- Email: [email protected]
Diurnal fluctuation in urine concentration in the little brown bat, Myotis lucifugus, in a natural roost
DIURNAL FLUCTUATION IN URINE CONCENTRATION IN THE LITTLE BROWN BAT, MYOTIS ~UC~~UGU~, IN A NATURAL ROOST KENNETH N. GELUSO* and EUGENE H. STUDIER? Museum of Southwestern Biology, University of New Mexico. Albuquerque, NM 87131, U.S.A. and Institute of Scientific Research, New Mexico, Highlands University, Las Vegas, NM 87701, U.S.A. (Received
1 Mar&
1978)
Urine collected periodically from naturally roosting Myotis lucijhgus on a rainy, overcast day and on a successive sunny, warm day showed no differences in concentration between sexes, age groups, or activity level (torpid or active). 2. Time of day at which samples were collected and the day of collection both showed significant effects on urine con~ntration. 3. Urine concentration dropped progressively on the rainy day from 2750mOsm/kg at predawn to 1890 mOsm/kg at sunset. 4. On the sunny day, midday samples were not different from the previous day; however, samples collected at dusk (2640 mOsm/kg) were significantly more concentrated. 5. Naturally roosting M. luc@gus excreted urine that was 275 mOsm/kg below their mean maximum urine concentration. Abstract-l.
MATERIALS AND
INTRODUCTION
The maximum concentrating capacity of the kidney indicates the degree to which mammals can conserve urinary water under natural conditions, but the extent to which these capabilities are actually employed in the field has only begun to be investigated. For example, MacMillen (1972) Bradford (1974) and Bakko (1975, 1977a) compared concentrations of field-collected urine in rodents during different seasons of the year with concentration values obtained from dehydrated, laboratory animals. We feel that another important approach to these field studies is to determine daily fluctuations in urine concentration, particularly during those days in an animal’s life cycle when demands for water conservation are great. Many bat species are ideal subjects for this approach. They are readily captured in certain maternity roosts, and urine can be collected easily from sunrise to sunset; thus, estimates of diurnal flux in urine concentration can be obtained for about I6 hr of the day. Behavioral effects on urine concentration due to handling are eliminated because urine can be collected immediately upon capture (see Bakko, i977b). For comparative purposes, laboratory data are now available on renal concentrating abilities of many bats (Carpenter, 1968, 1969; McFarland & Wimsatt, 1969; Vogel & Vogel, 1972; Geluso, 1975, 1978). To provide information on actual renal performance of bats under natural conditions, we report here some initial studies on diurnal fluctuations in urine concentration in naturally roosting Myotis lucif~glis.
* Present address: Department of Biology, University of Nebraska-Omaha, Omaha, NE 68101, U.S.A. t Present address: Department of Biology, University of Michigan-Flint, Flint, MI 48503, U.S.A. 471
On 24
METHODS
and 25 August, 1971, urine samples were collected periodically from naturally roosting Myotis lucifugus at a maternity roost located in the attic of Montezuma Seminary, Montezuma (elevation 2043 m). San Miguel Co., New Mexico. AIthough the third-floor attic is divided into many rooms, most of the 30&500 M. lucz~“uguswere located in four interconnected rooms which are generally orientated in a north-south direction along the east wing of the seminary. The rooms have a saddle roof with the apex 4.5 m above the floor. Urine samples were taken from animals roosting in clusters along the ridge board and rafters at the apex of the roof in rooms 1 and 2 in the schematic diagram of the attic (see O’Farrell & Studier, 1973). To collect urine, each bat was gently removed from its roosting site and quickly turned on its back. Usually within a few seconds the bat urinated. If not, a firm squeeze with the hand holding the bat and/or a gentle squeeze on the bladder often elicited urination. Urine was collected directly from the urethra with glass capillary tubes (1.5-2.0 mm dia.) which were immediately sealed with two Critocaps (Clay-Adams), labeled and frozen on dry ice. Sex, age, degree of activity or torpidity and time of collection (Mountain Daylight Savings Time) also were recorded for each bat before being released. Osmotic concentration of urine was measured with a freezing-point osmometer (Advanced Instruments, Inc., model 3W). All values are expressed in osmolality, milliosmols,kg water (mOsm/kg). All urine samples were volumetrically diluted by one of the following factors: 51-fold dilution (5 ~1 of urine in 250 11 of distilled water, final volume = 255 PI), 26fold (lOi1 in 250~1). lo-fold (25 ~1 in 225 ul) or S-fold (50 ul in 200 ~1).Identical dilutions were made using standards‘ of 500, bdo, 2000, 3000, 40 and 5000 mOsm/kg. Because consistent deviations in osmotic concentrations were found after diluting these standards and multiplying the measured osmolality by the proper dilution factor, a correction curve was constructed and used to correct the diluted samples. The range of repeatability when using the above dilutions was f2.8 mOsm/ kg.
