Evaporative Water Loss in Two Sympatric

J. Zool., Lond. (1995) 235, 269-278

Evaporative water loss in two sympatric species of vespertilionid bat, Plecotus auritus and Myotis daubentoni: relation to foraging mode and implications for roost site selection

P. I. WEBB*, J. R. SPEAKMAN AND P. A. RACEY Department of Zoology, University of Aberdeen, Aberdeen AB9 2TN, UK

(Accepted 3 December 1993)

(With 1 figure in the text)

Simultaneous measures of oxygen consumption and evaporative water loss (EWL) were made in two species of temperate-zone vespertilionid bat (Plecotus auritus and Myotis daubentoni; mean

body mass 9·12 and 10'12g, respectively) at ambient temperatures (Ta ) of 5,15 and 25°C and variable vapour pressure deficit. EWL was directly dependent on vapour pressure deficit and oxygen consumption and inversely dependent on Ta . EWL was significantly greater in P. auritus than in M. daubentoni. A model for EWL in P. auritus under a variety of environmental conditions (5-25°C and 20-80% relative humidity) suggested that EWL from bats in shallow summer torpor will be lowest at low Ta , and that, except at low « 50%) relative humidity, EWL from euthermic bats will be lowest at high Ta . At low relative humidity « 20%), resting bats could lose over 30% of body mass per day (24 h) through evaporation. At high 'T, (> 25 QC), EWL from euthermic bats could be over 65% lower at high (> 80%) compared to low « 20%) relative humidity. In bats in shallow summer torpor at low (5°C) T, the equivalent saving was > 96%. At low relative humidity predicted EWL from bats in shallow summer torpor was 34 to 81% of that from euthermic bats, and at low T, and high relative humidity was only 2%. In the wild, M. daubentoni has freer access to drinking water than does P. auritus and yet EWL at rest was higher in the latter species. We suggest that post-prandial dumping of urinary water by M. daubentoni leads to a limit in the amount of body water available to this species to cover evaporative losses once within the day roost, which in turn has led to an adaptation of physiology towards the minimization of EWL when at rest.

Contents

Page Introduction 270 Methods .. 270 Experimental animals . . 270 Experimental procedure 271 Results 274 Discussion. . 275 The importance of roost site and thermoregulatory status for evaporative water loss 275 Comparative evaporative water loss in P. auritus and M. daubentoni 276 References. . 276

• To whom all correspondence should be sent at: Mammal Research Institute, University of Pretoria, Pretoria 0002, South Africa 269 © 1995 The Zoological Society of London 270 P. 1. WEBB, J. R. SPEAKMAN AND P. A. RACEY Introduction

The small body size of vespertilionid bats means that they have a comparatively high surface area to volume ratio, which may be enhanced further by the possession of large areas of naked skin in the form of flight membranes. The surface area available for the potential loss of evaporative water is therefore high. Indeed, despite their apparent lack of sweat glands (Sisk, 1957), their ability to restrict blood flow to the wings (Nicoll & Webb, 1946), the tendency to fold away the flight membranes when at rest (e.g. Speakman, 1988), and the ability of bats to extract oxygen from the lungs with high efficiency (e.g. Chappell & Roverud, 1990), the rate of loss of water via evaporation from resting bats appears to be high in comparison with similar-sized terrestrial mammals and birds (Studier, 1970). In the summer, bats may be forced to spend up to 18 or more hours ofeach day confined within a roost in which drinking water is not available. As, in most bat species, no structural manipulation of the roost site (e.g. nest building) occurs (Kunz, 1982), there may be selective pressure for bats to select roost sites, either with environmental conditions that limit the rate at which individuals expend energy or lose water. Alternatively, there may be selective pressure for bats to choose as roosting sites structures which favour rapid beneficial modification of local air temperature and relative humidity once the bat (or bats) are in situ. Although evaporative water loss has been measured in a number of bat species (e.g. Macrotus californicus, Bell, Bartholomew & Nagy, 1986; Pizonyx vivesi, Carpenter, 1968; Leptonycteris sanborni, Eptesicus fuscus and Tadarida brasiliensis, Carpenter, 1969; Noctilio albiventris, Chappell & Roverud, 1990; Antrozous pallidus, Chew & White, 1960; E. juscus and T. brasiliensis, Herreid & Schmidt-Nielsen, 1966; T. brasiliensis, A. pallidus and Myotis yumanensis, Licht & Leitner, 1967; Natalus stramineus, Glossophaga soricina, Myotis nigri­ cans and Artibeus cinereus, Studier, 1970; Myotis thysanodes and M. lucifugus, Studier & O'Farrell, 1976; M. lucifugus, Procter & Studier, 1970), the relationship between the rate of evaporative water loss and environmental conditions such as ambient temperature and vapour pressure deficit has yet to be clarified. Our intention in the current paper is to establish these relationships in two sympatric species of temperate zone vespertilionid, the brown long-eared bat (Plecotus auritus) and Daubenton's bat (Myotis daubentoni). These two species are often found in the same roost but differ in other aspects of their water balance (Webb, Speakman & Racey, 1994). We then use these relationships to predict the pressure for, and the direction of roost site selection for, the restriction of evaporative water loss in P. auritus outside the hibernal period.

