The developmental order of bipedal locomotion in the jerboa (Jaculus orientalis): pivoting, creeping, quadrupedalism, and bipedalism.

This is a brief report on the postnatal development of locomotor behavior in the jerboa, a bipedal kangaroo-like rodent. Observations on one litter revealed three intriguing aspects of the postnatal development of the jerboa compared to other rodent species: (a) The weaning period is extended, (b) the developmental stage in which pivoting is the main locomotor activity is extended, and (c) locomotor performance is differently related to anatomical growth. Jerboa pups are born after a long pregnancy compared to other altricial rodents, but possess typical neonate morphology: The hindlegs and forelegs are of the same length, the tail is short, skin pigmentation and fur are absent, and the eyes and ears are closed. However, the neonate jerboa differs from other rodents in posture and activity: Its hindlegs extend laterally to the same side of the pelvis and it creeps with stepping of only the forelegs that drag the trunk while the hindlegs remain passive. Pivoting and creeping are preserved in the jerboa for 4 weeks, as compared to a few days in other species. Afterwords, quadruped locomotion emerges and the jerboa pup walks while folding its long hindlegs to the same functional length as the forelegs. Bipedal locomotion is acquired only in postnatal Day 47. These observations illustrate that further studies of the development of the jerboa, as well as other bipedal rodent species, may provide new perspectives on anatomy, histology, physiology, and motor behavior during postnatal development.