Figure 1.

Schema showing investment flow into male non-reproductive, pre- and post-ejaculatory expenditures, emphasizing sperm competition expenditures (within box with dotted outline); see text. Call these male expenditures (arrowheads) $j,k,\dots$, etc. If gain $G_i$ through each investment i follows monotonic diminishing returns, the ideal optimal overall strategy equalizes all marginal gains, i.e. $G_j^{\mathrm{\prime }}(j)=G_k^{\mathrm{\prime }}(k)=\hspace{0.17em}\dots .$, etc. Hence change in any one expenditure affects expenditure in all others, though the magnitudes of changes depend on the different forms of the $G_i$ curves. Note that $G_i$ is the average expenditure in trait i, which may vary with local information (e.g. sperm number). SFPs, seminal fluid proteins.