Methods

Experimental animals The brown long-eared bat (Plecotus auritus) and Daubenton's bat (Myotis daubentoni) are both temperate-zone members of the Vespertilionidae with body masses in the wild between 6 and 12g (Speakman et al., 1991a). Experiments were performed on 4 P. auritus and 3 M. daubentoni caught in summer roosts in central and north-east Scotland in August/September 1989 and 1990 under licence from the Nature Conservancy Council.Allanimalsweremaintainedunder a 12L: 12Dphotoperiod in a room in which they could fly freely, and wereprovidedwith unlimitedaccess to food (mealworms; Tenebrio sp.) and drinking water. EVAPORATIVE WATER LOSS IN BATS 271 lIxperirnentalprocedure Individuals were removed from the flight room, deprived of food for 12h, weighed and then placed in a perspex respirometry chamber (volume 190ml) containing a wooden wall on which they could hang. The chamber was connected into an open flow respirometry system through which air was drawn at one of 3 rates (8'00,14'9 or 26·8 em- s-l). Air flow rate was monitored using a flow meter (DM3A, Alexander Wright Ltd., Winchester, UK). These air flow rates are insufficient to cause significant changes in metabolic heat production in small mammals (e.g. Chappell & Holsclaw, 1984). Before entering the respirometry chamber, air was dried by passing it across silica gel. The relative humidity of gas leaving the chamber was determined using a humidity probe (Vaisala, Helsinki) connected to a data logger (SQ1201, Grant Instruments, Cambridge, UK) which logged a mean value once a minute. After drying, the oxygen content of gas leaving the chamber was determined on a paramagnetic oxygen analyser (series 1100, Servomex, Crowborough, UK) and mean values were logged automatically by a microcomputer (BBC B-series, Acorn Ltd.) once a minute. The lag time between a change in the oxygen content or relative humidity of gas within the chamber and that change being measured was less than 60 s. Similarly, the half-life for the turnover of gas within the system was less than 60 s. The respirometry chamber was housed in a cooled incubator (Gallenkamp type INL-401-01ON) in which ambient temperature was set to either 5,15 or 25°C. Five and 25 °C represent the approximate mean minimum nightly outside temperature and mean maximum daytime roost temperature for P. auritus roosts in north-east Scotland between May and August (Speakman & Racey, 1987). Measured air temperature within the respirometry chamber deviated from ambient temperature within the incubator by less than 0·2 0C. Each bat was allowed to settle within the chamber for 30 min after which the relative humidity and oxygen content of excurrent gas from the chamber were monitored simultaneously for 60 consecutive minutes. The bat was checked regularly during this period to ensure that it was not mobile. The bat was then removed from the chamber and replaced with its conspecifics. Oxygen consumption was determined from the depletion of oxygen in the gas leaving the chamber compared to ambient and the flow rate of gas through the system and was converted to standard temperature and pressure of dry air (STPD: O°C, 101325N· m-z, e.g. Webb et a!., 1992). Evaporative water loss was determined from the relative humidity of excurrent gas from the chamber, ambient temperature and the flow rate of air through the system (e.g. Buffenstein & Jarvis, 1985). The outlet pipe from the chamber was positioned adjacent to the wooden wall on which the bats hung so that vapour pressure deficit of gas leaving the chamber could be used as an estimate of that experienced by the bats. Values of oxygen consumption, evaporation and vapour pressure deficit were averaged over the 60-min duration of each experiment.

TABLE I Results of a full factorial ANCOVA of evaporative water loss (ul- min-I) on ambient temperature (To in °C), vapour 2 pressure deficit (VPdej in N.m- ) , oxygen consumption (V02 in ml- min-I) and body mass (g) in Plecotus auritus (n = 4 individuals each measured 12 times) and M. daubentoni (n = 3 individuals each measured 12 times) with species as a grouping factor. Non-significant parameters are not shown

Coefficient Variable df F P (mean ± 1 S.D.)

