Late Hibernation Research Articles

Specific alterations in complement protein activity of little brown myotis (Myotis lucifugus) hibernating in white-nose syndrome affected sites.

White-nose syndrome (WNS) is the most devastating condition ever reported for hibernating bats, causing widespread mortality in the northeastern United States. The syndrome is characterized by cutaneous lesions caused by a recently identified psychrophilic and keratinophylic fungus (Geomyces destructans), depleted fat reserves, atypical behavior, and damage to wings; however, the proximate cause of mortality is still uncertain. To assess relative levels of immunocompetence in bats hibernating in WNS-affected sites compared with levels in unaffected bats, we describe blood plasma complement protein activity in hibernating little brown myotis (Myotis lucifugus) based on microbicidal competence assays using Escherichia coli, Staphylococcus aureus and Candida albicans. Blood plasma from bats collected during mid-hibernation at WNS-affected sites had higher bactericidal ability against E. coli and S. aureus, but lower fungicidal ability against C. albicans when compared with blood plasma from bats collected at unaffected sites. Within affected sites during mid-hibernation, we observed no difference in microbicidal ability between bats displaying obvious fungal infections compared to those without. Bactericidal ability against E. coli decreased significantly as hibernation progressed in bats collected from an affected site. Bactericidal ability against E. coli and fungicidal ability against C. albicans were positively correlated with body mass index (BMI) during late hibernation. We also compared complement activity against the three microbes within individuals and found that the ability of blood plasma from hibernating M. lucifugus to lyse microbial cells differed as follows: E. coli>S. aureus>C. albicans. Overall, bats affected by WNS experience both relatively elevated and reduced innate immune responses depending on the microbe tested, although the cause of observed immunological changes remains unknown. Additionally, considerable trade-offs may exist between energy conservation and immunological responses. Relationships between immune activity and torpor, including associated energy expenditure, are likely critical components in the development of WNS.

Myostatin levels in skeletal muscle of hibernating ground squirrels

Myostatin, a negative regulator of muscle mass, is elevated during disuse and starvation. Mammalian hibernation presents a unique scenario, where animals are hypocaloric and in torpor, but the extent of muscle protein loss is minimized. We hypothesized that myostatin expression, which is usually increased early in disuse and under hypocaloric conditions, could be suppressed in this unique model. Skeletal muscle was collected from thirteen-lined ground squirrels, Spermophilus tridecemlineatus, at six time points during hibernation: control euthermic (CON); entrance into hibernation (ENT), body temperature (T(b)) falling; early hibernation (EHib), stable T(b) in torpor for 24 h; late hibernation (LHib), stable T(b) in torpor for 3 days; early arousal (EAr), T(b) rising; and arousal (AR), T(b) restored to 34-37°C for about 18 h. There was no significant increase of myostatin during ENT, EHib or LHib. Unexpectedly, there were approximately sixfold increases in myostatin protein levels as squirrels arose from torpor. The elevation during EAr remained high in AR, which represented an interbout time period. Mechanisms that could release the suppression or promote increased levels of myostatin were assessed. SMAD2 and phosphorylated SMAD2 were increased during EHib, but only the phosphorylated SMAD2 during AR mirrored increases in myostatin. Follistatin, a negative regulator of myostatin, did not follow the same time course as myostatin or its signaling pathway, indicating more control of myostatin at the signaling level. However, SMAD7, an inhibitory SMAD, did not appear to play a significant role during deep hibernation. Hibernation is an excellent natural model to study factors involved in the endogenous intracellular mechanisms controlling myostatin.

Changes in body condition of hibernating bats support the thrifty female hypothesis and predict consequences for populations with white-nose syndrome.

