Fig. 1: An evolutionary framework for non-breeding and cooperative behaviors based on Hamilton’s inequality.

The evolution of non-breeding and cooperative behaviors depends on the expected fitness associated with engaging in non-breeding and cooperative actions (the cooperative option) relative to alternative non-cooperative actions outside the group (the outside option) or inside the group (the inside option). The cooperative option can be favored by selection because of its beneficial effects on kin (e.g., via helping relatives) and in the future (e.g., via territory inheritance) and because of the low expected fitness associated with the outside option (e.g., due to ecological constraints) and the inside option (e.g., due to social constraints). In general, when we are trying to explain the evolution of non-breeding strategies, X_i on the left hand side and r_j Y_j, f_j Z_j, r_k Y_k, f_k Z_k on the right hand side (terms in light gray) are considered to be zero or trivially small compared to other terms (terms in black).