Species 58 4·90 0·031 (Intercept: P. auritus 35 -9,8 < 0·001 -0'282±0'172 M. daubentoni 26 -15,1 < 0·001 - 0·375 ± 0,129) VPdef 58 16·3 < 0·001 0·00107 ± 0·00026 V02 58 14-4 < 0·001 0·673 ± 0·178 VPdef X r, 58 4-7 0·034 -0,0000175 ± 0·0000081 272 P. I. WEBB, J. R. SPEAKMAN AND P. A. RACEY

3 (a)

o

2·5 cr o "" cr

C ~ 2 E. o ~ 0 0.co > ui

0·5

0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

2 Vapour pressure deficit (N/m )

2-4 (b)

2 0 0

c 0 1·6 ~ 0"2:,0 E. 0

0·4

O+------,------....,------,------j 0·0 0·2 0·4 0·6 0·8 Oxygen consumption (mllmin)

FIG. 1 EVAPORATIVE WATER LOSS IN BATS 273 Each individual was measured 12 times; once at each combination of air flow rate and ambient temperature. The dependence of evaporative water loss on oxygen consumption, ambient temperature, body mass and vapour pressure deficit was determined through a step-wise removal general linear model (GLM) analysis of covariance with species as a grouping factor (Minitab, Penn. State University). The best-fit multiple linear regression model describing evaporative water loss in resting bats generated by the GLM analysis was used to model evaporative water loss in P. auritus under a variety of environmental roosting conditions. Evaporative heat loss as a proportion of metabolic heat production and daily evaporative water loss as a proportion of body mass were estimated by assuming the calorific equivalent of oxygen to be 20·1 J . ml-1 (Elia & Livesey, 1988), the heat ofvapourization of water to be 2·45 J. g-I and using values for oxygen consumption by euthermic P. auritus and by P. auritus in shallow summer torpor when body temperature and oxygen consumption are reduced but not to the same extent as during hibernation (e.g. Speakman, Webb & Racey, 1991b; Webb, Speakman & Racey, 1993a), as predicted from empirical equations given by Webb et al. (1992).

1-4 (c) co 1·2 • Cl Cl

C

:§ Cl OJ £ Cl Ul Ul Cl • .2 0·8 coCD ... ~ Cl •• Q) > 0·6 Cl .~ . 0 . Cl c, eo Cl Cl > ill 0·4 Cl • Cl 0·2

Cl

Cl 0 0 10000 20000 30000 40000 "'"50000 60000 2 Ambienttemperature x Vapourpressure deficit(C.N/m )

FIG. 1. The dependence of evaporative water loss on three variables: (a) vapour pressure deficit; (b) oxygen consumption; and (c) vapour pressure deficit x ambient temperature in Plecotus auritus (D) and Myotis daubentoni (.) as determined through general linear model analysis (see text and Table I). In each case, evaporative water loss was corrected to the mean values across both species for two of the variables using the coefficients given in Table I, and then plotted against the third variable. The solid lines represent regression lines generated by taking mean slope across both species (coefficients in Table I) (in no case was the slope significantly different between the species) and passing this slope through the mean x and y value for each species independently. As EWL was significantly higher in P. auritus than it was in M. daubentoni (Table I), the lines for the former always fall above those for the latter. 274 P. 1. WEBB, 1. R. SPEAKMAN AND P. A. RACEY

TABLE II An empirically basedmodel showing evaporative heat loss (EHL) as a proportion ofmetabolic heat production (MHP) and daily evaporative water loss (DEWL) as a roportion ofbody mass ( BM) in resting Plecotus auritus (mean BM = 9·12 g). 1 EWL = evaporative water loss in ul- min- . Ta = ambient temperature in DC. VOl = oxygen consumption in ml- min- as predictedfrom Webb et al. (1992). RH = relative humidity as %

