Levers and linkages: Mechanical trade-offs in a power-amplified system

Mechanical redundancy within a biomechanical system (e.g., many-to-one mapping) allows morphologically divergent organisms to maintain equivalent mechanical outputs. However, most organisms depend on the integration of more than one biomechanical system. Here we test whether coupled mechanical systems follow a pattern of amplification (mechanical changes are congruent and evolve towards the same functional extreme) or independence (mechanisms evolve independently). We examined the correlated evolution and evolutionary pathways of the coupled 4-bar linkage and lever systems in mantis shrimp (Stomatopoda) ultrafast raptorial appendages. We examined models of character evolution in the framework of two divergent groups of stomatopods – “smashers” (hammer-shaped appendages) and “spearers” (bladed appendages). Smashers tended to evolve towards force amplification, while spearers evolved towards displacement amplification. These findings show that coupled biomechanical systems can evolve syne...