Abdominal muscle function in ventilation and locomotion in new world opossums and basal eutherians: Breathing and running with and without epipubic bones.

All tetrapods have the same four basic abdominal hypaxial muscle layers that wrap around the abdomen between the pelvis, ribcage, and spine. However, the marsupials and our immediate mammalian ancestors have epipubic bones extending anteriorly into the ventral hypaxial layers with two additional muscles connecting them to the ventral midline and femur. Studies of two marsupials have shown that all of the abdominal hypaxials play a part bilaterally in resting ventilation and during locomotion there is an asymmetrical pattern of activity as the hypaxial muscles form a cross-couplet linkage that uses the epipubic bone as a lever to provide long-axis support of the body between diagonal limb couplets during each step. The cross-couplet epipubic lever system defines the earliest mammals and is lost in placental mammals. To expand our understanding of the evolution of mammalian abdominal muscle function and loco-ventilatory integration we tested the generality of the cross-couplet system in marsupials and conducted the first formal studies of hypaxial abdominal motor patterns in generalized placental mammals focusing on a representative rodent and insectivore. These new data reveal 1) that continuous abdominal muscle tonus during resting ventilation and a 1:1 breath to step cycle during locomotion appear to be the basal condition for mammals, 2) that the loss of epipubic bones in eutherians is associated with a shift from the cross-couplet dominated motor pattern of marsupials to a shoulder-to-pelvis system with unilateral activation of abdominal muscles during locomotion and 3) that hypaxial function in generalized eutherians is more similar to marsupials than cursorial mammals.