Functional specialization and ontogenetic scaling of limb anatomy in Alligator mississippiensis

Musculoskeletal Geometry, Muscle Architecture and Functional Specialisations of the Mouse Hindlimb

Mice are one of the most commonly used laboratory animals, with an extensive array of disease models in existence, including for many neuromuscular diseases. The hindlimb is of particular interest due to several close muscle analogues/homologues to humans and other species. A detailed anatomical study describing the adult morphology is lacking, however. This study describes in detail the musculoskeletal geometry and skeletal muscle architecture of the mouse hindlimb and pelvis, determining the extent to which the muscles are adapted for their function, as inferred from their architecture. Using I₂KI enhanced microCT scanning and digital segmentation, it was possible to identify 39 distinct muscles of the hindlimb and pelvis belonging to nine functional groups. The architecture of each of these muscles was determined through microdissections, revealing strong architectural specialisations between the functional groups. The hip extensors and hip adductors showed significantly stronger adaptations towards high contraction velocities and joint control relative to the distal functional groups, which exhibited larger physiological cross sectional areas and longer tendons, adaptations for high force output and elastic energy savings. These results suggest that a proximo-distal gradient in muscle architecture exists in the mouse hindlimb. Such a gradient has been purported to function in aiding locomotor stability and efficiency. The data presented here will be especially valuable to any research with a focus on the architecture or gross anatomy of the mouse hindlimb and pelvis musculature, but also of use to anyone interested in the functional significance of muscle design in relation to quadrupedal locomotion.

How to build your dragon: scaling of muscle architecture from the world's smallest to the world's largest monitor lizard

Background

The functional design of skeletal muscles is shaped by conflicting selective pressures between support and propulsion, which becomes even more important as animals get larger. If larger animals were geometrically scaled up versions of smaller animals, increases in body size would cause an increase in musculoskeletal stress, a result of the greater scaling of mass in comparison to area. In large animals these stresses would come dangerously close to points of failure. By examining the architecture of 22 hindlimb muscles in 27 individuals from 9 species of varanid lizards ranging from the tiny 7.6 g Varanus brevicauda to the giant 40 kg Varanus komodoensis, we present a comprehensive dataset on the scaling of musculoskeletal architecture in monitor lizards (varanids), providing information about the phylogenetic constraints and adaptations of locomotor muscles in sprawling tetrapods.

Results

Scaling results for muscle mass, pennation and physiological cross-sectional area (PCSA), all suggest that larger varanids increase the relative force-generating capacity of femur adductors, knee flexors and ankle plantarflexors, with scaling exponents greater than geometric similarity predicts. Thus varanids mitigate the size-related increases in stress by increasing muscle mass and PCSA rather than adopting a more upright posture with size as is shown in other animals. As well as the scaling effects of muscle properties with body mass, the variation in muscle architecture with changes in hindlimb posture were also prominent. Within varanids, posture varies with habitat preference. Climbing lizards display a sprawling posture while terrestrial lizards display a more upright posture. Sprawling species required larger PCSAs and muscle masses in femur retractors, knee flexors, and ankle plantarflexors in order to support the body.

Conclusions

Both size and posture-related muscle changes all suggest an increased role in support over propulsion, leading to a decrease in locomotor performance which has previously been shown with increases in size. These estimates suggest the giant Pleistocene varanid lizard (Varanus megalania priscus) would likely not have been able to outrun early humans with which it co-habitated the Australian landmass with.

Electronic supplementary material

The online version of this article (doi:10.1186/s12983-016-0141-5) contains supplementary material, which is available to authorized users.

Differences in hindlimb morphology of ducks and chickens: effects of domestication and selection

Background

Poultry account for the most numerous species farmed for meat and have been subject to intense selection over approximately 60 generations. To assess morphological changes which have occurred in the avian leg due to selection for rapid growth and high meat yields, divergent lines of chicken (Gallus gallus) and duck (Anas platyrhynchos) were studied between 3 and 7 weeks of age. For each line, femoral and tibiotarsal morphology was recorded using computed tomography scanning and tibiotarsal bone quality measures (stiffness, bending stress and porosity) were assessed.

