Decision-making in shoaling: zebrafish integrate cues of familiarity and group size.

Social groups vary in the benefits that they offer to individuals through characteristics such as group size and composition, and consequently individual animals often exhibit preferences for groups with properties indicating greater benefits for members. Animals choosing between social groups may have to balance different preferences and integrate information about multiple group features to make optimal decisions and select the group that offers the greatest net benefit. We investigated how preferences for familiar individuals and for larger groups interact in the decision-making of zebrafish (Danio rerio) given a choice between two conspecific shoals. Adult subjects were given a series of choice tests with a shoal of four familiar fish, and a shoal of between four to eight unfamiliar fish. Subjects were tested on their preferences to spend time in proximity to the two shoals, and to interact with them. Zebrafish preferred to interact with the familiar shoal when both shoals comprised four individuals, however they did not show a preference for either shoal when choosing between four familiar fish and either five or six unfamiliar fish. When choosing between four familiar fish and either seven or eight unfamiliar fish, zebrafish showed clear preferences for the larger unfamiliar shoals over the familiar shoals. Our findings establish that zebrafish evaluate both the size and familiarity status of conspecific shoals, and integrate these multiple sources of information into social decision-making.