EHL DEWL T. RH Thermoregulatory status V02 EWL MHP BM

5 20 Euthermic 1·69 1·54±0·24 0·11 ± 0·02 0·24±0·04 Semi-torpid 0·18 0·53 ± 0·04 0·36 ± 0·03 0·08 ± 0·01 50 Euthermic 1·69 1'29 ± 0·25 0'09 ± 0·02 0·20 ±0·04 Semi-torpid 0·18 0·27 ± 0·05 0·18 ± 0·03 0·04 ± 0·01 80 Euthermic 1·69 1·03 ± 0·28 0·07 ± 0·02 0·16 ± 0·04 Semi-torpid 0'18 0·02 ±0,1O 0·01 ± 0·07 0·00 ± 0·02 15 20 Euthermic 1·23 1·65 ± 0·17 0·16 ± 0·02 0·26 ± 0·03 Semi-torpid 0·21 0·96 ±0·07 0·56 ± 0·04 0·15 ± 0·01 50 Euthermic 1·23 1·24±0·17 0·12 ± 0·02 0·20 ± 0·03 Semi-torpid 0·21 0·55 ± 0'03 0·32 ± 0·01 0·09 ± 0·00 80 Euthermic 1·23 0·83 ± 0·20 0·08 ± 0·02 0·13 ± 0·03 Semi-torpid 0'21 0·14 ± 0·09 0·08 ± 0·05 0·02 ± 0·01 25 20 Euthermic 0·77 1·83 ± 0·13 0·29 ± 0·02 0·29 ± 0·02 Semi-torpid 0'24 1·48 ± 0·07 0·75 ± 0·03 0·23 ± 0·01 50 Euthermic 0·77 1·24 ± 0·12 0·20 ± 0·02 0·20 ±0·02 Semi-torpid 0·24 0·88 ± 0·04 0-45 ± 0·02 0·14 ± 0·01 80 Euthermic 0·77 0·64 ± 0·15 0·10 ± 0·02 0·10 ± 0·02 Semi-torpid 0·24 0·28 ± 0·09 0·14 ± 0·05 0·04 ± 0·01

Results

Mean body mass of the P. auritus was 9·12 g (S.D. = 0'38, range = 8'69-9'59, n = 4) while that of the M. daubentoniwas lO'l2g (S.D. = 1'12, range = 8,97-11'20, n = 3). The results ofa GLM multivariate analysis of covariance for the dependence of evaporative water loss on species as a grouping factor and ambient temperature, body mass, oxygen consumption, and vapour pressure deficit as independent variables are shown in Table I. The model explained 72·1% of the variation in evaporative water loss (F3,59 = 50'8, P < 0'001). There was a significant direct linear dependence of evaporative water loss on vapour pressure deficit and oxygen consumption, and a significant inverse linear dependence ofevaporative water loss on the product of vapour pressure deficit and ambient temperature. Body mass had no significant linear effect on evaporative water loss in either species. For no variable was the coefficient significantly different between the two species, thus the coefficients quoted in Table I are mean values across both species. The intercept of the multiple linear regression equation generated by the GLM analysis was significantly different between the two species (Table I), implying that evaporative water loss was on average 0·0928 mg . min - [ greater in P. auritus than it was in M. daubentoni. The independent effects of vapour pressure deficit, oxygen consumption and vapour pressure deficit x ambient temperature on evaporative water loss were visualized by correcting evapora­ tive water loss to the mean values (across both species) of two of the three variables and then plotting these corrected values against the third variable (Figs l a, b and c). An empirically based model showing the effect of ambient temperature (5, IS or 25 "C), relative humidity (20,50 or 80%) and thermoregulatory status (euthermic vs. shallow summer torpor) on evaporative water loss in P. auritus is given in Table II. The model implies that at low « 20%) EVAPORATIVE WATER LOSS IN BATS 275 ambient relative humidities, bats could lose up to 30% of their body mass per day (24 h) as evaporative water (Table II). The rate of evaporative water loss could, however, be reduced by more than 65% in euthermic bats at 25 DC and more than 96% in bats in shallow summer torpor at 5 DC by selecting a roost site of high (> 80%) rather than low « 20%) relative humidity (Table II).

