Hydrostatic locomotion in a limbless tetrapod

Biphasic burrowing in Atlantic hagfish (Myxine limosa)

Myxine limosa is a burrowing species of hagfish that occurs in the western North Atlantic in areas with muddy substrate and at depths generally greater than 100 meters. Burrowing of M. limosa has been observed from submersibles, but little is known about the behavior of these animals within the substrate or the biomechanical mechanisms involved. Here, we investigated burrowing in M. limosa by observing individuals as they burrowed through transparent gelatin. A photoelastic setup using crossed polarizers allowed us to visualize stress development in the gelatin as the hagfish moved through it. We found that M. limosa created U-shaped burrows in gelatin using a stereotyped, two-phase burrowing behavior. In the first ('thrash') phase, hagfish drove their head and their anterior body into the substrate using vigorous sinusoidal swimming movements, with their head moving side-to-side. In the second ('wriggle') phase, swimming movements ceased, with propulsion coming exclusively from the anterior, submerged portion of body. The wriggle phase involved side-to-side head movements and movements of the submerged part of the body that resembled the internal concertina strategy used by caecilians and uropeltid snakes. The entire burrowing process took on average 7.6 min to complete and ended with the hagfish's head protruding from the substrate and the rest of its body generally concealed. Understanding the burrowing activities of hagfishes could lead to improved understanding of sediment turnover in marine benthic habitats, new insights into the reproductive behavior of hagfishes, or even inspiration for the design of burrowing robots.

Functional diversity of snake locomotor behaviors: A review of the biological literature for bioinspiration

Organismal solutions to natural challenges can spark creative engineering applications. However, most engineers are not experts in organismal biology, creating a potential barrier to maximally effective bioinspired design. In this review, we aim to reduce that barrier with respect to a group of organisms that hold particular promise for a variety of applications: snakes. Representing >10% of tetrapod vertebrates, snakes inhabit nearly every imaginable terrestrial environment, moving with ease under many conditions that would thwart other animals. To do so, they employ over a dozen different types of locomotion (perhaps well over). Lacking limbs, they have evolved axial musculoskeletal features that enable their vast functional diversity, which can vary across species. Different species also have various skin features that provide numerous functional benefits, including frictional anisotropy or isotropy (as their locomotor habits demand), waterproofing, dirt shedding, antimicrobial properties, structural colors, and wear resistance. Snakes clearly have much to offer to the fields of robotics and materials science. We aim for this review to increase knowledge of snake functional diversity by facilitating access to the relevant literature.

The anatomy of the head muscles in caecilians (Amphibia: Gymnophiona): Variation in relation to phylogeny and ecology?

In limbless fossorial vertebrates such as caecilians (Gymnophiona), head-first burrowing imposes severe constraints on the morphology and overall size of the head. As such, caecilians developed a unique jaw-closing system involving the large and well-developed m. interhyoideus posterior, which is positioned in such a way that it does not significantly increase head diameter. Caecilians also possess unique muscles among amphibians. Understanding the diversity in the architecture and size of the cranial muscles may provide insights into how a typical amphibian system was adapted for a head-first burrowing lifestyle. In this study, we use dissection and non-destructive contrast-enhanced micro-computed tomography (μCT) scanning to describe and compare the cranial musculature of 13 species of caecilians. Our results show that the general organization of the head musculature is rather constant across extant caecilians. However, the early-diverging Rhinatrema bivittatum mainly relies on the 'ancestral' amphibian jaw-closing mechanism dominated by the m. adductores mandibulae, whereas other caecilians switched to the use of the derived dual jaw-closing mechanism involving the additional recruitment of the m. interhyoideus posterior. Additionally, the aquatic Typhlonectes show a greater investment in hyoid musculature than terrestrial caecilians, which is likely related to greater demands for ventilating their large lungs, and perhaps also an increased use of suction feeding. In addition to three-dimensional interactive models, our study provides the required quantitative data to permit the generation of accurate biomechanical models allowing the testing of further functional hypotheses.

Triassic stem caecilian supports dissorophoid origin of living amphibians

Living amphibians (Lissamphibia) include frogs and salamanders (Batrachia) and the limbless worm-like caecilians (Gymnophiona). The estimated Palaeozoic era gymnophionan-batrachian molecular divergence¹ suggests a major gap in the record of crown lissamphibians prior to their earliest fossil occurrences in the Triassic period^2-6. Recent studies find a monophyletic Batrachia within dissorophoid temnospondyls^7-10, but the absence of pre-Jurassic period caecilian fossils^11,12 has made their relationships to batrachians and affinities to Palaeozoic tetrapods controversial^1,8,13,14. Here we report the geologically oldest stem caecilian-a crown lissamphibian from the Late Triassic epoch of Arizona, USA-extending the caecilian record by around 35 million years. These fossils illuminate the tempo and mode of early caecilian morphological and functional evolution, demonstrating a delayed acquisition of musculoskeletal features associated with fossoriality in living caecilians, including the dual jaw closure mechanism^15,16, reduced orbits¹⁷ and the tentacular organ¹⁸. The provenance of these fossils suggests a Pangaean equatorial origin for caecilians, implying that living caecilian biogeography reflects conserved aspects of caecilian function and physiology¹⁹, in combination with vicariance patterns driven by plate tectonics²⁰. These fossils reveal a combination of features that is unique to caecilians alongside features that are shared with batrachian and dissorophoid temnospondyls, providing new and compelling evidence supporting a single origin of living amphibians within dissorophoid temnospondyls.

