MATCHING GAS-EXCHANGE IN THE BAT FROM FLIGHT TO TORPOR

Many microchiropteran bats can reduce their metabolic rate three order s of magnitude during heterothermic torpor, This extraordinary range p rovides a unique insight into the adaptability of mammalian ventilator y control and function, To enable powered flight, bats have developed the highest capacity gas exchange system among mammals. However, starv ing during winter may account for the greatest mortality among bats th at hibernate, thus imposing a strong selective pressure to decrease me tabolic cost during torpor, This high capacity gas exchange system mus t therefore operate efficiently at very reduced rates, despite conflic ting 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 breath ing rates and tidal volumes near predicted optimal levels, In addition , a passive oxygen influx coupled with a high acidotic tolerance facil itates longer intervals between the breathing bouts, The acidotic tole rance supports the endurance of these apneas because the passive efflu x of carbon dioxide does not match the rate of oxygen influx, The acid otic tolerance further helps by allowing carbon dioxide to enrich the alveolar gas during apnea to levels above that of a nonacidotic, conti nuous pattern of breathing, Thus, the bat's carbon dioxide load can be cleared in fewer breaths when breathing resumes, By efficiently contr olling a high capacity gas exchange system to meet the minuscule deman ds during torpor, the bat demonstrates how physiological control strat egies can adapt to overcome limitations imposed by conflicting selecti on pressures.