EVAPORATIVE WATER-LOSS IN 2 SYMPATRIC SPECIES OF VESPERTILIONID BAT, PLECOTUS-AURITUS AND MYOTIS-DAUBENTONI - RELATION TO FORAGING MODE ANDIMPLICATIONS FOR ROOST SITE SELECTION

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 (T-a) of 5, 15 and 25 degr ees C and variable vapour pressure deficit. EWL was directly dependent on vapour pressure deficit and oxygen consumption and inversely depen dent on T-a. EWL was significantly greater in P. auritus than in M. da ubentoni. A model for EWL in P. auritus under a variety of environment al conditions (5-25 degrees C and 20-80% relative humidity) suggested that EWL from bats in shallow summer torpor will be lowest at low T-a, and that, except at low (< 50%) relative humidity, EWL from euthermic bats will be lowest at high T-a. At low relative humidity (< 20%), re sting bats could lose over 30% of body mass per day (24h) through evap oration. At high T-a (> 25 degrees C), EWL from euthermic bats could b e over 65% lower at high (> 80%) compared to low (< 20%) relative humi dity. In bats in shallow summer torpor at low (5 degrees C) T-a the eq uivalent saving was > 96%. At low relative humidity predicted EWL from bats in shallow summer torpor was 34 to 81% of that from euthermic ba ts, and at low T-a and high relative humidity was only 2%. In the wild , M. daubentoni has freer access to drinking water than does P. auritu s and yet EWL at rest was higher in the latter species. We suggest tha t post-prandial dumping of urinary water by M. daubentoni leads to a l imit in the amount of body water available to this species to cover ev aporative losses once within the day roost, which in turn has led to a n adaptation of physiology towards the minimization of EWL when at res t.