Discrete element models for understanding the biomechanics of fossorial animals

The morphological features of fossorial animals have continuously evolved in response to the demands of survival. However, existing methods for animal burrowing mechanics are not capable of addressing the large deformation of substrate. The discrete element method (DEM) is able to overcome this limitation. In this study, we used DEM to develop a general model to simulate the motion of an animal body part and its interaction with the substrate. The DEM also allowed us to easily change the forms of animal body parts to examine how those different forms affected the biomechanical functions. These capabilities of the DEM were presented through a case study of modeling the burrowing process of North American Badger. In the case study, the dynamics (forces, work, and soil displacements) of burrowing were predicted for different forms of badger claw and manus, using the model. Results showed that when extra digits are added to a manus, the work required for a badger to dig increases considerably, while the mass of soil dug only increases gradually. According to the proposed efficiency index (ratio of the amount of soil dug to the work required), the modern manus with 5 digits has indeed biomechanical advantage for their fossorial lifestyle, and the current claw curvature (25.3 mm in radius) is indeed optimal. The DEM is able to predict biomechanical relationships between functions and forms for any fossorial animals. Results can provide biomechanical evidences for explaining how the selective pressures for functions influence the morphological evolution in fossorial animals.

Is vertebral shape variability in caecilians (Amphibia: Gymnophiona) constrained by forces experienced during burrowing?

Caecilians are predominantly burrowing, elongate, limbless amphibians that remain relatively poorly studied. Although it has been suggested that the sturdy and compact skulls of caecilians are an adaptation to their head-first burrowing habits, no clear relationship between skull shape and burrowing performance appears to exist. However, the external forces encountered during burrowing are transmitted by the skull to the vertebral column, and as such, may impact vertebral shape. Additionally, the muscles that generate the burrowing forces attach onto the vertebral column and consequently may impact vertebral shape that way as well. Here, we explore the relationships between vertebral shape and maximal in vivo push forces in 13 species of caecilian amphibians. Our results show that the shape of the two most anterior vertebrae, as well as the shape of the vertebrae at 90% of the total body length, are not correlated with peak push forces. Conversely, the shape of the third vertebrae, and the vertebrae at 20% and 60% of the total body length, do show a relationship to push forces measured in vivo. Whether these relationships are indirect (external forces constraining shape variation) or direct (muscles forces constraining shape variation) remains unclear and will require quantitative studies of the axial musculature. Importantly, our data suggest that mid-body vertebrae may potentially be used as proxies to infer burrowing capacity in fossil representatives.

Regional differences in vertebral shape along the axial skeleton in caecilians (Amphibia: Gymnophiona)

Caecilians are elongate, limbless and annulated amphibians that, as far as is known, all have an at least partly fossorial lifestyle. It has been suggested that elongate limbless vertebrates show little morphological differentiation throughout the postcranial skeleton. However, relatively few studies have explored the axial skeleton in limbless tetrapods. In this study, we used μCT data and three-dimensional geometric morphometrics to explore regional differences in vertebral shape across a broad range of caecilian species. Our results highlight substantial differences in vertebral shape along the axial skeleton, with anterior vertebrae being short and bulky, whereas posterior vertebrae are more elongated. This study shows that despite being limbless, elongate tetrapods such as caecilians still show regional heterogeneity in the shape of individual vertebrae along the vertebral column. Further studies are needed, however, to understand the possible causes and functional consequences of the observed variation in vertebral shape in caecilians.

How to build single-celled cnidarians with worm-like motility: Lessons from Myxozoa

Metazoans with worm-like morphologies across diverse and disparate groups typically demonstrate motility generated by hydrostatic skeletons involving tissue layers (muscles and epithelia). Here we present representative morphological, behavioural and molecular data for parasitic cnidarians (myxozoans) that demonstrate unprecedented variation in form and function, developing as cellular hydrostats. Motile elongate plasmodia characterise a remarkable radiation of species in the genus Ceratomyxa. The vermiform plasmodia inhabit gall bladders of a range of South American freshwater fish and exhibit undulatory motility reminiscent of nematodes but achieved at the cellular level. Collective insights from ultrastructure, confocal and light microscopy along with videos depicting movements highlight key features that we propose explain the unique motility of the plasmodia. These features include cytoskeletal elements (net forming microfilaments and microtubules), a large internal vacuole, a relatively rigid outer glycocalyx and peripherally arranged mitochondria. These constituents provide collective evidence for repurposing of the cnidarian epitheliomuscular cell to support worm-like motility at the cellular level. The apparent restriction of vermiform ceratomyxids to South American freshwaters suggests an origination via Cretaceous or Miocene marine transgressions and subsequent radiation.

