Figure 1.
Torpid and euthermic metabolic rates measured by open flow respirometry.
Silver-haired bats were held at a range of temperatures similar to local conditions. Open circles and dashed regression line indicate resting metabolic rate (RMR) and filled circles and solid regression line indicate torpid metabolic rate (TMR).
Figure 2.
Temperature profiles of free-living silver-haired bats during autumn migration.
Vertical lines indicate sunrise and sunset, horizontal dashed line indicates the individually determined threshold below which the bat was considered torpid (see Materials and Methods), black dots indicate skin temperature, and gray dots indicate ambient temperature. The amount of time spent torpid depended on ambient conditions. Mean ambient temperature over the period of observation was 27.4°C (Sept 3), 21.9°C (Aug 28), and 18.3°C (Sept 8) in panels (a), (b), and (c) respectively. Accordingly, in (a) the bat spent only a brief period of the morning in torpor, whereas at cooler temperatures bats spent most (b) or all (c) of the day torpid.
Figure 3.
Relative energy savings of daytime torpor use during stopover.
(a) On cooler days, when the energetic cost of defending normal body temperature would be greater, bats spent more time in torpor. (b) Bats saved 12–91% of the estimated energy required to remain euthermic, and saved more energy on cooler days.
Figure 4.
Estimated field metabolic rate of migrating bats calculated for the entire daytime period of observation.
Metabolic rate was determined from ambient temperature and the regression lines from respirometry trials (Fig. 1) accounting for periods of torpor and euthermia. The lines correspond to the expected field metabolic rate if the bats had remained strictly euthermic (dashed line), or torpid (solid line) based on respirometry trials. The secondary y-axis converts metabolic rate to the mass of fat required.