Results

In chicken and duck, divergence in hindlimb morphology has occurred in the commercial meat lines compared to their lighter conspecifics. As expected, the differences were largest between species. Leg development nears completion much earlier in ducks than in chickens. Duck tibiotarsi showed a large degree of lateral curvature, which is expected to affect foot position during swimming and walking, and thus to influence gait. All lines have adapted their tibiotarsal morphology to suit the loading forces they experience; however bone quality was found to be poorer in chickens.

Conclusions

We demonstrate that intensive selection for growth rate in both chickens and ducks has resulted in leg morphology changes, which are likely to influence gait. Ducks represent an interesting compromise of adaptation for efficient locomotion in two media—on land and in water. Some aspects of bone morphology in the duck, such as lateral curvature of the tibiotarsus, may result from adaptation to swimming, which potentially imposes limitations on terrestrial locomotion.

Electronic supplementary material

The online version of this article (doi:10.1186/s12711-015-0166-9) contains supplementary material, which is available to authorized users.

Ontogenetic scaling patterns and functional anatomy of the pelvic limb musculature in emus (Dromaius novaehollandiae)

Emus (Dromaius novaehollandiae) are exclusively terrestrial, bipedal and cursorial ratites with some similar biomechanical characteristics to humans. Their growth rates are impressive, as their body mass increases eighty-fold from hatching to adulthood whilst maintaining the same mode of locomotion throughout life. These ontogenetic characteristics stimulate biomechanical questions about the strategies that allow emus to cope with their rapid growth and locomotion, which can be partly addressed via scaling (allometric) analysis of morphology. In this study we have collected pelvic limb anatomical data (muscle architecture, tendon length, tendon mass and bone lengths) and calculated muscle physiological cross sectional area (PCSA) and average tendon cross sectional area from emus across three ontogenetic stages (n = 17, body masses from 3.6 to 42 kg). The data were analysed by reduced major axis regression to determine how these biomechanically relevant aspects of morphology scaled with body mass. Muscle mass and PCSA showed a marked trend towards positive allometry (26 and 27 out of 34 muscles respectively) and fascicle length showed a more mixed scaling pattern. The long tendons of the main digital flexors scaled with positive allometry for all characteristics whilst other tendons demonstrated a less clear scaling pattern. Finally, the two longer bones of the limb (tibiotarsus and tarsometatarsus) also exhibited positive allometry for length, and two others (femur and first phalanx of digit III) had trends towards isometry. These results indicate that emus experience a relative increase in their muscle force-generating capacities, as well as potentially increasing the force-sustaining capacities of their tendons, as they grow. Furthermore, we have clarified anatomical descriptions and provided illustrations of the pelvic limb muscle–tendon units in emus.

Comparative architectural properties of limb muscles in Crocodylidae and Alligatoridae and their relevance to divergent use of asymmetrical gaits in extant Crocodylia

Crocodiles and their kin (Crocodylidae) use asymmetrical (bounding and galloping) gaits when moving rapidly. Despite being morphologically and ecologically similar, it seems alligators and their kin (Alligatoridae) do not. To investigate a possible anatomical basis for this apparent major difference in locomotor capabilities, we measured relative masses and internal architecture (fascicle lengths and physiological cross-sectional areas) of muscles of the pectoral and pelvic limbs of 40 individuals from six representative species of Crocodylidae and Alligatoridae. We found that, relative to body mass, Crocodylidae have significantly longer muscle fascicles (increased working range), particularly in the pectoral limb, and generally smaller muscle physiological cross-sectional areas (decreased force-exerting capability) than Alligatoridae. We therefore hypothesise that the ability of some crocodylians to use asymmetrical gaits may be limited more by the ability to make large, rapid limb motions (especially in the pectoral limb) than the ability to exert large limb forces. Furthermore, analysis of scaling patterns in muscle properties shows that limb anatomy in the two clades becomes more divergent during ontogeny. Limb muscle masses, fascicle lengths and physiological cross-sectional areas scale with significantly larger coefficients in Crocodylidae than Alligatoridae. This combination of factors suggests that inter-clade disparity in maximal limb power is highest in adult animals. Therefore, despite their apparent morphological similarities, both mean values and scaling patterns suggest that considerable diversity exists in the locomotor apparatus of extant Crocodylia.

