Mechanics of lung ventilation in a post-metamorphic salamander, Ambystoma tigrinum

The mechanics of lung ventilation in frogs and aquatic salamanders has been well characterized, whereas lung ventilation in terrestrial-phase (post-me tamorphic) salamanders has received little attention. We used electromyogra phy (EMG), X-ray videography, standard videography and buccal and body cavi ty pressure measurements to characterize the ventilation mechanics of adult (post-metamorphic) tiger salamanders (Ambystoma tigrinum), Three results e merged: (i) under terrestrial conditions or when floating at the surface of the water, adult A. tigrinum breathed through their nares using a two-stro ke buccal pump; (ii) in addition to this narial two-stroke pump, adult tige r salamanders also gulped air in through their mouths using a modified two- stroke buccal pump when in an aquatic environment; and (iii) exhalation in adult tiger salamanders is active during aquatic gulping breaths, whereas e xhalation appears to be passive during terrestrial breathing at rest. Activ e exhalation in aquatic breaths is indicated by an increase in body cavity pressure during exhalation and associated EMG activity in the lateral hypax ial musculature, particularly the M, transversus abdominis, In terrestrial breathing, no EMG activity in the lateral hypaxial muscles is generally pre sent, and body cavity pressure decreases during exhalation, In aquatic brea ths, tidal volume is larger than in terrestrial breaths, and breathing freq uency is much lower (approximately 1 breath 10 min(-1) versus 4-6 breaths m in(-1)). The use of hypaxial muscles to power active exhalation in the aqua tic environment may result from the need for more complete exhalation and l arger tidal volumes when breathing infrequently. This hypothesis is support ed by previous findings that terrestrial frogs ventilate their lungs with s mall tidal volumes and exhale passively, whereas aquatic frogs and salamand ers use large tidal volumes and and exhale actively.