412 HESL’LTS
On 24 August 1971, a light rain fell from 0530 until 1510 hr. and the rest of the day remained overcast. Based on data from many earlier studies of one of us (EHS), the range in roost temperature throughout that day would lie between 20 and 25°C with ambient water vapor pressure (WVP) ranging from 4 to 8 mm Hg in those areas occupied by hats. The next day. 25 August 1971, was a typical hot. sunny day. Roost temperatures of 16-20 ‘C at dawn and 3G40 C at dusk at the apex of the roof were typical. Although temperatures at the apex of the roof would rise rapidly to 3&4O’C (Studier & O’Farrell. 1972). M. I~c~$cytt.s behaviorally thermoregulate by choosing roosting areas where the temperature remains about 32 C. Table 1 shows the WVP and temperature of the microclimate occupied by M. IuL.~~~~~~u.s on these sunny days. Eighty-four urine samples were obtained from M. /tcc$ryus. Postlactating females, adult males and flying juvemics (I : I .86, males: females) provided 58.3, 17.9 and X8”,,. respectively. of the total samples collected. On 24 August, urine was collected during four periods, tw-o in the morning and one each in the aftcrnoon and evening. On 25 August. urine was collected once in the afternoon and once in the evening. Torpid animals were found only during the morning and afternoon collections on 24 August. A five-way analysis of variance (Nie et al.. 197.5) showed that urine concent~tions were not different between sexes (P = 0.99). ages (P = O.Y9) or torpid and active animals (P = 0.23): however, the time of day and day of collection both had a significant effect on urine concentration (P < 0.02). All interactions were not significant (P > 0.07). Mean values of urine concentration for each collection period during both days were determined by averaging all values obtained from animals regardless of sex, age or degree of activity (Fig. 1). On the cool. rainy day (24 August). urine concentrations gradually decreased from 2750 mOsm/kg at predawn to 2350 mOsm/kg at midday to 1890mOsm,kg at dusk. On the hot, sunny day (25 August). morning samples were not collected, but mid-day levels were not different from the previous day (P > 0.5, t-test). However, unlike the previous day, urine concentrations increased from midday (2410 mOsm/kg) to dusk (2640mOsm/kg). The mean urine concentrations reached by the end of the two days were significantly different (P < 0.001, r-test). Table 1. Temperature and water vapor pressure (WVP) of the microclimate occupied by roosting AJI. lucijugus throughout a typical sunny day [modified from Studier & Ewing (1971)]
Time 0500 0800
I100 1400 1700 2000
Temp.
WVP
(’ C)
(mm HE)
16.6
< 0.7
20.0 25.6 31.6 32.2 32.2
< 0.9 < 1.3 < 1.x 4.6 7.2
Fig. I. Diurnal fluctuation in urine concentration of naturally roosting M.~i.s /uc~fuqu.s on a rainy day. August 24 (open rectangles) and on a sunny day. August 25 (shaded rectangles) 1971. Vertical lines represent ranges, horizontal lines indicate means and rectangles enclose the interval ji + fo.cs S.E. Sample sires are given beside each Dicegram. Brackets indicate the range in time (Mountain Daylight Saving Time) in which samples were collected on August 24 (bottom brackets) and August 75 (top brackets).
