Figure 1. Group-level relationship between the proportion of males among helpers and (a) proactive prosociality and (b) and social tolerance.

Cotton-top tamarin and golden-headed lion tamarin groups are labelled according to Table 1, all other data points represent common marmoset groups. The dotted lines in (b) connect data points from groups that have been tested a second time after the elapse of several years, after several changes in group composition had occurred.