Maintenance of the pectoralis muscle during hibernation in the big brown bat, Eptesicus fuscus

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1 J Cmp Physil (183) 152:7-104 Jurnal f Cmparative Physilgy, B Springer-Verlag 183 Maintenance f the pectralis muscle during hibernatin in the big brwn bat, Eptesicus fuscus Marshall E. Yace* Divisin f Bilgical Sciences, University f Michigan, nn rbr, Michigan 4810, US ccepted December 2, 182 Summary. The relatinship between pectralis muscle mass and bdy mass is examined thrughut the annual bdy mass cycle in Eptesicusfuscus in rder t evaluate muscle maintenance during hibernatin. E. fuscus underges large fluctuatins in bdy mass during the year due t pregnancy, parturitin, prehibernatin fattening, and hibernatin (Table 1). Parallel changes ccur in pectralis muscle mass and ttal pectralis prtein mass (Table 2). The strng crrelatin between lg pectralis mass and lg bdy mass (Fig. 3) and the lack f crrelatin between pectralis mass and frearm length (Fig. 1, 2) suggest that the seasnal variatin in pectralis muscle mass represents a cmpensatry respnse t changing bdy mass. In active bats this relatinship clsely resembles the cmpensatry respnse predicted by flight thery. Bth pectralis muscle mass and bdy mass decrease significantly during fur mnths f hibernatin (Tables 1, 2). lthugh pectralis mass and bdy mass are nt significantly crrelated after fur mnths f hibernatin the values fall within the range f bservatins in active summer bats (Fig. 3), indicating an apprximate maintenance f the pectralis mass/bdy mass relatinship. The lack f crrelatin in hibernating bats may result frm differences in the functin f pectralis muscle during activity (pwering flight) and hibernatin (thermgenesis and supply f glucnegenic precursrs). The significant elevatin f pectralis muscle prtein cncentratin during prehibernatin fattening and its prgressive decrease during hibernatin (Fig. 4) supprt the idea that the pectralis muscle serves as an imprtant surce f glucnegenic precursrs during hibernatin. * Present address : Marine Bilgy Divisin -002, Scripps Institutin f ceangraphy, University f Califrnia, San Dieg, La Jlla, Califrnia 203, US Intrductin During summer the big brwn bat, Eptesicusfuscus, is a ncturnal, aerial insectivre. Fraging flights require a sustained, high level f pwer utput, virtually all f which is prvided by the tw large pectralis muscles. Pectralis muscle mass and bdy mass are strngly crrelated acrss a wide size range in bats (Greenewalt 162), implying that a certain amunt f muscle is needed t supprt flight in a bat f a given bdy mass. This relatinship is predicted in flight thery (Pennycuick 175) and supprted by numerus bservatins in birds (Greenewalt 162; Tucker 173; Greenewalt 175; Marsh and Strer 181). It fllws that in rder t retain the ability t fly, the relatinship between pectralis mass and bdy mass must als be maintained thrughut hibernatin. Eptesicus fuscus hibernates fr up t five mnths in sutheastern Michigan due t the absence f its primary fd, flying insects. The energy requirements f hibernatin are supplied mainly by fat (Beer and Richards 156; Ddgen and Bld 156). Hwever, mammals cannt live n fat alne. The xidatin f fats in the citric acid cycle depends n a cnstant input f 3- and 4-carbn intermediates (Spydevld et al. 176; Lee and Davis 17), which can nly be derived frm glucse, amin acids, r glycerl (Lehninger 175). In additin, the central nervus system requires glucse even during prlnged fasting (wen et al. 167). Hibernatrs which d nt cache fd must prvide these requirements frm endgenus surces. Since glycgen stres are small and d nt underg net depletin during hibernatin in the mammals which have been studied (Ddgen and Bld 156; Lenard and Wimsatt 15; Tryer 15; Galster and Mrrisn 170, 175;