DISCUSSION
Each evening during late summer, Iittle brown bats leave their maternity roost at Montezuma Seminary and proceed directly to the nearby river to drink and forage. These bats also have been observed drinking at this river throughout the night and during predawn hours (Studier, unpublished data). Bats re-enter the seminary before dawn. Most bats defecate shortly after their return to the roost indicating that digestive activity is in progress (Studier ef ai., 1970); alternatively, some bats seem to dispense with digestive activity after reentering the roost, for their stomachs are found nearly full at the end of the day (Studier & O’Farrell, 1972; O’Farrell & Studier, 1976). The difference in diurnal fluctuation of urine concentration between 24 and 25 August apparently resulted from daily differences in roost temperature and WVP; temperatures were higher and WVPs generally lower during the sunny day. Increases in heat and dryness will cause increases in evaporative water loss, as well as subsequent dehydration and rise in blood solute concentration (Studier, 1970; Studier et ul., 1970; Procter & Studier, 1970; Studier & Ewing, 1971). Such increases in blood concentration would stimulate the osmoreceptors of the hypothalamus to release more antidiuretic hormone (ADH) and thus cause a greater reduction in urinary water during the sunny day. This phenomenon can be further substantiated as follows. In a study of diurnal fluctuation in blood ion concentration in naturally roosting M. it~ifuyus on a hot day, Studier & Ewing (1971) found
Roosting urine concentration
a rise in serum sodium and chloride levels of approximately 6.5 m-equiv/l from 1400 to 2000 hr. During this same time span, urine osmotic pressure rose by 230 mOsm/kg (Fig. 1). Thus, the rise in urine osmotic pressure allows for urinary water retention and minimal rise in blood electrolyte levels on hot, dry days. The decrease in urine concentration from predawn to dusk on the rainy day is comparable to the gradual decline in concentration shown by M. lucifugus from 7 to 24 hr after feeding and drinking in the laboratory (Geluso, 1975). The initial rise in urine osmotic pressure seen in the laboratory probably occurred prior to our first urine collection in the roost. The higher, initial ~on~ntration of urine found in roosting bats (2750mOsm/kg, Fig. I) as compared to laboratory animals (2375 mOsm/kg, Geluso, 1975) is most likely related to the slightly lower WVP in the roost than in the laboratory (temperatures were about the same). A slight decrease in WVP alone (for example, 9.5 to 3.7 mm Hg) can cause an increase in urine concentration up to 500 mOsm/kg in M. ll{c~~gMsthat have eaten and drunk (Geluso, unpublished data). As explained above, this increase in dryness can lead to an eventual reduction in urinary water. A second factor that also may be involved is that the naturally feeding bats may have had a greater protein intake. The mean maximum urine concentration of M. ~~{e~~g~sis 3025 mOsm/kg (Geiuso, 1975). This concentration is reached only for a certain period directly after feeding when animals are undergoing water stress. Even under water stress, urine osmolality will eventually decline after feeding as urea concentration falls due to its excretion. The effect of urea from protein metabolism on urine con~ntrating ability is well documented [for example, see Levinsky & Berliner (1959) and Schmidt-Nielsen & Robinson (1970); for a possible mechanism to account for this phenomenon, see Schmidt-Nielsen (1977)]. When urea concentration in the interstitial fluid of the renal medulla was markedly low, M. luc~fu~us undergoing water stress in the laboratory produced a mean urine concentration of 1625mOsm/kg 32 hr after feeding (Geluso, 1975). Thus, the concentration level of 2640mOsm/kg produced by M. lucijiigus at the end of the hot day probably represents the highest concentration attainable by the bats with the amount of urea and ADH present in their system at dusk. At predawn, naturally roosting M. 1ucI~ugus excreted urine with a mean concentration that was 275 mOsm/kg below their mean maximum concentration. Furthermore, the possibility exists that just prior to this collecting period concentrations may have been even higher. This small differential between naturally produced urine and the m~imum level attainable by this species would indicate that under natural conditions, M. tucifugus would face problems with water balance only when they eat and do not drink prior to returning to the roost. The cessation of protein digestion, which sometimes occurs under natural conditions
(Studier & O’Farrell,
1972; O’Far-
rell & Studier, 1976), may help certain bats avoid a solute load that may ultimately result in a net loss of water at dangerous
levels (see Geluso,
1975).