Scaling the Feeding Mechanism of Captive Alligator mississippiensis from Hatchling to Juvenile

Small changes in size can lead to potential performance consequences and may influence an organism’s ability to utilize resources in its environment. As the American alligator (Alligator mississippiensis) transitions between neonate, juvenile and adult habitats (ontogenetic niche shifts), there are inevitably dynamic changes in their feeding performance. This study sought to investigate the scaling of the feeding mechanism and its performance from hatchling to juvenile size classes in A. mississippiensis. Feeding events were recorded during March 2011 at Rockefeller Wildlife Refuge (Grand Chenier, Louisiana). Thirty-six captive individuals were randomly sampled, ranging from 30.5 cm to 91.5 cm total length, and feeding events were recorded using a high speed camera at a rate of 300 fps. Results indicated that many linear, angular and timing kinematic variables scale allometrically with cranium length; whereas maximum gape velocity and duration of feeding bout do not scale with cranium length and remain constant between these size classes. Although it has been shown that there is an isometric relationship between cranial elements and body size in A. mississippiensis, this relationship is not transferred to linear and timing variables of prey-capture events. These allometric relationships echo other investigations of scaling relationships such as bite-force production and terrestrial locomotion.

Three-dimensional skeletal kinematics of the shoulder girdle and forelimb in walking Alligator

Crocodylians occupy a key phylogenetic position for investigations of archosaur locomotor evolution. Compared to the well-studied hindlimb, relatively little is known about the skeletal movements and mechanics of the forelimb. In this study, we employed manual markerless XROMM (X-ray Reconstruction Of Moving Morphology) to measure detailed 3-D kinematics of the shoulder girdle and forelimb bones of American alligators (Alligator mississippiensis) walking on a treadmill. Digital models of the interclavicle, scapulocoracoid, humerus, radius and ulna were created using a 3-D laser scanner. Models were articulated and aligned to simultaneously recorded frames of fluoroscopic and standard light video to reconstruct and measure joint motion. Joint coordinate systems were established for the coracosternal, glenohumeral and elbow joints. Our analysis revealed that the limb joints only account for about half of fore/aft limb excursion; the remaining excursion results from shoulder girdle movements and lateral bending of the vertebral column. Considerable motion of each scapulocoracoid relative to the vertebral column is consistent with coracosternal mobility. The hemisellar design of the glenohumeral joint permits some additional translation, or sliding in the fore-aft plane, but this movement does not have much of an effect on the distal excursion of the bone.

Mechanical and energetic scaling relationships of running gait through ontogeny in the ostrich (Struthio camelus)

It is unclear whether small animals, with their high stride frequency and crouched posture, or large animals, with more tendinous limbs, are more reliant on storage and return of elastic energy during locomotion. The ostrich has a limb structure that appears to be adapted for high-speed running with long tendons and short muscle fibres. Here we investigate biomechanics of ostrich gait through growth and, with consideration of anatomical data, identify scaling relationships with increasing body size, relating to forces acting on the musculoskeletal structures, effective mechanical advantage (EMA) and mechanical work. Kinematic and kinetic data were collected through growth from running ostriches. Joint moments scaled in a similar way to the pelvic limb segments as a result of consistent posture through growth, such that EMA was independent of body mass. Because no postural change was observed, relative loads applied to musculoskeletal tissues would be predicted to increase during growth, with greater muscle, and hence tendon, load allowing increased potential for elastic energy storage with increasing size. Mass-specific mechanical work per unit distance was independent of body mass, resulting in a small but significant increase in the contribution of elastic energy storage to locomotor economy in larger ostriches.

Computational modelling of locomotor muscle moment arms in the basal dinosaur Lesothosaurus diagnosticus: assessing convergence between birds and basal ornithischians

