Fig. 1: Experimental setup and trial sequence.

Experimental set-up and trial sequence. A illustrates the setup. There was a dual tank in which two fish shared the same water environment and could see each other but were separated by a partition that prevented food or fish from moving from one side to the other. The acting fish was required to choose between two targets, presented on a computer screen positioned above it. The two targets (prosocial, non-prosocial) differed in their outcome for the receiving fish. B is a photo of the actual set-up and a demonstration of the fish spitting in the laboratory setting. C shows the sequence of events in a typical trial. At the beginning of the trial, two black location markers flickered for 200 ms. After a 600 ms blank interval, the markers appeared again. This was repeated three times. The targets then appeared at one of three locations for 5500 ms (see Method section). After the fish spat or the target interval ended, a blank screen was shown for 5500 ms before the next trial began. At this time, the fish received the outcome according to its choice of target.