Acknowledgement-We thank Dr R. W. Dapson for his critical evaluation of the manuscript.
in M. ~~c~~g~s
473
REFERENCES
BAKKO E. B. (1975) A field water balance study of gray
squirrels (Sciurus ciurus
and red squirrels (TamiasBiochem. Physiol. 51A.
carolinensis)
hudsonicus).
Comp.
759-768. BAKKO E. B. (1977~) Field water balance performance
prairie dogs
(Cynomys Comp. Biochrm. Physiol.
i~cur~s
and C.
in
~udo~icianus).
56A. 443-451. BAKKOE. B. (1977h) Influence of collecting techniques on estimate of natural renal function in red squirrels. Am. Midl.
Nat. 97, 502-504.
BRADFORDD. F. (1974) Water stress of free-living myscus truei.
Ecology
Pero-
55, 1407--i 414.
CARPENTER R. E. (1968) Salt and water metabolism in the marine fish-eating bat, Pizonq;x oivesi. Comp. Bioehrm. Physiol. 24. 95 i-964. CARPENTER R. E. (1969) Structure and function of the kidney and the water balance of desert bats. Physiol. Zool. 42, 288.-302. GELUSOK. N. (1975) Urine concentration cycles of insectivorous bats in the laboratory. J. camp. Physiol. 99, 309-319. GELLISOK. N. (1978) Urine concentrating ability and renal structure of insectivorous bats. J. Mammal. 59, 312-323. LEVINSKYN. G. & BERLINERR. W. (1959) The role of urea in the urine concentrating mechanism. J. din. Inrest. 38, 741-748. MACMILLENR. E. (1972) Water economy of nocturnal desert rodents. Symp. zool. Sot. Land. 31, 147-174. MCFARLANDW. N. & WIMSA~ W. A. (1969) Renal function and its relation to the ecology of the vampire bat, Desmodus 1006.
rotundus.
NIE N. H., HULL C.
Comp.
Biochem.
Physiol.
28,
985-
H., JENKINSJ. G., STEINRRENNER K.
& BENTD. H.
(1975) SPSS: Social Sciences, pp. 398-433.
Statistical
Package for
the
McGraw-Hill, New York.
O’FARRELLM. J. & STUDIERE. H. (1973) Reproduction, growth and development in Mvotis f~ysanodes and M. Iucifugus (Chiroptera: Vespertiiionidae) in northeastern New Mexico. Ecology 54, -18-30. O’FARRELLM. J. & STUDIERE. H. (1976) Cvclicai chances in flight characteristics, body &mpo&ibn and or&n weights in Myoris thysanodes and M. lucifugus (Chirop-
tera: Vespertilionidae). Bull.
South
Calif:
Acad.
Sci, 75,
258-266.
PROCTERJ. W. & STUDIERE. H. (1970) Effects of ambient temperature and water vapor pressure on evaporative water loss in Mvotis lucifuaus. J. Mammal. 51. 799--804. SCHMIDT-NIELSEN ‘B. (1977) Excretion in mammals: role of the renal pelvis in the modification of the urinary concentration and composition. Federation Proceedings 36, 2493-2503.
SCHMIDT-NIELSEN B. & ROBINSONR. R. (1970) Contribution of urea to urinary concentrating ability in the dog. Am. J. Physiol.
218, 1363-1369.
STUDIERE. H. (1970) Evaporative water loss in bats. Biochem.
Physiol.
Comp.
35, 935-943.
STUDIERE. H. & EWING W. G. (1971) Diurnal fluctuation in weight and blood composition in Mbaootisnigricans and Iif: ~uc~ugus. Comp. 3j~chern~ P~ysiol.'~A, ii9- 139, STUDIERE. H. & O’FARRELL M. J. (19721 Bioloav of Myotis thysanades and M. lucifugus (Ciiroptera: V&pertilionidae)--I. Thermoreguiation. Comp. Biochem. Physiol. 41A,
567-596.
