Hypaxial musculature is recruited during increased respiratory drive in anuran amphibians

Inspiration in anuran amphibians occurs by a positive‐pressure buccal pump. The expiratory phase of breathing is thought to rely exclusively on passive recoil of the lungs owing to the greater intra‐pulmonary pressure relative to atmospheric pressure. However, some evidence suggests that anurans are capable of recruiting the hypaxial musculature to assist expiration during specific behaviors, such as calling. This study examined the role of hypaxial muscle contraction and its contribution to expiration in bullfrogs ( Rana catesbeiana ) and cane toads ( Bufo marinus ). We recorded activity of hypaxial muscles (abdominal and external/internal oblique) with electromyography (EMG) while recording breathing cycles using buccal pressures. Similar to previous studies, we observed little or no EMG activity during quiet, resting conditions. By contrast, hypaxial muscles were actively recruited just prior to an inhalation cycle when breathing frequency increased in response to exercise or ‘handling stress’. These data indicate that hypaxial musculature is capable of recruitment during behaviors that are associated with an increase in overall respiratory drive and increased respiratory frequency. Active expiration under these circumstances may be necessary to empty the lungs sufficiently prior to inspiration. Supported by NIH‐SO6 GM48135 and NSF‐IBN 0110713.