2 8 M.E. Yace: Maintenance f muscle during hibernatin in the bat Yace 182; Zimmerman 182), glucse and citric acid cycle intermediates must be synthesized frm amin acids and/r glycerl. Hwever, the availability f glycerl is limited by the rate f fat xidatin. It is inherent in the chemical cmpsitin f triglycerides that glycerl can prvide nly a small cntributin t verall metablism. Demand in excess f the amunt which can be supplied by glycerl must be met by amin acids derived frm tissue prteins. Skeletal muscle is by far the largest prtein reserve and the mst imprtant surce f glucnegenic precursrs during prlnged fasting in nrmthermic mammals (Snell 180). Hwever, the catablism f muscle prtein results in atrphy and lss f functin. The present study examines the extent t which the degradatin f skeletal muscle prtein may meet metablic needs during hibernatin in E. fuscus. It was hypthesized that successful hibernatin might depend n the balance between the use f prtein fr metablic needs and the maintenance f sufficient muscle functin t supprt nrmal activity upn arusal in the spring. The extent f muscle prtein catablism during hibernatin is evaluated here in light f the general decrease in bdy mass during hibernatin (Beer and Richards 156). The relatinship between pectralis muscle mass and bdy mass is examined thrughut the annual bdy mass cycle in E.fuscus and is cmpared with that predicted by flight thery (Pennycuick 175). These cmparisns are used t determine the extent t which shrt term changes in bdy mass (as in pregnancy and prehibernatin fattening) are cmpensated by changing muscle mass. Deviatins f the bserved relatinship frm the predicted ne shuld indicate the extent t which muscle mass may vary frm the predicted value and still be capable f supprting flight. The relatinship between pectralis muscle mass and bdy mass in actively flying bats is then used as an index fr evaluating the maintenance f muscle during hibernatin. Finally, muscle prtein cntent is measured t prvide an indicatin f the extent f prtein catablism during hibernatin. Materials and methds nimals ll f the bats used in this study were cllected in attics and barns in the vicinity f nn rbr, Michigan. 'Summer' animals were cllected frm early June thrugh the middle f ugust. Bats used fr the prehibernatin and hibernatin measurements were cllected in late September and early ctber. nimals used fr prehibernatin measurements were sampled immediately. The remaining bats were hused in cages in a darkened chamber at 5-7 ~ and 100 % relative humidity. These cnditins apprximate thse reprted fr natural hibernacula (Beer and Richards 156) and bats held in this manner fllw a similar curse f mass lss t that f free living animals in hibernatin (Beer and Richards 156). The bats used here had been hibernating fr at least three mnths prir t sampling. Physical measurements Bats were kilted by a blw t the head. Bdy mass was measured t 0.01 g using a Mettler tp lading balance. The left pectralis muscle was rapidly excised, weighed t 0.01 g and used fr prtein determinatins. The right pectralis muscle was carefully dissected and weighed t g using a Mettler analytical balance. The pectralis mass reprted here is twice the measured mass f the right pectralis. Frearm length was recrded t 0.5 ram. Bdy cmpsitin Fat mass was determined by the methd f Carey et al. (178). Carcasses were dried t cnstant mass using a Virtis lyphilizer. The dry carcasses were weighed t the nearest 0.01 g, shredded with scissrs, and packed int cellulse extractin thimbles. Carcass neutral lipids (predminantly fats) were extracted with petrleum ether using a Sxhlet apparatus. The extracted material was then ven-dried fr a week at 0 ~ and weighed t 0.01 g. Fat mass was taken t be the difference in carcass mass with fat extractin. Fetal mass was determined directly by dissectin f bth fetuses and the placenta. The cmbined mass f this material was measured t 0.01 g. Lean mass was calculated by subtracting fat mass (and fetus