Fig. 2.

Experimental setup of the dyadic games. See also video S2. The donor individual in the right-hand compartment can pull the upper or lower tray within reach of the recipient individual in the left-hand compartment (who is grasping for the food reward in the upper left food bowl, on the tray that is being pulled by the donor). The recipient cannot pull the trays because she has no access to the handle. An individual’s prosociality refers to the pulling frequency when a partner is present minus the pulling frequency in the control condition when no partner is present. Note that once a tray is pulled, it stays close to the wire mesh, so the recipient has access to the food bowl