adiposity during prehibernatory fattening in little brown bats

Hibernating animals deposit adipose tissue before hibernation to withstand long periods of reduced energy intake. Normally, adiposity is positively correlated with increased secretion from adipose tissue of the satiety hormone, leptin. During the prehibernatory phase of the little brown bat, Myotis lucifugus, body mass and adiposity increased to a maximum within 12 days. Leptin secretion from adipose tissue in vitro and plasma leptin, however, increased before the increase in adiposity, then significantly decreased when adiposity increased. Basal metabolic rate (BMR) decreased when plasma leptin was increasing. This was followed by an increase in nonshivering thermogenic capacity and brown adipose tissue mass. We conclude that in the early prehibernatory phase, BMR decreases despite increasing plasma leptin levels, suggesting a state of relative leptin resistance at that time. At later stages, adiposity increases as BMR continues to decrease, and plasma leptin becomes dissociated from adiposity. Thus, in M. lucifugus, hibernation may be achieved partly by removing the metabolic signal of leptin during the fattening period of prehibernation.