Figure 2. Rats of the SD strain helped trapped rats of the LE strain only if they were familiar with an LE individual.
Three experimental conditions are diagrammed at top. The free rat was always from the SD strain and was housed with a cagemate, denoted at top, from either the SD (white) or LE (black and white) strain. The free rat was then tested with an LE rat that was either the cagemate (‘a’) or a stranger (‘?’). Note that in the LE familiar condition (right), rats had previously housed with an LE rat (illustrated) but were housed with an SD rat (not illustrated) at the time of testing. Across the days of testing, the median latency to door-opening (bottom) decreased for SD rats tested with trapped LE cagemates (left), but not those tested with LE strangers (middle). Like rats in the LE cagemate condtion, rats in the LE familiar condition (right) also became openers.