Lennart Eigen

Elephants use their trunks, muscular hydrostats, to perform a plethora of tasks. Trunk tip morphology as well as grasping behavior differ between elephant species. While African savanna elephants ( Loxodonta africana ) use their dorsal and ventral finger for pinching movements, Asian elephants ( Elephas maximus ) prefer to wrap around objects with their one dorsal finger and ventral bulb trunk tip lip. Moreover, E. maximus can flip their ventral bulb backwards to clamp objects behind the trunk tip. Whether trunk tip musculature differs between elephant species and muscle architecture is reflected by preferred grasping behavior is, however, not clear. In this study, we performed dense muscle fascicle reconstruction of three L. africana and three E. maximus hemi-trunk tips using a combination of manual and automated segmentation of high-resolution microfocus tomography (microCT) scans. We distinguish three types of muscle fascicles: longitudinal (bending and shortening), radial (elongating) and transversal muscle fascicles (elongating). We found that trunk tips of L. africana consist to one third of longitudinal and two thirds radial/transversal muscle fascicles, likely aiding in their grasping behavior, while E. maximus trunk tips consist to two thirds of longitudinal and one third radial/transversal muscle fascicles, which is advantageous for their wrapping and backward clamping behavior.

Humans are drawn to patterns and hierarchies in nature, mimicking them particularly in decoration and architecture. Natural patterns, however, are never purely esthetic and, since evolution works on a variety of factors simultaneously, natural structural systems are intrinsically multifunctional. In order to understand the roles that structural patterns play in biology (and therefore their potential capabilities and utilization in design, architecture and engineering), we need to catalog and encapsulate the diversity of examples and the materials involved. Here, we provide a first classification of biological “tilings,” tessellated natural architectures that involve the repeated pattern of geometric, discrete elements bound by a joint material. By examining 100 examples across the Tree of Life, we reveal this natural structural motif is unexpectedly prevalent: we cover a huge taxonomic diversity, eight orders of magnitude in size scale, and myriad morphologies and functions ranging from optics to armor, allowing us to construct a hierarchical system of eight variables to classify form, function, and materiality in biological tilings. Using diverse means of data analysis (including multiple correspondence analysis), we show this database can be explored to reveal fundamental links among anatomical characteristics and functions as well as connections among and within taxonomic groups. Our resulting collection of “tessellated materials” and its companion website act therefore as a multidisciplinary meeting point (e.g. for biologists, designers, engineers, architects). In this way, our database offers windows for exploring selective pressures and trade-offs and a launchpad for future research and collaborative, cross-disciplinary, bioinspired projects.

Tessellations (surface architectures of arrays of hard tiles) are common in natural and man-made designs. Boxfishes (Ostracioidea) are almost completely encased in a tessellated armor and have evolved a plethora of cross-sectional carapace shapes, yet whether the scutes constructing these exhibit comparable variation is unknown. Using high-resolution microCT and semi-automatic segmentation algorithms, we quantitatively examined thousands of scutes from 13 species of diverse body form. A cluster analysis revealed that certain scute types are associated with specific carapace regions independent of carapace shape. Scute types differentiate between carapace edges and flat regions, as well as between the head region with many carapace openings and the more consistently closed abdominal region, pointing at a constructional commonality or constraint shared by all boxfish species. However, the dimensions of edge scutes varied systematically with carapace shape (e.g., scute aspect ratio tended to increase with decreasing carapace height). This suggests that protection is maintained across body forms by managing scute- and carapace-level mechanisms for increasing bending resistance. Future studies on other taxa are necessary to understand whether these architectural principles are specific evolutionary solutions for building a boxfish carapace or whether they are shared by other biological systems that serve a similar protective function.

Behavior and innervation suggest a high tactile sensitivity of elephant trunks. To clarify the tactile trunk periphery we studied whiskers with the following findings. Whisker density is high at the trunk tip and African savanna elephants have more trunk tip whiskers than Asian elephants. Adult elephants show striking lateralized whisker abrasion caused by lateralized trunk behavior. Elephant whiskers are thick and show little tapering. Whisker follicles are large, lack a ring sinus and their organization varies across the trunk. Follicles are innervated by ~90 axons from multiple nerves. Because elephants don’t whisk, trunk movements determine whisker contacts. Whisker-arrays on the ventral trunk-ridge contact objects balanced on the ventral trunk. Trunk whiskers differ from the mobile, thin and tapered facial whiskers that sample peri-rostrum space symmetrically in many mammals. We suggest their distinctive features—being thick, non-tapered, lateralized and arranged in specific high-density arrays—evolved along with the manipulative capacities of the trunk.

