IS HYPERTHERMIA A CONSTRAINT ON THE DIURNAL ACTIVITY OF BATS

Bats are normally nocturnal, despite some potential advantages of bein g active during the day. A possible constraint on diurnal activity is hyperthermia. We evaluated the hyperthermia hypothesis by constructing a biophysical model which considered all the heat fluxes across a bat 's wing during diurnal flight and predicted a critical ambient air tem perature (T-acrit) above which a bat would be unable to fly without ex periencing fatal hyperthermia. Many factors had important influences o n T-acrit, including time of day, latitude, cloud cover and foliage co ver. Ground surface temperature and ground albedo had significant but mutually opposed effects. Important organismal factors included body m ass (larger bats were more susceptible to overheating), aspect ratio ( lower aspect ratios more susceptible), flight speed (slower more susce ptible), and the albedo and transmittance of the wing membranes (darke r more susceptible). Using the expected latitudinal variation in the e nvironmental components we suggest hyperthermia will constrain the diu rnal flight of large bats (c. 900 g) at about 85% of sites between 40 degrees S and 40 degrees N. For intermediate sized bats (90 g) hyperth ermia will constrain diurnal activity at 50-60% of sites between 20 an d 30 degrees N and degrees S but is less important around the equator (constrained at only 10% of sites). For small bats (9 g) hyperthermia will constrain diurnal activity at about 30-40% of sites between 20 an d 30 degrees N and degrees S, but less than 1% of sites at the equator . For all sizes of bats hyperthermia probably constrains flight in the day at less than 1% of sites above 50 degrees N or degrees S.