REILLY, Stephen M.; WHITE, Thonmas D.; Ohio University, Athens; Buffalo State College, Buffalo: Active exhalation, loco-ventilatory integration and size constraints on locomotor scope in mammals

We demonstrate for the first time the active role of the abdominal muscles in ventilation and the speed dynamics of locomotor-ventilatory integration in small mammals, which reveals a critical size constraint that has significant implications for the origin of mammals. In marsupial mammals, abdominal hypaxial muscles are actively involved in both resting and locomotory ventilation but in a size-dependent manner. Because of relatively higher natural frequencies of the ventilatory mechanism in small mammals (Pygmy opossums) they are limited to high ventilation rates at both rest and during locomotion. This size constraint greatly limits the locomotor scope of small mammals that actively exhale at ~10 Hz at rest AND during locomotion. Their hypaxial muscles are used to actively exhale at ~10 Hz (with shallow diaphragmatic movements) and locomotion is constrained to limb cycle rates that match 10 Hz (5 trotting couplets/s). This pattern of abdominal hypaxial function in exhalation appears to represent the primitive condition for Mammaliaformes. Larger mammals (Virginia opossums), with relatively lower natural frequencies, ventilate at ~2Hz at rest (with deep diaphragmatic ventilation) and are able to increase ventilatory rates during locomotion. Their hypaxial muscles act to pressurize the body cavity at rest (with mild tonus) but act in focussed bursts as part of the �cross-couplet locomotor linkage� used to stiffen the body during locomotion. We propose that the evolution of increased size in mammals during the Cenozoic may have been to escape the ventilatory limitations of small size.