THE DEVELOPMENT OF SENSORIMOTOR REFLEXES IN THE BRAZILIAN OPOSSUM MONODELPHIS-DOMESTICA

The development of a number of sensorimotor reflexes was studied in th e Brazilian opossum Monodelphis domestica. At birth, an opossum's fore limbs execute rhythmic, alternate movements which resemble swimming, w hereas the hindlimbs are little more than embryonic buds that do not m ove independent of the trunk. It is possible, therefore, to witness th e entire development of hindlimb motility, the advent of coordination between forelimbs and hindlimbs, and the development of ambulation. Th e following sequence in the appearance and maturation of the reflexes was observed: rooting, an innate reflex in mammals which disappears ov er time; withdrawal of the forelimbs followed by withdrawal of the hin dlimbs; crossed extension of the forelimbs and then of the hindlimbs; grasp, the forelimbs preceding the hindlimbs; body righting on a surfa ce; forward hopping of the forelimbs followed by the hindlimbs; latera l and medial hopping of the forelimbs, then the hindlimbs; chin tactil e placing; body righting in the air; and, finally, visual placing. Lim b tactile placing is not significantly expressed, even in the adult, T his behavioral sequence generally matches the sequence of somatic (tru nk, limbs, head) and neural (spinal cord and brain) structures involve d in the control of these behaviors.