The effects of experimental bipedalism and upright posture in the rat and their significance for the study of human evolution

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Abstract
In the present study rats with front-leg, hind-leg, and asymmetrical bipedalism produced by complete amputation and their normal controls were analyzed in normal, upright and upside-down positions in order to establish certain physiological reactions and the effect of hyperfunction and hypofunction on the development of bone. Hyper- and hypofunction was further achieved by unilateral front and hind-leg paralysis and bilateral hind-leg exarticulation. The following observations were made: In permanently upright and upside-down position gravitational ptosis of the stomach and blood-pressure changes remain within the normal range. Only in the initial phase of the postural change is a temporary momentum experienced. Stress is increased or measurable only in upside-down position. These findings seem to indicate a considerable physiological ease in the acquisition of upright posture, not paralleled however by the osteological response. They also seem to have a bearing on the problem of human physiological adaptability to conditions of space flight. Hind-leg elongation of man can be paralleled in experimental hind-leg bipedalism; and the forearm elongation of brachiation can be duplicated relatively, though not absolutely, in the front-leg bipedalism of "brachiating" rats. The failure to reproduce absolute forearm and metacarpal length increase in experimental "brachiation" is probably due to reduced activity in the experimental form of locomotion differing from the increased activity of natural brachiation. The size of the humeral and femoral head and of the auricular surface of the pelvis and sacrum depend on function, as does the talus neck angle. Otic capsule rotation, which occurs in front-leg and hind-leg bipedalism, does not occur in upside-down position because of the "righting reflexes" of the rat. Certain features of the interior structure of bone are probably not altered by hypofunction as such, but by the mechanical factors resulting from the morphological changes produced by hypofunction. Hind-leg bipedalism produces an intensification of cervical lordosis, thoracic kyphosis and slight lumbar lordosis, as well as intervertebral disc herniation; and asymmetrical bipedalism produces a double scoliosis from head to tail. Scapulo-clavicular fusion is frequent in front-leg amputation, and femoro-tibial fusion in straightened hind-leg fixation. However, the frequency of internal diseases is in no way increased in any of the various experimental situations. Morphological changes produced experimentally by function and paralleling phylogenetic changes in primate and human evolution as well as in race formation, probably represent biomechanical advantages responsible for their selection.