© 1997 by The Society for Integrative and Comparative Biology
Matching Gas Exchange in the Bat from Flight to Torpor1
Deep Springs College
SYNOPSIS. Many microchiropteran bats can reduce their metabolic rate three orders of magnitude during heterothermic torpor. This extraordinary range provides a unique insight into the adaptability of mammalian ventilatory control and function. To enable powered flight, bats have developed the highest capacity gas exchange system among mammals. However, starving during winter may account for the greatest mortality among bats that hibernate, thus imposing a strong selective pressure to decrease metabolic cost during torpor. This high capacity gas exchange system must therefore operate efficiently at very reduced rates, despite conflicting mechanical constraints imposed by an enormous functional overhead. The bat surmounts this dilemma by adjusting its control strategy to breathe intermittently during torpor. This allows instantaneous breathing rates and tidal volumes near predicted optimal levels. In addition, a passive oxygen influx coupled with a high acidotic tolerance facilitates longer intervals between the breathing bouts. The acidotic tolerance supports the endurance of these apneas because the passive efflux of carbon dioxide does not match the rate of oxygen influx. The acidotic tolerance further helps by allowing carbon dioxide to enrich the alveolar gas during apnea to levels above that of a nonacidotic, continuous pattern of breathing. Thus, the bat's carbon dioxide load can be cleared in fewer breaths when breathing resumes. By efficiently controlling a high capacity gas exchange system to meet the minuscule demands during torpor, the bat demonstrates how physiological control strategies can adapt to overcome limitations imposed by conflicting selection pressures.