Ornithischia (the 'bird-hipped' dinosaurs) encompasses bipedal, facultative quadrupedal and quadrupedal taxa. Primitive ornithischians were small bipeds, but large body size and obligate quadrupedality evolved independently in all major ornithischian lineages. Numerous pelvic and hind limb features distinguish ornithischians from the majority of other non-avian dinosaurs. However, some of these features, notably a retroverted pubis and elongate iliac preacetabular process, appeared convergently in maniraptoran theropods, and were inherited by their avian descendants. During maniraptoran/avian evolution these pelvic modifications led to significant changes in the functions of associated muscles, involving alterations to the moment arms and the activation patterns of pelvic musculature. However, the functions of these features in ornithischians and their influence on locomotion have not been tested and remain poorly understood. Here, we provide quantitative tests of bipedal ornithischian muscle function using computational modelling to estimate 3D hind limb moment arms for the most complete basal ornithischian, Lesothosaurus diagnosticus. This approach enables sensitivity analyses to be carried out to explore the effects of uncertainties in muscle reconstructions of extinct taxa, and allows direct comparisons to be made with similarly constructed models of other bipedal dinosaurs. This analysis supports some previously proposed qualitative inferences of muscle function in basal ornithischians. However, more importantly, this work highlights ambiguities in the roles of certain muscles, notably those inserting close to the hip joint. Comparative analysis reveals that moment arm polarities and magnitudes in Lesothosaurus, basal tetanuran theropods and the extant ostrich are generally similar. However, several key differences are identified, most significantly in comparisons between the moment arms of muscles associated with convergent osteological features in ornithischians and birds. Craniad migration of the iliofemoralis group muscles in birds correlates with increased leverage and use of medial femoral rotation to counter stance phase adduction moments at the hip. In Lesothosaurus the iliofemoralis group maintains significantly higher moment arms for abduction, consistent with the hip abduction mode of lateral limb support hypothesized for basal dinosaurs. Sensitivity analysis highlights ambiguity in the role of musculature associated with the retroverted pubis (puboischiofemoralis externus group) in ornithischians. However, it seems likely that this musculature may have predominantly functioned similarly to homologous muscles in extant birds, activating during the swing phase to adduct the lower limb through lateral rotation of the femur. Overall the results suggest that locomotor muscle leverage in Lesothosaurus (and by inference basal ornithischians in general) was more similar to that of other non-avian dinosaurs than the ostrich, representing what was probably the basal dinosaur condition. This work thereby contradicts previous hypotheses of ornithischian-bird functional convergence.

Disparity and convergence in bipedal archosaur locomotion

This study aims to investigate functional disparity in the locomotor apparatus of bipedal archosaurs. We use reconstructions of hindlimb myology of extant and extinct archosaurs to generate musculoskeletal biomechanical models to test hypothesized convergence between bipedal crocodile-line archosaurs and dinosaurs. Quantitative comparison of muscle leverage supports the inference that bipedal crocodile-line archosaurs and non-avian theropods had highly convergent hindlimb myology, suggesting similar muscular mechanics and neuromuscular control of locomotion. While these groups independently evolved similar musculoskeletal solutions to the challenges of parasagittally erect bipedalism, differences also clearly exist, particularly the distinct hip and crurotarsal ankle morphology characteristic of many pseudosuchian archosaurs. Furthermore, comparative analyses of muscle design in extant archosaurs reveal that muscular parameters such as size and architecture are more highly adapted or optimized for habitual locomotion than moment arms. The importance of these aspects of muscle design, which are not directly retrievable from fossils, warns against over-extrapolating the functional significance of anatomical convergences. Nevertheless, links identified between posture, muscle moments and neural control in archosaur locomotion suggest that functional interpretations of osteological changes in limb anatomy traditionally linked to postural evolution in Late Triassic archosaurs could be constrained through musculoskeletal modelling.

A computational analysis of limb and body dimensions in Tyrannosaurus rex with implications for locomotion, ontogeny, and growth

