Breathing with your belly: Abdominal exhalation, loco-ventilatory integration and size constraints on locomotion in small mammals

In mammals, diaphragmatic contractions control inhalation while contraction of some thoracic hypaxial muscles and the transversus abdominis muscle contribute to exhalation. Additional thoracic hypaxial muscles are recruited as accessory ventilatory muscles to improve inhalation and exhalation during locomotion. However, the contribution of abdominal hypaxial muscles to resting and locomotor ventilation is little understood in mammals and loco-ventilatory integration has not been studied in small basal mammals. We show for the first time that all of the abdominal hypaxial muscles actively contribute to both resting and locomotory ventilation in mammals but in a size-dependent manner. In large opossums (Didelphis), hypaxial muscles exhibit uniform mild tonus during resting ventilation (pressurizing the gut to aid in exhalation) and shift to phasic bursts of activity during each exhalation during locomotion. Smaller opossums (Monodelphis) actively exhale by firing the abdominal hypaxial muscles at ∼10 Hz at both rest and at preferred locomotor speeds. Furthermore, the large opossums entrained ventilation to limb cycling as speed increased while the small opossums entrained limb cycling to the resting ventilation rate during locomotion. Differences in these species are related to size effects on the natural frequency of the ventilatory system and increasing resting ventilation rates at small size. Large mammals, with lower resting ventilation rates, can increase ventilatory rates during locomotion, while the high resting ventilation rates of small mammals limits their ability to increase ventilation rates during locomotion. We propose that increase in mammalian body size during the Cenozoic may have been an adaptation or exaptation to overcome size effects on ventilation frequency.