Sharper, Straighter, Stiffer, Stronger: Bill shape of male Green Hermits (Phaethornis guy) confers superior biomechanical performance during stabbing simulations.

In hummingbirds, bill sexual dimorphism has been mainly related to differential use of floral resources between the sexes (i.e., intersexual resource partitioning). However, intrasexual selection has a potential role in driving hermit bill morphology. Males of Phaethornis longirostris possess weaponized bills, sharp and elongated dagger-like bill-tips, that enhance puncturing ability and territory defense during male-to-male combat at leks. In this study, we employed 3D modelling and finite element analysis to explore bill dimorphism and biomechanical stabbing performance in Phaethornis guy. We found that P. guy also exhibit a dimorphic weapon, with males displaying significantly sharper bill-tips than females. Additionally, we demonstrated a greater degree in biomechanical performance during horizontal stabbing in the straighter bills of male P. guy through a reduction in the energy expended in deformation (strain energy) and the risk of breakage (von Mises stress). Our findings indicate another example of bill-tip weapons and support the potential role of sexual selection in the evolution of hummingbird bill dimorphism.