The large theropod dinosaur Tyrannosaurus rex underwent remarkable changes during its growth from <10 kg hatchlings to >6000 kg adults in <20 years. These changes raise fascinating questions about the morphological transformations involved, peak growth rates, and scaling of limb muscle sizes as well as the body's centre of mass that could have influenced ontogenetic changes of locomotion in T. rex. Here we address these questions using three-dimensionally scanned computer models of four large, well-preserved fossil specimens as well as a putative juvenile individual. Furthermore we quantify the variations of estimated body mass, centre of mass and segment dimensions, to characterize inaccuracies in our reconstructions. These inaccuracies include not only subjectivity but also incomplete preservation and inconsistent articulations of museum skeletons. Although those problems cause ambiguity, we conclude that adult T. rex had body masses around 6000-8000 kg, with the largest known specimen ("Sue") perhaps ∼9500 kg. Our results show that during T. rex ontogeny, the torso became longer and heavier whereas the limbs became proportionately shorter and lighter. Our estimates of peak growth rates are about twice as rapid as previous ones but generally support previous methods, despite biases caused by the usage of scale models and equations that underestimate body masses. We tentatively infer that the hindlimb extensor muscles masses, including the large tail muscle M. caudofemoralis longus, may have decreased in their relative size as the centre of mass shifted craniodorsally during T. rex ontogeny. Such ontogenetic changes would have worsened any relative or absolute decline of maximal locomotor performance. Regardless, T. rex probably had hip and thigh muscles relatively larger than any extant animal's. Overall, the limb "antigravity" muscles may have been as large as or even larger than those of ratite birds, which themselves have the most muscular limbs of any living animal.

When hatchlings outperform adults: locomotor development in Australian brush turkeys (Alectura lathami, Galliformes)

Within Galliformes, megapods (brush turkey, malleefowl, scrubfowl) exhibit unique forms of parental care and growth. Hatchlings receive no post-hatching parental care and exhibit the most exaggerated precocial development of all extant birds, hatching with fully developed, flight-capable forelimbs. Rather than flying up to safety, young birds preferentially employ wing-assisted incline running. Newly hatched Australian brush turkeys (Alectura lathami) are extraordinarily proficient at negotiating all textured inclined surfaces and can flap-walk up inclines exceeding the vertical. Yet, as brush turkeys grow, their forelimb-dependent locomotor performance declines. In an attempt to elucidate how hatchlings perform so well, we analysed hindlimb forces and forelimb kinematics. We measured ground reaction forces (GRFs) for animals spanning the entire growth range (110-2000 g) as they ascended a variably positioned inclined ramp that housed a forceplate. These data are compared with a similar dataset for a chukar partridge (Alectoris chukar) that exhibit a growth strategy typical of most other Galliformes and that demonstrate improved incline performance with increasing age. The brush turkeys' ontogenetic decline in incline running performance is accompanied by loss of traction at steep angles, reduced GRFs and increased wing-loading. We hypothesize that Australian brush turkeys, in contrast to other Galliformes, develop from forelimb-dominated young that exploit a variable terrain (e.g. mound nests, boulders, embankments, cliffs, bushes and trees) into hindlimb-dominated adults dependent on size and running speed to avoid predation.

The effects of selective breeding on the architectural properties of the pelvic limb in broiler chickens: a comparative study across modern and ancestral populations

Intensive artificial selection has led to the production of the modern broiler chicken, which over the last few decades has undergone a dramatic increase in growth rate and noticeable changes in body conformation. Unfortunately, this has been associated with musculoskeletal abnormalities which have altered the walking ability of these birds, raising obvious welfare concerns, as well as causing economic losses. Here we present a comparative study of ancestral and derived muscle anatomy in chickens to begin to tease apart how evolutionary alterations of muscle form in chickens have influenced their locomotor function and perhaps contributed to lameness. We measured the muscle architectural properties of the right pelvic limb in 50 birds, including the Giant Junglefowl, a commercial strain broiler and four pureline commercial broiler breeder lines (from which the broiler populations are derived) to identify which features of the broiler's architectural design have diverged the most from the ancestral condition. We report a decline in pelvic limb muscle mass in the commercial line birds that may compromise their locomotor abilities because they carry a larger body mass. This greater demand on the pelvic limb muscles has mostly led to changes in support at the hip joint, revealing significantly larger abductors and additionally much larger medial rotators in the broiler population. Differences were seen within the commercial line bird populations, which are likely attributed to different selection pressures and may reflect differences in the walking ability of these birds. In addition, Junglefowl seem to have both greater force-generating capabilities and longer, presumably faster contracting muscles, indicative of superior musculoskeletal/locomotor function. We have provided baseline data for generating hypotheses to investigate in greater depth the specific biomechanical constraints that compromise the modern broiler's walking ability and propose that these factors should be considered in the selection for musculoskeletal health in the chickens of the future. Our new anatomical data for a wide range of domestic and wild-type chickens is useful in a comparative context and for deeper functional analysis including computer modelling/simulation of limb mechanics.