White-nose syndrome (WNS) is a new disease of bats that has devastated populations in eastern North America. Infection with the fungus, Geomyces destructans, is thought to increase the time bats spend out of torpor during hibernation, leading to starvation. Little is known about hibernation in healthy, free-ranging bats and more data are needed to help predict consequences of WNS. Trade-offs presumably exist between the energetic benefits and physiological/ecological costs of torpor, leading to the prediction that the relative importance of spring energy reserves should affect an individual's use of torpor and depletion of energy reserves during winter. Myotis lucifugus mate during fall and winter but females do not become pregnant until after spring emergence. Thus, female reproductive success depends on spring fat reserves while male reproductive success does not. Consequently, females should be “thrifty” in their use of fat compared to males. We measured body condition index (BCI; mass/forearm length) of 432 M. lucifugus in Manitoba, Canada during the winter of 2009/2010. Bats were captured during the fall mating period (n = 200), early hibernation (n = 125), and late hibernation (n = 128). Adult females entered hibernation with greater fat reserves and consumed those reserves more slowly than adult males and young of the year. Consequently, adult females may be more likely than males or young of the year to survive the disruption of energy balance associated with WNS, although surviving females may not have sufficient reserves to support reproduction.

Energy homeostasis regulatory peptides in hibernating grizzly bears

Grizzly bears ( Ursus arctos horribilis) are inactive for up to 6 months during hibernation. They undergo profound seasonal changes in food intake, body mass, and energy expenditure. The circa-annual regulation of metabolism is poorly understood. In this study, we measured plasma ghrelin, leptin, obestatin, and neuropeptide-Y (NPY) levels, hormones known to be involved in the regulation of energy homeostasis, in ten grizzly bears. Blood samples were collected during the active summer period, early hibernation and late hibernation. Plasma levels of leptin, obestatin, and NPY did not change between the active and the hibernation periods. Plasma total ghrelin and desacyl-ghrelin concentrations significantly decreased during the inactive winter period compared to summer levels. The elevated ghrelin levels may help enhance body mass during pre-hibernation, while the low plasma ghrelin concentrations during hibernation season may contribute to the maintenance of hypophagia, low energy utilization and behavioral inactivity. Our results suggest that ghrelin plays a potential role in the regulation of metabolic changes and energy homeostasis during hibernation in grizzly bears.

Overwinter body temperature patterns in captive jerboas (Jaculus orientalis): influence of sex and group

The jerboa (Jaculus orientalis) has been described in the past as a hibernator, but no reliable data exist on the daily and seasonal rhythmicity of body temperature (T (b)). In this study, T (b) patterns were determined in different groups of jerboas (isolated males and females, castrated males and grouped animals) maintained in captivity during autumn and winter, and submitted to natural variations of light and ambient temperature (T (a)). T (b) and T (a) variations were recorded with surgically implanted iButton temperature loggers at 30-min intervals for two consecutive years. About half (6/13) of isolated female jerboas hibernated with a T (b)<33°C, with hibernation bouts interspersed with short periods of normothermy from November to February. Hibernation bout durations were longer (4-5days) than those of normothermia phases (1-4days). During hibernation, the minimum T (b) was low (T (b)min ~10.7°C). In contrast, one of the 12 isolated males showed short hibernation bouts of ca. 2days late in the hibernation season, February-March. The males had T (b)min values of 15.1°C. In contrast to predictions, no castrated males hibernated. When jerboas were grouped, females and males exhibited concomitant torpor bouts. In males, the longest bouts were observed during the late hibernation season. These data suggest complex regulation of hibernation in jerboas.