mass) frm ttal bdy mass. Prtein determinatins Pectralis muscle prtein cntent was estimated using the biuret prcedure described by Watters (178). Muscle samples were hmgenized in 10 ml/g tissue f 100 mm ptassium phsphate, 2 mm EDT, ph 7.3 fr 10 s at a setting f 50 n a Tekmar Tissuemizer. Duplicate diluted prtins f this hmgenate were used fr the prtein determinatin and each value reprted is the mean f tw independent measurements. Values fr the ttal mass f pectralis muscle prtein represent the prduct f prtein cncentratin and tissue mass. Statistics Multiple cmparisns f mean values were made by analysis f variance, except where sample variances differed significantly. In thse cases, multiple cmparisns were made by the Kruskal-Wallis test and pairwise cmparisns were made by the Wilcxn rank sum test. The relatinships f pectralis mass with bdy mass and ttal pectralis prtein with bdy mass were evaluated using analysis f cvariance. Null hyptheses were rejected at the 0.05 level. Results Eptesicus fuscus underges cntinuus, large fluctuatins in bdy mass during the year (Table 1). Hwever, fr purpses f simplicity the yearly bdy mass cycle is brken int fur perids: preg-

3 M.E. Yace: Maintenance f muscle during hibernatin in the bat Table 1. Physical characteristics f Eptesicusfuscus during the annual bdy mass cycle Parameter Pregnant Nnpregnant Prehibernatin Hibernatin Ttal bdy mass (g) a b 1.12_ a 14.10_ b (n = 17) (n = 28) (n = 11) (n = 14) Fat mass (g) 0.88 _ a b " 1.45 _ u (n = 17) (n = 18) (n = 11) (n = 14) Fetal mass (g) (n - 17) Lean mass (g) 16.13_+0.25 a b " 12.65_+0.2 b (n= 17) (n= 18) (n= 11) (n- 14) Lean dry mass (g) 4.82_+0.16" a 3.8_+0.15 b (n= 17) (n= 18) (n= 11) (n = 14) % Water ~ _+0.7 (n = 17) (n = 18) (n = 11) (n = 14) Frearm length(ram) _ _+0.5 (n = 17) (n = 18) (n ~ 11) (n = 14) II values are presented as mean_+ SEM, 'n' dentes sample size " Significantly different frm the summer nnpregnant value (P<0.01) b Significantly different frm the prehibernatin value (P<0.01) ~ The percent f lean mass accunted fr by water nant, summer nnpregnant, prehibernatin fattening, and hibernatin. Mean bdy mass varies significantly amng these fur grups (P<0.001). Pregnant females were captured in early June when they were very near term in rder t see the maximum effect f pregnancy n ttal bdy mass and related parameters. The mean bdy mass f pregnant females (22.35 _ g) is significantly greater (P< 0.001) than that f nnpregnant summer animals (12.86_+0.37g). The summer nnpregnant grup is rather hetergeneus, including pstpartum females, males, and sme first year animals captured in late ugust. The bdy mass f adult males, pst-partum females, and first year bats d nt differ significantly frm ne anther. nimals in the prehibernatin grup were captured in late September and early ctber when they were quite fat but still active. The mean bdy mass (1.12_+1.05 g) in prehibernatin bats is significantly greater (P<0.001) than in nnpregnant summer animals. Hibernating bats were sacrificed after 3-4 mnths in hibernatin. Bdy mass in this grup (14.10_+0.46g) is significantly less (P< 0.001) than in prehibernatin animals. The seasnal cycle in ttal bdy mass is largely accunted fr by changes in fat mass and fetal mass (Table 1). Fat mass reaches a peak during prehibernatin fattening ( g) and then gradually decreases during hibernatin. Fetal mass a few days prir t parturitin ( g) als represents a large fractin f bdy mass. Hwever, the changes in bdy mass shwn in Table 1 als include significant seasnal changes in lean mass (P < 0.001). Bth pectralis muscle mass and ttal pectralis prtein mass are significantly greater during pregnancy (P<0.001) and prehibernatin fattening (P<0.001) than in nnpregnant summer bats (Table 2). fter fur mnths f hibernatin bth pectralis mass (P<0.001) and ttal pectralis prtein mass (P<0.001) are significantly reduced relative t prehibernatin