STUDIERE. H., PROCTERJ. W. & HOWELLD. J. (1970) Diurnal body weight loss and tolerance of weight loss in five species of Myotis. ~...~arnrna~. 51, 302-309. VOGELV. & VOGELW. (1972) Uber das Konzentrationsvermogen der Nieren zweier Fledermausarten (Rhinopoma hardwickei und Rhinolophus ferrum equinum) mit unterschiedlich ianger Nierenpapille. 2. uergl. Physiol. 76, 358-371.
1 Mar&
1978)
Urine collected periodically from naturally roosting Myotis lucijhgus on a rainy, overcast day and on a successive sunny, warm day showed no differences in concentration between sexes, age groups, or activity level (torpid or active). 2. Time of day at which samples were collected and the day of collection both showed significant effects on urine con~ntration. 3. Urine concentration dropped progressively on the rainy day from 2750mOsm/kg at predawn to 1890 mOsm/kg at sunset. 4. On the sunny day, midday samples were not different from the previous day; however, samples collected at dusk (2640 mOsm/kg) were significantly more concentrated. 5. Naturally roosting M. luc@gus excreted urine that was 275 mOsm/kg below their mean maximum urine concentration. Abstract-l.
MATERIALS AND
INTRODUCTION
The maximum concentrating capacity of the kidney indicates the degree to which mammals can conserve urinary water under natural conditions, but the extent to which these capabilities are actually employed in the field has only begun to be investigated. For example, MacMillen (1972) Bradford (1974) and Bakko (1975, 1977a) compared concentrations of field-collected urine in rodents during different seasons of the year with concentration values obtained from dehydrated, laboratory animals. We feel that another important approach to these field studies is to determine daily fluctuations in urine concentration, particularly during those days in an animal’s life cycle when demands for water conservation are great. Many bat species are ideal subjects for this approach. They are readily captured in certain maternity roosts, and urine can be collected easily from sunrise to sunset; thus, estimates of diurnal flux in urine concentration can be obtained for about I6 hr of the day. Behavioral effects on urine concentration due to handling are eliminated because urine can be collected immediately upon capture (see Bakko, i977b). For comparative purposes, laboratory data are now available on renal concentrating abilities of many bats (Carpenter, 1968, 1969; McFarland & Wimsatt, 1969; Vogel & Vogel, 1972; Geluso, 1975, 1978). To provide information on actual renal performance of bats under natural conditions, we report here some initial studies on diurnal fluctuations in urine concentration in naturally roosting Myotis lucif~glis.
* Present address: Department of Biology, University of Nebraska-Omaha, Omaha, NE 68101, U.S.A. t Present address: Department of Biology, University of Michigan-Flint, Flint, MI 48503, U.S.A. 471
On 24
METHODS
and 25 August, 1971, urine samples were collected periodically from naturally roosting Myotis lucifugus at a maternity roost located in the attic of Montezuma Seminary, Montezuma (elevation 2043 m). San Miguel Co., New Mexico. AIthough the third-floor attic is divided into many rooms, most of the 30&500 M. lucz~“uguswere located in four interconnected rooms which are generally orientated in a north-south direction along the east wing of the seminary. The rooms have a saddle roof with the apex 4.5 m above the floor. Urine samples were taken from animals roosting in clusters along the ridge board and rafters at the apex of the roof in rooms 1 and 2 in the schematic diagram of the attic (see O’Farrell & Studier, 1973). To collect urine, each bat was gently removed from its roosting site and quickly turned on its back. Usually within a few seconds the bat urinated. If not, a firm squeeze with the hand holding the bat and/or a gentle squeeze on the bladder often elicited urination. Urine was collected directly from the urethra with glass capillary tubes (1.5-2.0 mm dia.) which were immediately sealed with two Critocaps (Clay-Adams), labeled and frozen on dry ice. Sex, age, degree of activity or torpidity and time of collection (Mountain Daylight Savings Time) also were recorded for each bat before being released. Osmotic concentration of urine was measured with a freezing-point osmometer (Advanced Instruments, Inc., model 3W). All values are expressed in osmolality, milliosmols,kg water (mOsm/kg). All urine samples were volumetrically diluted by one of the following factors: 51-fold dilution (5 ~1 of urine in 250 11 of distilled water, final volume = 255 PI), 26fold (lOi1 in 250~1). lo-fold (25 ~1 in 225 ul) or S-fold (50 ul in 200 ~1).Identical dilutions were made using standards‘ of 500, bdo, 2000, 3000, 40 and 5000 mOsm/kg. Because consistent deviations in osmotic concentrations were found after diluting these standards and multiplying the measured osmolality by the proper dilution factor, a correction curve was constructed and used to correct the diluted samples. The range of repeatability when using the above dilutions was f2.8 mOsm/ kg.
