Fig. 4

Hypothetic model of the evolution of digit ratio dimorphism modulated by multi-trait hormonal effects on developmental pathways. Acquisition of higher sensitivity to sex steroids in specific digits (indicated by stars, Digit II or IV) is initially independent of hormonal concentrations per se (see blue arrows), and the identity of dimorphic digits seems labile among tetrapod lineages (small black arrows among digits). The advent of sexual dimorphism in digit ratios evolves independently of variation in the hormonal milieu (blue arrows), and such trajectory results on a diversity of patterns of digit ratio dimorphism in Tetrapoda. Once the sensibility of a dimorphic digit to steroids is present in the lineage, the length of the digit (and consequently the digit ratio) changes in response to variations in hormonal concentrations and together with others traits that are also hormonally-regulated (doted black arrows).Multi-trait effects of circulating steroids during development (orange lines and arrows) establish correlated variation between digit ratios and other dimorphic traits