Abstract
One of nature's premier illustrations of adaptive evolution concerns the tight correspondence in birds between beak morphology and feeding behavior. In seed-crushing birds, beaks have been suggested to evolve at least in part to avoid fracture. Yet, we know little about mechanical relationships between beak shape, stress dissipation, and fracture avoidance. This study tests these relationships for Darwin's finches, a clade of birds renowned for their diversity in beak form and function. We obtained anatomical data from micro-CT scans and dissections, which in turn informed the construction of finite element models of the bony beak and rhamphotheca. Our models offer two new insights. First, engineering safety factors are found to range between 1 and 2.5 under natural loading conditions, with the lowest safety factors being observed in species with the highest bite forces. Second, size-scaled finite element (FE) models reveal a correspondence between inferred beak loading profiles and observed feeding strategies (e.g. edge-crushing versus tip-biting), with safety factors decreasing for base-crushers biting at the beak tip. Additionally, we identify significant correlations between safety factors, keratin thickness at bite locations, and beak aspect ratio (depth versus length). These lines of evidence together suggest that beak shape indeed evolves to resist feeding forces.
Highlights
The often tight correspondence between bird beaks and plant morphology well-illustrates the power and precision of natural selection [1]
First we examine how natural loadings are reflected in safety factors for a subset of species for which physiological finite element (FE) models could be created
The estimated maximum von Mises (vM) stresses, near the nasal hinge, were 4–44% lower than maximum vM stresses calculated in prior models that did not take into account the keratinous rhamphotheca [24]
Summary
The often tight correspondence between bird beaks and plant morphology well-illustrates the power and precision of natural selection [1]. Subtle variations in beak morphology can affect foraging efficiency [2], and in some cases tip the balance between survival and starvation [3]. For seed-crushing birds, analyses of feeding capacity require a broader view than just beak morphology per se. This is because a bird’s ability to crush seeds is determined mainly by bite force capacity, which in turn depends primarily on the orientatio
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