The role of natural selection in the evolution of trait complexity can be characterized by testing hypothesized links between complex forms and their functions across species. Predatory venoms are traits composed of multiple proteins that collectively function to incapacitate prey. Venom complexity fluctuates considerably over evolutionary timescales, with apparent increases and decreases in complexity, yet the evolutionary causes of this variation is unclear. Here, we tested alternative hypotheses for the link between venom complexity and...