Cave visitation by temperate zone bats: effects of climatic factors

We investigated the effects of climatic variables on the flight activity of bats at the entrance of a hibernaculum (Kateřinská cave, Moravian Karst, CZ). Activity was recorded automatically using a double infrared-light logging system. Climatic factors influenced not only seasonal but also night-to-night and overnight patterns in cave visitation. The effect of individual variables and their contribution to variability in activity levels changed during the year. (1) Flight activity during late hibernation (5 March–14 April) was positively affected by the mean ambient temperature (Tavg) and negatively affected by previous day minimal temperature. (2) During the departure period (15 April–4 June), nightly activity correlated with Tavg and Pavg (mean barometric pressure). Previous day rainfall caused a decline in the activity levels. (3) Summer activity (5 June–26 July) increased as the range of daily temperature (Tdif max−min) increased and was suppressed by previous day rainfall. In contrast, a higher amount of rainfall (>10 mm) in the study day caused an increase in activity, likely due to bats sheltering. (4) During swarming (5 September–14 November), activity was positively related to Tavg, Pavg and the amount of rainfall. (5) During hibernation (15 November–4 March), temperature (Tavg and Tdif max−min) was the best predictor of the activity level. The percentage of nights on which activity occurred increased with increasing temperature during hibernation and late hibernation. Activity occurred even at temperatures<0 °C (Tmin=−13.2 °C). The recordings were all positive at Tmax≥6.2 °C. The activity within corresponding temperature groups was significantly lower during hibernation than during late hibernation. We review possible explanations for the patterns observed.

Ambient temperature and annual timing affect torpor bouts and euthermic phases of hibernating European ground squirrels (Spermophilus citellus)

The low body temperature state of hibernating mammals is interrupted regularly by short euthermic phases. The frequency of these euthermic phases changes according to the phase of hibernation. This typical pattern, common among ground squirrels (genus Spermophilus F. Cuvier, 1825), can be regulated by internal factors and affected by outside temperature. To evaluate the effects of internal annual timing and ambient temperature, we monitored nest temperatures of overwintering European ground squirrels ( Spermophilus citellus (L., 1766)) under three temperature conditions during late hibernation (9, 5, 0 °C). Our results showed that in spite of constant thermal conditions, an annual timing effect changed torpor-bout length (TBL) at 9 and 5 °C but not at 0 °C. Ambient temperature had an independent influence on TBL and euthermic phases. The loss in body mass was higher at higher ambient temperatures and was affected by TBL and euthermic phases. Thus, the hibernation pattern of European ground squirrels is regulated by both an annual timing pattern and the ambient temperature in the hibernacula.

Hibernating black bears ( Ursus americanus) experience skeletal muscle protein balance during winter anorexia

Black bears spend four to seven months every winter confined to their den and anorexic. Despite potential for skeletal muscle atrophy and protein loss, bears appear to retain muscle integrity throughout winter dormancy. Other authors have suggested that bears are capable of net protein anabolism during this time. The present study was performed to test this hypothesis by directly measuring skeletal muscle protein metabolism during the summer, as well as early and late hibernation periods. Muscle biopsies were taken from the vastus lateralis of six free-ranging bears in the summer, and from six others early in hibernation and again in late winter. Protein synthesis and breakdown were measured on biopsies using 14C-phenylalanine as a tracer. Muscle protein, nitrogen, and nucleic acid content, as well as nitrogen stable isotope enrichment, were also measured. Protein synthesis was greater than breakdown in summer bears, suggesting that they accumulate muscle protein during periods of seasonal food availability. Protein synthesis and breakdown were both lower in winter compared to summer but were equal during both early and late denning, indicating that bears are in protein balance during hibernation. Protein and nitrogen content, nucleic acid, and stable isotope enrichment measurements of the biopsies support this conclusion.

Plasma levels of ursodeoxycholic acid in black bears, Ursus americanus: Seasonal changes