Under pressure: the relationship between cranial shape and burrowing force in caecilians (Gymnophiona)

Caecilians are elongate, limbless and annulated amphibians that, with the exception of one aquatic family, all have an at least partly fossorial lifestyle. It has been suggested that caecilian evolution resulted in sturdy and compact skulls with fused bones and tight sutures, as an adaptation to their head-first burrowing habits. However, although their cranial osteology is well described, relationships between form and function remain poorly understood. In the present study, we explored the relationship between cranial shape and in vivo burrowing forces. Using micro-computed tomography (µCT) data, we performed 3D geometric morphometrics to explore whether cranial and mandibular shapes reflected patterns that might be associated with maximal push forces. The results highlight important differences in maximal push forces, with the aquatic Typhlonectes producing a lower force for a given size compared with other species. Despite substantial differences in head morphology across species, no relationship between overall skull shape and push force could be detected. Although a strong phylogenetic signal may partly obscure the results, our conclusions confirm previous studies using biomechanical models and suggest that differences in the degree of fossoriality do not appear to be driving the evolution of head shape.

Burrowing in blindsnakes: A preliminary analysis of burrowing forces and consequences for the evolution of morphology

Burrowing is a common behavior in vertebrates. An underground life-style offers many advantages but also poses important challenges including the high energetic cost of burrowing. Scolecophidians are a group of morphologically derived subterranean snakes that show great diversity in form and function. Although it has been suggested that leptotyphlopids and anomalepidids mostly use existing underground passageways, typhlopids are thought to create their own burrows. However, the mechanisms used to create burrows and the associated forces that animals may be able to generate remain unknown. Here, we provide the first data on push forces in scolecophidians and compare them with those in some burrowing alethinophidian snakes. Our results show that typhlopids are capable of generating higher forces for a given size than other snakes. The observed differences are not due to variation in body diameter or length, suggesting fundamental differences in the mechanics of burrowing or the way in which axial muscles are used. Qualitative observations of skull and vertebral shape suggest that the higher forces exerted by typhlopids may have impacted the evolution of their anatomy. Our results provide the basis for future studies exploring the diversity of form and function in this fascinating group of animals. Quantitative comparisons of the cranial and vertebral shape in addition to collecting functional and ecological data on a wider array of species would be particularly important to test the patterns described here.

Trade-offs between burrowing and biting force in fossorial scincid lizards?

Trade-offs are thought to be important in constraining evolutionary divergence as they may limit phenotypic diversification. The cranial system plays a vital role in many functions including defensive, territorial, predatory and feeding behaviours in addition to housing the brain and sensory systems. Consequently, the morphology of the cranial system is affected by a combination of selective pressures that may induce functional trade-offs. Limbless, head-first burrowers are thought to be constrained in their cranial morphology as narrow heads may provide a functional advantage for burrowing. However, having a wide and large head is likely beneficial in terms of bite performance. We used 15 skink species to test for the existence of trade-offs between maximal push and bite forces, and explored the patterns of covariation between external head and body morphology and performance. Our data show that there is no evidence of a trade-off between bite and burrowing in terms of maximal force. Species that generate high push forces also generate high bite forces. Our data also show that overall head size covaries with both performance traits. However, future studies exploring trade-offs between force and speed or the energetic cost of burrowing may reveal other trade-offs.

Tunnel-tube and Fourier methods for measuring three-dimensional medium reaction force in burrowing animals

Subterranean digging behaviors provide opportunities for protection, access to prey, and predator avoidance for a diverse array of vertebrates, yet studies of the biomechanics of burrowing have been limited by the technical challenges of measuring kinetics and kinematics of animals moving within a medium. We describe a new system for measuring 3D reaction forces during burrowing, called a 'tunnel-tube', which is composed of two, separately instrumented plastic tubes: an 'entry tube' with no medium, in series with a 'digging tube' filled with medium. Mean reaction forces are measured for a digging bout and Fourier analysis is used to quantify the amplitude of oscillatory digging force as a function of frequency. In sample data from pocket gophers digging in artificial and natural media, the mean ground reaction force is constant, whereas Fourier analysis resolves a reduced amplitude of oscillatory force in the artificial medium with lower compaction strength.