bats (Table 2). The differences in muscle mass and prtein mass between grups are nt due t sampling different sized bats, since mean frearm length des nt differ significantly between grups (Table 1). There is n verall crrelatin between pectralis mass and frearm length (Fig. 1). Hwever, within grups differences in frearm length seem t accunt fr part f the variatin in muscle mass. Pectralis muscle mass is significantly crrelated with frearm length in pregnant (r = 0.60, P< 0.01) and hibernating (r=0.64, P<0.01) bats, but nt in nnpregnant r prehibernatin animals. The extent f seasnal differences in muscle mass and prtein mass (between prehibernatin and hibernatin bats) are cmpared with the effect f bdy size n these parameters in Fig. 2. Pectralis muscle mass and ttal pectralis prtein mass are significantly crrelated with frearm length in hibernating bats, but the seasnal changes in these parameters far utweigh the differences related t variatin in bdy size. Pectralis muscle mass clsely fllws bdy

4 100 M.E. Yace: Maintenance f muscle during hibernatin in the bat Table 2. Characteristics f the pectralis muscle during the annual bdy mass cycle Parameter Pregnant Nnpregnant Prehibernatin Hibernatin Pectralis mass (g) c 1.26_ a 0.87 ~ 0.04 b 1.20 _+ 0.06" 0.88 _ b (n = 18) (n= 18) (n= 11) (n= 14) Ttal pectralis prtein (g) 0.31 _ a b a 0.22_ b (n = 18) (n = 18) (n= 11) (n = 14) Prtein cntent (mg/g) _ b " _+ 5.7 "b (n= 18) (n= 18) (n = 11) (n = 14) % f bdy mass a 5.77 _+ 0.12"b _ (n = 18) (n = 28) (n = 11) (n = 14) ll values are presented as mean SEM, "n' dentes sample size a Significantly different frm the summer nnpregnant value (P < 0.01) b Significantly different frm the prehibernatin value (P < 0.01) c Twice the mass f the right pectralis muscle (see Materials and Methds) d (Pectralis mass/ttal bdy mass) x 100. Bdy mass values are as reprted in Table ~x 1.5. c c~ c " ~ ~ p" a ~ ~ i i p ~ 0.4 Frearm Length, mm :; c Fig. 1. The mass f the pectralis muscle as a functin f frearm length. The pen circles are nnpregnant summer bats, the clsed circles are pregnant bats, the pen triangles are bats during prehibernatin fattening, and the clsed triangles are bats after fur mnths f hibernatin ~ 0.2 mass thrughut the year. Despite a large variatin in ttal bdy mass, the prprtin f bdy mass represented by pectralis muscle mass des nt differ significantly between the nnpregnant summer, prehibernatin, and hibernatin grups (Table 2). Hwever, this prprtin is significantly lwer (P<0.001) in pregnant females. The relatinship between pectralis muscle mass and bdy mass is shwn in mre detail in Fig. 3. There is a strng verall crrelatin between pectralis mass and bdy mass (r=0.1), but this relatinship differs between grups f animals. The simple linear regressin f lg pectralis mass (rap) n lg bdy mass (mb) in nnpregnant summer bats ~...~/ I 1 I I 1 I //I i I I I I Frearm Length, mm Fig. 2. Factrs influencing ttal pectralis muscle mass and ttal pectralis prtein mass. Upper panel: The relatinship between pectralis muscle mass and frearm length in prehibernatin (pen triangles) and hibernating (clsed triangles) bats. The line represents the simple linear regressin f pectralis mass n frearm length in hibernating bats. The regressin equatin is: my=0.04 L-1.13 (r=0.64, P<0.01), where mp=pectralis muscle mass in g and L = frearm length in mm. There is n significant crrelatin between these parameters in prehibernatin bats. Lwer panel: The relatinship between ttal pectralis prtein mass and frearm length in prehibernatin (pen triangles) and hibernating (clsed triangles) bats. The line represents the simple linear regressin f prtein mass n frearm length in hibernating bats. It is described by the equatin, mprt=0.01 L-0.28 (r=0.63, P<0,01), where mprt=ttal pectralis prtein mass in g and L = frearm length in mm. There is n significant crrelatin in prehibernatin bats lh B.

