Billfishes (marlins, spearfishes, sailfishes and swordfish) are one of the fastest and largest marine apex predators, and perhaps their most recognizable attribute is their bill or rostrum. The proposed function for this novel structure has ranged from hydrodynamic enhancement to defensive weaponry. However, the most supported hypothesis for its function has been linked to feeding. Billfishes have been observed to subdue their prey with their rostrum, either stunning or cutting them into pieces before ingestion. Due to their large body sizes and pelagic lifestyles a thorough investigation of the function of this structure has been logistically challenging. The goal of my dissertation is to investigate the role of the rostrum during feeding from a functional, mechanical and morphological standpoint. By the use of interdisciplinary approaches that blend engineering with biology, the function of the rostrum and billfish putative feeding behavior was investigated. By the use of different approaches that involve morphological characterizations, histology, estimation of performance measurements such as bite force and the investigation architectural tradeoffs from geometric morphometrics analysis, my dissertation aims to characterized the role of the rostrum in billfishes as a possible adaptation for feeding. Results showed that the rostrum in billfishes is mechanically capable of acting as a feeding weapon; continuous stress distribution along its length suggest no particular point that could lead to breakage during feeding. Finite element analysis, as well as bending experiments suggest feeding behavior may be species specific and strictly associated with rostrum morphology. While istiophorids may be morphologically suited to strike their prey with a wide range of motions, swordfish appear to be specialized from a mechanical and hydrodynamic standpoint to hit their prey with lateral strikes. Biting performance is relatively low in these top predators compared to other non-billfish species suggesting the rostrum may facilitate prey processing reducing the need for powerful biting. However contrary to my expectations rostrum length was not a predictor of bite force. Skull variation was evident among billfish species. Swordfish, the species with the longest rostrum, had the smallest head and the lowest relative bite force whereas blue marlin, the species with the stiffer, most compact rostrum, had the largest head and one of the greatest relative bite forces. The shortbill spearfish showed a relatively low bite force indicating predatory success in this species may be linked to an extended lower jaw that may facilitate a speed efficient jaw during prey capture. Whether the rostrum in billfishes has evolved as an adaptation for feeding, remains uncertain. However results from this study demonstrate that rostrum material properties, morphology and head architecture, in addition to relatively low biting performance in billfishes, favor a role of prey capture for the rostrum.

Bio-structures owe their remarkable mechanical properties to their hierarchical geometrical arrangement as well as heterogeneous material properties. This dissertation presents an integrated, interdisciplinary approach that employs computational mechanics combined with flow network analysis to gain fundamental insights into the failure mechanisms of high performance, light-weight, structured composites by examining the stress flow patterns formed in the nascent stages of loading for the rostrum of the paddlefish. The data required for the flow network analysis was generated from the finite element analysis of the rostrum. The flow network was weighted based on the parameter of interest, which is stress in the current study. The changing kinematics of the structural system was provided as input to the algorithm that computes the minimum-cut of the flow network. The proposed approach was verified using two classical problems – three- and four-point bending of a simply-supported concrete beam. The current study also addresses the methodology used to prepare data in an appropriate format for a seamless transition from finite element binary database files to the abstract mathematical domain needed for the network flow analysis. A robust, platform-independent procedure was developed that efficiently handles the large datasets produced by the finite element simulations. Results from computational mechanics using Abaqus and complex network analysis are presented. The complex network strategy successfully identified failure mechanisms in the bio-structure by identifying strain localization in regions of tension, and buckling/crushing in regions of compression. The transdisciplinary strategy used in this study identified the failure mechanisms early, when the material was still in the linearly elastic regime, thereby tremendously reducing the computational time and cost as compared to running a finite element analysis to failure. This work also developed five proof-of-concept, bio-inspired models with varying lattice complexity based on the rostrum. Performance of these bio-inspired models was analyzed with respect to the stress and deformation. Numerical experiments were carried out on one of the bio-inspired model to demonstrate the application of newly developed similitude laws for blast loading. This research has laid the groundwork for an efficient design-test-build cycle for rapid prototyping of novel bio-inspired structures by using flow network analysis, finite element analysis, and similitude laws.

abstract: Weevils are among the most diverse and evolutionarily successful animal lineages on Earth. Their success is driven in part by a structure called the rostrum, which gives weevil heads a characteristic "snout-like" appearance. Nut weevils in the genus Curculio use the rostrum to drill holes into developing fruits and nuts, wherein they deposit their eggs. During oviposition this exceedingly slender structure is bent into a straightened configuration - in some species up to 90° - but does not suffer any damage during this process. The performance of the snout is explained in terms of cuticle biomechanics and rostral curvature, as presented in a series of four interconnected studies. First, a micromechanical constitutive model of the cuticle is defined to predict and reconstruct the mechanical behavior of each region in the exoskeleton. Second, the effect of increased endocuticle thickness on the stiffness and fracture strength of the rostrum is assessed using force-controlled tensile testing. In the third chapter, these studies are integrated into finite element models of the snout, demonstrating that the Curculio rostrum is only able to withstand repeated, extreme bending because of modifications to the composite structure of the cuticle in the rostral apex. Finally, interspecific differences in the differential geometry of the snout are characterized to elucidate the role of biomechanical constraint in the evolution of rostral morphology for both males and females. Together these studies highlight the significance of cuticle biomechanics - heretofore unconsidered by others - as a source of constraint on the evolution of the rostrum and the mechanobiology of the genus Curculio. / Dissertation/Thesis / Doctoral Dissertation Evolutionary Biology 2009