412 HESL’LTS
On 24 August 1971, a light rain fell from 0530 until 1510 hr. and the rest of the day remained overcast. Based on data from many earlier studies of one of us (EHS), the range in roost temperature throughout that day would lie between 20 and 25°C with ambient water vapor pressure (WVP) ranging from 4 to 8 mm Hg in those areas occupied by hats. The next day. 25 August 1971, was a typical hot. sunny day. Roost temperatures of 16-20 ‘C at dawn and 3G40 C at dusk at the apex of the roof were typical. Although temperatures at the apex of the roof would rise rapidly to 3&4O’C (Studier & O’Farrell. 1972). M. I~c~$cytt.s behaviorally thermoregulate by choosing roosting areas where the temperature remains about 32 C. Table 1 shows the WVP and temperature of the microclimate occupied by M. IuL.~~~~~~u.s on these sunny days. Eighty-four urine samples were obtained from M. /tcc$ryus. Postlactating females, adult males and flying juvemics (I : I .86, males: females) provided 58.3, 17.9 and X8”,,. respectively. of the total samples collected. On 24 August, urine was collected during four periods, tw-o in the morning and one each in the aftcrnoon and evening. On 25 August. urine was collected once in the afternoon and once in the evening. Torpid animals were found only during the morning and afternoon collections on 24 August. A five-way analysis of variance (Nie et al.. 197.5) showed that urine concent~tions were not different between sexes (P = 0.99). ages (P = O.Y9) or torpid and active animals (P = 0.23): however, the time of day and day of collection both had a significant effect on urine concentration (P < 0.02). All interactions were not significant (P > 0.07). Mean values of urine concentration for each collection period during both days were determined by averaging all values obtained from animals regardless of sex, age or degree of activity (Fig. 1). On the cool. rainy day (24 August). urine concentrations gradually decreased from 2750 mOsm/kg at predawn to 2350 mOsm/kg at midday to 1890mOsm,kg at dusk. On the hot, sunny day (25 August). morning samples were not collected, but mid-day levels were not different from the previous day (P > 0.5, t-test). However, unlike the previous day, urine concentrations increased from midday (2410 mOsm/kg) to dusk (2640mOsm/kg). The mean urine concentrations reached by the end of the two days were significantly different (P < 0.001, r-test). Table 1. Temperature and water vapor pressure (WVP) of the microclimate occupied by roosting AJI. lucijugus throughout a typical sunny day [modified from Studier & Ewing (1971)]
Time 0500 0800
I100 1400 1700 2000
Temp.
WVP
(’ C)
(mm HE)
16.6
< 0.7
20.0 25.6 31.6 32.2 32.2
< 0.9 < 1.3 < 1.x 4.6 7.2
Fig. I. Diurnal fluctuation in urine concentration of naturally roosting M.~i.s /uc~fuqu.s on a rainy day. August 24 (open rectangles) and on a sunny day. August 25 (shaded rectangles) 1971. Vertical lines represent ranges, horizontal lines indicate means and rectangles enclose the interval ji + fo.cs S.E. Sample sires are given beside each Dicegram. Brackets indicate the range in time (Mountain Daylight Saving Time) in which samples were collected on August 24 (bottom brackets) and August 75 (top brackets).