5 M.E. Yace: Maintenance f muscle during hibernatin in the bat 101 u) s 13) Pregnant Nnpregnant 1.50 ~ Prehlbernatln 1.25 Hibernatin ~ 1.00 ~a //, 10 ~g I I I Bdy Mass, g Fig. 3. The relatinship between pectralis muscle mass and bdy mass thrugh the yearly bdy mass cycle in Eptesicus fitscus. Bth scales are lgarithmic.=_- s 13-.ffl "5 _ I ,1.//i 10 Pregnant 0 Nnpregnant ~- PrehlbernatJn Hlbernat~n -- zl 0 Q, 0 0 I I I I I I I I Bdy Mass, g Fig. 4. The relatinship between ttal pectralis muscle prtein mass and bdy mass thrugh the yearly bdy mass cycle in Eptesicus fuscus is described by the equatin: lg mp = 1.37 lg m b- 1.5 (1) (r=0.3, P<0.001). s bdy mass increases during pregnancy and prehibernatin fattening, pectralis mass des nt increase at the rate predicted by Eq. 1. The slpe f the regressin f lg pectralis mass n lg bdy mass in pregnant animals: lg mp = 0.62 lg rb (2) (r = 0.61, P < 0.01) is significantly lwer (P < 0.001) than in nnpregnant summer bats. Similarly, the slpe f the regressin f lg pectralis mass n lg bdy mass in prehibernatin bats: lg mp = 0.82 lg rb--0.8 (3) (r=0.3, P<0.001) is significantly lwer (P< 0.005) than in nnpregnant summer bats. Pectralis muscle mass is nt significantly crrelated with bdy mass in hibernating bats (r = 0.50, P ). Hwever, the distributin f values is very similar t that fr nnpregnant summer bats (Fig. 3). dditinally, neither mean pectralis mass (Table 1), mean bdy mass (Table 1), nr the rati f pectralis mass t bdy mass (Table 2) differ significantly between nnpregnant summer bats and hibernating bats. Pectralis muscle prtein cncentratin in prehibernatin bats (336.1 mg.gmuscle -1) is significantly greater than in pregnant ( mg.g muscle-i; P<0.001) and nn- pregnant summer bats (24.7 _+ 6.6 mg g muscle- 1., P<0.001). fter fur mnths f hibernatin, pectralis muscle prtein cncentratin (302.2_+ 5.7 mg.g muscle- 1) is significantly less than in prehibernatin bats (P< 0.01), but it is still greater than in summer bats (P<0.01). The relatinship between ttal pectralis prtein mass and ttal bdy mass is shwn in Fig. 4. Nt surprisingly, this is very similar t the relatinship between pectralis muscle mass and ttal bdy mass (Fig. 3). Hwever, as a result f the increased pectralis muscle prtein cncentratin, the slpe f the regressin f ttal pectralis prtein mass (mprt) n bdy mass (rb) in prehibernatin bats: rnp~t = m b (4) (r=0.80, P<0.005) is identical with that fr nnpregnant summer bats: /T/prt = m b (5) (r=0.7, P<0.001). Ttal pectralis prtein and bdy mass are nt significantly crrelated in hibernating bats (r= 0.50, P= 0.066). Hwever the distributin f values falls within the range f bservatins fr nnpregnant summer bats (Fig. 4). The regressin f ttal pectralis prtein mass n bdy mass in pregnant bats: mprt = m b (6) (r = 0.64, P < 0.005) differs significantly with Eqs. (4) and (5).

6 102 M.E. Yace: Maintenance f muscle during hibernatin in the bat Discussin This paper addresses the hypthesis that successful hibernatin in Eptesicus fuscus depends partly n the balance between the use f pectralis muscle prtein fr metablic needs and the maintenance f sufficient muscle t supprt flight upn arusal in the spring. Hwever, the relevance f this hypthesis is dependent n the extent t which muscle prtein is catablized during hibernatin. Bth pectralis muscle mass and ttal pectralis prtein mass are significantly lwer in bats sampled after fur mnths f hibernatin than in prehibernatin bats (Table 2). These data indicate that muscle mass is decreased during hibernatin due t prtein catablism. The prehibernatin and hibernating grups d nt differ in bdy size (frearm length= _+0.5 mm, respectively). Mrever, there is n verall relatinship between pectralis mass and frearm length (Fig. 1), indicating that the extent f seasnal changes in pectralis mass is much larger than the variatin due t differences in bdy size. The nature f the changes in pectralis mass and prtein mass during hibernatin are further illustrated in Fig. 2. Bats in all size classes lse muscle mass and prtein during hibernatin. Prtein metablism during hibernatin has received very little attentin. The data presented here are the first t shw that muscle mass and prtein decrease during hibernatin. Hwever, bservatins that hibernating black