To date, no other studies have examined the seasonal changes in circulating levels of various bile acids in the plasma of wild North American black bears, Ursus americanus. Using gas chromatography, bile acid concentrations were measured in plasma samples obtained during either early or late hibernation, and during summer active periods. Thus, specific compositional changes from individual animals were examined through a given year. Total bile acid concentrations in the plasma of these normal animals were found to range between 0.2 and 3.1 μmol/L (0.9 ± 0.2 μmol/L, mean ± SEM). Cholic, ursodeoxycholic and chenodeoxycholic acids were the major bile acid species identified. Ursodeoxycholic acid represented 28.0 ± 2.6% of the total bile acid pool. Deoxycholic and lithocholic acids were found only in small amounts. In addition, total bile acid concentrations were lower in plasma samples obtained during hibernation compared with those obtained during summer active periods (0.6 ± 0.1 and 1.2 ± 0.4 μmol/L, respectively; p < 0.05). However, the relative proportion of ursodeoxycholic acid, was significantly greater in winter than in summer (31.5 ± 3.2% and 22.2 ± 4.5%, p < 0.05). Finally, taurine-conjugated bile acids were the predominant species in bear plasma, accounting for > 67% of the total bile acids. These data demonstrate that ursodeoxycholic acid is a major bile acid in black bear plasma, mostly conjugated with taurine. Further, the finding of seasonal variation in plasma bile acid composition provides evidence to support the possible role that ursodeoxycholic acid may play in cellular protection in hibernating black bears.

Hibernating black bears have blood chemistry and plasma amino acid profiles that are indicative of long-term adaptive fasting

Blood samples were drawn from six black bears (Ursus americanus Pallas, 1780) active in the summer and six others in early and late hibernation. Plasma urea:creatinine ratios and concentrations of amino acids, alanine aminotransferase, and aspartate aminotransferase dropped during the winter denning season, suggesting a decreased protein breakdown. Fifteen amino acids (3 branched chain and 12 glucogenic) were lower in the early winter than in the summer, but 6 of these amino acids rose back to summer levels by the late denning season. Hydroxyproline and glycine were also elevated during late winter, suggesting an increase in collagen breakdown. This profile suggests a dynamic process of adaptive fasting and protein conservation during the winter with a mobilization of non-myofibrilar collagen and perhaps smooth muscle protein reserves to augment a potential but slight increased breakdown of skeletal muscle during the late winter.

Seasonal Variations in Thyroid Hormone Levels in Free-Living Echidnas (Tachyglossus aculeatus)

Hulbert and Augee (1982) have suggested that the thyroid has little effect on energy metabolism in the echidna. In order to investigate whether thyroid status changes during hibernation, when metabolism drops dramatically, we measured levels of thyroid hormones in 31 free-living echidnas at various times during the year. Unlike eutherian hibernators, in which thyroid hormone levels may rise to seasonally high values in late hibernation, total and free T4 and T3 were all significantly depressed throughout hibernation. TT4 from nonhibernating echidnas was 11.8 ± 0.9 ng/ml (n=23), confirming previously published values, but fell to half this level (5.9±0.7 ng/ml, n=8) during hibernation. By contrast to the low TT4 values, nonhibernating FT4, TT3, and FT3 values were similar to normal values for eutherian mammals. Differences in the seasonal pattern of variation in thyroid hormones between echidnas and hibernating eutherians may be due to differences in thyroid hormone transporting proteins.

Seasonal Fluctuations in Pituitary Gland and Plasma Levels of Gonadotropic Hormones inRana

Changes in plasma and pituitary levels of two gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH), in male and female Korean frogs (Rana dybowskiiandRana nigromaculata) were examined. Plasma levels of LH and FSH were measured by radioimmunoassay using antibodies raised against bullfrog LH and FSH. In male and femaleR. dybowskii,plasma concentrations of LH were low in early hibernation (October–January) and increased to high levels by late hibernation, when breeding occurs (late February–early March). Plasma FSH levels were also higher in the breeding period than during hibernation in male and female animals, but absolute levels of FSH were much lower than those of LH. In females, pituitary LH levels were higher in early hibernation, whereas pituitary FSH in females and pituitary LH and FSH in males changed little during hibernation. Plasma LH levels of frogs having uterine eggs or in amplexus were much higher (>25 ng/ml) than those of frogs with unovulated follicles (December) (<8 ng/ml) or frogs that had already spawned (<11 ng/ml). InR. nigromaculata,plasma LH and FSH levels of females collected during hibernation (October–May) were relatively low; however, following hibernation, plasma LH and FSH levels were markedly elevated for a short period. Thus, these animals exhibit a clear gonadotropin surge prior to ovulation and spawning. Soon after spawning, however, gonadotropin levels dropped to basal levels. Following spawning in females, levels of FSH increased steadily and rapid follicle growth occurred until August. By September, FSH had dropped to basal levels. In males, a sharp elevation of LH and FSH levels occurred during the short breeding period followed by a second increase in August, when early stages of spermatogenesis were evident.

