| Abstract: | SYNOPSIS. Many microchiropteran bats can reduce their metabolicrate three orders of magnitude during heterothermic torpor.This extraordinary range provides a unique insight into theadaptability of mammalian ventilatory control and function.To enable powered flight, bats have developed the highest capacitygas exchange system among mammals. However, starving duringwinter may account for the greatest mortality among bats thathibernate, thus imposing a strong selective pressure to decreasemetabolic cost during torpor. This high capacity gas exchangesystem must therefore operate efficiently at very reduced rates,despite conflicting mechanical constraints imposed by an enormousfunctional overhead. The bat surmounts this dilemma by adjustingits control strategy to breathe intermittently during torpor.This allows instantaneous breathing rates and tidal volumesnear predicted optimal levels. In addition, a passive oxygeninflux coupled with a high acidotic tolerance facilitates longerintervals between the breathing bouts. The acidotic tolerancesupports the endurance of these apneas because the passive effluxof carbon dioxide does not match the rate of oxygen influx.The acidotic tolerance further helps by allowing carbon dioxideto enrich the alveolar gas during apnea to levels above thatof a nonacidotic, continuous pattern of breathing. Thus, thebat's carbon dioxide load can be cleared in fewer breaths whenbreathing resumes. By efficiently controlling a high capacitygas exchange system to meet the minuscule demands during torpor,the bat demonstrates how physiological control strategies canadapt to overcome limitations imposed by conflicting selectionpressures. |