DISCUSSION
Each evening during late summer, Iittle brown bats leave their maternity roost at Montezuma Seminary and proceed directly to the nearby river to drink and forage. These bats also have been observed drinking at this river throughout the night and during predawn hours (Studier, unpublished data). Bats re-enter the seminary before dawn. Most bats defecate shortly after their return to the roost indicating that digestive activity is in progress (Studier ef ai., 1970); alternatively, some bats seem to dispense with digestive activity after reentering the roost, for their stomachs are found nearly full at the end of the day (Studier & O’Farrell, 1972; O’Farrell & Studier, 1976). The difference in diurnal fluctuation of urine concentration between 24 and 25 August apparently resulted from daily differences in roost temperature and WVP; temperatures were higher and WVPs generally lower during the sunny day. Increases in heat and dryness will cause increases in evaporative water loss, as well as subsequent dehydration and rise in blood solute concentration (Studier, 1970; Studier et ul., 1970; Procter & Studier, 1970; Studier & Ewing, 1971). Such increases in blood concentration would stimulate the osmoreceptors of the hypothalamus to release more antidiuretic hormone (ADH) and thus cause a greater reduction in urinary water during the sunny day. This phenomenon can be further substantiated as follows. In a study of diurnal fluctuation in blood ion concentration in naturally roosting M. it~ifuyus on a hot day, Studier & Ewing (1971) found
Roosting urine concentration
a rise in serum sodium and chloride levels of approximately 6.5 m-equiv/l from 1400 to 2000 hr. During this same time span, urine osmotic pressure rose by 230 mOsm/kg (Fig. 1). Thus, the rise in urine osmotic pressure allows for urinary water retention and minimal rise in blood electrolyte levels on hot, dry days. The decrease in urine concentration from predawn to dusk on the rainy day is comparable to the gradual decline in concentration shown by M. lucifugus from 7 to 24 hr after feeding and drinking in the laboratory (Geluso, 1975). The initial rise in urine osmotic pressure seen in the laboratory probably occurred prior to our first urine collection in the roost. The higher, initial ~on~ntration of urine found in roosting bats (2750mOsm/kg, Fig. I) as compared to laboratory animals (2375 mOsm/kg, Geluso, 1975) is most likely related to the slightly lower WVP in the roost than in the laboratory (temperatures were about the same). A slight decrease in WVP alone (for example, 9.5 to 3.7 mm Hg) can cause an increase in urine concentration up to 500 mOsm/kg in M. ll{c~~gMsthat have eaten and drunk (Geluso, unpublished data). As explained above, this increase in dryness can lead to an eventual reduction in urinary water. A second factor that also may be involved is that the naturally feeding bats may have had a greater protein intake. The mean maximum urine concentration of M. ~~{e~~g~sis 3025 mOsm/kg (Geiuso, 1975). This concentration is reached only for a certain period directly after feeding when animals are undergoing water stress. Even under water stress, urine osmolality will eventually decline after feeding as urea concentration falls due to its excretion. The effect of urea from protein metabolism on urine con~ntrating ability is well documented [for example, see Levinsky & Berliner (1959) and Schmidt-Nielsen & Robinson (1970); for a possible mechanism to account for this phenomenon, see Schmidt-Nielsen (1977)]. When urea concentration in the interstitial fluid of the renal medulla was markedly low, M. luc~fu~us undergoing water stress in the laboratory produced a mean urine concentration of 1625mOsm/kg 32 hr after feeding (Geluso, 1975). Thus, the concentration level of 2640mOsm/kg produced by M. lucijiigus at the end of the hot day probably represents the highest concentration attainable by the bats with the amount of urea and ADH present in their system at dusk. At predawn, naturally roosting M. 1ucI~ugus excreted urine with a mean concentration that was 275 mOsm/kg below their mean maximum concentration. Furthermore, the possibility exists that just prior to this collecting period concentrations may have been even higher. This small differential between naturally produced urine and the m~imum level attainable by this species would indicate that under natural conditions, M. tucifugus would face problems with water balance only when they eat and do not drink prior to returning to the roost. The cessation of protein digestion, which sometimes occurs under natural conditions
(Studier & O’Farrell,
1972; O’Far-
rell & Studier, 1976), may help certain bats avoid a solute load that may ultimately result in a net loss of water at dangerous
levels (see Geluso,
1975).
Acknowledgement-We thank Dr R. W. Dapson for his critical evaluation of the manuscript.
in M. ~~c~~g~s
473
REFERENCES
BAKKO E. B. (1975) A field water balance study of gray
squirrels (Sciurus ciurus
and red squirrels (TamiasBiochem. Physiol. 51A.
carolinensis)
hudsonicus).
