Developpement de la locomotion chez le mulot sylvestre (Apodemus sylvaticus L.)

This longitudinal study describes the functional development of locomotion in the woodmouse, Apodemus sylvaticus . It shows that postnatal day (PND) 11 is a critical age in the acquisition of independence. Locomotion develops in successive stages before reaching the adult pattern (walking) at PND 11. Intermediate patterns are determined by the degree of coordination between the different parts of the body. Some of these patterns overlap and do not necessarily appear in every individual in our experiments. The young woodmouse respond optimally in three sensorimotor tests on PND 10 (contact righting, negative geotaxis) and on PND 11 (cliff avoidance). Immature reflexes, which may have specific functions in a short period of the early development have then disappeared. Simultaneously, there is a transition from stereotyped to oriented reactions (exploration, flight, etc.) which depend more and more on nonspecific stimulations (manipulation, etc.) rather than on specific ones (slope, cliff). These decisive changes are accomplished two days before eye opening. We can parallel them to the drastic ones that we reported elsewhere in the production of ultrasonic vocalisations. These signals elicit the retrieval of the pups by the mother, thereby compensating for the incapacity of the pup to move in an organised way. This study gives a basis for comparisons with other Rodents. In general, the woodmouse matures earlier than the rat by about two days, but later than the housemouse by two days. However, this delay might vary according to the given task. We discuss our results from a methodological point of view, and underline the advantages of measuring a continuous variable against an arbitrary criterion.