The development of sensorimotor reflexes in the Brazilian opossum Monodelphis domestica

The development of a number of sensorimotor reflexes was studied in the Brazilian opossum Monodelphis domestica. At birth, an opossum's forelimbs execute rhythmic, alternate movements which resemble swimming, whereas the hindlimbs are little more than embryonic buds that do not move independent of the trunk. It is possible, therefore, to witness the entire development of hindlimb motility, the advent of coordination between forelimbs and hindlimbs, and the development of ambulation. The following sequence in the appearance and maturation of the reflexes was observed: rooting, an innate reflex in mammals which disappears over time; withdrawal of the forelimbs followed by withdrawal of the hindlimbs; crossed extension of the forelimbs and then of the hindlimbs; grasp, the forelimbs preceding the hindlimbs; body righting on a surface; forward hopping of the forelimbs followed by the hindlimbs; lateral and medial hopping of the forelimbs, then the hindlimbs; chin tactile placing; body righting in the air; and, finally, visual placing. Limb tactile placing is not significantly expressed, even in the adult. This behavioral sequence generally matches the sequence of somatic (trunk, limbs, head) and neural (spinal cord and brain) structures involved in the control of these behaviors.