On caudal prehensility and phylogenetic constraint in lizards: The influence of ancestral anatomy on function in Corucia and Furcifer

We examined caudal anatomy in two species of prehensile‐tailed lizards, Furcifer pardalis and Corucia zebrata. Although both species use their tails to grasp, each relies on a strikingly different anatomy to do so. The underlying anatomies appear to reflect phylogenetic constraints on the consequent functional mechanisms. Caudal autotomy is presumably the ancestral condition for lizards and is allowed by a complex system of interdigitating muscle segments. The immediate ancestor of chameleons was nonautotomous and did not possess this specialized anatomy; consequently, the derived arrangement in the chameleon tail is unique among lizards. The limb functions as an articulated linkage system with long tendinous bands originating from longitudinal muscles to directly manipulate vertebrae. Corucia is incapable of autotomy, but it is immediately derived from autotomous ancestors. As such, it has evolved a biomechanical system for prehension quite different from that of chameleons. The caudal anatomy in Corucia is very similar to that of lizards with autotomous tails, yet distinct differences in the ancestral pattern and its relationship to the subdermal tunic are derived. Instead of the functional unit being individual autotomy segments, the interdigitating prongs of muscle have become fused with an emphasis on longitudinal stacks of muscular cones. The muscles originate from the vertebral column and a subdermal collagenous tunic and insert within the adjacent cone. However, there is remarkably little direct connection with the bones. The muscles have origins more associated with the tunic and muscular septa. Like the axial musculature of some fish, the tail of Corucia utilizes a design in which these collagenous elements serve as an integral skeletal component. This arrangement provides Corucia with an elegantly designed system capable of a remarkable variety of bending movements not evident in chameleon tails. J. Morphol. 239:143–155, 1999. © 1999 Wiley‐Liss, Inc.     

Gripping during climbing of arboreal snakes may be safe but not economical

On the steep surfaces that are common in arboreal environments, many types of animals without claws or adhesive structures must use muscular force to generate sufficient normal force to prevent slipping and climb successfully. Unlike many limbed arboreal animals that have discrete gripping regions on the feet, the elongate bodies of snakes allow for considerable modulation of both the size and orientation of the gripping region. We quantified the gripping forces of snakes climbing a vertical cylinder to determine the extent to which their force production favoured economy or safety. Our sample included four boid species and one colubrid. Nearly all of the gripping forces that we observed for each snake exceeded our estimate of the minimum required, and snakes commonly produced more than three times the normal force required to support their body weight. This suggests that a large safety factor to avoid slipping and falling is more important than locomotor economy.     

Role of the prehensile tail during ateline locomotion: experimental and osteological evidence

The dynamic role of the prehensile tail of atelines during locomotion is poorly understood. While some have viewed the tail of Ateles simply as a safety mechanism, others have suggested that the prehensile tail plays an active role by adjusting pendulum length or controlling lateral sway during bimanual suspensory locomotion. This study examines the bony and muscular anatomy of the prehensile tail as well as the kinematics of tail use during tail-assisted brachiation in two primates, Ateles and Lagothrix. These two platyrrhines differ in anatomy and in the frequency and kinematics of suspensory locomotion. Lagothrix is stockier, has shorter forelimbs, and spends more time traveling quadrupedally and less time using bimanual suspensory locomotion than does Ateles. In addition, previous studies showed that Ateles exhibits greater hyperextension of the tail, uses its tail to grip only on alternate handholds, and has a larger abductor caudae medialis muscle compared to Lagothrix. In order to investigate the relationship between anatomy and behavior concerning the prehensile tail, osteological data and kinematic data were collected for Ateles fusciceps and Lagothrix lagothricha. The results demonstrate that Ateles has more numerous and smaller caudal elements, particularly in the proximal tail region. In addition, transverse processes are relatively wider, and sacro-caudal articulation is more acute in Ateles compared to Lagothrix. These differences reflect the larger abductor muscle mass and greater hyperextension in Ateles. In addition, Ateles shows fewer side-to-side movements during tail-assisted brachiation than does Lagothrix. These data support the notion that the prehensile tail represents a critical dynamic element in the tail-assisted brachiation of Ateles, and may be useful in developing inferences concerning behavior in fossil primates.     

