An endpoint, descriptive, and kinematic comparison of skilled reaching in mice (Mus musculus) with rats (Rattus norvegicus).

The forelimb and paw movements of mice were compared with those of rats in two skilled reaching for food tasks. Both species could use a single forelimb to reach for food, as measured in two different tasks, although performance was better and more consistent in rats than in mice. Both species located the food using olfaction, and moved from a diagonal supporting pattern in which support and postural adjustments came mainly from the contralateral to reaching forelimb and the ipsilateral to reaching hindlimb. Forelimb movements in both species could be divided into 3 components: (1) a transport component in which the limb is lifted, aimed, advanced, and pronated over the food using mainly movements of the upper limb; (2) a manipulatory component in which the digits are pronated over the food in an arpeggio movement and in which the digits grasp the food using grips that are scaled to the size of the food; and (3) a withdrawal component, in which the paw is supinated by adduction by the upper arm and by a movement about the wrist, as the food is withdrawn and presented to the mouth. Kinematic measures showed that the initial velocity of reaching in mice was slower than for rats, but otherwise the shape of the velocity curve and the trajectory of the limb movement, above and beyond the food pellet, were similar in both species. The results suggest that the motor control of reaching in the two rodent species is almost the same despite an evolutionary divergence that occurred about 20 million years ago. This profile of movement could be useful for investigating motor systems in normal mice and it could be applied to the analysis of motor systems in mutant and transgenic mice suspected of having motor abnormalities.