Comp.
759-768. BAKKO E. B. (1977~) Field water balance performance
prairie dogs
(Cynomys Comp. Biochrm. Physiol.
i~cur~s
and C.
in
~udo~icianus).
56A. 443-451. BAKKOE. B. (1977h) Influence of collecting techniques on estimate of natural renal function in red squirrels. Am. Midl.
Nat. 97, 502-504.
BRADFORDD. F. (1974) Water stress of free-living myscus truei.
Ecology
Pero-
55, 1407--i 414.
CARPENTER R. E. (1968) Salt and water metabolism in the marine fish-eating bat, Pizonq;x oivesi. Comp. Bioehrm. Physiol. 24. 95 i-964. CARPENTER R. E. (1969) Structure and function of the kidney and the water balance of desert bats. Physiol. Zool. 42, 288.-302. GELUSOK. N. (1975) Urine concentration cycles of insectivorous bats in the laboratory. J. camp. Physiol. 99, 309-319. GELLISOK. N. (1978) Urine concentrating ability and renal structure of insectivorous bats. J. Mammal. 59, 312-323. LEVINSKYN. G. & BERLINERR. W. (1959) The role of urea in the urine concentrating mechanism. J. din. Inrest. 38, 741-748. MACMILLENR. E. (1972) Water economy of nocturnal desert rodents. Symp. zool. Sot. Land. 31, 147-174. MCFARLANDW. N. & WIMSA~ W. A. (1969) Renal function and its relation to the ecology of the vampire bat, Desmodus 1006.
rotundus.
NIE N. H., HULL C.
Comp.
Biochem.
Physiol.
28,
985-
H., JENKINSJ. G., STEINRRENNER K.
& BENTD. H.
(1975) SPSS: Social Sciences, pp. 398-433.
Statistical
Package for
the
McGraw-Hill, New York.
O’FARRELLM. J. & STUDIERE. H. (1973) Reproduction, growth and development in Mvotis f~ysanodes and M. Iucifugus (Chiroptera: Vespertiiionidae) in northeastern New Mexico. Ecology 54, -18-30. O’FARRELLM. J. & STUDIERE. H. (1976) Cvclicai chances in flight characteristics, body &mpo&ibn and or&n weights in Myoris thysanodes and M. lucifugus (Chirop-
tera: Vespertilionidae). Bull.
South
Calif:
Acad.
Sci, 75,
258-266.
PROCTERJ. W. & STUDIERE. H. (1970) Effects of ambient temperature and water vapor pressure on evaporative water loss in Mvotis lucifuaus. J. Mammal. 51. 799--804. SCHMIDT-NIELSEN ‘B. (1977) Excretion in mammals: role of the renal pelvis in the modification of the urinary concentration and composition. Federation Proceedings 36, 2493-2503.
SCHMIDT-NIELSEN B. & ROBINSONR. R. (1970) Contribution of urea to urinary concentrating ability in the dog. Am. J. Physiol.
218, 1363-1369.
STUDIERE. H. (1970) Evaporative water loss in bats. Biochem.
Physiol.
Comp.
35, 935-943.
STUDIERE. H. & EWING W. G. (1971) Diurnal fluctuation in weight and blood composition in Mbaootisnigricans and Iif: ~uc~ugus. Comp. 3j~chern~ P~ysiol.'~A, ii9- 139, STUDIERE. H. & O’FARRELL M. J. (19721 Bioloav of Myotis thysanades and M. lucifugus (Ciiroptera: V&pertilionidae)--I. Thermoreguiation. Comp. Biochem. Physiol. 41A,
567-596.
STUDIERE. H., PROCTERJ. W. & HOWELLD. J. (1970) Diurnal body weight loss and tolerance of weight loss in five species of Myotis. ~...~arnrna~. 51, 302-309. VOGELV. & VOGELW. (1972) Uber das Konzentrationsvermogen der Nieren zweier Fledermausarten (Rhinopoma hardwickei und Rhinolophus ferrum equinum) mit unterschiedlich ianger Nierenpapille. 2. uergl. Physiol. 76, 358-371.