Force–velocity trade‐off in Darwin's finch jaw function: a biomechanical basis for ecological speciation?

Summary 1. Biomechanical trade-offs have been proposed to constrain trajectories of evolutionary diversification. In songbirds, however, one such trade-off may facilitate diversification; adaptations that enhance bite force capacity are assumed to constrain vocal performance by hampering velocities of beak gape modulations required for vocal resonance tracking during song production. Resulting divergence in vocal mating signals may thus generate mating isolation between groups that eat foods of differing size and hardness. 2. We tested for a force‐velocity trade-off in jaw function in Darwin’s finches, by measuring bite forces and jaw movements during song production in birds on Santa Cruz Island. Bite force and speed of jaw closing varied broadly in our sample, and were negatively correlated both within and among species. Moreover, these correlations were largely independent of overall body size and phylogenetic relationships. 3. Adaptations to varying food types thus appear to drive divergence not only in beak size and bite force, but also in jaw closing velocity and vocal performance capacity. These results support a biomechanical link between adaptive divergence and mating signal divergence, the two key features that were assumed to have driven this radiation.