Structural and environmental constraints on reduction of paired appendages among vertebrates

Burrowing habits or complex environments have generally been considered as potential drivers acting on reduction and loss of the appendicular skeleton among vertebrates. Herein, we suggest that this might be the case for lissamphibians and squamates, but that fin loss in fishes is usually prevented by important structural constraints, because pectoral fins are commonly used to control rolling and pitching. We provide an overview of the distribution of paired appendage reduction across vertebrates while examining the ecological affinities of finless and limbless clades. We analysed the correlation between lifestyle and fin or limb loss using the discrete comparative analysis. The resulting Bayesian factors indicate strong evidence of correlation between: (1) pectoral-fin loss and coexistence of anguilliform elongation and burrowing habits or complex habitat in teleost fishes; and (2) limb loss and a burrowing or grass-swimming lifestyle in squamate reptiles and lissamphibians. These correlations suggest that a complex environment or a fossorial habit is a driving force leading to appendage loss. The only style of locomotion that is functional even in the absence of paired appendages is the undulatory one, which is typical of all elongated reptiles and lissamphibians, but certainly less common in teleost fishes.

Burrowing with a kinetic snout in a snake (Elapidae: Aspidelaps scutatus)

Of the few elongate, fossorial vertebrates that have been examined for their burrowing mechanics, all were found to use an akinetic, reinforced skull to push into the soil, powered mostly by trunk muscles. Reinforced skulls were considered essential for head-first burrowing. In contrast, I found that the skull of the fossorial shield-nosed cobra (Aspidelaps scutatus) is not reinforced and retains the kinetic potential typical of many non-fossorial snakes. Aspidelaps scutatus burrows using a greatly enlarged rostral scale that is attached to a kinetic snout that is independently mobile with respect to the rest of the skull. Two mechanisms of burrowing are used: (1) anteriorly directed head thrusts from a loosely bent body that is anchored against the walls of the tunnel by friction, and (2) side-to-side shovelling using the head and rostral scale. The premaxilla, to which the rostral scale is attached, lacks any direct muscle attachments. Rostral scale movements are powered by, first, retractions of the palato-pterygoid bar, mediated by a ligament that connects the anterior end of the palatine to the transverse process of the premaxilla and, second, by contraction of a previously undescribed muscle slip of the m. retractor pterygoidei that inserts on the skin at the edge of the rostral scale. In derived snakes, palatomaxillary movements are highly conserved and power prey capture and transport behaviors. Aspidelaps scutatus has co-opted those mechanisms for the unrelated function of burrowing without compromising the original feeding functions, showing the potential for evolution of functional innovations in highly conserved systems.

Fascial hierarchies and the relevance of crossed-helical arrangements of collagen to changes in the shape of muscles

Muscles are composite structures consisting of contractile myofibres surrounded by complex hierarchies of collagen-reinforced fascial sheaths. They are essentially flexible cylinders that change in shape, with the particular alignment of collagen fibres within their myofascial walls reflecting the most efficient distribution of mechanical stresses and coordinating these changes. However, while the functional significance of this crossed-helical fibre arrangement is well established in other species and in different parts of the body, relatively little attention has been given to this within the fascia of humans; and the relevance of this geometric configuration to muscles and surrounding fascial tissues is described.

Burrowing by small polychaetes - mechanics, behavior and muscle structure of Capitella sp

Worms of different sizes extend burrows through muddy sediments by fracture, applying dorso-ventral forces that are amplified at the crack tip. Smaller worms displace sediments less than larger worms and therefore are limited in how much force they can apply to burrow walls. We hypothesized that small worms would exhibit a transition in burrowing mechanics, specifically a lower limit in body size for the ability to burrow by fracture, corresponding with an ontogenetic transition in muscle morphology. Kinematics of burrowing in a mud analog, external morphology and muscle arrangement were examined in juveniles and adults of the small polychaete Capitella sp. We found that it moves by peristalsis, and no obvious differences were observed among worms of different sizes; even very small juveniles were able to burrow through a clear mud analog by fracture. Interestingly, we found that in addition to longitudinal and circular muscles needed for peristaltic movements, left- and right-handed helical muscles wrap around the thorax of worms of all sizes. We suggest that in small worms helical muscles may function to supplement forces generated by longitudinal muscles and to maintain hydrostatic pressure, enabling higher forces to be exerted on the crack wall. Further research is needed, however, to determine whether surficial sediments inhabited by small worms fail by fracture or plastically deform under forces of the magnitudes applied by Capitella sp.