bears have decreased but significant rates f urine frmatin and nitrgen excretin (Nelsn et al. 173) and that arctic grund squirrels underg a significant lss f lean bdy mass during hibernatin (Galster and Mrrisn 176) are cnsistent with the idea that prtein is catablized during hibernatin. This is further strengthened by studies f humans which shw significant, althugh greatly reduced, rates f prtein degradatin and nitrgen excretin during prlnged fasting (Cahill 176; Felig 17; Millward 17). Prtein catablism during hibernatin may serve tw majr functins. First, it culd prvide precursrs fr glucnegenesis. Glucse is a required nutrient in nrmthermic mammals (Cahill 176; Felig 17; wen et al. 167), althugh its necessity during hibernatin is uncertain (Zimmerman 182). Since glucse is nly stred in small quantities as liver and muscle glycgen in E.fuscus (Yace 182) and these stres are nt depleted during hibernatin, glucse use shuld depend n glucnegenesis. Secnd, prtein catablism culd prvide a surce f citric acid cycle intermediates. Mitchndria are unable t xidize fatty acids in the absence f citric acid cycle intermediates. The maintenance f cnstant levels f intermediates invlves cntinuus flux int the cycle (Spydevld et al. 176; Lee and Davis 17). Input f intermediates may cme frm the carbxylatin f pyruvate and prprinate (Lee and Davis 17) r the deaminatin f amin acids (Lehninger 175). ll f these precursrs culd be derived frm prtein during hibernatin. Catablism f muscle prtein may prvide sme f the nutrient requirements f hibernatin but it results in decreased muscle mass. The effect this wuld have n the ptential fr flight depends n the resulting relatinship between pectralis mass and bdy mass. Thery predicts that as an individual underges changes in bdy mass the pwer required fr level flapping flight will vary with the 1.5 pwer f bdy mass (Pennycuick 175). The difference in pwer requirements caused by changing bdy mass may be cmpensated by changes in kinematic parameters such as wing beat frequency and sweep angle f the wing. Cmpensatin may als invlve changes in pectralis muscle pwer utput deriving frm changes in muscle mass. If muscle is t fully cmpensate fr changing bdy mass the mass f the pectralis muscle shuld vary with the 1.5 pwer f bdy mass (Marsh and Strer 181; Pennycuick 175). In E. fuscus the relatinship f pectralis mass t bdy mass differs thrugh the annual bdy mass cycle (Fig. 3). In nnpregnant summer animals the slpe (1.37) f the simple linear regressin relating lg pectralis mass, lg rap, t lg bdy mass, lg mb, (Eq. (I)) des nt differ significantly frm the predicted value (1.5) fr ttal cmpensatin. Hwever, during pregnancy and prehibernatin fattening the slpes f the regressins f lg rnp n lg m b are significantly less than 1.5 (Eqs. (2), (3)). fter fur mnths f hibernatin lg mp is nt significantly crrelated with lg mb. This nnunifrmity may result frm differences between grups with respect t 1) the time curse ver which muscle adaptatins ccur and 2) the causal factr fr the maintenance f a particular muscle mass. The pwer requirement f flight is the causal factr fr the relatinship between pectralis muscle mass and bdy mass in active bats (Pennycuick 175). Thus, bdy mass is a majr determinant f pectralis muscle mass in nnpregnant summer, pregnant, and prehibernatin in bats. s a result, lg rnp is strngly crrelated with lg mu in each f these grups. The differences between these grups may reflect the perid f time ver which adaptatin has ccurred. The relatinship between

7 M.E. Yace: Maintenance f muscle during hibernatin in the bat 103 pectralis mass and bdy mass in n0npregnant summer animals develps ver a relatively lng perid f time during which pectralis mass and bdy mass change very little. This is analgus t the situatin described in Cper's hawks, ccipiler cperii, (Marsh and Strer 181) in which bdy mass changes slwly ver a lng perid f time. s in the nnpregnant summer bats, changes in bdy mass are ttally cmpensated by changes in muscle mass. In cntrast, the changes in bdy mass during pregnancy and prehibernatin fattening are large and ccur ver a perid f nly a few weeks. The expnents relating pectralis mass t bdy mass in pregnant and prehibernatin bats are significantly lwer than the predicted value (1.5) fr