Figure 1: Basic model.

Arrows indicate evolutionary trajectories (assuming traits evolve at rates proportional to sex-specific selection gradients) whose stable end points are marked as black dots. Dashed lines indicate equal caring by both sexes. If the sexes differ only in care duration (the null model, (a)), trajectories end on a line of equilibria along which there is no selection, at a given total amount of care. If males face stronger sexual selection (b), or lower parentage certainty (c), or both (d), female-only care evolves. Parameter values: k=n=M=r=1; ; α=0.1.