Influence of fossoriality on inner ear morphology: insights from caecilian amphibians

It is widely accepted that a relationship exists between inner ear morphology and functional aspects of an animal's biology, such as locomotor behaviour. Animals that engage in agile and spatially complex behaviours possess semicircular canals that morphologically maximise sensitivity to correspondingly complex physical stimuli. Stemming from the prediction that fossorial tetrapods require a well-developed sense of spatial awareness, we investigate the hypothesis that fossoriality leads to inner ear morphology that is convergent with other spatially adept tetrapods. We apply morphometrics to otic capsule endocasts of 26 caecilian species to quantify aspects of inner ear shape, and compare these with a sample of frog and salamander species. Our results reveal caecilians (and also frogs) possess strongly curved canals, a feature in common with spatially adept species. However, significantly shorter canals in caecilians suggest reduced sensitivity, possibly associated with reduced reliance on vestibulo-ocular reflexes in this group of visually degenerate tetrapods. An elaboration of the sacculus of caecilians is interpreted as a unique adaptation among amphibians to increase sensitivity to substrate-borne vibrations transmitted through the head. This study represents the first quantitative analyses of inner ear morphology of limbless fossorial tetrapods, and identifies features within a new behavioural context that will contribute to our understanding of the biological consequences of physical stimuli on sensory function and associated morphological evolution.

Giant negative linear compressibility in zinc dicyanoaurate

The counterintuitive phenomenon of negative linear compressibility (NLC) is a highly desirable but rare property exploitable in the development of artificial muscles, actuators and next-generation pressure sensors. In all cases, material performance is directly related to the magnitude of intrinsic NLC response. Here we show the molecular framework material zinc(II) dicyanoaurate(I), Zn[Au(CN)(2)](2), exhibits the most extreme and persistent NLC behaviour yet reported: under increasing hydrostatic pressure its crystal structure expands in one direction at a rate that is an order of magnitude greater than both the typical contraction observed for common engineering materials and also the anomalous expansion in established NLC candidates. This extreme behaviour arises from the honeycomb-like structure of Zn[Au(CN)(2)](2) coupling volume reduction to uniaxial expansion, and helical Au…Au 'aurophilic' interactions accommodating abnormally large linear strains by functioning as supramolecular springs.

Is solid always best? Cranial performance in solid and fenestrated caecilian skulls

Caecilians (Lissamphibia: Gymnophiona) are characterized by a fossorial lifestyle that appears to play a role in the many anatomical specializations in the group. The skull, in particular, has been the focus of previous studies because it is driven into the substrate for burrowing. There are two different types of skulls in caecilians: (1) stegokrotaphic, where the squamosal completely covers the temporal region and the jaw closing muscles, and (2) zygokrotaphic, with incomplete coverage of the temporal region by the squamosal. We used 3-D imaging and modeling techniques to explore the functional consequences of these skull types in an evolutionary context. We digitally converted stegokrotaphic skulls into zygokrotaphic skulls and vice versa. We also generated a third, akinetic skull type that was presumably present in extinct caecilian ancestors. We explored the benefits and costs of the different skull types under frontal loading at different head angles with finite element analysis (FEA). Surprisingly, the differences in stress distributions and bending between the three tested skull types were minimal and not significant. This suggests that the open temporal region in zygokrotaphic skulls does not lead to poorer performance during burrowing. However, the results of the FEA suggest a strong relationship between the head angle and skull performance, implying there is an optimal head angle during burrowing.

Why colour in subterranean vertebrates? Exploring the evolution of colour patterns in caecilian amphibians

The proximate functions of animal skin colour are difficult to assign as they can result from natural selection, sexual selection or neutral evolution under genetic drift. Most often colour patterns are thought to signal visual stimuli; so,their presence in subterranean taxa is perplexing. We evaluate the adaptive nature of colour patterns in nearly a third of all known species of caecilians, an order of amphibians most of which live in tropical soils and leaf litter. We found that certain colour pattern elements in caecilians can be explained based on characteristics concerning above-ground movement. Our study implies that certain caecilian colour patterns have convergently evolved under selection and we hypothesize their function most likely to be a synergy of aposematism and crypsis, related to periods when individuals move overground. In a wider context, our results suggest that very little exposure to daylight is required to evolve and maintain a varied array of colour patterns in animal skin.

Deciphering morphological variation in the braincase of caecilian amphibians (Gymnophiona)

High levels of morphological homoplasy have hindered progress in understanding morphological evolution within gymnophione lissamphibians. Stemming from the hypothesis that the braincase has the potential to yield phylogenetic information, the braincases of 27 species (23 genera) of gymnophione amphibians were examined using high-resolution micro-computed tomography and histologically prepared specimens. Morphology of the brain and its relationship to features of the braincase is described, and it is shown that eight different patterns exist in the distribution of foramina in the antotic region. The distribution of variants is congruent with molecule-based phylogeny. Additionally, all variants are shown to correspond directly to stages along developmental continua, suggesting that the evolutionary truncation of development in the antotic region at various stages has driven the evolution of morphology in this region. Attempts to correlate the observed morphology with proxies of putative heterochronic events (including those attributable to burrowing, life history, and size) fail to explain the distribution of morphology if each proxy is considered separately. Thus, it is concluded that either currently unrecognized causes of heterochrony or combinations thereof have influenced morphology in different lineages independently. These data identify clades whose morphology can now be reconsidered in light of previously unrecognized heterochronic events, thereby providing a foundation for future analyses of the evolution of morphology within Gymnophiona as a whole. Most significantly, these data confirm, for the first time in a lissamphibian group, that the braincase can preserve important phylogenetic information that is otherwise obscured in regions of the skull that experience strong influences from functional constraints.