Differential effects of gonadotropin and orthovanadate on oocyte maturation, ovulation, and prostaglandin synthesisby Rana ovarian follicles in vitro

Effects of gonadotropin and sodium orthovanadate on oocyte maturation, ovulation, prostaglandin, and progesterone synthesis were examined during in vitro culture of Rana ovarian follicles obtained from hibernating animals. Frog pituitary homogenates (FPH, 0.05 gland/ml) effectively induced oocyte maturation (germinal vesicle breakdown, GVBD) and ovulation (> 90%) in ovarian follicles obtained in mid-hibernation (mid-December through late January). In contrast, orthovanadate induced a limited amount (< 45%) of ovulation of some oocytes without concomitant induction of maturation during mid-hibernation. In late hibernation (February to early March), considerable spontaneous maturation and ovulation occurred in cultured ovarian follicles. During this period both FPH (0.0005-0.05 gland/ml) or orthovanadate (0.01-1 mM) treatment markedly increased the incidence of ovulation and accelerated the onset of ovulation in a dose dependent manner. When examined after 12 h of culture, few (< 5%) oocytes ovulated in response to orthovanadate had undergone GVBD whereas most (> 90%) of those ovulated in response to FPH had matured. However, the majority of oocytes ovulated (72%) within 6 h of exposure to FPH had intact GVs, indicating that GVBD is also not a prerequisite for ovulation induction in response to FPH. Orthovanadate stimulated PGF2 alpha in a dose dependent manner but failed to stimulate progesterone production whereas FPH stimulated secretion of both PGF2 alpha and progesterone. The amount and time course of PGF2 alpha secretion in response to orthovanadate were similar to results produced with FPH stimulation. Treatment of PGF2 alpha to follicles obtained in late hibernation also accelerated the ovulation of the oocytes. Taken together, the data suggest that orthovanadate enhanced ovulation of immature oocytes was mediated via enhanced PGF2 alpha production and that oocyte maturation is not essential or prerequisite for in vitro oocyte ovulation in Rana.

Seasonal influences on the control of plasma sex hormone-binding globulin by T4 in male little brown bats

Male little brown bats showed a well-defined seasonal cycle of plasma thyroxine (T4). Total T4 concentrations increased during hibernation, reached peak levels in May at spring emergence, and subsequently declined to lowest levels in September. Thyroxine-binding globulin (TBG) activity increased only in summer, and seasonal changes in non-TBG-bound T4 generally paralleled those of total T4. During the fall and early hibernation period, bats studied under feral conditions or following photoperiod manipulation showed coincident changes in plasma T4 and sex hormone-binding globulin (SHBG) levels. In contrast, males studied during late hibernation or following spring arousal exhibited declining plasma T4 levels during periods of increased SHBG activity. Additionally, the prehibernation decline in SHBG did not depend on changing environmental conditions. Bats maintained under summerlike conditions during the fall showed normal seasonal reductions in SHBG and T4. These studies indicate that T4 is the likely physiological regulator of plasma SHBG activity during the fall and early hibernation, but that other factors modulate the timing of SHBG induction in the spring.

Steroidogenic shift by cultured ovarian follicles of Rana dybowskii at breeding season.