Constraints on muscle performance provide a novel explanation for the scaling of posture in terrestrial animals

Larger terrestrial animals tend to support their weight with more upright limbs. This makes structural sense, reducing the loading on muscles and bones, which is disproportionately challenging in larger animals. However, it does not account for why smaller animals are more crouched; instead, they could enjoy relatively more slender supporting structures or higher safety factors. Here, an alternative account for the scaling of posture is proposed, with close parallels to the scaling of jump performance. If the costs of locomotion are related to the volume of active muscle, and the active muscle volume required depends on both the work and the power demanded during the push-off phase of each step (not just the net positive work), then the disproportional scaling of requirements for work and push-off power are revealing. Larger animals require relatively greater active muscle volumes for dynamically similar gaits (e.g. top walking speed)—which may present an ultimate constraint to the size of running animals. Further, just as for jumping, animals with shorter legs and briefer push-off periods are challenged to provide the power (not the work) required for push-off. This can be ameliorated by having relatively long push-off periods, potentially accounting for the crouched stance of small animals.     

Positional behavior in the Hominoidea

Quantitative studies on the positional behavior of members of the Hominoidea are compared in order (1) to identify consistencies across the superfamily, (2) to contrast ape positional behavior with that of Old World monkeys (forest-livingPapio anubis were chosen for study to reduce body size effects), and (3) to identify distinctive behaviors in each of the ape taxa. Differences in the way behaviors were sampled in the various studies necessitated considering posture and locomotion separately. Unimanual arm-hanging and vertical climbing were the most distinctive shared postural and locomotor modes among the apes (the gorilla excepted), constituting ≥5.0% and ≥4.9% of all behavior in each species. Arm-hanging and brachiation (sensu stricto) frequencies were the highest by far in hylobatids. Hand-foot hanging, bipedal posture, and clambering, an orthograde suspensory locomotion assisted by the hindlimbs, were more common in orangutans than in any other hominoid. Sitting and walking were observed in the highest frequencies in the African apes but were no more common than in the baboon. Relatively high frequencies of brachiation (sensu stricto) were reported for all apes except chimpanzees and gorillas. Brachiation and arm-hanging were kinematically different in apes and baboons, involving complete humeral abduction only in the former, whereas vertical climbing appeared to be kinematically similar in apes and baboons. It is concluded that the morphological specializations of the apes may be adaptations to (1) the unique physical demands of arm-hanging and (2) less kinematically distinct, but still quantitatively significant, frequencies of vertical climbing.     

Volumetric imaging of shark tail hydrodynamics reveals a three-dimensional dual-ring vortex wake structure

Understanding how moving organisms generate locomotor forces is fundamental to the analysis of aerodynamic and hydrodynamic flow patterns that are generated during body and appendage oscillation. In the past, this has been accomplished using two-dimensional planar techniques that require reconstruction of three-dimensional flow patterns. We have applied a new, fully three-dimensional, volumetric imaging technique that allows instantaneous capture of wake flow patterns, to a classic problem in functional vertebrate biology: the function of the asymmetrical (heterocercal) tail of swimming sharks to capture the vorticity field within the volume swept by the tail. These data were used to test a previous three-dimensional reconstruction of the shark vortex wake estimated from two-dimensional flow analyses, and show that the volumetric approach reveals a different vortex wake not previously reconstructed from two-dimensional slices. The hydrodynamic wake consists of one set of dual-linked vortex rings produced per half tail beat. In addition, we use a simple passive shark-tail model under robotic control to show that the three-dimensional wake flows of the robotic tail differ from the active tail motion of a live shark, suggesting that active control of kinematics and tail stiffness plays a substantial role in the production of wake vortical patterns.     

