Figure 1. Helping behavior for adult rats tested with ingroup and outgroup members.

Adult rats selectively helped ingroup members. (A) Diagram of the helping behavior test (HBT) with a trapped rat. The trapped rat was either a cagemate of the same strain (ingroup, left) or a stranger of an unfamiliar strain (outgroup, right). (B, C) Rats released ingroup members but not outgroup members, as expressed by % door-openings (B) and mean (± SEM) latency to open (C) across testing sessions. The dashed line indicates the half-way door-opening by the experimenter. (D–F) Representative movement patterns of rats tested with an ingroup or outgroup member, depicted by a heatmap of the rat’s location along the session. Rats were more active (G–I) and spent more time around the restrainer (J–L) in the presence of a trapped ingroup member than an outgroup member. Lighter dots indicate non-openers, dark dots are openers.