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Session Schedule & Abstracts

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Sunday 3rd July, 2016

Locomotion 7

Room: Salon F   2:30 pm–4:00 pm

Moderator(s): Abourachid A, Allen V
LOC7-1  2:30 pm  Does exaggerated morphology constrain locomotor performance in the peacock, Pavo cristatus (Aves: Galliformes). Thavarajah N K*, Manchester University; Codd J R, Manchester University
Abstract: The evolution of exaggerated sexually selected morphology is paradoxical, as although these traits are thought to enhance reproductive success, they are widely presumed to be costly to possess. The costs associated with sexually selected traits are suggested to handicap their possessors, causing a negative impact on survival and acting as an honest signal of quality to potential mates. Exaggerated traits such as the peacock train, represent an additional load that must be carried by the animal and therefore may influence locomotor performance, which in turn could influence predation risk. To examine the effect of the train morphology on the energetics and kinematics of locomotion, we conducted respirometry experiments on peacocks with and without their trains and on peahens, which do not produce a train. In addition, we performed jumping experiments on peacocks with and without trains to assess the traits effect on initial take-off performance. We demonstrate that peacocks with fully-grown trains had a lower absolute and mass specific metabolic cost of locomotion when compared to peacocks that had shed their trains. Furthermore, when controlling for size, peacocks had a lower mass specific metabolic rate during walking than peahens. Peacocks also had improved jump performance with trains than without. Our findings indicate that the sexually selected train does not compromise locomotor performance. We suggest that adaptations to mitigate any costs associated with exaggerated morphology are central in the evolution of sexually selected traits.

LOC7-2  2:45 pm  Fifty ways to measure a moment arm: cadaveric analysis of emu toe joints using XROMM. Sustaita D*, Brown University; Roberts TJ, Brown University; Gatesy SM, Brown University
Abstract: The torque generated by a muscle is dependent upon the muscle’s force-generating capacity and the leverage with which it acts about a joint – its moment arm. The moment arms of muscles spanning proximal hindlimb joints of terrestrial birds have been studied in detail. However, those of the toes, which interact directly with the substrate, have received less attention. Three prevailing approaches have been used to obtain measurements of moment arms (after Tsaopoulos et al. 2006; Clinical Biomechanics 21: 657–667): tendon excursion (the distance a tendon translates as a function of joint angle), 2D or 3D geometric measurement (the linear distance between the tendon and the axis of joint rotation), and by direct load (computed from static equilibrium equations based on variable known in- and out-forces). We used XROMM to obtain moment arm measurements of the deep digital flexor tendons about the tarsometatarso-phalangeal joints of emu feet based on all three methods. Because each of these techniques assumes an axis of joint rotation, we calculated joint kinematics using two different approaches: an axis fit geometrically to the joint condyles, and an axis normal to the motion path of a distal marker. Despite variation within and among techniques, there was broad agreement among methodologies for digit III, which experiences little ab- or adduction during flexion and extension. Although similar to digit III in the overall magnitude of its moment arm, the greater complexity of joint motion in digit IV resulted in greater variation in moment arm length with joint angle, greater discordance among methods, and greater sensitivity to the axis of rotation used. We discuss how differences in the variability of moment arms between middle and side toes, yet similarity in their overall magnitudes, may be mediated by the interaction among the subarticular cartilages, condyles, and flexor tendons, despite observable differences in joint condyle diameters.

LOC7-3  3:00 pm  Patella mechanics in avian terrestrial locomotion . Allen V*, Royal Veterinary College; Kambic R, Harvard University; Gatesy SM, Brown University; Hutchinson JR, Royal Veterinary College
Abstract: Patellar sesamoid bones are found in the knee extensor tendons of a broad range of tetrapod vertebrates, yet their mechanics have only been studied in few of these taxa. Based on ex vivo study, the patellae of humans are thought to increase the leverage of the knee extensor muscles during walking and running (0-20° flexion from vertical), reducing the forces that they otherwise would need to generate. Due to lack of comparative data, it is currently unclear if the patellae of other taxa may perform a similar function. Inspired by this, we used dual x-ray video analysis and biomechanical modelling to estimate the effects of avian patellae on knee extensor leverage during terrestrial locomotion. We considered avian patellae as an ideal starting place for comparative study, for as obligate bipeds, they place similar locomotor demands on their hindlimbs to humans, but their similarly-positioned patellae (when present) are non-homologous with those of humans and other mammals. Our results indicate that avian patellae enhance the output force of the knee extensors muscles to a varying degree during the stance and swing phases of terrestrial locomotion. Because increasing leverage about a joint also decreases the speed of induced rotation, this indicates that avian patellae also variably retard the rate and extent of knee extension during locomotion. We find the patellae increase leverage considerably (~150%) during early-mid stance, when they may assist the knee extensors in controlling active knee flexion under high ground reaction force loads, and in late stance/early swing, when they may instead prevent excessive knee extension as the biarticular knee extensors/hip flexors flex the hip to protract the limb. When rapid knee extension is required in mid-stance and especially early-mid swing, we find that the leverage-enhancing effect of the patella (and so its retarding of extension rate), is minimised. Based on these findings we speculate that, as in humans, avian patellae perform important functions tuning knee extension mechanics during locomotion.