The steroid secretory activity of cultured ovarian follicles of Rana dybowskii and the activities of relevant steroidogenic enzymes were examined during the natural hibernation period (October-February). Enzyme activities were measured indirectly by monitoring the conversion of exogenous substrates to products by isolated follicles. Follicles were incubated for 6 h in amphibian Ringer in the presence or absence of frog pituitary homogenate (FPH, 0.1 pituitary/2 ml) and/or various steroid precursors. Progesterone (P4), 17 alpha-hydroxyprogesterone (17 alpha-OHP) or testosterone (T) secreted by the follicles into the medium were measured by RIA. In the presence of FPH, high levels of P4 were produced by follicles at the early and mid-hibernation period (695 and 898 pg/follicle, respectively) whereas markedly elevated levels of P4 were produced during late hibernation (1,393 pg/follicle) (just prior to or breeding season, February). In contrast, high levels of T were produced by the follicles early in hibernation (1,206 pg/follicle) while negligible levels were produced in late hibernation (69 pg/follicle). Higher levels of 17 alpha-OHP were produced by follicles at early and mid hibernation (594 and 705 pg/follicle, respectively) than in later hibernation (221 pg/follicle). Addition of exogenous pregnenolone markedly increased P4 levels in a dose-dependent manner when added to follicles at early and late stages of hibernation: FPH addition further enhanced conversion of pregnenolone. Similarly, addition of androstenedione (AD) increased T levels in a dose-dependent manner by these follicles. However, following addition of exogenous P4, less 17 alpha-OHP was produced by follicles collected in late hibernation as compared to those collected earlier.(ABSTRACT TRUNCATED AT 250 WORDS)

Serum Chemistry and Hematology of Black Bears: Physiological Indices of Habitat Quality or Seasonal Patterns?

We evaluated the use of serum chemistry and hematological variables as indices of habitat quality for adult (≥4 yr) female black bears ( Ursus americanus ) in northeastern Minnesota. Blood samples ( n = 182) collected from 48 adult females during 1970–1976 showed seasonal variation ( P < 0.05) for four of five hematologic characters and seven of 13 serum chemistries. Body mass, red-blood-cell count, mean corpuscular-volume, serum-urea nitrogen, uric acid, thyroxine, and Cortisol were different ( P < 0.05) among years of differing food availability. Discriminant functions developed from samples from late summer, autumn, and early hibernation provided 75–80% accuracy in classifying observations into year classes of good and poor food production. Our results suggested that blood characteristics can be indicators of nutritional condition and habitat quality after seasonal rhythms are controlled. We recommend use of blood-profile data to empirically test the utility of our discriminant functions.

Changes in renal morphology and renin secretion in the golden-mantled ground squirrel (Spermophilus lateralis) during activity and hibernation

Chronological changes in renal glomerular morphology and plasma renin activity were investigated during active and hibernating periods in the golden-mantled ground squirrel Spermophilus lateralis. The objective of this study was to determine whether the glomerular endothelium, visceral epithelium (podocytes), basement membrane, mesangial cells, proximal convoluted tubule cells and plasma renin activity exhibit measurable sequential differences between as well as within active and hibernating states at various time points. Limitations in the size of the experimental population prevented an evaluation of changes in these parameters during other important periods such as periodic arousal between hibernation bouts. In this study, glomerular endothelial pore number and epithelial filtration slit number significantly decreased by early hibernation when compared to those during summer activity, and then they increased back toward summer levels by late hibernation. In contrast, podocytic pedicel width along the glomerular basement membrane increased from summer activity to early hibernation, before significantly decreasing again by late hibernation. Mesangial cell and proximal convoluted tubule cell activity appeared increased during hibernation as compared to summer activity, whereas the width of the glomerular basement membrane showed no significant alterations throughout. Plasma renin activity significantly increased during early hibernation and mid-hibernation when compared to summer levels but had decreased by late hibernation toward summer values. The glomerular and plasma renin activity changes observed in this study clearly illustrate the drastic structural and functional adjustments which hibernating species make during torpor and also correlate well with the reported decrease in renal perfusion pressure and urine formation during hibernation.(ABSTRACT TRUNCATED AT 250 WORDS)