Burrowing and subsurface locomotion in anguilliform fish: behavioral specializations and mechanical constraints

Fish swimming is probably one of the most studied and best understood locomotor behaviors in vertebrates. However, many fish also actively exploit sediments. Because of their elongate body shape, anguilliform fishes are not only efficient swimmers but also very maneuverable. Consequently, many species live in complexly structured environments near the bottom and many are known to burrow into the sediment. To better understand burrowing and subsurface locomotion in anguilliform fish we provide descriptive kinematic data on subsurface locomotion in a burrowing eel (Pisodonophis boro) using videofluoroscopy. We also measured the maximal forces that can be exerted by this species during head-first and tail-first burrowing, and explored the implications of head-first burrowing on mechanical stress distribution in the skull. Our data show that P. boro uses lateral undulation to penetrate and move in sandy sediments under water. The kinematics of subsurface locomotion are different from those observed during swimming and are characterized by a very high slip factor. These observations differ considerably from recently published data in terrestrial sand-swimming lizards, and suggest that the sediment behaves like a solid rather than a frictional fluid. Finally, our finite element models show that the cranial shape and structure in the head-first burrowing P. boro is mechanically more suited for head-first burrowing than that of an obligate tail-first burrowing species, Heteroconger hassi.

Vertebral microanatomy in squamates: structure, growth and ecological correlates

The histological study of vertebrae in extant squamates shows that the internal vertebral structure in this group differs from that of other tetrapods. Squamate vertebrae are lightly built and basically composed of two roughly concentric osseous tubes--one surrounding the neural canal and the other constituting the peripheral cortex of the vertebra--connected by few thin trabeculae. This structure, which characteristically evokes that of a tubular bone, results from a peculiar remodelling process characterised by an imbalance between local bone resorption and redeposition; in both periosteal and endosteo-endochondral territories, bone is extensively resorbed but not reconstructed in the same proportion by secondary deposits. This process is particularly intense in the deep region of the centrum, where originally compact cortices are made cancellous, and where the endochondral spongiosa is very loose. This remodelling process starts at an early stage of development and remains active throughout subsequent growth. The growth of squamate centra is also strongly asymmetrical, with the posterior (condylar) part growing much faster than the anterior (cotylar) part. Preliminary analyses testing for associations between vertebral structure and habitat use suggest that vertebrae of fossorial taxa are denser than those of terrestrial taxa, those in aquatic taxa being of intermediate density. However, phylogenetically informed analyses do not corroborate these findings, thus suggesting a strong phylogenetic signal in the data. As our analyses demonstrate that vertebrae in snakes are generally denser than those of lizards sensu stricto, this may drive the presence of a phylogenetic signal in the data. More comprehensive sampling of fossorial and aquatic lizards is clearly needed to more rigorously evaluate these patterns.

Are palaeoscolecids ancestral ecdysozoans?

The reconstruction of ancestors is a central aim of comparative anatomy and evolutionary developmental biology, not least in attempts to understand the relationship between developmental and organismal evolution. Inferences based on living taxa can and should be tested against the fossil record, which provides an independent and direct view onto historical character combinations. Here, we consider the nature of the last common ancestor of living ecdysozoans through a detailed analysis of palaeoscolecids, an early and extinct group of introvert-bearing worms that have been proposed to be ancestral ecdysozoans. In a review of palaeoscolecid anatomy, including newly resolved details of the internal and external cuticle structure, we identify specific characters shared with various living nematoid and scalidophoran worms, but not with panarthropods. Considered within a formal cladistic context, these characters provide most overall support for a stem-priapulid affinity, meaning that palaeoscolecids are far-removed from the ecdysozoan ancestor. We conclude that previous interpretations in which palaeoscolecids occupy a deeper position in the ecdysozoan tree lack particular morphological support and rely instead on a paucity of preserved characters. This bears out a more general point that fossil taxa may appear plesiomorphic merely because they preserve only plesiomorphies, rather than the mélange of primitive and derived characters anticipated of organisms properly allocated to a position deep within animal phylogeny.

