Size-mediated trade-offs involving bridging ability. How a size-mediated trade-off between fecundity and bridging ability (females) and a size-mediated trade-off between other male competitive abilities (e.g. walking on the ground, male-male contest competition) and bridging ability (males) can lead to the evolution of extreme SSD and the reversal to monomorphism. Black circles, ancentral bi-variate trait; Red circles, trait values after the response to opposite selection in females and males; Blue circles, values after response to selection converging to similar female and male sizes. The horizontal dotted arrows denote the strength of selective forces (i.e. favouring bridging) decreasing body size and the vertical solid arrows denote the strength of selective forces increasing body size. The red and blue arrows denote the trajectories of the response to selection along the trade-off line. When fecundity selection in females is stronger than selection for dispersal, female gigantism evolves. When selection for dispersal by bridging is stronger in males, male dwarfism evolves. The join effect is the evolution of extreme SSD. Similarly, when dispersal is favoured over fecundity in females and other male competitive abilities are favoured over bridging in males, reversal to monomorphism will evolve.