Geotaxis in 2-week-old Norway rats (Rattus norvegicus): A reevaluation.

In 1926, Crozier and Pincus first reported that 2-week-old rats placed head-down on an inclined plane orient in a head-up direction; this response is called negative geotaxis. In Experiment 1, we replicated this finding by testing 12- to 14-day-old rats on an inclined plane covered with wire mesh. Pups oriented in a head-up direction and avoided the head-down direction at inclines of 45 degrees but not 30 degrees. Because pups in Experiment 1 appeared to grasp the wire mesh with their claws, pups in Experiment 2 were now tested on a smooth but high-friction substrate. At inclines of 30 degrees, 35 degrees, and 40 degrees, pups did not exhibit significant tendencies to orient in a head-up direction or avoid a head-down direction. Finally, in Experiment 3, the effect of substrate on geotaxis was tested further by comparing pups' behaviors at 40 degrees with the inclined plane covered with either wire mesh or the high-friction substrate. Pups' orientation behaviors differed on the two substrates. Taken together, these data suggest that testing substrate affects the orientation behaviors of young rats and raise questions about the plausibility of applying the concept of geotaxis to young mammals, at least when tested on an inclined plane.