These panels illustrate the construction of maturation reaction norms (MRNs) for von Bertalanffy growth.

This figure corresponds to Figs 2 and 3 for linear growth. R₀ is the underlying fitness function for the top two panels (A, B); r is the underlying fitness function for the bottom two (C, D). The y-axis corresponds to log-length in the two panels on the left (A, C) and to linear length in the two panels on the right (B, D). The graphs of the von Bertalanffy growth curves are represented by black dotted lines in all 4 panels. The iso-fitness curves (from Fig 1) are solid grey lines in the top 2 panels and solid red lines in the bottom two. For each growth curve, one finds the highest iso-fitness curve that intersects it. The locus of such points, as growth varies, traces out the MRN (solid black curve). In the top two panels the MRN(R₀) is a vertical line bowed to the right; in the bottom two panels the MRN(r) is mostly horizontal. The blue curve marks the R₀ = 1 or r = 0 iso-fitness curve.