Fig. 1. Alternative evolutionary models of social evolution.

Arrows represent permitted transitions between different social organizations for each model. (A) Unstructured models: Under this model, all possible transitions are allowed. ER: All rates are fixed to a single optimized rate parameter; SYM Forward and reverse rates between two states are identical; ARD: Rates are fully independent. (B) Increasing complexity (IC) model: Transitions are only allowed between solitary and pair living, pair living and unimale groups, and unimale groups and multimale organization. (C) Shultz et al. (4): Transitions are allowed from solitary to multimale and from multimale to pair living and to unimale and back. (D) Reversible-jump–derived model (RJ-MCMC): Transitions are permitted from solitary to pair living, from pair living to multimale, and from multimale to unimale. All transitions are reversible. The RJ-MCMC (dashed box) was the preferred model using Bayes factor (see Table 1).