ttal cmpensatin. Similarly, in the gray catbird, Dumetella carlinensis (Marsh 181), muscle mass des nt increase sufficiently during premigratry fattening t cmpensate fully fr increased bdy mass. Data taken frm Marsh (17) yield the relatinship: lg mp= 0.65 lg rb (7) (r=0.73; P<0.001; n=2). The incmplete cmpensatin in these species may be due t physical limitatins n the maximum extent f muscle enlargement pssible in small flying animals. They may als be the result f a lag in muscle respnse. Interestingly, in the much larger species, the piedbilled grebe, Pdilymbus pdiceps, the increased bdy mass assciated with premigratry fattening is ttally cmpensated by increased pectralis mass (R.L. Marsh, persnal cmmunicatin). Based n the abve arguments, the pectralis muscle mass needed t supprt flight upn arusal f E. fuscus shuld depend n bdy mass upn emergence. Since bdy mass is significantly reduced during hibernatin due t the xidatin f fat (Beer and Richards 156; Table 1), the necessary muscle mass after fur mnths shuld be less than at the utset f hibernatin. Hwever, during hibernatin bdy mass is nt a causal factr in the maintenance f a particular muscle mass. Muscle mass is determined by the rate f prtein catablism. Therefre, it is nt surprising that after fur mnths f hibernatin lg pectralis mass is nt significantly crrelated with lg bdy mass. Hwever, it is remarkable that after fur mnths f hibernatin the values fr pectralis mass and bdy mass verlap with thse f summer bats (Fig. 3; Table 2). It appears that the rate f muscle prtein catablism is such that the distributin f values fr pectralis mass and bdy mass after fur mnths f hibernatin is similar t that bserved in nnpregnant summer bats (Fig. 3). The lack f crrelatin between lg pectralis mass and lg bdy mass in hibernating bats suggests that the maintenance f the ability t fly may be nly partially dependent n the maintenance f a specific relatinship between pectralis mass and bdy mass. These animals are capable f flight after fur mnths f hibernatin in the labratry. Therefre, an ability t adjust the kinematic parameters f flight such as wing beat frequency and sweep angle may play an imprtant rle in flight immediately upn arusal in the spring, Prtein is the central cmpnent f muscle, bth in its cntractile rle and in its rle as a stre f glucnegenic precursrs. The changes in muscle mass during the yearly bdy mass cycle in E. fuscus are accmpanied by changes in ttal pectralis prtein mass (Table 2). Hwever, ttal pectralis prtein mass des nt remain in cnstant relatin t pectralis muscle mass thrughut the year (Table 2). The muscle enlargement ccurring in prehibernatin fattening differs qualitatively with that in pregnancy. The cncentratin f prtein in the pectralis muscles f pregnant ( rag- g muscle- 1) and nnpregnant summer bats (24.7_+ 6.6 rag. g muscle- 2) remains cnstant despite the large change in muscle mass (Table 2). Muscle prtein cncentratin during prehibernatin fattening ( mg.g muscle -1) is significantly greater (P < 0.001) than in summer bats. This reflects a very large elevatin f ttal prtein mass during prehibernatin fattening (Table 2; Fig. 4). During hibernatin bth muscle mass and muscle prtein cncentratin decrease significantly (Table 2). The increased prtein cncentratin in pectralis muscle during prehibernatin fattening is puzzling. Typically, muscle hypertrphy ccurs with n change in muscle prtein cncentratin (fr example, see Gldberg et al. 175; Ianuzz and Chen 17). In fact, the muscle enlargement during pregnancy in E. fuscus des nt invlve changes in prtein cncentratin (Table 2). The increased pectralis muscle prtein cncentratin in prehibernatin bats is nt due t verall dehydratin (Table 1). Therefre, it appears that the prtein cncentratin f the pectralis muscle is preferentially increased during prehibernatin fattening. Perhaps this may act as a prtein stre which is used during hibernatin, presumably fr glucnegenesis. cknwledgements. This wrk was supprted by a blck grant frm the Hrace H. Rackham Schl f Graduate Studies, University f Michigan. I wuld like t thank W.R. Dawsn, R.L. Marsh, P. Myers, and M.L. Zimmerman fr their interest in this prject and fr their criticism f the manuscript.

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