Biological armors derive their mechanical integrity in part from their geometric architectures, often involving tessellations: individual structural elements tiled together to form surface shells. The carapace of boxfish, for example, is composed of mineralized polygonal plates, called scutes, arranged in a complex geometric pattern and nearly completely encasing the body. In contrast to artificial armors, the boxfish exoskeleton grows with the fish; the relationship between the tessellation and the gross structure of the armor is therefore critical to sustained protection throughout growth. To clarify whether or how the boxfish tessellation is maintained or altered with age, we quantify architectural aspects of the tessellated carapace of the longhorn cowfish Lactoria cornuta through ontogeny (across nearly an order of magnitude in standard length) and in a high-throughput fashion, using high-resolution microCT data and segmentation algorithms to characterize the hundreds of scutes that cover each individual. We show that carapace growth is canalized with little variability across individuals: rather than continually adding scutes to enlarge the carapace surface, the number of scutes is surprisingly constant, with scutes increasing in volume, thickness, and especially width with age. As cowfish and their scutes grow, scutes become comparatively thinner, with the scutes at the edges (weak points in a boxy architecture) being some of the thickest and most reinforced in younger animals and thinning most slowly across ontogeny. In contrast, smaller scutes with more variable curvature were found in the limited areas of more complex topology (e.g., around fin insertions, mouth, and anus). Measurements of Gaussian and mean curvature illustrate that cowfish are essentially tessellated boxes throughout life: predominantly zero curvature surfaces comprised of mostly flat scutes, and with scutes with sharp bends used sparingly to form box edges. Since growth of a curved, tiled surface with a fixed number of tiles would require tile restructuring to accommodate the surface's changing radius of curvature, our results therefore illustrate a previously unappreciated advantage of the odd boxfish morphology: by having predominantly flat surfaces, it is the box-like body form that in fact permits a relatively straightforward growth system of this tessellated architecture (i.e., where material is added to scute edges). Our characterization of the ontogeny and maintenance of the carapace tessellation provides insights into the potentially conflicting mechanical, geometric, and developmental constraints of this species but also perspectives into natural strategies for constructing mutable tiled architectures.

Callitrichidae are small, arboreal New World primates that utilize a variety of locomotor behaviors including trunk-to-trunk leaping (TTL) and horizontal locomotion which involve differential functional demands. Little is known about the relationship between the preferred locomotor behavior and musculoskeletal architecture of these primates. In this study, we compared the musculoskeletal architecture of selected shoulder muscles in two cadavers each of the trunk-to-trunk leaperCebuella pygmaeaand the mainly pronograde quadrupedally movingSaguinus imperator subgrisescens. Contrast-enhanced microfocus computed tomography (µCT) was used to virtually dissect the cadavers, produce muscle maps, and create 3D reconstructions for an image-based analysis of the muscles. Muscle lengths, muscle volumes, and osteological muscle moment arms were measured, and the anatomical cross-sectional areas (ACSA) were calculated. We expected the muscles of the forelimb ofS. imperatorto be larger in volume and to be relatively shorter with a larger ACSA due to a higher demand for powerful extension in the forelimbs of this horizontally locomoting species. ForC. pygmaea, we expected relatively larger moment arms for the triceps brachii, supraspinatus, infraspinatus and subscapularis, as larger moment arms present an advantage for extensive vertical clinging on the trunk. The muscles ofS. imperatorwere relatively larger in volume than inC. pygmaeaand had a relatively larger ACSA. Thus, the shoulder muscles ofS. imperatorwere suited to generate relatively larger forces than those ofC. pygmaea. Contrary to our expectations, there were only slight differences between species in regard to muscle lengths and moment arms, which suggests that these properties are not dependent on the preferred locomotor mode. The study of this limited dataset demonstrates that some but not all properties of the musculoskeletal architecture reflect the preferred locomotor behavior in the two species of Callitrichidae examined.