Torpor Bout Duration during the Hibernation Season of Two Sciurid Rodents: Interrelations with Temperature and Metabolism

Torpor bouts of mammalian hibernators are generally shorter at the beginning and end and are consistently longer during the main part of the hibernation season. Because it is not known why the duration of torpor bouts changes at the beginning and end of the hibernation season, we studied this phenomenon in two sciurid rodents: Spermophilus saturatus (200–300g) and Eutamias amoenus (45-60g). We examined the seasonal change in torpor bout duration during hibernation at a constant air temperature (Ta) of 2° C in relation to (1) the minimum body temperature (minimum Tb) to which the animals could be experimentally cooled before they maintained a constant Tb or began to arouse, (2) Ta at the time of minimum Tb (minimum Ta), and (3) oxygen consumption (V̇o2) of torpid individuals at Ta 2° C. Average duration of torpor bouts during the main part of the hibernation season was about 11 d in S. saturatus and 8–9 d in E. amoenus; in response to experimental cooling, minimum Tb of both species declined as low as −0.2° C. In early and late hibernation, when torpor bouts were short, minimum Tb, minimum Ta, and V̇o2 during torpor were higher than in the main part of the hibernation season. Regression analyses suggest that minimum Tb and minimum Ta are more strongly correlated with torpor bout duration than V̇o2 in both species.

Seasonal variations of DNA-synthesis in intestinal epithelial cells of hibernating animals—2. DNA-synthesis in intestinal epithelial cells of ground squirrel ( Citellus undulatus) during autumn and late hibernation season

1. 1. DNA-synthesis was found to be gradually reduced in the intestinal crypt cells of ground squirrel during autumn at the time of preparation for hibernation and gradually rose during late hibernation season in February–March at the time of preparation for arousal. 2. 2. A conclusion is made on the seasonal variations in cell replacement of true hibernators.

Plasma progesterone levels during pregnancy in the little brown bat Myotis lucifugus (Vespertilionidae).

Plasma progesterone was measured by radioimmunoassay in individual female Myotis lucifugus throughout pregnancy and lactation. Progesterone levels, which averaged 6.7 +/- 0.7 ng/ml in late hibernation, rose to a mean of 18.9 +/- 6.7 ng/ml in unimplanted bats collected in the first two weeks after arrival at a maternity roost. Analysis of progesterone levels in bats in which the developmental stage of the embryo was known revealed two sharp, transient increases in plasma progesterone during the preimplantation period. The first, with values of 30-45 ng/ml, occurred at ovulation. The second, with values of 20-30 ng/ml, coincided with blastocyst formation. Progesterone levels increased exponentially from a mean of 7.4 +/- 1.0 ng/ml during early implantation to peak values of 100-200 ng/ml (means = 136.2 +/- 15.6) in the last two weeks of pregnancy, and showed no evidence of either a midpregnancy or prepartum decline. Despite involution of the corpus luteum at the end of pregnancy, progesterone levels averaged 9.0 +/- 1.0 ng/ml during lactation and did not decline until the end of lactation. In bats undergoing abortion, mean levels of plasma progesterone were already less than 6 ng/ml, equivalent to levels in nonbreeding females. The results indicate that the progesterone profile of pregnant M. lucifugus, though generally resembling those of other bats, exhibits several distinctive features. The sharp rise in plasma progesterone coinciding with blastocyst formation has not been reported in other mammals and suggests a possible role of progesterone in the cavitation process. In addition, peak values of plasma progesterone in late pregnancy were conspicuously higher than levels reported in other verpertilionid bats. The levels did not appear to fall before parturition, although such falls have been reported in other bats.