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.