LOC7-4  3:15 pm  The locomotion of the Hoatzin chick. Abourachid A*, Museum National d'Histoire Naturelle CNRS; Herrel A, Museum NAtional d'Histoire Naturelle CNRS; Garcia Amado M A, Instituto Venezolano de Investigaciones Cientificas; Decamps T, Museum National d'Histioire Naturelle CNRS
Abstract: The Hoatzin (Opisthocomus hoatzin) is a strange South-American bird that lives in bushes along the streams of the Amazonian Basin. This bird has a suite of unique features among which features most prominently their ability to digest leaves using foregut fermentation. This unique feature has important consequences on their anatomy: the crop is so large that the place available for the sternal keel is reduced which impacts its flight ability. However, the wings are strong enough to allow locomotion in both adults and the chicks. The juveniles uniquely have two functional claws on the wing. Young reported in 1888 that "as soon as the young escape from the egg, they creep about with the assistance of these hands, stretching out their wings and digging these claws into hooking on whatever the meet". Futhermore, they escape by jumping into the river under the nest, swim back to the vegetation and then climb back to the nest. This behaviour is well known, often reported, but has never been studied since the late 1880's. Here we present novel data obtained in the field in Venezuela on the locomotion of juvenile Hoatzin and discuss how these results may improve our understanding of the evolution of the birds.

LOC7-5  3:30 pm  Rapid growth backfires: Biomechanical simulations of broiler chicken gait reveal the effects of intrinsic pelvic limb muscle weakness on locomotion. Paxton H*, The Royal Veterinary College; Rankin JW, The Royal Veterinary College; Hutchinson JR, The Royal Veterinary College
Abstract: Estimates show that 2.52 million broiler chickens are affected by lameness or poor walking ability (‘leg weakness’) every year. Broilers have a unique anatomy and an extremely rapid growth rate, associated with muscle damage and metabolic stress. In addition, our previous work has shown that maximal isometric stresses of the pelvic limb muscles are low (15-80kNm-2), 5-35% of typical avian/vertebrate values. In broilers, low muscle forces may indicate a greater reliance on passive structures (ligaments and bony processes) during locomotion and may partly explain the high incidence of joint-related pathologies. In this study, we aim to determine the effects that weak muscles (i.e., having low isometric stress) may have on passive tissue mechanical demands in broiler chickens. We developed a 3D musculoskeletal model (male broiler chicken, 42 days old) using anatomical data (muscle-tendon unit lengths and architectural measurements) obtained via dissection and CT scans. Experimental kinematic and kinetic data were then used with the model to generate a number of stance phase simulations and estimate muscle activity, force and passive tissue contributions to the movement. Comparisons between simulations using an upper limit for muscle force (predicted from muscle architecture data) and a lower limit (based on our muscle physiology data) were used to determine how muscle weakness influences passive force mechanical demand. Our results indicate that at extremely low muscle stress values the reliance on passive tissues increases by ~20%. Rapid growth has likely led to changes in pelvic limb muscle, which ultimately means the broiler must rely more on passive structures through locomotion resulting in detrimental effects for broiler chicken leg health.

LOC7-6  3:45 pm  Intraspecific scaling of the minimum metabolic cost of transport in leghorn chickens (Gallus gallus domesticus): links with limb kinematics, morphometrics and posture. Rose K. A.*, University of Manchester; Nudds R. L., University of Manchester; Codd J. R., University of Manchester
Abstract: The minimum metabolic cost of transport (CoTmin; J kg-1 m-1) scales negatively with increasing body mass (Mb-1/3) across species from a wide range of taxa associated with marked differences in body plan. At the intraspecific level, or between closely related species, however, CoTmin does not always scale with Mb. Similarity in physiology, dynamics of movement, skeletal geometry and posture between closely related individuals is thought to be responsible for this phenomenon, despite the fact that energetic, kinematic and morphometric data are rarely collected together. We examined the relationship between these integrated components of locomotion in leghorn chickens (Gallus gallus domesticus) selectively bred for large and bantam (miniature) varieties. Interspecific allometry predicts a CoTmin 16% greater in bantams compared with the larger variety. However, despite 38% and 23% differences in Mb and leg length, respectively, the two varieties shared an identical walking CoTmin, independent of speed and equal to the allometric prediction derived from interspecific data for the larger variety. Furthermore, the two varieties moved with dynamic similarity and shared geometrically similar appendicular and axial skeletons. Hip height, however, did not scale geometrically and the smaller variety had more erect limbs, contrary to interspecific scaling trends. The lower than predicted CoTmin in bantams for their Mb was associated with both the more erect posture and a lower cost per stride (J kg-1 stride-1). Therefore, our findings are consistent with the notion that a more erect limb is associated with a lower CoTmin and with the previous assumption that similarity in skeletal shape, inherently linked to walking dynamics, is associated with similarity in CoTmin.

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