Positional behavior of female bornean orangutans (Pongo pygmaeus)

Observational data were collected on the positional behavior of habituated adult female orangutans in the rain forest of the Kutai National Park, East Kalimantan, Indonesia. Results revealed the following about locomotion during travel: movement was concentrated in the understory and lower main canopy; and brachiation (without grasping by the feet) accounted for 11% of travel distance, quadrupedalism for 12%, vertical climbing for 18%, tree-swaying for 7%, and clambering for 51%. In climbing and clambering, the animal was orthograde and employed forelimb suspension with a mixture of hindlimb suspension and support. Thus suspension by the forelimbs occurred in at least 80% of travel. Locomotion in feeding trees resembled that during travel but with more climbing and less brachiation. Feeding was distributed more evenly among canopy levels than was travel, and use of postures (by time) included sitting 50%, suspension with the body vertical 11%, and suspension by hand and foot with the body horizontal 36%. The traditional explanation of the evolution of the distinctive hominoid postcranium stresses brachiation. More recently it has been proposed that climbing, broadly defined and partly equivalent to clambering in this study, is the most significant behavior selecting for morphology. The biomechanical similarity of brachiation and the orthograde clambering of orangutans precludes the present results from resolving the issue for the evolution of Pongo. The orangutan is by far the largest mammal that travels in forest canopy, and a consideration of the ways that its positional behavior solves problems posed by habitat structure, particularly the tapering of branches and gaps between trees, indicates that suspensory capacities have been essential in permitting the evolution and maintenance of its great body size.     

Mechanics of posture and gait of some large dinosaurs

Dimensions of dinosaur bones and of models of dinosaurs have been used as the basis for calculations designed to throw light on the posture and gaits of dinosaurs.
Estimates of the masses of some dinosaurs, obtained from the volumes of models, are compared with previous estimates. The positions of dinosaurs' centres of mass, derived from models, show that some large quadrupedal dinosaurs supported most of their weight on their hind legs and were probably capable of rearing up on their hind legs.
Distributions of bending moments along the backs of large dinosaurs are derived from measurements on models. The tensions required in epaxial muscles to enable Diplodocus to stand are calculated. It is likely that the long neck of this dinosaur was supported by some structure running through the notches in the neural spines of its cervical and dorsal vertebrae. The nature of this hypothetical structure is discussed.
An attempt is made to reconstruct the walking gait of sauropod dinosaurs, from the pattern of footprints in fossil tracks.
The dimensions of dinosaur leg bones are compared to predictions for mammals of equal body mass, obtained by extrapolation of allometric equations. Their dimensions are also used to calculate a quantity which is used as an indicator of strength in bending. Comparisons with values for modern animals lead to speculations about the athletic performance of dinosaurs.
Estimates of pressures exerted on the ground by the feet of dinosaurs are used in a discussion of the ability of dinosaurs to walk over soft ground.     

Functional aspects of strepsirrhine lumbar vertebral bodies and spinous processes

The relationship between form and function in the lumbar vertebral column has been well documented among platyrrhines and especially catarrhines, while functional studies of postcranial morphology among strepsirrhines have concentrated predominantly on the limbs. This morphometric study investigates biomechanically relevant attributes of the lumbar vertebral morphology of 20 species of extant strepsirrhines. With this extensive sample, our goal is to address the influence of positional behavior on lumbar vertebral form while also assessing the effects of body size and phylogenetic history. The results reveal distinctions in lumbar vertebral morphology among strepsirrhines in functional association with their habitual postures and primary locomotor behaviors. In general, strepsirrhines that emphasize pronograde posture and quadrupedal locomotion combined with leaping (from a pronograde position) have the relatively longest lumbar regions and lumbar vertebral bodies, features promoting sagittal spinal flexibility. Indrids and galagonids that rely primarily on vertical clinging and leaping with orthograde posture share a relatively short (i.e., stable and resistant to bending) lumbar region, although the length of individual lumbar vertebral bodies varies phylogenetically and possibly allometrically. The other two vertical clingers and leapers, Hapalemur and Lepilemur, more closely resemble the pronograde, quadrupedal taxa. The specialized, suspensory lorids have relatively short lumbar regions as well, but the lengths of their lumbar regions are influenced by body size, and Arctocebus has dramatically longer vertebral bodies than do the other lorids. Lumbar morphology among galagonids appears to reflect a strong phylogenetic signal superimposed on a functional one. In general, relative length of the spinous processes follows a positively allometric trend, although lorids (especially the larger-bodied forms) have relatively short spinous processes for their body size, in accordance with their positional repertoire. The results of the study broaden our understanding of postcranial adaptation in primates, while providing an extensive comparative database for interpreting vertebral morphology in fossil primates.