Mechanical stress, fracture risk and beak evolution in Darwin's ground finches (Geospiza)

摘要

Darwin's finches have radiated from a common ancestor into 14 descendent species, each specializ-ing on distinct food resources and evolving divergent beak forms. Beak morphology in the groundfinches (Geospiza) has been shown to evolve via natural selection in response to variation in foodtype, food availability and interspecific competition for food. From a mechanical perspective, how-ever, beak size and shape are only indirectly related to birds' abilities to crack seeds, and beak formis hypothesized to evolve mainly under selection for fracture avoidance. Here, we test the fracture-avoidance hypothesis using finite-element modelling. We find that across species, mechanicalloading is similar and approaches reported values of bone strength, thus suggesting pervasive selec-tion on fracture avoidance. Additionally, deep and wide beaks are better suited for dissipating stressthan are more elongate beaks when scaled to common sizes and loadings. Our results illustrate thatdeep and wide beaks in ground finches enable reduction of areas with high stress and peak stressmagnitudes, allowing birds to crack hard seeds while limiting the risk of beak failure. These resultsmay explain strong selection on beak depth and width in natural populations of Darwin's finches.