The ontogenic development of sensorimotor reflexes and spontaneous locomotion in the Mongolian gerbil (Meriones unguiculatus).

Most behavioral studies on the ontogeny of sensorimotor reflexes and locomotion were done in quadrupedal species with equally developed forelimbs (FL) and hindlimbs (HL). In contrast, the Mongolian gerbil has long and strong HL but relatively small FL, indicating their differential use for locomotion. We have used the gerbil to study the ontogeny of a number of reflexes and locomotor acts to see if their sequence of appearance, their maturation, as well as their intensity of expression, differed from some other mammals. The following sequence was observed: forward FL hopping, FL grasp, forward HL hopping, surface body righting, chin tactile placing, lateral FL hopping, lateral HL hopping, medial FL hopping, medial HL hopping, visual placing, air body righting, and FL and HL tactile placing. When comparing FL and HL, a given reflex does not necessarily appear earlier in the limb that expresses it more strongly. The results are discussed in relation to the development of the central nervous system, the limb, and the locomotor behavior.