Computational models of heart pumping efficiencies based on contraction waves in spiral elastic bands

We present a framework for modeling biological pumping organs based on coupled spiral elastic band geometries and active wave-propagating excitation mechanisms. Two pumping mechanisms are considered in detail by way of example: one of a simple tube, which represents a embryonic fish heart and another more complicated structure with the potential to model the adult human heart. Through finite element modeling different elastic contractions are induced in the band. For each version the pumping efficiency is measured and the dynamics are evaluated. We show that by combining helical shapes of muscle bands with a contraction wave it is possible not only to achieve efficient pumping, but also to create desired dynamics of the structure. As a result we match the function of the model pumps and their dynamics to physiological observations.

Dynamic properties of a locomotory muscle of the tobacco hornworm Manduca sexta during strain cycling and simulated natural crawling

Caterpillars are soft-bodied terrestrial climbers that perform a wide variety of complex movements with several hundred muscles and a relatively small number of neurons. Control of movements is therefore expected to place unusual demands on the mechanical properties of the muscles. The muscles develop force slowly (1-6 s to peak) yet over a strain range extending from under 60% to more than 160% of resting length, with a length-tension relationship resembling that of supercontracting or cross-striated muscle. In passive and active sinusoidal strain cycling, muscles displayed viscoelastic qualities, with very low and stretch-velocity dependent resilience; there was a positive linear relationship between stretch velocity and the fraction of work dissipation attributable to passive muscle properties (20-80%). In linear stretches of unstimulated muscles at velocities bracketing those encountered in natural crawling, the rise in tension showed a distinct transition to a lower rate of increase, with transition tension dependent upon stretch velocity; peak force was exponentially related to stretch velocity. When stretching ceased, force decayed exponentially, with slower decay associated with lower stretch velocities; the decay time constant was exponentially related to stretch velocity. From the kinematics of caterpillars crawling horizontally we determined that the ventral interior lateral muscle (VIL) of the third abdominal segment (A3) is at or near resting length for most of the crawl cycle, with a fairly linear shortening by 25-30% and re-lengthening occupying about 45% of cycle duration. Synchronized kinematic and EMG recordings showed that during horizontal crawling A3 VIL is stimulated as the muscle shortens from about 95% to 75% of its resting length. We subjected in vitro VIL preparations to strain cycling and stimulus phase and duration similar to that of natural crawling. The resulting work loops were figure-eight shaped, with the muscle performing work during the shortest 45-65% of the strain cycle but dissipating work during the rest of the cycle. The muscle remained in the ascending limb of its length-tension relationship throughout the crawl cycle. Peak force occurred at the end of re-lengthening, nearly a full second after stimulation ceased, underscoring the importance of understanding passive muscle properties to explain caterpillar locomotion. Whether A3 VIL functions as an actuator at all during simulated natural strain cycling is highly sensitive to stimulus timing but far less so to stimulus duration. The muscle's elastomer-like properties appear to play a major role in its function.

Kinematics of soft-bodied, legged locomotion in Manduca sexta larvae

Caterpillar crawling is distinct from that of worms and molluscs; it consists of a series of steps in different body segments that can be compared to walking and running in animals with stiff skeletons. Using a three-dimensional kinematic analysis of horizontal crawling in Manduca sexta, the tobacco hornworm, we found that the phase of vertical displacement in the posterior segments substantially led changes in horizontal velocity and the segments appeared to pivot around the attached claspers. Both of the motions occur during vertebrate walking. In contrast, vertical displacement and horizontal velocity in the anterior proleg-bearing segments were in phase, as expected for running gaits coupled by elastic storage. We propose that this kinematic similarity to running results from the muscular compression and release of elastic tissues. As evidence in support of this proposal, the compression and extension of each segment were similar to harmonic oscillations in a spring, although changes in velocity were 70 degrees out of phase with displacement, suggesting that the spring was damped. Measurements of segment length within, and across, intersegmental boundaries show that some of these movements were caused by folding of the body wall between segments. These findings demonstrate that caterpillar crawling is not simply the forward progression of a peristaltic wave but has kinetic components that vary between segments. Although these movements can be compared to legged locomotion in animals with stiff skeletons, the underlying mechanisms of caterpillar propulsion, and in particular the contribution of elastic tissues, remain to be discovered.

Penises as Variable-Volume Hydrostatic Skeletons

Penises are inflatable intromittent organs that transfer sperm to a female during copulation. Most of the time, males store their penises in a flexible detumesced state, but they can rapidly inflate them with blood when an opportunity for reproductive behavior arises. In mammals, the primary erectile tissue is called the corpus cavernosum; its anatomy is a close match to a model hydroskeleton reinforced by an axial orthogonal fiber array. The wall of the corpus cavernosum contains layers of highly organized collagen fibers arranged at 0 degrees and 90 degrees to the penile long axis. Flaccid wall tissue is folded. Collagen fiber straightening during erection expands the tunica albuginea and increases both its stiffness and its second moment of area. These changes make the entire penis larger and harder to bend. Axial orthogonal fiber reinforcement affects the mechanical behavior of the erect corpus cavernosum, making it resistant to tensile, compressive, and bending forces.