Discussion

The importance of roost site and thermoregulatory status for evaporative water loss The results show that evaporative water loss was directly dependent on vapour pressure deficit and oxygen consumption and inversely dependent on ambient temperature in both species studied. These results implicate a definite cost, in terms of a high rate of evaporative water loss, for individuals that select roosting sites with certain environmental or structural properties. The daily rates of evaporative water loss reported here (Table II), and the consequent pressure for the selection of roost sites that minimize evaporative water loss, could be even higher when the potentially high rate of evaporative water loss during flight is taken into account (Carpenter, 1969). Total daily water influx in free-living insectivorous bats in the summer has been estimated at between 19 and 69% of body mass (Carpenter, 1969; Bell et al., 1986; Kurta et al., 1989) and of this between 28 and 44% is attributable to evaporative water loss (Carpenter, 1969, Kurta et al., 1989). Studier, Procter & Howell (1970) found three species of vespertilionid bat to lose approximately 15 to 16% of body mass during a l2-hour roosting period in natural roosts. In a separate study, Studier & Ewing (1971) found the body water content of two species of vespertilionid to remain constant during the roosting period. As body water content of bats is approximately 70% (Studier & Ewing, 1971), this implies that approximately 70% of body mass loss was attributable to the loss of body water. As Studier et al. (1970) found the LDso for four species ofvespertilionid to range between 23 and 32% of body mass loss, it seems likely that roost site selection and thermoregulatory behaviour within the roost are both important factors ensuring survival of non-hibernal bats. Nevertheless, vespertilionid bats can be found in roost sites of both high (> 75%) and low « 5%) relative humidities in the wild (e.g. Studier & Ewing, 1971). Only at low ambient humidity « 50%) will it be beneficial, in terms of reducing evaporative water loss, for a euthermic individual to select a roost oflow ambient temperature, and even then the savings are marginal (Table II). At high relative humidities (> 50%), a roost site of higher temperature is preferential. However, when in shallow summer torpor, evaporative water loss is always lower at low ambient temperature regardless of the relative humidity (Table II). This implies that, except when euthermic at low relative humidity, the environmental conditions that lead to a low rate of energy expenditure within the roost (i.e. low ambient temperature when in shallow summer torpor, high ambient temperature when euthermic; e.g. Herreid & Schmidt­ Nielsen, 1966; Speakman & Racey, 1987; Speakman et al., 1991b; Webb et al., 1992) also lead to a low rate of evaporative water loss. The predicted reduction in evaporative water loss when in shallow summer torpor compared to when euthermic was highest (96%) at low ambient temperature (5°C) and high relative humidity (80%) (Table II). The differential in energy expenditure between euthermic bats and bats in shallow summer torpor is also greatest at low ambient temperature (e.g. Webb et al., 1992). Our predictions for the savings achieved at low relative humidity were 20 to 70%, a range which 276 P. L WEBB, J. R. SPEAKMAN AND P. A. RACEY includes estimates made by Herreid & Schmidt-Nielsen (1966) of one-half to two-thirds in Tadarida brasiliensis.

Comparative evaporative water loss in P. auritus and M. daubentoni Under any given set of environmental conditions (ambient temperature and vapour pressure deficit), evaporative water loss was greater in P. auritus than in M. daubentoni (Table I). Myotis daubentoni is a species with a strong association with open water in terms of the location of both roost sites (Richardson, 1989; Speakman et al., 1991a) and foraging sites (Miller & Degn, 1981; Swift & Racey, 1983) often gaffing prey from the water surface (Jones & Rayner, 1988). In contrast, P. auritus is a species normally associated with woodland habitat (Boyd & Stebbings, 1989) gleaning a high proportion of its prey from tree surfaces (Anderson & Racey, 1991; Shiel, McAney & Fairley, 1991). We might therefore expect access to free-water to be limited in P. auritus and thus, in apparent contradiction to the data presented here, that conservation of body water and minimization of evaporative water loss will be of greater priority in P. auritus than in M. daubentoni. However, access to water is only higher for M. daubentoni when it is foraging outside the roost. Thus, unless temporary storage of water in some form occurs within the body, it is only when outside the roost that we might expect water conservation to be greater in P. auritus than in M. daubentoni. Although the rate of post-prandial faecal water loss is the same in both species when fed an equivalent meal size (Webb, Speakman & Racey, 1993b), post­ prandial urine loss in water-deprived bats is over twice as great in M. daubentoni than it is in P. auritus (Webb et al., 1994). This high urinary loss may be an adaption to enable dumping of excess water taken in when foraging (Webb et al., 1994). However, such a high post-prandial sensible water loss in M. daubentoni may lead to a reduced availability of body water for evaporative water loss once the bats have returned to the day roost. The data presented here suggest that M. daubentoni do have a physiology that, when at rest within the day roost, leads to a reduced rate of evaporative water loss compared to P. auritus under equivalent environmental conditions. Myotis daubentoni could also reduce water loss within the roost by behavioural selection of roost sites that minimize evaporative water loss (see Table II). Alternatively, a high rate of urine loss by M. daubentoni within the roost could lead to elevated relative humidity, thus reducing the requirement to select for roost of high ambient humidity. However, although there are no comparative data available on the temperature and humidity of roosts occupied by P. auritus and M. daubentoni, both species sometimes occupy the same roost (Swift & Racey, 1983; Speakman et aI., 1991a), suggesting that such selectivity may not be particularly strong.

PIW was supported by a NERC post-graduate studentship. We are grateful to Tom Kunz and Dominic Houlihan for constructive comments on an earlier draft of the manuscript.

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