Cranial musculature in the larva of the caecilian,Ichthyophis kohtaoensis (Lissamphibia: Gymnophiona)

Within the Gymnophiona (caecilians) oviparous species with biphasic life-cycles possess a free living semi-aquatic larval stage that feeds in aquatic habitats. The larvae pass through a metamorphosis to a purely terrestrial adult stage. It is likely that the cranial morphology of caecilian larvae has specializations for aquatic feeding. However, little is known about the cranial morphology, and the cranial musculature is especially neglected in the literature. This study provides a detailed description of the jaw and hyobranchial musculature in larval stages of a caecilian. We studied late embryonic and early larval specimens of Ichthyophis kohtaoensis. Furthermore, we compared and homologized the cranial muscles found in larval I. kohtaoensis with the muscles described for adult caecilians. Most cranial muscles of larval I. kohtaoensis are also present in the adult, except for the m. levator mandibulae externus and the m. subarcualis obliquus II. Our results were compared with the data available for larval frogs and salamanders in order to hypothesize the cranial musculature in the larva of the most recent common ancestor of the Lissamphibia. Larval caecilians, frog tadpoles, and salamander larvae share many characters in their cranial musculature, which, consequently, can be assigned to the lissamphibian ground pattern. However, the m. pterygoideus and the m. levator quadrati are unique to the Gymnophiona.

Lateral bending of the lumbar spine during quadrupedalism in strepsirhines

Much research has been devoted to spinal kinematics of nonmammalian vertebrates, while comparatively little is known about the locomotor role of spinal movements in mammals, especially primates. This study, conducted at the Duke University Primate Center, examines the function of lateral spinal bending during quadrupedal walking among a diverse sample of strepsirhines. The taxa studied include Loris tardigradus (1), Nycticebus coucang (1), N. pygmaeus (1), Cheirogaleus medius (2), Varecia variegata (2), Eulemur fulvus (2), and a total sample size of 261 strides. Lateral bending varies among the taxa with respect to both magnitude and effects of velocity, and does not appear to be correlated with body size. In addition, the timing of lateral bending during a stride appears to differ from that reported for other (nonmammalian) tetrapods. On average, maximum lateral flexion occurs just after ipsilateral foot touchdown, which may be functionally associated with touchdown of the contralateral forelimb during diagonal sequence gait. For some of the taxa, lateral flexion coincides more closely with foot touchdown as velocity increases, suggesting a functional role in increasing hindlimb stride length. Both of these timing patterns contrast with those reported for lizards. Finally, although lorids as a group have been described as having a "sinuous" gait, this study shows more pronounced lateral flexion in Nycticebus than in Loris.

Ontogenetic scaling of burrowing forces in the earthworm Lumbricus terrestris

In hydrostatic skeletons, it is the internal fluid under pressure surrounded by a body wall in tension (rather than a rigid lever) that enables the stiffening of the organism, the antagonism of muscles and the transmission of force from the muscles to the environment. This study examined the ontogenetic effects of body size on force production by an organism supported with a hydrostatic skeleton. The earthworm Lumbricus terrestris burrows by forcefully enlarging crevices in the soil. I built a force-measuring apparatus that measured the radial forces as earthworms of different sizes crawled through and enlarged pre-formed soil burrows. I also built an apparatus that measured the radial and axial forces as earthworms of different sizes attempted to elongate a dead-end burrow. Earthworms ranging in body mass m(b) from hatchlings (0.012 g) to adults (8.9 g) exerted maximum forces (F, in N) during active radial expansion of their burrows (F=0.32 m(b)(0.43)) and comparable forces during axial elongation of the burrow (F=0.26 m(b)(0.47)). Both these forces were almost an order of magnitude greater than the radial anchoring forces during normal peristalsis within burrows (F=0.04 m(b)(0.45)). All radial and axial forces scaled as body mass raised to the 2/5 power rather than to the 2/3 power expected by geometric similarity, indicating that large worms exert greater forces than small worms on an absolute scale, but the difference was less than predicted by scaling considerations. When forces were normalized by body weight, hatchlings could push 500 times their own body weight, while large adults could push only 10 times their own body weight.

Materials with negative compressibilities in one or more dimensions

Rare crystal phases that expand in one or more dimensions when hydrostatically compressed are identified and shown to have negative Poisson's ratios. Some of these crystals (i) decrease volume and expand in two dimensions when stretched in a particular direction and (ii) increase surface area when hydrostatically compressed. Possible mechanisms for achieving such negative linear and area compressibilities are described for single crystals and composites, and sensor applications are proposed. Materials with these properties may be used to fabricate porous solids that either expand in all directions when hydrostatically compressed with a penetrating fluid or behave as if they are incompressible.